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Basics of Radio Astronomy - Goldstone-Apple Valley Radio Telescope (GAVRT):

This is a JPL Presentation of the basics of radio astronomy. This is an ASP version of the original PDF on-line book.

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Basics of Radio Astronomy
for the
Goldstone-Apple Valley
Radio Telescope

Prepared by
Diane Fisher Miller
Advanced Mission Operations Section
Also available on the Internet at URL
http://www.jpl.nasa.gov/radioastronomy
April 1998
JPL D-13835

Table of Contents:

Preface Faraday Rotation
Introduction Chapter Five
Acknowledgments Doppler Effect
Assumptions and Disclaimers Gravitational Red Shifting
Chapter One Gravitational Lensing
Jansky's Experiment Superluminal Velocities
Reber's Prototype Radio Telescope Occultations
So What is a Radio Telescope? Chapter Six
What's the GAVRT Classifying the Source
Chapter Two Star Sources
Frequency and Wavelength Variable Stars
Inverse-Square Law of Propagation Pulsars
The Electromagnetic Spectrum Our Sun
Wave Polarization Galactic and Extra-Galactic Sources
Chapter Three Planetary Sources and Their Satellites
Thermal Radiation The Jupiter System
Blackbody Characteristics Sources of Interference
Spectral Line Emissions from Atoms and Molecules Chapter Seven
Non-Thermal Mechanisms Earth's Coordinate System
Masers Revolution of Earth
Chapter Four Precession of the Earth Axis
Atmospheric Windows Astronomical Coordinate System
Absorption and Emission Lines Horizon Coordinate System
Reflection Equatorial Coordinate System
Refraction Ecliptic Coordinate System
Superposition Galactic Coordinate System
Phase Chapter Eight
Interference The Universe in Six Steps
Diffraction The Search for Extraterrestrial Intelligence
Scintillation  

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Preface - Back to Table of Contents

In a collaborative effort, the Science and Technology Center (in Apple Valley, California), the
Apple Valley Unified School District, the Jet Propulsion Laboratory, and NASA have converted a
34-meter antenna at NASA's Deep Space Network's Goldstone Complex into a unique interactive
research and teaching instrument available to classrooms throughout the United States, via the
Internet. The Science and Technology Center is a branch of the Lewis Center for Educational
Research.

The Goldstone-Apple Valley Radio Telescope (GAVRT) is located in a remote area of the
Mojave Desert, 40 miles north of Barstow, California. The antenna, identified as DSS-12, is a 34-
meter diameter dish, 11 times the diameter of a ten-foot microwave dish used for satellite television
reception. DSS-12 has been used by NASA to communicate with robotic space probes for
more than thirty years. In 1994, when NASA decided to decommission DSS-12 from its operational
network, a group of professional scientists, educators, engineers, and several community
volunteers envisioned a use for this antenna and began work on what has become the GAVRT
Project.

The GAVRT Project is jointly managed by the Science and Technology Center and the DSN
Science Office, Telecommunications and Mission Operations Directorate, at the Jet Propulsion
Laboratory.

This workbook was developed as part of the training of teachers and volunteers who will be
operating the telescope. The students plan observations and operate the telescope from the Apple
Valley location using Sun workstations. In addition, students and teachers in potentially 10,000
classrooms across the country will be able to register with the center’s Web site and operate the
telescope from their own classrooms.

Introduction - Back to Table of Contents

This module is the first in a sequence to prepare volunteers and teachers at the Science and
Technology Center to operate the Goldstone-Apple Valley Radio Telescope (GAVRT). It covers
the basic science concepts that will not only be used in operating the telescope, but that will make
the experience meaningful and provide a foundation for interpreting results.

Acknowledgements - Back to Table of Contents

Many people contributed to this workbook. The first problem we faced was to decide which of
the overwhelming number of astronomy topics we should cover and at what depth in order to
prepare GAVRT operators for the radio astronomy projects they would likely be performing.
George Stephan generated this initial list of topics, giving us a concrete foundation on which to
begin to build. Thanks to the subject matter experts in radio astronomy, general astronomy, and
physics who patiently reviewed the first several drafts and took time to explain some complex
subjects in plain English for use in this workbook. These kind reviewers are Dr. M.J. Mahoney,
Roger Linfield, David Doody, Robert Troy, and Dr. Kevin Miller (who also loaned the project
several most valuable books from his personal library). Special credit goes to Dr. Steve Levin,
who took responsibility for making sure the topics covered were the right ones and that no known
inaccuracies or ambiguities remained. Other reviewers who contributed suggestions for clarity
and completeness were Ben Toyoshima, Steve Licata, Kevin Williams, and George Stephan.

Assumptions and Disclaimers - Back to Table of Contents

This training module assumes you have an understanding of high-school-level chemistry, physics,
and algebra. It also assumes you have familiarity with or access to other materials on general
astronomy concepts, since the focus here is on those aspects of astronomy that relate most
specifically to radio astronomy.

This workbook does not purport to cover its selected topics in depth, but simply to introduce them
and provide some context within the overall disciplines of astronomy in general and radio astronomy
in particular. It does not cover radio telescope technology, nor details of radio astronomy
data analysis.

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Chapter 1 - Back to Table of Contents

Overview: Discovering an Invisible Universe

Objectives: Upon completion of this chapter, you will be able to describe the general principles
upon which radio telescopes work.

Before 1931, to study astronomy meant to study the objects visible in the night sky. Indeed, most
people probably still think that’s what astronomers do—wait until dark and look at the sky using
their naked eyes, binoculars, and optical telescopes, small and large. Before 1931, we had no
idea that there was any other way to observe the universe beyond our atmosphere.
In 1931, we did know about the electromagnetic spectrum. We knew that visible light included
only a small range of wavelengths and frequencies of energy. We knew about wavelengths
shorter than visible light—Wilhelm Röntgen had built a machine that produced x-rays in 1895.
We knew of a range of wavelengths longer than visible light (infrared), which in some circumstances
is felt as heat. We even knew about radio frequency (RF) radiation, and had been developing
radio, television, and telephone technology since Heinrich Hertz first produced radio waves
of a few centimeters long in 1888. But, in 1931, no one knew that RF radiation is also emitted by
billions of extraterrestrial sources, nor that some of these frequencies pass through Earth’s
atmosphere right into our domain on the ground.

All we needed to detect this radiation was a new kind of “eyes.”

Jansky’s Experiment - Back to Table of Contents

As often happens in science, RF radiation from outer space was first discovered while someone
was looking for something else. Karl G. Jansky (1905-1950) worked as a radio engineer at the
Bell Telephone Laboratories in Holmdel, New Jersey. In 1931, he was assigned to study radio
frequency interference from thunderstorms in order to help Bell design an antenna that would
minimize static when beaming radio-telephone signals across the ocean. He built an awkward
looking contraption that looked more like a wooden merry-go-round than like any modern-day
antenna, much less a radio telescope. It was tuned to respond to radiation at a wavelength of 14.6
meters and rotated in a complete circle on old Ford tires every 20 minutes. The antenna was
connected to a receiver and the antenna’s output was recorded on a strip-chart recorder.


Jansky’s Antenna that First Detected Extraterrestrial RF Radiation

 

He was able to attribute some of the static (a term used by radio engineers for noise produced by
unmodulated RF radiation) to thunderstorms nearby and some of it to thunderstorms farther away,
but some of it he couldn’t place. He called it “ . . . a steady hiss type static of unknown origin.”
As his antenna rotated, he found that the direction from which this unknown static originated
changed gradually, going through almost a complete circle in 24 hours. No astronomer himself, it
took him a while to surmise that the static must be of extraterrestrial origin, since it seemed to be
correlated with the rotation of Earth.

He at first thought the source was the sun. However, he observed that the radiation peaked about
4 minutes earlier each day. He knew that Earth, in one complete orbit around the sun, necessarily
makes one more revolution on its axis with respect to the sun than the approximately 365 revolutions
Earth has made about its own axis. Thus, with respect to the stars, a year is actually one day
longer than the number of sunrises or sunsets observed on Earth. So, the rotation period of Earth
with respect to the stars (known to astronomers as a sidereal day) is about 4 minutes shorter than
a solar day (the rotation period of Earth with respect to the sun). Jansky therefore concluded that
the source of this radiation must be much farther away than the sun. With further investigation,
he identified the source as the Milky Way and, in 1933, published his findings.

Reber’s Prototype Radio Telescope - Back to Table of Contents

Despite the implications of Jansky’s work, both on the design of radio receivers, as well as for
radio astronomy, no one paid much attention at first. Then, in 1937, Grote Reber, another radio
engineer, picked up on Jansky’s discoveries and built the prototype for the modern radio telescope
in his back yard in Wheaton, Illinois. He started out looking for radiation at shorter wavelengths,
thinking these wavelengths would be stronger and easier to detect. He didn’t have much
luck, however, and ended up modifying his antenna to detect radiation at a wavelength of 1.87
meters (about the height of a human), where he found strong emissions along the plane of the
Milky Way.


Reber’s Radio Telescope

Reber continued his investigations during the early 40s, and in 1944 published the first radio
frequency sky maps. Up until the end of World War II, he was the lone radio astronomer in the
world. Meanwhile, British radar operators during the war had detected radio emissions from the
Sun. After the war, radio astronomy developed rapidly, and has become of vital importance in
our observation and study of the universe.

So What’s a Radio Telescope? - Back to Table of Contents

RF waves that can penetrate Earth’s atmosphere range from wavelengths of a few millimeters to
nearly 100 meters. Although these wavelengths have no discernable effect on the human eye or
photographic plates, they do induce a very weak electric current in a conductor such as an antenna.
Most radio telescope antennas are parabolic (dish-shaped) reflectors that can be pointed
toward any part of the sky. They gather up the radiation and reflect it to a central focus, where
the radiation is concentrated. The weak current at the focus can then be amplified by a radio
receiver so it is strong enough to measure and record. See the discussion of Reflection in Chapter
4 for more about RF antennas.

Electronic filters in the receiver can be tuned to amplify one range (or “band”) of frequencies at a
time. Or, using sophisticated data processing techniques, thousands of separate narrow frequency
bands can be detected. Thus, we can find out what frequencies are present in the RF radiation
and what their relative strengths are. As we will see later, the frequencies and their relative
powers and polarization give us many clues about the RF sources we are studying.
The intensity (or strength) of RF energy reaching Earth is small compared with the radiation
received in the visible range. Thus, a radio telescope must have a large “collecting area,” or
antenna, in order to be useful. Using two or more radio telescopes together (called arraying) and
combining the signals they simultaneously receive from the same source allows astronomers to
discern more detail and thus more accurately pinpoint the source of the radiation. This ability
depends on a technique called radio interferometry. When signals from two or more telescopes
are properly combined, the telescopes can effectively act as small pieces of a single huge telescope.
A large array of telescopes designed specifically to operate as an array is the Very Large Array
(VLA) near Socorro, New Mexico. Other radio observatories in geographically distant locations
are designed as Very Long Baseline Interferometric (VLBI) stations and are arrayed in varying
configurations to create very long baseline arrays (VLBA). NASA now has four VLBI tracking
stations to support orbiting satellites that will extend the interferometry baselines beyond the
diameter of Earth.

Since the GAVRT currently operates as a single aperture radio telescope, we will not further
discuss interferometry here.

What’s the GAVRT? - Back to Table of Contents

The technical details about the GAVRT telescope will be presented in the GAVRT system course
in the planned training sequence. However, here’s a thumbnail sketch.
GAVRT is a Cassegrain radio telescope (explained in Chapter 4) located at Goldstone, California,
with an aperture of 34 meters and an hour-angle/declination mounting and tracking system
(explained in Chapter 7). It has S-band and X-band solid-state, low-noise amplifiers and receivers.
Previously part of the National Aeronautics and Space Administration’s (NASA’s ) Deep
Space Network (DSN), and known as Deep Space Station (DSS)-12, or “Echo,” it was originally
built as a 26-meter antenna in 1960 to serve with NASA’s Echo project, an experiment that
transmitted voice communications coast-to-coast by bouncing the signals off the surface of a
passive balloon-type satellite. In 1979, its aperture was enlarged to 34 meters, and the height of
its mounting was increased to accommodate the larger aperture. It has since provided crucial
support to many deep-space missions, including Voyager in the outer solar system, Magellan at
Venus, and others. In 1996, after retiring DSS-12 from the DSN, NASA turned it over to
AVSTC (associated with the Apple Valley, California, School District) to operate as a radio
telescope. AVSTC plans to make the telescope available over the internet to classrooms across
the country for radio astronomy student observations. NASA still retains ownership, however,
and responsibility for maintenance.

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Chapter 2 - Back to Table of Contents

The Properties of Electromagnetic Radiation

Objectives: When you have completed this chapter, you will be able to define the term
“electromagnetic spectrum,” explain the relationship between frequency and
wavelength, and give the relationship between energy received and distance from
the source. You will be able to describe the limits of the “S-band” and “X-band”
of the electromagnetic spectrum. You will be able to describe wave polarization.
What is Electromagnetic Radiation?


Field is a physics term for a region that is under the influence of some force that can act on matter
within that region. For example, the Sun produces a gravitational field that attracts the planets in
the solar system and thus influences their orbits.

Stationary electric charges produce electric fields, whereas moving electric charges produce both
electric and magnetic fields. Regularly repeating changes in these fields produce what we call
electromagnetic radiation. Electromagnetic radiation transports energy from point to point. This
radiation propagates (moves) through space at 299,792 km per second (about 186,000 miles per
second). That is, it travels at the speed of light. Indeed light is just one form of electromagnetic
radiation.

Some other forms of electromagnetic radiation are X-rays, microwaves, infrared radiation, AM
and FM radio waves, and ultraviolet radiation. The properties of electromagnetic radiation
depend strongly on its frequency. Frequency is the rate at which the radiating electromagnetic
field is oscillating. Frequencies of electromagnetic radiation are given in Hertz (Hz), named for
Heinrich Hertz (1857-1894), the first person to generate radio waves. One Hertz is one cycle per
second.

Frequency and Wavelength - Back to Table of Contents

As the radiation propagates at a given frequency, it has an associated wavelength— that is, the
distance between successive crests or successive troughs. Wavelengths are generally given in
meters (or some decimal fraction of a meter) or Angstroms (Å, 10-10 meter).
Since all electromagnetic radiation travels at the same speed (in a vacuum), the number of crests
(or troughs) passing a given point in space in a given unit of time (say, one second), varies with
the wavelength. For example, 10 waves of wavelength 10 meters will pass by a point in the same
length of time it would take 1 wave of wavelength 100 meters. Since all forms of electromagnetic
energy travel at the speed of light, the wavelength equals the speed of light divided by the
frequency of oscillation (moving from crest to crest or trough to trough).

In the drawing below, electromagnetic waves are passing point B, moving to the right at the speed
of light (usually represented as c, and given in km/sec). If we measure to the left of B a distance
D equal to the distance light travels in one second (2.997 x 105 km), we arrive at point A along
the wave train that will just pass point B after a period of 1 second (moving left to right). The
frequency f of the wave train—that is, the number of waves between A and B—times the length
of each, l, equals the distance D traveled in one second.

Since we talk about the frequency of electromagnetic radiation in terms of oscillations per
second and the speed of light in terms of distance traveled per second, we can say

Inverse-Square Law of Propagation - Back to Table of Contents

As electromagnetic radiation leaves its source, it spreads out, traveling in straight lines, as if it
were covering the surface of an ever expanding sphere. This area increases proportionally to the
square of the distance the radiation has traveled. In other words, the area of this expanding
sphere is calculated as 4pR2 , where R is the distance the radiation has travelled, that is, the
radius of the expanding sphere. This relationship is known as the inverse-square law of (electromagnetic)
propagation. It accounts for loss of signal strength over space, called space loss. For
example, Saturn is approximately 10 times farther from the sun than is Earth. (Earth to sun
distance is defined as one astronomical unit, AU). By the time the sun’s radiation reaches Saturn,
it is spread over 100 times the area it covers at one AU. Thus, Saturn receives only 1/100th the
solar energy flux (that is, energy per unit area) that Earth receives.

The inverse-square law is significant to the exploration of the universe. It means that the concentration
of electromagnetic radiation decreases very rapidly with increasing distance from the
emitter. Whether the emitter is a spacecraft with a low-power transmitter, an extremely powerful
star, or a radio galaxy, because of the great distances and the small area that Earth covers on the
huge imaginary sphere formed by the radius of the expanding energy, it will deliver only a small
amount of energy to a detector on Earth.

The Electromagnetic Spectrum - Back to Table of Contents

Light is electromagnetic radiation at those frequencies to which human eyes (and those of most
other sighted species) happen to be sensitive. But the electromagnetic spectrum has no upper or
lower limit of frequencies. It certainly has a much broader range of frequencies than the human
eye can detect. In order of increasing frequency (and decreasing wavelength), the electromagnetic
spectrum includes radio frequency (RF), infrared (IR, meaning “below red”), visible light,
ultraviolet (UV, meaning “above violet”), X-rays, and gamma rays. These designations describe
only different frequencies of the same phenomenon: electromagnetic radiation.
The frequencies shown in the following two diagrams are within range of those generated by
common sources and observable using common detectors. Ranges such as microwaves, infrared,
etc., overlap. They are categorized in spectrum charts by the artificial techniques we use to
produce them.

Electromagnetic radiation with frequencies between about 5 kHz and 300 GHz is referred to as
radio frequency (RF) radiation. Radio frequencies are divided into ranges called “bands,” such as
“S-band,” “X-band,” etc. Radio telescopes can be tuned to listen for frequencies within certain
bands.

The GAVRT can observe S-band and X-band frequencies. Much of radio astronomy involves
studies of radiation well above these frequencies.

Wave Polarization - Back to Table of Contents

If electromagnetic waves meet no barriers as they travel through an idealized empty space, they
travel in straight lines. As mentioned at the beginning of this chapter, stationary electric charges
produce electric fields, and moving electric charges produce magnetic fields. Thus, there are two
components to an electromagnetic wave—the electric field and the magnetic field. In free space,
the directions of the fields are at right angles to the direction of the propagation of the wave.

The drawing below shows part of a wavefront as it would appear to an observer at the point
indicated in the drawing. The wave is moving directly out of the page. One-half a period later,
the observer will see a similar field pattern, except that the directions of both the electric and the
magnetic fields will be reversed.

The magnetic field is called the magnetic vector, and the electric field is called the electric vector.
A vector field has both a magnitude and a direction at any given point in space. The polarization
of electromagnetic waves is defined as the direction of the electric vector. If the electric vector
moves at a constant angle with respect to the horizon, the waves are said to be linearly polarized.
In radio wave transmission, if the polarization is parallel to Earth’s surface, the wave is said to be
horizontally polarized. If the wave is radiated in a vertical plane, it is said to be vertically polarized.
Waves may also be circularly polarized, whereby the angle of the electric (or magnetic)
vector rotates around an (imaginary) line traveling in the direction of the propagation of the wave.
The rotation may be either to the right or left.

Radio frequency radiation from extraterrestrial sources may be linearly or circularly polarized, or
anything in between, or unpolarized. The polarization of the waves gives astronomers additional
information about their source.

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Chapter 3 - Back to Table of Contents

The Mechanisms of Electromagnetic Emissions

Objectives: Upon completion of this chapter, you will be able to describe the difference
between thermal and non-thermal radiation and give some examples of each. You
will be able to distinguish between thermal and non-thermal radiation curves.
You will be able to describe the significance of the 21-cm hydrogen line in radio
astronomy.


If the material in this chapter is unfamiliar to you, do not be discouraged if you don’t understand
everything the first time through. Some of these concepts are a little complicated and few nonscientists
have much awareness of them. However, having some familiarity with them will make
your radio astronomy activities much more interesting and meaningful.

What causes electromagnetic radiation to be emitted at different frequencies? Fortunately for us,
these frequency differences, along with a few other properties we can observe, give us a lot of
information about the source of the radiation, as well as the media through which it has traveled.
Electromagnetic radiation is produced by either thermal mechanisms or non-thermal mechanisms.
Examples of thermal radiation include:

  • Continuous spectrum emissions related to the temperature of the object or
    material.
  • Specific frequency emissions from neutral hydrogen and other atoms and molecules.

Examples of non-thermal mechanisms include:

  • Emissions due to synchrotron radiation.
  • Amplified emissions due to astrophysical masers.

Thermal Radiation - Back to Table of Contents

Did you know that any object that contains any heat energy at all emits radiation? When you’re
camping, if you put a large rock in your campfire for a while, then pull it out, the rock will emit
the energy it has absorbed as radiation, which you can feel as heat if you hold your hand a few
inches away. Physicists would call the rock a “blackbody” because it absorbs all the energy that
reaches it, and then emits the energy at all frequencies (although not equally) at the same rate it
absorbs energy.

All the matter in the known universe behaves this way.

Some astronomical objects emit mostly infrared radiation, others mostly visible light, others
mostly ultraviolet radiation. The single most important property of objects that determines the
radiation they emit is temperature.

In solids, the molecules and atoms are vibrating continuously. In a gas, the molecules are really
zooming around, continuously bumping into each other. Whatever the amount of molecular
motion occurring in matter, the speed is related to the temperature. The hotter the material, the
faster its molecules are vibrating or moving.

Electromagnetic radiation is produced whenever electric charges accelerate—that is, when they
change either the speed or direction of their movement. In a hot object, the molecules are continuously
vibrating (if a solid) or bumping into each other (if a liquid or gas), sending each other
off in different directions and at different speeds. Each of these collisions produces electromagnetic
radiation at frequencies all across the electromagnetic spectrum. However, the amount of
radiation emitted at each frequency (or frequency band) depends on the temperature of the
material producing the radiation.

It turns out that the shorter the wavelength (and higher the frequency), the more energy the
radiation carries. When you are out in the sun on a hot day and your skin starts to feel hot, that
heat is not what you need to worry about if you get sunburned easily. Most of the heat you feel is
the result of infrared radiation striking the surface of your skin. However, it is the higher frequency—
thus higher energy—ultraviolet radiation penetrating the skin’s surface that stimulates
the deeper layers to produce the melanin that gives fair complected folks the nice tan—or bad
sunburn. X-rays, at still higher frequencies, have enough energy to pass right through skin and
other soft tissues. That is how bone and soft tissues of varying densities can be revealed by the xray
imaging techniques used by medicine.|

Any matter that is heated above absolute zero generates electromagnetic energy. The intensity of
the emission and the distribution of frequencies on the electromagnetic spectrum depend upon the
temperature of the emitting matter. In theory, it is possible to detect electromagnetic energy from
any object in the universe. Visible stars radiate a great deal of electromagnetic energy. Much of
that energy has to be in the visible part of the spectrum—otherwise they would not be visible
stars! Part of the energy has to be in the microwave (short wave radio) part of the spectrum, and
that is the part astronomers study using radio telescopes.

Blackbody Characteristics - Back to Table of Contents

Blackbodies thus have three characteristics:

  1. A blackbody with a temperature higher than absolute zero emits some energy at
    all wavelengths.
  2. A blackbody at higher temperature emits more energy at all wavelengths than
    does a cooler one.
  3. The higher the temperature, the shorter the wavelength at which the maximum
    energy is emitted.

To illustrate, at a low temperature setting, a burner on an electric stove emits infrared radiation,
which is transferred to other objects (such as pots and food) as heat. At a higher temperature, it
also emits red light (lower frequency end of visible light range). If the electrical circuit could
deliver enough energy, as the temperature increased further, the burner would turn yellow, or
even blue-white.

The sun and other stars may, for most purposes, be considered blackbodies. So we can estimate
temperatures of these objects based on the frequencies of radiation they emit—in other words,
according to their electromagnetic spectra.

For radiation produced by thermal mechanisms, the following table gives samples of wavelength
ranges, the temperatures of the matter emitting in that range, and some example sources of such
thermal radiation.

The hotter the object, the shorter is the wavelength of the radiation it emits. Actually, at hotter
temperatures, more energy is emitted at all wavelengths. But the peak amount of energy is
radiated at shorter wavelengths for higher temperatures. This relationship is known as Wien’s
Law.

A beam of electromagnetic radiation can be regarded as a stream of tiny packets of energy called
photons. Planck’s Law states that the energy carried by a photon is directly proportional to its
frequency. To arrive at the exact energy value, the frequency is multiplied by Planck’s Constant,
which has been found experimentally to be 6.625 x 10-27 erg sec. (The erg is a unit of energy.)

If we sum up the contributions from all parts of the electromagnetic spectrum, we obtain the total
energy emitted by a blackbody over all wavelengths. That total energy, emitted per second per
square meter by a blackbody at a given temperature is proportional to the fourth power of its
absolute temperature. This relationship is known as the Stefan-Boltzmann Law. If the sun, for
example, were twice as hot as it is and the same size, that is, if its temperature were 11,600 K, it
would radiate 24, or 16, times more energy than it does now.

The flux density of the radiation is defined as the energy received per unit area per unit of frequency
bandwidth. Astronomers also consider the radiation’s brightness, which is a more
mathematically precise calculation of the energy received per unit area, for a particular frequency
bandwidth, and also taking into consideration the angle of incidence on the measuring surface and
the solid angle of sky subtended by the source. The brightness of radiation received (at all
frequencies) is thus related to temperature of the emitting object and the wavelength of the
received radiation.

The variation of brightness with frequency is called the brightness spectrum. The spectral power
is the energy observed per unit of time for a specific frequency bandwidth.

A plot of a brightness spectrum shows the brightness of the radiation received from a source as it
varies by frequency and wavelength. In the plot below, the brightness of blackbodies at various
temperatures is plotted on the vertical scale and wavelengths are plotted on the horizontal scale.

The main thing to notice about these plots is that the curves never cross each other. Therefore, at
any frequency, there is only one temperature for each brightness. So, if you can measure the
brightness of the energy at a given frequency, you know the temperature of the emitting object!

Despite their temperatures, not all visible stars are good radio frequency emitters. We can detect
stars at radio frequencies only

         if they emit by non-thermal mechanisms (described next), or

         if they are in our solar system (that is, our sun), or

         if there is gas beyond the star which is emitting (for example, a stellar wind).

As it turns out, the hottest and brightest stars emit more energy at frequencies above the visible
range than below it. Such stars are known for their x-ray and atomic particle radiation. However,
intense thermal generators such as our own sun emit enough energy in the radio frequencies to
make them good candidates for radio astronomy studies. The Milky Way galaxy emits both
thermal and non-thermal radio energy, giving radio astronomers a rich variety of data to ponder.

Our observations of radiation of thermal origin have two characteristics that help distinguish it
from other types of radiation. Thermal radiation reproduces on a loudspeaker as pure static hiss,
and the energy of radiation of thermal origin usually increases with frequency.
Continuum Emissions from Ionized Gas

Thermal blackbody radiation is also emitted by gases. Plasmas are ionized gases and are considered
to be a fourth state of matter, after the solid, liquid, and gaseous states. As a matter of fact,
plasmas are the most common form of matter in the known universe (constituting up to 99% of
it!) since they occur inside stars and in the interstellar gas. However, naturally occurring plasmas
are relatively rare on Earth primarily because temperatures are seldom high enough to produce
the necessary degree of ionization. The flash of a lightning bolt and the glow of the aurora
borealis are examples of plasmas. But immediately beyond Earth’s atmosphere is the plasma
comprising the Van Allen radiation belts and the solar wind.

An atom in a gas becomes ionized when another atom bombards it with sufficient energy to
knock out an electron, thus leaving a positively charged ion and a negatively charged electron.
Once separated, the charged particles tend to recombine with their opposites at a rate dependent
on the density of the electrons. As the electron and ion accelerate toward one another, the electron
emits electromagnetic energy. Again, the kinetic energy of the colliding atoms tends to
separate them into electron and positive ion, making the process continue indefinitely. The gas
will always have some proportion of neutral to ionized atoms.

As the charged particles move around, they can generate local concentrations of positive or
negative charge, which gives rise to electric and magnetic fields. These fields affect the motion
of other charged particles far away. Thus, elements of the ionized gas exert a force on one
another even at large distances. An ionized gas becomes a plasma when enough of the atoms are
ionized so that the gas exhibits collective behavior.

Whenever a vast quantity of free and oppositely charged ions coexist in a relatively small space,
the combination of their reactions can add up to intense, continuous, wideband radio frequency
radiation. Such conditions prevail around stars, nebulae, clusters of stars, and even planets—
Jupiter being at least one we know of.

Spectral Line Emissions from Atoms and Molecules - Back to Table of Contents

While the mechanism behind thermal-related energy emissions from ionized gases involves
electrons becoming detached from atoms, line emissions from neutral hydrogen and other atoms
and molecules involves the electrons changing energy states within the atom, emitting a photon of
energy at a wavelength characteristic of that atom. Thus, this radiation mechanism is called line
emission, since the wavelength of each atom occupies a discrete “line” on the electromagnetic
spectrum.

In the case of neutral (not ionized) hydrogen atoms, in their lower energy (ground) state, the
proton and the electron spin in opposite directions. If the hydrogen atom acquires a slight amount
of energy by colliding with another atom or electron, the spins of the proton and electron in the
hydrogen atom can align, leaving the atom in a slightly excited state. If the atom then loses that
amount of energy, it returns to its ground state. The amount of energy lost is that associated with
a photon of 21.11 cm wavelength (frequency 1428 MHz).

Hydrogen is the key element in the universe. Since it is the main constituent of interstellar gas,
we often characterize a region of interstellar space as to whether its hydrogen is neutral, in which
case we call it an H I region, or ionized, in which case we call it an H II region.
Some researchers involved in the search for extra-terrestrial intelligence (see Chapter 8) have
reasoned that another intelligent species might use this universal 21-cm wavelength line emission
by neutral hydrogen to encode a message; thus these searchers have tuned their antennas specifically
to detect modulations to this wavelength. But, perhaps more usefully, observations of this
wavelength have given us much information about the interstellar medium and locations and
extent of cold interstellar gas.

Non-thermal Mechanisms - Back to Table of Contents

Radiation is also produced by mechanisms unrelated to the temperature of object (that is, thermal
radiation). Here we discuss some examples of non-thermal radiation.

Synchrotron Radiation

Notwithstanding the vast number of sources of thermal emissions, much of the radiation from our
own galaxy, particularly the background radiation first discovered by Jansky, and most of that
from other galaxies is of non-thermal origin. The major mechanism behind this type of radiation
has nothing to do with temperature, but rather with the effect of charged particles interacting with
magnetic fields. When a charged particle enters a magnetic field, the field compels it to move in a
circular or spiral path around the magnetic lines of force. The particle is thus accelerated and
radiates energy. Under non-relativistic conditions (that is, when particle velocities are well-below
the speed of light), this cyclotron radiation is not strong enough to have much astronomical
importance. However, when the speed of the particle reaches nearly the speed of light, it emits a
much stronger form of cyclotron radiation called synchrotron radiation.

Quasars (described in Chapter 6) are one source of synchrotron radiation not only at radio wavelengths,
but also at visible and x-ray wavelengths.

An important difference in radiation from thermal versus non-thermal mechanisms is that while
the intensity (energy) of thermal radiation increases with frequency, the intensity of non-thermal
radiation usually decreases with frequency.

Masers - Back to Table of Contents

Astronomical masers are another source of non-thermal radiation. “Maser” is short for microwave-
amplified stimulated emission of radiation. Masers are very compact sites within molecular
clouds where emission from certain molecular lines can be enormously amplified. The interstellar
medium contains only a smattering of molecular species such as water (H2O), hydroxl radicals
(OH), silicon monoxide (SiO), and methanol (CH3OH). Normally, because of the scarcity of
these molecules, their line emissions would be very difficult to detect with anything but very
crude resolution. However, because of the phenomenon of “masing,” these clouds can be
detected in other galaxies!

In simplified terms, masing occurs when clouds of these molecules encounter an intense radiation
field, such as that from a nearby source such as a luminous star, or when they collide with the far
more abundant H2 molecules. What is called a “population inversion” occurs, in which there are
more molecules in an excited state (that is, their electrons have “jumped” to a higher energy
level), than in a stable, ground state. This phenomenon is called pumping. As the radiation
causing the pumping travels through the cloud, the original ray is amplified exponentially,
emerging at the same frequency and phase as the original ray, but greatly amplified. Some
masers emit as powerfully as stars! This phenomenon is related to that of spectral line emissions,
explained in Chapter 4.

Incidentally, this same principle is used in a device called a maser amplifier, which is installed as
part of some radio telescopes (not in the GAVRT, however) to amplify the signal received by the
antenna.

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Chapter 4 - Back to Table of Contents

Effects of Media

Objectives: When you have completed this chapter, you will be able to describe several
important variables in the media through which the radiation passes and how they
affect the particles/waves arriving at the telescope. You will be able to describe
atmospheric “windows” and give an example. You will be able to describe the
effects of absorbing and dispersing media on wave propagation. You will be able
to describe Kirchhoff’s laws of spectral analysis, and give examples of sources of
spectral lines. You will be able to define reflection, refraction, superposition,
phase, interference, diffraction, scintillation, and Faraday rotation.

Electromagnetic radiation from space comes in all the wavelengths of the spectrum, from gamma
rays to radio waves. However, the radiation that actually reaches us is greatly affected by the
media through which it has passed. The atoms and molecules of the medium may absorb some
wavelengths, scatter (reflect) other wavelengths, and let some pass through only slightly bent
(refracted).

Atmospheric “Windows” - Back to Table of Contents

Earth’s atmosphere presents an opaque barrier to much of the electromagnetic spectrum. The
atmosphere absorbs most of the wavelengths shorter than ultraviolet, most of the wavelengths
between infrared and microwaves, and most of the longest radio waves. That leaves only visible
light, some ultraviolet and infrared, and short wave radio to penetrate the atmosphere and bring
information about the universe to our Earth-bound eyes and instruments.

The main frequency ranges allowed to pass through the atmosphere are referred to as the radio
window and the optical window. The radio window is the range of frequencies from about 5
MHz to over 300 GHz (wavelengths of almost 100 m down to about 1 mm). The low-frequency
end of the window is limited by signal absorption in the ionosphere, while the upper limit is
determined by signal attenuation caused by water vapor and carbon dioxide in the atmosphere.

The optical window, and thus optical astronomy, can be severely limited by atmospheric conditions
such as clouds and air pollution, as well as by interference from artificial light and the
literally blinding interference from the sun’s light. Radio astronomy is not hampered by most of
these conditions. For one thing, it can proceed even in broad daylight. However, at the higher
frequencies in the atmospheric radio window, clouds and rain can cause signal attenuation. For
this reason, radio telescopes used for studying sub-millimeter wavelengths are built on the highest
mountains, where the atmosphere has had the least chance for attenuation. (Conversely, most
radio telescopes are built in low places to alleviate problems with human-generated interference,
as will be explained in Chapter 6.)

Absorption and Emission Lines - Back to Table of Contents

As described in Chapter 3, a blackbody object emits radiation of all wavelengths. However,
when the radiation passes through a gas, some of the electrons in the atoms and molecules of the
gas absorb some of the energy passing through. The particular wavelengths of energy absorbed
are unique to the type of atom or molecule. The radiation emerging from the gas cloud will thus
be missing those specific wavelengths, producing a spectrum with dark absorption lines.

The atoms or molecules in the gas then re-emit energy at those same wavelengths. If we can
observe this re-emitted energy with little or no back lighting (for example, when we look at
clouds of gas in the space between the stars), we will see bright emission lines against a dark
background. The emission lines are at the exact frequencies of the absorption lines for a given
gas. These phenomena are known as Kirchhoff’s laws of spectral analysis:

  1. When a continuous spectrum is viewed through some cool gas, dark spectral
    lines (called absorption lines) appear in the continuous spectrum.
  2. If the gas is viewed at an angle away from the source of the continuous spectrum,
    a pattern of bright spectral lines (called emission lines) is seen against an otherwise
    dark background.

The same phenomena are at work in the non-visible portions of the spectrum, including the radio
range. As the radiation passes through a gas, certain wavelengths are absorbed. Those same
wavelengths appear in emission when the gas is observed at an angle with respect to the radiation
source.

Why do atoms absorb only electromagnetic energy of a particular wavelength? And why do they
emit only energy of these same wavelengths? What follows here is a summarized explanation,
but for a more comprehensive one, see Kaufmann’s Universe, pages 90-96.

The answers lie in quantum mechanics. The electrons in an atom may be in a number of allowed
energy states. In the atom’s ground state, the electrons are in their lowest energy states. In order
to jump to one of a limited number of allowed higher energy levels, the atom must gain a very
specific amount of energy. Conversely, when the electron “falls” to a lower energy state, it
releases a very specific amount of energy. These discrete packets of energy are called photons.

Thus, each spectral line corresponds to one particular transition between energy states of the
atoms of a particular element. An absorption line occurs when an electron jumps from a lower
energy state to a higher energy state, extracting the required photon from an outside source of
energy such as the continuous spectrum of a hot, glowing object. An emission line is formed
when the electron falls back to a lower energy state, releasing a photon.

The diagram on the next page demonstrates absorption and emission of photons by an atom using
the Neils Bohr model of a hydrogen atom, where the varying energy levels of the electron are
represented as varying orbits around the nucleus. (We know that this model is not literally true,
but it is useful for describing electron behavior.) The varying series of absorption and emission
lines represent different ranges of wavelengths on the continuous spectrum. The Lyman series,
for example, includes absorption and emission lines in the ultraviolet part of the spectrum.

Emission and absorption lines are also seen when oppositely charged ions recombine to an
electrically neutral state. The thus formed neutral atom is highly excited, with electrons
transitioning between states, emitting and absorbing photons. The resulting emission and absorption
lines are called recombination lines. Some recombination lines occur at relatively low
frequencies, well within the radio range, specifically those of carbon ions.

Molecules, as well as atoms, in their gas phase also absorb characteristic narrow frequency bands
of radiation passed through them. In the microwave and long wavelength infrared portions of the
spectrum, these lines are due to quantized rotational motion of the molecule. The precise frequencies
of these absorption lines can be used to determine molecular species. This method is
valuable for detecting molecules in our atmosphere, in the atmospheres of other planets, and in
the interstellar medium. Organic molecules (that is, those containing carbon) have been detected
in space in great abundance using molecular spectroscopy. Molecular spectroscopy has become
an extremely important area of investigation in radio astronomy.

As will be discussed in Chapter 5, emission and absorption lines in all spectra of extraterrestrial
origin may be shifted either toward higher (blue) or lower (red) frequencies, due to a variety of
mechanisms.

Reflection - Back to Table of Contents

RF radiation generally travels through space in a straight line. RF waves can be reflected by
certain substances, much in the same way that light is reflected by a mirror. The angle at which a
radio wave is reflected from a smooth metal surface, for example, will equal the angle at which it
approached the surface. In other words, the angle of reflection of RF waves equals their angle of
incidence.

This principle of RF reflection is used in antenna design to focus transmitted waves into a narrow
beam and to collect and concentrate received RF signals for a receiver. If a reflector is designed
with the reflecting surface shaped like a paraboloid, electromagnetic waves approaching parallel
to the axis of the antenna will be reflected and will focus above the surface of the reflector at the
feed horn. This arrangement is called prime focus and provides the large aperture (that is,
antenna surface area) necessary to receive very weak signals.

However, a major problem with prime focus arrangements for large aperture antennas is that the
equipment required at the prime focus is heavy and the supporting structure tends to sag under the
weight of the equipment, thus affecting calibration. A solution is the Cassegrain focus arrangement.
Cassegrain antennas add a secondary reflecting surface to “fold” the electromagnetic
waves back to a prime focus near the primary reflector. The DSN’s antennas (including the
GAVRT) are of this design because it accommodates large apertures and is structurally strong,
allowing bulky equipment to be located nearer the structure’s center of gravity.

The reflective properties of electromagnetic waves have also been used to investigate the planets
using a technique called planetary radar. With this technique, electromagnetic waves are transmitted
to the planet, where they reflect off the surface of the planet and are received at one or
more Earth receiving stations. Using very sophisticated signal processing techniques, the receiving
stations dissect and analyze the signal in terms of time, amplitude, phase, and frequency.
JPL’s application of this radar technique, called Goldstone Solar System Radar (GSSR), has been
used to develop detailed images and measurements of several main belt and near-Earth asteroids.

Refraction - Back to Table of Contents

Refraction is the deflection or bending of electromagnetic waves when they pass from one kind of
transparent medium into another. The index of refraction is the ratio of the speed of electromagnetic
energy in a vacuum to the speed of electromagnetic energy in the observed medium. The
law of refraction states that electromagnetic waves passing from one medium into another (of a
differing index of refraction) will be bent in their direction of travel.

Usually, substances of higher densities have higher indices of refraction. The index of refraction
of a vacuum, by definition, is 1.0. The index of refraction of air is 1.00029, water is 1.3, glass
about 1.5, and diamonds 2.4. Since air and glass have different indices of refraction, the path of
electromagnetic waves moving from air to glass at an angle will be bent toward the perpendicular
as they travel into the glass. Likewise, the path will be bent to the same extent away from the
perpendicular when they exit the other side of glass.

In a similar manner, electromagnetic waves entering Earth's atmosphere from space are slightly
bent by refraction. Atmospheric refraction is greatest for radiation from sources near the horizon
(below about 15° elevation) and causes the apparent altitude of the source to be higher than the
true height. As Earth rotates and the object gains altitude, the refraction effect decreases, becoming
zero at zenith (directly overhead). Refraction's effect on sunlight adds about 5 minutes to the
daylight at equatorial latitudes, since the sun appears higher in the sky than it actually is.

Superposition - Back to Table of Contents

Many types of waves, including electromagnetic waves, have the property that they can traverse
the same space independently of one another. If this were not the case, we would be unable to
see anything. Imagine you are standing at one end of a room in an art museum, trying to view a
painting on the far wall and this property did not apply. The light waves from all of the paintings
on the side walls crossing back and forth would disrupt the light waves coming directly to you
from the painting you were trying to view, and the world would be nothing but a blur. All these
electromagnetic waves do, in fact, additively combine their electric fields at the point where they
cross. Once they have crossed, each wave resumes in its original direction of propagation with its
original wave form. This phenomenon is called the property of superposition.

Phase - Back to Table of Contents

As applied to waves of electromagnetic radiation, phase is the relative measure of the alignment
of two wave forms of similar frequency. They are said to be in phase if the peaks and troughs of
the two waves match up with each other in time. They are said to be out of phase to the extent
that they do not match up. Phase is expressed in degrees from 0 to 360.

Interference - Back to Table of Contents

When two waves of the same frequency and moving in the same direction meet, the resulting
wave is the additive combination of the two waves. In the special case where two waves have the
same electric field amplitude (wave height) and are in phase, the resulting amplitude is twice the
original amplitude of each wave and the resulting frequency is the same as the original frequency
of each wave. This characteristic is called constructive interference. In the special case where
two waves have the same electric field amplitude and are 180° out of phase, the two waves cancel
each other out. This characteristic is called destructive interference.

Diffraction - Back to Table of Contents

When an electromagnetic wave passes by an obstacle in space, the wave is bent around the object.
This phenomenon is known as diffraction. The effects of diffraction are usually very small, so we
seldom notice it.

However, you can easily see the effect of diffraction for yourself. All you need is a source of
light, such as a fluorescent or incandescent light bulb. Hold two fingers about 10 cm in front of
one eye and make the space between your fingers very small, about 1 mm. Now look through the
space between your fingers at the light source. With a little adjustment of the spacing, you will
see a series of dark and light lines. These are caused by constructive and destructive interference
of light diffracting around your fingers.

The reason diffraction occurs is not exactly obvious. Christian Huygens in the mid-1600s offered
an explanation that, strange though it may seem, still nicely explains the observations. You will
recall the inverse-square law of electromagnetic propagation from Chapter 2. Electromagnetic
energy may be considered to propagate from a point source in plane waves. (The illustration of
reflection on page 34 shows the RF waves as planes.) The inverse square law applies not only to
the source of the energy but also to any point on a plane wave. That is, from any point on the
plane wave, the energy is propagated as if the point were the source of the energy. Thus, waves
may be considered to be continuously created from every point on the plane and propagated in
every direction.

For an infinite plane wave, the sideways propagation from each point is balanced by the propagation
from its neighbors, so the wave continues on as a plane. However, when a wave encounters
an object, the effect at the edges of the object is that the path of the radiation is slightly bent.

Now suppose the radiation (for example, light) is then intercepted by a surface (such as a screen)
a short distance from the object. Then, compared to the parallel waves that have passed farther
from the object’s edge (for example “waves B, C, and D” in the illustration below), the waves
that have bent around the edges of the object (“waves A and E” for example) will have travelled a
shorter or longer distance from the object to any given point on the screen.

The effect is that the light waves are out of phase when they arrive at any given point on the
surface. If they are 180° out of phase, they cancel each other out (destructive interference) and
produce a dark line. If they are in phase, they add together (constructive interference) and
produce a bright line.

Diffraction is most noticeable when an electromagnetic wave passing around an obstacle or
through an opening in an obstacle (such as the slit between your fingers) is all of the same
frequency, or monochromatic.

The picture below shows a typical diffraction pattern seen when observing a star through a
telescope with a lens that focuses the light to a point (a converging lens).

Scintillation - Back to Table of Contents

As electromagnetic waves travel through Earth’s atmosphere, they pass through areas of varying
pressure, temperature, and water content. This dynamic medium has rapidly varying indices of
refraction, causing the waves to take different paths through the atmosphere. The consequence is
that at the point of observation, the waves will be out of phase and appear to be varying in
intensity. The effect in the visual range is that stars appear to twinkle and distant scenes on the
horizon appear to shimmer (for example, when we see distant “water” mirages in the hot desert).
In the radio range, the same phenomenon is called scintillation. The interplanetary and interstellar
media can have a similar effect on the electromagnetic waves passing through them.

A star will scintillate or twinkle most violently when it is low over the horizon, as its radiation
passes through a thick layer of atmosphere. A planet, which appears as a small disk, rather than a
point, will usually scintillate much less than a star, because light waves from one side of the disk
are “averaged” with light waves coming from other parts of the disk to smooth out the overall
image.

Technology has been developed for both radio and optical telescopes to significantly cancel out
the phase changes observed for a given source, thus correcting the resulting distortion. This
technology is not implemented on the GAVRT.

Faraday Rotation - Back to Table of Contents

Faraday rotation (or Faraday effect) is a rotating of the plane of polarization of the linearly
polarized electromagnetic waves as they pass through a magnetic field in a plasma. A linearly
polarized wave may be thought of as the sum of two circularly polarized waves of opposite hand.
That is, one wave is polarized to the right and one wave is polarized to the left. (Both waves are
at the same frequency.) When the linearly polarized wave passes through a magnetic field, the
right polarized wave component travels very slightly faster than the left polarized wave component.
Over a distance, this phenomenon has the effect of rotating the plane of the linearly polarized
wave. A measure of the amount of rotation can give a value of the density of a plasma.

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Chapter 5 - Back to Table of Contents

Effects of Motion and Gravity

Objectives: When you have completed this chapter, you will be able to describe the Doppler
effect on the frequency of the received particles/waves; describe the significance
of spectral red shifting and blue shifting; describe the effects of gravity on
electromagnetic radiation; describe superluminal expansion; and define occultation.


Doppler Effect - Back to Table of Contents

Regardless of the frequency of electromagnetic waves, they are subject to the Doppler effect.
The Doppler effect causes the observed frequency of radiation from a source to differ from the
actual radiated frequency if there is motion that is increasing or decreasing the distance between
the source and the observer. The same effect is readily observable as variation in the pitch of
sound between a moving source and a stationary observer, or vice versa.

When the distance between the source and receiver of electromagnetic waves remains constant,
the frequency of the source and received wave forms is the same. When the distance between the
source and receiver of electromagnetic waves is increasing, the frequency of the received wave
forms is lower than the frequency of the source wave form. When the distance is decreasing, the
frequency of the received wave form will be higher than the source wave form.

The Doppler effect is very important to both optical and radio astronomy. The observed spectra
of objects moving through space toward Earth are shifted toward the blue (shorter wavelengths),
while objects moving through space away from Earth are shifted toward the red. The Doppler
effect works at all wavelengths of the electromagnetic spectrum. Thus, the phenomenon of
apparent shortening of wavelengths in any part of the spectrum from a source that is moving
toward the observer is called blue shifting, while the apparent lengthening of wavelengths in any
part of the spectrum from a source that is moving away from the observer is called red shifting.

Relatively few extraterrestrial objects have been observed to be blue shifted, and these, it turns
out, are very close by, cosmically speaking. Examples are planets in our own solar system with
which we are closing ranks due to our relative positions in our orbits about the sun, some other
objects in our galaxy, some molecular clouds, as well as some galaxies in what is termed the
local group of galaxies.

Almost all other distant objects are red shifted. The red shifting of spectra from very distant
objects is due to the simple fact that the universe is expanding. Space itself is expanding between
us and distant objects, thus they are moving away from us. This effect is called cosmic red
shifting, but it is still due to the Doppler effect.

Distances to extragalactic objects can be estimated based in part on the degree of red shifting of
their spectra. As the universe expands, all objects recede from one another at a rate proportional
to their distances. The Hubble Constant relates the expansion velocity to the distance and is most
important for estimating distances based on the amount of red shifting of radiation from a source.
Our current estimate for the Hubble Constant is 60-80 km/s per million parsecs (1 parsec = 3.26
light years).

The spectra from quasars, for example, are quite red-shifted. Along with other characteristics,
such as their remarkable energy, this red shifting suggests that quasars are the oldest and most
distant objects we have observed. The most distant quasars appear to be receding at over 90% the
speed of light!

Gravitational Red Shifting - Back to Table of Contents

Red shifting, of course, indicates an elongating of the wavelength. An elongated wavelength
indicates that the radiation has lost some of its energy from the instant it left its source.
As predicted by Einstein, radiation also experiences a slight amount of red shifting due to gravitational
influences. Gravitational red shifting is due to the change in the strength of gravity and
occurs mostly near massive bodies. For example, as radiation leaves a star, the gravitational
attraction near the star produces a very slight lengthening of the wavelengths, as the radiation
loses energy in its effort to escape the pull of gravity from the large mass. This red shifting
diminishes in effect as the radiation travels outside the sphere of influence of the source’s gravity.

Gravitational Lensing - Back to Table of Contents

Einstein’s theory of general relativity predicts that space is actually warped around massive
objects.

In 1979, astronomers noticed two remarkably similar quasars very close together. They had the
same magnitude, spectra, and red shift. They wondered if the two images could actually represent
the same object. It turned out that a galaxy lay directly in the path between the two quasars
and Earth, much closer to Earth than the quasars. The geometry and estimated mass of the
galaxy were such that it produced a gravitational lens effect—that is, a warping of the light as it
passes through the space around the galaxy.

Many other instances of gravitational lensing have now been detected. Gravitational lensing can
produce more than two images, or even arcs. Images produced by point-like gravitational lenses
can appear much brighter than the original source would appear in the absence of the gravitational
lens.

Superluminal Velocities - Back to Table of Contents

Some discrete (defined in the next chapter) sources within quasars have been observed to change
positions over a brief period. Their motion generally appears to the observer to be radially
outward from the center of the quasar image. The apparent velocities of these objects have been
measured, and if the red shifts actually do represent the distance and recession velocities of the
quasar, then these discrete objects are moving at speeds greater than the speed of light! We call
these apparent speeds superluminal velocities or superluminal expansion.

Well, we know this is impossible, right? So astronomers had to come up with a more reasonable
explanation. The most widely accepted explanation is that the radiation emitted from the object
at the first position (A in the diagram below) has traveled farther and thus taken longer to reach
Earth than the radiation emitted from the second position (B), 5 LY from A.


Suppose A is 4 light years (LY) farther from Earth than B (that is, AC is 4 LY). Moving just a bit
under the speed of light, the object takes just over 5 LY to travel from A to B. However, the
radiation it emitted at A reaches C in 4 years. As that radiation continues toward Earth, it is one
year ahead of the radiation emitted toward us by the object when it arrived at B. When it finally
(after several billion years) reaches Earth, the radiation from A is still one year ahead of the
radiation from B. It appears to us that the object has moved tangentially out from the center of
the quasar, from C to B and (from the Pythagorean theorem) has gone 3 LY in just over one year!
That the object appears to travel at nearly three times light speed is only because of the projection
effect, with its radiation traveling from A to C in 4 years, while the object itself went from A to
B in 5 years.

Occultations - Back to Table of Contents

When one celestial body passes between Earth and another celestial body, we say that the object
that is wholly or partially hidden from our view is occulted. Examples of occultations are the
moon passing in front of a star or a planet, or a planet passing in front of a star, or one planet
passing in front of another planet, such as in 1590 when Venus occulted Mars.

An occultation can provide an unparalleled opportunity to study any existing atmosphere on the
occulting planet. As the radiation from the farther object passes through the atmosphere at the
limb of the nearer object, that radiation will be influenced according to the properties of that
atmosphere. The degree of refraction of the radiation gives information about the atmosphere’s
density and thickness. Spectroscopic studies give information about the atmosphere’s composition.

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Chapter 6 - Back to Table of Contents

Sources of Radio Frequency Emissions

Objectives: Upon completion of this chapter, you will be able to define and give examples of
a “point source,” a “localized source,” and an “extended source” of radio frequency
emissions; distinguish between “foreground” and “background” radiation;
describe the theoretical source of “cosmic background radiation”; describe a
radio star, a flare star, and a pulsar; explain why pulsars are sometimes referred
to as standard clocks; describe the relationship between pulsar spin down and
age; describe “normal” galaxies and “radio” galaxies; describe the general
characteristics of the emissions from Jupiter, Io, and the Io plasma torus; describe
the impact of interference on radio astronomy observations; and describe a major
source of natural interference and of human made interference.


Classifying the Source - Back to Table of Contents

Radiation whose direction can be identified is said to originate from a discrete source. A discrete
source often can be associated with a visible (whether by the naked eye or by optical
telescope) object. For example, a single star or small group of stars viewed from Earth is a
discrete source. Our sun is a discrete source. A quasar is a discrete source. However, the
definition of “discrete,” in addition to the other terms used to describe the extent of a source,
often depends upon the beam size of the radio telescope antenna being used in the observation.
Discrete sources may be further classified as point sources, localized sources, and extended
sources.

A point source is an idealization. It is defined as a source that subtends an infinitesimally small
angle. All objects in reality subtend at least a very tiny angle, but often it is mathematically
convenient for astronomers to regard sources of very small extent as point sources. Objects that
appear smaller than the telescope’s beam size are often called “unresolved” objects and can
effectively be treated as point sources. A localized source is a discrete source of very small
extent. A single star may be considered a localized source.

Emitters of radiation that covers a relatively large part of the sky are called extended sources.
An example of an extended source of radiation is our Milky Way galaxy, or its galactic center
(called Sagittarius A) from which radiation emissions are most intense.

An optical analogy to the extended source would be the view of a large city at night from an
airplane at about 10 km altitude. All the city lights would tend to blend together into an apparently
single, extended source of light. On the other hand, a single searchlight viewed from the
same altitude would stand out as a single object, analogous to a localized or point source.

The terms localized and extended are relative and depend on the precision with which the telescope
observing them can determine the source.

Background radiation is radio frequency radiation that originates from farther away than the
object being studied, whereas foreground radiation originates from closer than the object being
studied. If an astronomer is studying a specific nearby star, the radiation from the Milky Way
may be considered not merely an extended source, but background radiation. Or, if it is a distant
galaxy being observed, the Milky Way may be considered a pesky source of foreground radiation.
Background and foreground radiation may consist of the combined emissions from many discrete
sources or may be a more or less continuous distribution of radiation from our galaxy.

Cosmic background radiation, on the other hand, is predicted to remain as the dying glow from
the big bang. It was first observed by Arno Penzias and Robert Wilson in 1965. (They won a
Nobel Prize for this discovery in 1978). As discussed in Chapter 3, much of background and
foreground radiation tends to be of non-thermal origin. The cosmic background radiation,
however, is thermal.

In the group of pictures below (from Griffith Observatory and JPL), the entire sky is shown at (a)
radio, (b) infrared, (c) visible, and (d) X-ray wavelengths. Each illustration shows the Milky Way
stretching horizontally across the picture. It is clear that radio wavelengths give us a very different
picture of our sky.

Star Sources - Back to Table of Contents

Many thousands of visible stellar objects have been discovered to also be strong emitters of radio
frequency radiation. All such stars may be called radio stars.

It is helpful in discussing star types and activities to review stellar evolution. For a discussion of
star birth, maturation, old age, and death, please read Chapters 20-22 in Universe, by William J.
Kaufmann III, or Chapters 28-30 in Abell’s Exploration of the Universe, by David Morrison,
Sidney Wolff, and Andrew Fraknoi.

Variable Stars - Back to Table of Contents

Stars do not shine uniformly brightly all the time. Stars that show significant changes in brightness
over periods we short-lived humans can perceive are of great importance to astronomy
because of what we can surmise from those changes. And fortunately for radio astronomy, it has
been discovered that stars whose output of visible radiation varies over short periods, either
regularly or irregularly, have corresponding variations in their output of radio frequency emissions.

Some variable stars , such as Cepheids (SEE-fee-ids), are absolutely regular in their cyclic
changes, varying from a few days to a few weeks. It has been found that stars with longer regular
periods are always more luminous (emitting more energy) than those with shorter regular periods.

Variable stars with very short periods (1.25 to 30 hours) are called RR Lyrae variables. None of
these shorter period variables is bright enough to see with the naked eye. Because the intrinsic
luminosities of Cepheids and RR Lyraes with similar periods are comparable, variable stars such
as these can be used to work out interstellar and even intergalactic distances.

Other variable stars have much longer periods, are less regular in their cycles, and vary by a much
greater magnitude. These are called semi-regular variables. The red giant Betelgeuse in the
Orion constellation is an example. No period-luminosity relationship has been found for semiregular
variables.

Irregular variables have no set periods at all. They usually are young stars and their luminosities
may vary over a very large range.

Flare stars are faint red dwarf stars (older and feebler than white dwarfs) that exhibit sudden
increases in brightness over a period of a few minutes due to intense flare activity, fading back to
their usual brightness within an hour or so. Typical flare stars are UV Ceti and AD Leonis.
Binary (double) stars may produce apparently regularly varying radiation if the two stars eclipse
one another in their orbits. Also, radio emissions from binaries are more common than for single
stars. The interaction of stellar winds and magnetospheres, bow shocks, and tidal effects may
contribute to the conditions producing radio frequency emissions.

Pulsars - Back to Table of Contents

Sometimes when a star goes supernova, all that is left after this most violent of processes is a
cloud of expanding gas and the tiny remnant of extremely dense material only a few tens of
kilometers in diameter. The supernova implosion is so intense that the protons and electrons in
the atoms of the star are jammed together, thus canceling out their electrical charges and forming
neutrons. This neutron star may be 1014 times as dense as water! It will have extremely powerful
magnetic fields and may rotate very rapidly. Because the magnetic axis may not correspond
to the spin axis, a beam of radiation emitted from the magnetic poles may seem to an observer to
pulse like a rotating searchlight. Thus we call these rotating neutron stars pulsars. Although
some pulsars are seen at visible and x-ray frequencies, many more are seen at radio frequencies.

Since 1967, when the first pulsar was detected by Jocelyn Bell, hundreds of pulsars have been
discovered. The Crab pulsar spins at 30 times per second. The pulsar 1937+21 in Cygnus pulses
642 times per second. We receive this emission on Earth as if it were a signal produced by a
cosmic clock. Over the brief period we have been observing them, however, they all them seem
to be gradually slowing down. Their energy is dissipating with age. After correction for this
effect, some millisecond pulsars are at least as accurate at timekeeping as the best atomic clocks.
The rate at which pulsars slow down has been helpful in confirming aspects of Einstein’s theory
of general relativity. Also, the timing of pulsars can be useful in determining properties of the
interstellar medium.

Our Sun - Back to Table of Contents

The strongest extraterrestrial radio source we experience here on Earth is our own star. The Sun
is a very ordinary star—not particularly massive or small, not particularly hot or cold, not particularly
young or old. Perhaps we are fortunate it is so typical because from it we can learn much
about stars in general.

The photosphere is the part of the sun’s atmosphere that emits most of the visible light, while the
corona, the sun’s outer atmosphere, is much less dense and emits only a very small amount of
visible light. The chromosphere, cool and dim compared to the photosphere, forms the boundary
between the photosphere and the corona.

The sun seems to have about an 11-year cycle of activity. When the sun is in a quiet phase, radio
emissions from the photosphere (the part that also emits radiation in the visible wavelength) are
in the wavelength range of 1 cm, while radio emissions from the corona approach a wavelength
of one meter. The size of the radio solar disk appears only slightly larger than the optical solar
disk as long as the telescope is tuned to only the 1-cm to 10-cm wavelength range. But at the
longer wavelengths, the radio solar disk is much larger, including, as it does, the corona, which
extends millions of kilometers above the photosphere.

Sunspots are darker appearing areas on the photosphere, and, as mentioned above, they seem to
fluctuate in frequency over about an 11-year cycle. They appear darker because they are a “cool”
4,000°C relative to the surrounding 6,000°C surface. They are the centers of magnetic fields,
apparently related to the sun’s magnetic field. It is possible that the sun’s magnetic lines of force
periodically get “tangled” and destabilized since the sun’s rate of rotation varies from the equator
to the poles. Solar flares breaking out of the sun’s upper atmosphere are usually associated with
sunspot groups.

Solar flares emit short bursts of radio energy, with wavelengths observable from the ground from
about 1 to 60 m (300-5 MHz). Sometimes during intense flares, a stream of high-energy cosmic
ray particles is emitted, traveling at over 500-1000 km per sec. When these charged particles
reach Earth’s magnetic field, we get magnetic storms and the aurora. The pattern of radio emissions
from solar flares appears to originate from a larger area of the solar surface than does the
pattern of visible-range radiation, but it is still apparent that they are the result of the same
activity.

The radiation associated with solar flares is circularly polarized, rather than randomly polarized
as is usual from extraterrestrial sources. This polarization may be caused by electrons gyrating in
the localized, intense magnetic field of the flare.

The sun is studied by radio astronomers both directly, by observing the actual radio emissions
from the sun, and indirectly, by observing the effect of the sun’s radiation on Earth’s ionosphere.

Galactic and Extragalactic Sources - Back to Table of Contents

We can think of extra-terrestrial radio emissions as originating either within our galaxy or outside
our galaxy. Inside our galaxy, remnants of supernova explosions are strong sources of radio
emissions.

Outside our galaxy, we find great variation in the radio emissions from different galaxies. So we
have arbitrarily divided these other galaxies into “normal” and “active” galaxies.

Normal galaxies are not very strong sources. For example, the Great Andromeda Spiral, the
largest galaxy in our so-called local group of galaxies, emits 1032 watts of power. In contrast,
Cygnus A, over half a billion light years from Earth, is one of the most conspicuous radio sources
in the sky, with a power output of 1038 watts. (See figures at end of Chapter 8 for a rough idea of
the locations of these galaxies.)

Active galaxies include radio galaxies, quasars, blasars, and Seyfert Galaxies.
Radio galaxies emit a very large quantity of radio waves.

Quasars, coined from the phrase “quasi-stellar radio source,” may be pouring out energy a
million times more powerfully than a normal galaxy. Quasars are the most distant objects we
have detected, some approaching 15 billion light years distant—their radiation requiring nearly
the age of the universe to reach us. And some seem to be receding from us at a rate 90% the
speed of light.

Blasars are galaxies with extremely bright centers, whose luminosity seems to vary markedly
over a very short period.

Seyfert galaxies are also intense sources of radiation whose spectra include emission lines.

In all these, the predominant radiation-producing mechanism is synchrotron radiation. An active
galaxy may radiate 1,000,000 times more powerfully in the radio frequencies than a normal
galaxy. Much of the radiation often seems to come from the nucleus of the galaxy. Astronomers
are now investigating the plausibility of a “unified theory of active galaxies,” which would
account for the varying behavior observed by all these types of active galaxies. It may be that
these galaxies have a black hole or a supermassive black hole at their centers, and their appearance
to us depends on the angle at which we are observing them.

Please read Chapter 27 of Universe, by Kaufmann, for more information, including many color
photos, about these fascinating and mysterious objects.

Planetary Sources and Their Satellites - Back to Table of Contents

Unlike stars, the radio energy observed from planets and their satellites (except the Jupiter system
and, to a small extent, Saturn) is mostly thermal blackbody radiation. The wavelengths of
radiation observed from these bodies gives us fairly precise indications of their temperatures, both
at their surfaces and at varying depths beneath their surfaces.

The Jupiter System - Back to Table of Contents

By far the most interesting planet for radio astronomy studies is Jupiter. As beautiful and fascinating
as it is visually, it is even more fascinating and complex to observe in the radio frequency
range. Most of the radiation from the Jupiter system is much stronger at longer wavelengths than
would be expected for thermal radiation. In addition, much of it is circularly or elliptically
polarized—not at all typical of thermal radiation. Thus, it must be concluded that non-thermal
processes similar to those taking place in galaxies are at work. That is, ions and electrons accelerated
by the planet’s spinning magnetic field are generating synchrotron radiation.

Jupiter is 318 times as massive as Earth. Its magnetic axis is tilted 15° from its rotational axis
and offset from the planet’s center by 18,000 km. Its polarity is opposite that of Earth (that is, a
compass needle would point south).

Jupiter’s surface magnetic field is 20 to 30 times as strong as that of Earth. The magnetosphere
of a planet is the region around it in which the planet’s magnetic field dominates the interplanetary
field carried by the solar wind. If we could see Jupiter’s magnetosphere from Earth, it
would appear as large as our moon!

The farther a planet is from the sun, the weaker will be the pressure from the solar wind on the
planet’s magnetosphere. Thus, Jupiter’s magnetic field, already quite intense, has considerably
less pressure holding it close to the planet than does Earth’s magnetic field. Jupiter’s magnetosphere
expands and contracts with variations in the solar wind. Its upstream (closest to the sun)
boundary (called the bowshock) varies from 50 to 100 Jupiter radii and envelopes Jupiter’s four
large Galilean satellites. (Sixteen Jupiter satellites have been discovered; the Galilean satellites
are by far the largest).

The magnetosphere of a planet traps plasma, as magnetic lines of force catch protons and electrons
carried on the solar wind and atoms that escape upward from the planet’s atmosphere. In
the case of Jupiter, since the magnetosphere is so large, it also traps atoms from the surfaces of
the satellites orbiting within it. Io, the innermost Galilean satellite, is an especially rich source of
oxygen and sulfur ions from its many violently active volcanoes. Io is estimated to contribute 10
tons of material to the magnetosphere per second!

As a matter of fact, a predominant feature of Jupiter’s magnetosphere is the plasma torus that
surrounds the planet, corresponding closely with the orbit of Io, which is at about five Jupiter
radii. It is an intensely radiating plasma within a slightly less active outer plasma. To add to the
adventure, as Io orbits through the magnetic field lines, an electric current of up to 5 million
Amps is generated between Io and the planet! Where this current reaches the atmosphere of
Jupiter, it generates strong radio frequency emissions that can be associated with the orbital
position of Io. The current also generates auroras in the upper atmosphere of Jupiter.

The Goldstone-Apple Valley radio telescope will be used to measure time variable radio frequency
emissions from Jupiter’s magnetic field. These observations can provide new information
about the magnetosphere, the plasma torus, and the rotation of Jupiter’s core and how it differs
from the rotation of the visible atmosphere.

Sources of Interference - Back to Table of Contents

Radio frequency “noise” complicates the task of the radio astronomer, at times making it difficult
to distinguish emissions from an object under study from extraneous emissions produced by other
nearby sources. Interference comes from both natural and artificial sources, the latter ones
becoming a bigger problem every day. By international agreement (the World Administrative
Radio Conference), certain frequencies have been allocated strictly for radio astronomy (Kraus, p.
A 24). However, there is disagreement about how far beyond the restricted limits is acceptable
“spillover” (for example, radio broadcasters may think 10mm over their wavelength limit is
acceptable, while radio astronomers may think .001 mm is too much). In some countries, the
restrictions are not enforced, so may as well not exist.

Natural sources of interference include:

  • Radio emissions from the Sun
  • Lightning
  • Emissions from charged particles (ions) in the upper atmosphere

Among the growing list of human-made sources of interference are:

  • Power-generating and transforming facilities
  • Airborne radar
  • Ground-based radio and television transmitters (which are getting more powerful
    all the time)
  • Earth-orbiting satellite transmitters and transponders, including Global Positioning
    Satellites (GPS)
  • Cellular phones

Human-generated interference that originates on the ground (such as radio and television transmissions)
travels along the ground and over the horizon. It used to be that such interference
tended to be weak at ground level, increasing in strength with height above ground. For this
reason, most radio telescopes have been situated in valleys or other low places, unlike optical
telescopes which are often built on mountain tops. (The exceptions are radio telescopes built for
studying sub-millimeter wavelengths, as mentioned in Chapter 4). However, more and more,
interference at ground level is becoming a problem even for low-lying radio telescopes.

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Chapter 7 - Back to Table of Contents

Mapping the Sky

Objectives: When you have completed this chapter, you will be able to describe the terrestrial
coordinate system; define and describe the relationship among the terms commonly
used in the “horizon” coordinate system, the “equatorial” coordinate
system, the “ecliptic” coordinate system, and the “galactic” coordinate system;
and describe the difference between an azimuth-elevation antenna and hour
angle-declination antenna.

In order to explore the universe, coordinates must be developed to consistently identify the
locations of the observer and of the objects being observed in the sky.
Because space is observed from Earth, Earth’s coordinate system must be established before
space can be mapped. Earth rotates on its axis daily and revolves around the sun annually. These
two facts have greatly complicated the history of observing space. However, once known, accurate
maps of Earth could be made using stars as reference points, since most of the stars’ angular
movements in relationship to each other are not readily noticeable during a human lifetime.
Although the stars do move with respect to each other, this movement is observable for only a
few close stars, using instruments and techniques of great precision and sensitivity.

Earth’s Coordinate System - Back to Table of Contents

A great circle is an imaginary circle on the surface of a sphere whose center is at the center of the
sphere. The equator is a great circle. Great circles that pass through both the north and south
poles are called meridians, or lines of longitude. For any point on the surface of Earth a meridian
can be defined. The prime meridian, the starting point measuring the east-west locations of other
meridians, marks the site of the old Royal Observatory in Greenwich, England. Longitude is
expressed in degrees, minutes, and seconds of arc from 0 to 180 degrees eastward or westward
from the prime meridian. For example, the GAVRT is located at 116.805 degrees, or 116 degrees,
48 minutes, 18 seconds of arc westward of the prime meridian: 116deg. 48' 18" W.

The starting point for measuring north-south locations on Earth is the equator (the equator is the
imaginary circle around the Earth which is everywhere equidistant from the poles). Circles in
parallel planes to that of the equator define north-south measurements called parallels, or lines of
latitude. Latitude is also expressed in degrees, minutes, and seconds of the arc subtended from the
center of the Earth. The GAVRT is located at 35.300 degrees, or 35 degrees, 18 minutes of arc
north of the equator: 35deg. 18' 00" N.

Revolution of Earth - Back to Table of Contents

Earth revolves around the sun in 365 days, 6 hours, 9 minutes with reference to the stars. Its mean
orbital speed is about 100,000 km per hour. The 6 hours, 9 minutes adds up to about an extra day
every fourth year, which is designated a leap year, with the extra day added as February 29th.
Solar vs. Sidereal Day

The Earth rotates on its axis relative to the sun every 24.0 hours mean solar time, with an inclination
of 23.5 degrees from the plane of its orbit around the sun. Mean solar time represents an
average of the variations caused by Earth's non-circular orbit. Earth’s rotation period relative to
the other stars (sidereal time) is 3 minutes 56.55 seconds shorter than the mean solar day.

The following figure explains this apparent discrepancy. Suppose the day starts when Earth’s
orbital position is at A and the Sun on the meridian (that is, directly above the local southern
horizon) of an observer at point O on Earth’s surface. When Earth has completed one rotation
with respect to the distant stars (C), the Sun will not yet have advanced to the meridian for the
observer at point O due to the movement of Earth along its orbit about the sun from A to B. To
complete a solar day, Earth must rotate an additional 1/365 of a full turn, which takes nearly 4
minutes. Thus the solar day is about 4 minutes longer than the sidereal day. Therefore, a clock
geared to sidereal time, in the space of one year, falls behind a regular clock by an amount equal
to about one solar day (24 hours).

Precession of the Earth Axis - Back to Table of Contents

Like a spinning top with a slow wobble, Earth’s axis slowly wobbles, or precesses, relative to its
orbital plane. The moon's gravity, primarily, and to a lesser degree the sun's gravity, acting on
Earth's oblateness tries to move Earth's axis perpendicular to the plane of Earth's orbit. However,
due to gyroscopic action, Earth's poles do not “right themselves” to a position perpendicular to
the orbital plane. Instead, they precess at 90 degrees to the force applied. This precession causes
the axis of Earth to describe a circle having a 23.5 degree radius relative to a fixed point in space
over about 26,000 years.

Astronomical Coordinate Systems  - Back to Table of Contents

Mapping locations on Earth is easy because, except for an occasional earthquake and the slow
glide of the tectonic plates, things stay put on Earth’s surface. But in the sky, things are moving,
and of course we are constantly moving with respect to the sky. Therefore, coordinate systems
for locating objects in the sky have to take all this movement into account. Several systems have
been devised to describe positions of celestial objects relative to Earth. The choice of which one
to use depends on what you are observing and how.

Horizon Coordinate System - Back to Table of Contents

The horizon is defined as the dividing line between the Earth and the sky, as seen by an observer
on the ground. In the horizon coordinate system the astronomical horizon is the hypothetical
interface between Earth and sky, as would be seen by the observer if the surrounding terrain were
perfectly flat (as out on a calm ocean).

Referring to the drawing below, zenith is the point straight overhead, perpendicular to the horizon
plane, and nadir is the point directly under the observer. A vertical circle through an object in
the sky and the zenith is the object circle. The coordinates of the object are given by the azimuth,
which is the horizontal angle from north clockwise to the object circle, and the altitude or elevation
angle, which is measured upward from the horizon to the object. The great circle through the
north and south points on the horizon and the zenith is called the meridian.

A horizon mask is a diagram that maps in silhouette the horizon in 360° of azimuth as actually
seen by the observer, including hills, valleys, mountains, buildings, trees, and anything else that
would hide from view any part of the sky that would be visible if the terrain were perfectly flat.
A horizon mask for the GAVRT is shown on the next page.

In the horizon system, the coordinates of an object in the sky change throughout the day with
Earth’s rotation. While the azimuth and elevation angles are convenient for positioning a radio
telescope antenna that rotates around horizontal and vertical axes (AZ-EL mounted), they are not
so convenient for specifying the position of a celestial object. Better for this purpose are systems
using fixed coordinates, such as the equatorial coordinate system.

Equatorial Coordinate System - Back to Table of Contents

In the equatorial coordinate system, Earth’s equator is the plane of reference. Earth’s axis of
rotation points to the north and south celestial poles. The celestial sphere is as large as the
known universe, and Earth is at the center of this sphere. The celestial poles do not move as
Earth rotates. For an observer standing at Earth’s equator, the celestial poles are on opposite
horizons at exactly the north and south points, and the celestial equator passes overhead going
exactly from the east to west horizon.

To describe an object’s location in the sky, we imagine a great circle through the celestial poles
and the object and call this the object’s hour circle. Where the sun in its path crosses the celestial
equator each year around March 21 is called the vernal equinox, and it is this line that is the
reference for the east-west coordinate of the object. Because Earth’s axis is inclined 23.5° to the
plane of its orbit around the sun, objects in the solar system (such as the planets and, from our
perspective, the sun) move across the celestial sphere not along the equator, but rather in their
own orbits, most of which are in nearly the same plane as Earth’s orbit. This imaginary path of
the sun’s apparent motion, called the ecliptic, is a curving line that runs around the sphere ranging
between 23.5° north and 23.5° south. The constellations of the zodiac all lie on the ecliptic. The
object’s elevation above the celestial equator is called declination.

The coordinates of the object, then, are given by its declination and its right ascension or hour
angle between the object’s hour circle and the vernal equinox. Declination is expressed in
degrees (0° to +90° if north of the equator, 0° to -90° if south of the equator). Right ascension is
expressed either in degrees (0° to 360° measured eastward from the vernal equinox) or, more
commonly, in hours, minutes, and seconds of time (0 to 24 hours).

Declination and right ascension are absolute coordinates regardless of the observer’s location on
Earth or the time of observation. The only exception is due to the slight 26,000-year cyclic
change in the equatorial coordinates because of the precession of Earth’s axis. Precession causes
the stars to appear to shift west to east at the rate of .01 degree (360 degrees/26,000 years) with
respect to the vernal equinox each year. For example, in the time of ancient Rome, the vernal
equinox was located in the constellation of Aries. It is now moving into the constellation of
Aquarius.

For this reason, sky almanacs identify the date and time of the instant used as the date of reference,
or epoch, and provide equations for updating data based on the almanac to the current date.
Epochs are named in 50-year increments. The 1950 epoch, for example, gives the coordinates
precisely as they were on January 1, 1950.

Coordinates may also be given relative to the observer’s location and time of observation. The
great circle that passes through the celestial poles and the zenith is called the meridian circle.
The angle between the object’s hour circle and the meridian circle is the object’s hour angle. The
hour angle increases with time, being negative before the object’s hour circle crosses the meridian
and positive afterwards. Hour angle is measured either in degrees or hours, minutes, and seconds
of sidereal time.

Radio telescopes are designed with mountings that are engineered to take best advantage of either
the hour angle-declination (HA-DEC) coordinate system or the azimuth-elevation (AZ-EL)
system (also called the altitude-elevation system, or ALT-EL). In an HA-DEC system, the HA
axis is parallel to Earth’s axis of rotation. In this way, since Earth is turning toward the east, the
telescope is mounted to turn toward the west (backwards) on an axis exactly parallel to Earth’s,
thus canceling out the east-west motion and simplifying the task of tracking objects in the sky.
Thus, the mounting built for a telescope near the equator would appear different from one built
for use in high latitudes. AZ-EL systems would appear the same no matter where on Earth they
are being used.

HA-DEC antenna mounting systems require an asymmetrical structural design unsuited to the
support of very heavy structures. Their advantage is that motion is required mostly in only one
axis to track an object as Earth rotates, although this advantage has largely been obviated by the
use of digital computers that can drive both axes of AZ-EL systems properly while they track.
The GAVRT uses an HA-DEC mount.

Ecliptic Coordinate System - Back to Table of Contents

In the ecliptic coordinate system, the reference is the plane of the ecliptic—that is, the plane
formed by Earth’s orbit around the sun. The orbits of the other planets in the solar system, with
the except of Pluto, lie within 7° of this plane. (Pluto’s orbit is inclined 17° to the ecliptic.) The
coordinates of an object are given as celestial longitude, measured eastward along the ecliptic
from the vernal equinox, and celestial latitude, measured north (+) or south (-) from the ecliptic.
This system is handy for studying the solar system.

Galactic Coordinate System - Back to Table of Contents

In the galactic coordinate system, the reference is a plane through the sun parallel to the mean
plane of the galaxy. By specifying the orientation of the north galactic pole in terms of its
equatorial coordinates, equatorial coordinates can be converted into galactic coordinates, and vice
versa, using transformation equations on a pocket calculator.

Latitude in this system is given in degrees, + toward the north galactic pole, - toward the south
galactic pole. Longitude is measured along the galactic equator to the east from the galactic
center, with 0° at the intersection of the galactic equator with the celestial equator.

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Chapter 8 - Back to Table of Contents

Our Place in the Universe

Objectives: When you complete this chapter, you will be able to describe the relative position
of Earth in the solar system and our solar system in the galaxy; describe the
approximate size of the known universe; and summarize the major issues astronomers
have considered in the search for extraterrestrial intelligence.

The Universe in Six Steps - Back to Table of Contents

The vastness of the universe is unimaginable for us humans. Perhaps the best we can do is to try
to conceive a model of the universe that begins to show us our relative size and position, at least
in our local neighborhood.

Gareth Wynn-Williams in The Fullness of Space uses the following analogy to help demonstrate
some of these distances:

Some idea of the vastness of the Universe may be gained by considering a model in
which everything has been scaled down by a factor of a billion. In this model the Earth
would have the dimensions of a grape. The Moon would resemble a grapeseed 40 cm
away while the Sun would be a 1.4-meter diameter sphere at a distance of 150 meters.
Neptune would be more than 4 km away. On this one-billionth scale, the nearest star
would be at a distance of 40,000 km – more than the actual diameter of the Earth. One
would have to travel five thousand times farther yet to reach the center of the Milky Way
Galaxy, another 80 times farther to reach the next nearest spiral galaxy, and another
several thousand times farther still to reach the limits of the known Universe.

To further make our point, the following drawings (from Kraus, 1986) represent the universe in
six steps: (1) the Earth Moon system, (2) the solar system, (3) the solar neighborhood, (4) our
galaxy, (5) the galactic neighborhood, and (6) the universe.

The Search for Extraterrestrial Intelligence - Back to Table of Contents

Perhaps the most urgent question the human species ever asks itself is “are we alone?” Scientists,
philosophers, and “ordinary” people address the question in unique ways, some optimistic, some
pessimistic, some very certain the answer is “No,” despite a dearth of physical evidence or
likelihood. Unable to travel interstellar distances, humans have only one tool currently capable of
answering this question, and that is the radio telescope. So let’s have a closer look at these
endeavors.

Even the most objective attempts to calculate the likely number of planets in our galaxy that
could produce an intelligent life form with whom we might communicate come up with estimates
of anywhere from 1 (us) to 10 million planets. These planets include those that

(a) could support life as we know it,

(b) have evolved a species with enough intelligence to have a technology,

(c) are in a period of the planet’s history when this intelligent species has the capability
of transmitting electromagnetic signals into space,

(d) is in a period of the planet’s history before that intelligent species goes extinct or
otherwise loses its technology, and

(e) the planet is at the right distance from us for their signals to be reaching us about
now.

Since the early 1980s, several projects have been undertaken to search for some sort of signal
from outer space that could be a message from another civilization. Complicating this type of
search is the possibility that another species might choose any frequency along the entire electromagnetic
spectrum to carry its signal. However, frequencies within the radio band would be the
most reasonable choices for communication because a minimum of energy is required to transmit
signals in this range. Furthermore, frequencies within the 1-10 GHz ranges, known as the “microwave
window,” are considered likely candidates since they would stand out from the galactic
background radiation. In addition to searching over a considerable range of frequencies, there is
the problem of where to look. Even within the Milky Way galaxy, the number of target stars with
possible planets is in the billions.

In 1960, radio astronomer Frank Drake conducted the first radio frequency search, Project Ozma,
for signals from other planetary systems. He aimed his 85-foot radio telescope toward two close
by sun-like stars, and tuned it to the frequency of radiation emitted by neutral hydrogen, assuming
that another intelligent species might select this frequency because of its astronomical importance.
He didn’t find anything, but his attempt spurred others to take up the search for extraterrestrial
intelligence (SETI).

The Soviet Union dominated SETI in the 1960s, turning their antennas in almost every direction,
hoping that at least a few advanced civilizations might be radiating very strong signals. Then, in
the early 1970s, NASA’s Ames Research Center in Mountain View, California, did a comprehensive
study called Project Cyclops. This project produced an analysis of the scientific and technological
issues involved in SETI that has been the basis of much of the SETI work since.

During the 1970s, many American radio astronomers conducted searches using existing antennas
and receivers. A few efforts from that era continue to this day. By the late-’70s, SETI programs
had been established at NASA’s Ames Research Center and at the Jet Propulsion Laboratory
(JPL). These two labs developed a dual strategy for a large scale study. Ames would examine
1,000 sun-like stars in a targeted search capable of detecting weak or sporadic signals. JPL, on
the other hand, would systematically survey the sky in all directions. In 1992, NASA had formally
adopted and funded this strategy, and observations began. This project was called the High
Resolution Microwave Survey (HRMS). However, within a year, Congress terminated the
funding.

Since then, SETI programs have continued with private funding. The SETI Institute, founded in
1984, helps coordinate research and find funding for numerous SETI researchers and projects.
The most comprehensive SETI project ever undertaken is Project Phoenix. Project Phoenix will
“listen” for radio signals from 1000 nearby, Sun-like stars, using the largest antennas in the
world. In addition, The Planetary Society, based in Pasadena, California, also has an active SETI
program.

NASA has recently initiated its new Origins Program, which takes a different approach in addressing
the question of life elsewhere in the Universe. The Origins Program seeks to learn how
stars and planets are formed. In the process, very advanced technology telescopes will use the
techniques of astrometry and direct imaging to look for evidence of planets around other stars.
The assumption is that a planet is the first requirement for life to emerge and evolve. If we
discover that planets are very common, then we will at least be encouraged in our other techniques
for detecting extraterrestrial intelligence.

Back to Table of Contents

This concludes the ASP version of this JPL text. An appendix, quizzes and quiz answers as well as a list of references is available at the homepage for this material.

 

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