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Image of Sun in Hα
At the heart of our Solar System is the Sun. Every day, the Sun rises and sets and ancient astronomers used this consistency to erect temples to predict seasonal changes for the harvest. The warmth of the Sun is vital to life on Earth because of photosynthesis and its warmth heats the surface to help with weather changes. But what is the Sun made of? How does the Sun generate its energy? Will the Sun shine forever? The answers are within....

Image of Sun in white light
Mean Distance from Earth: 149,598,000 km
Mean angular diameter: 32 arcmin
Radius: 696,109 km
Mass: 1.9891 x 1030 kg
Composition: 74% Hydrogen
  25% Helium
  1% Other
Mean Density: 1410 kg/m3
Mean Temperature: 5800K
Luminosity: 3.86 x 1026 Watts
Orbit about Galaxy: 220 million years
Orbital Speed: 220 km/s

Using a variety of imaging techniques, we have only been able to view only the surface layers of the Sun. There are only three layers of the Sun in which we can view (but never with the un-aided eye!):

Photosphere This is the lowest of the three levels, and this is the level we actually can see directly. Solar granulation occurs at this level and Sun Spots appear here.
Chromosphere The level above the photosphere is the chromosphere. To view this layer, it is best to use something called a Hydrogen-Alpha (Hα) filter to view this very thin layer. Filaments and spicules are visible with the Hα.
Corona The corona is the outermost layer of the Sun that is very dim, and is seen best during a total solar eclipse. The corona is the source of solar winds

So what do we know about our Sun?

Our Sun is an average star with a spectral class of G2. It sits near the center of the main-sequence (which is a part of a very important diagram called the Hertzsprung-Russell diagram). It is a big ball of hydrogen and helium gas that has collapsed to a point that hydrogen is forced to fuse deep in the core.

The Sun produces its energy through what is called the Proton-Proton Chain, which is the act of nuclear fusion - the fusion of hydrogen atoms to form a helium atom. The energy released is E = mc2, the famous Einstein equation. The Sun converts 600 million metric tons of hydrogen to helium every second. Regardless, the Sun is only middle-aged at about 4.6 billion years old. The Sun maintains its steady burning cycle by a process called hydrostatic equilibrium. This means the force of the energy exerted by the core of the Sun is equal to the amount of gravity collapsing toward the core.

The Sun does not rotate like a solid body (because it is not a solid body!). The equatorial regions rotate faster than the polar regions - this is called differential rotation. Solar rotation is around 27 days at the equator and around 33 days at the poles. It is this rotation that is believed to be the source of Sunspots as well as the sunspot maximum and sunspot minimum. The Sun has a 22 year cycle resulting from the twisting of magnetic fields because of differential rotation. That cycle is divided by 2 to indicate an 11 year cycle from average sunspot population to maximum or minimum.

Sometimes the Sun releases built-up energy (presumably from the magnetic fields) in the form of prominences, solar flares, and coronal mass ejections. These alterations in the Sun's magnetic field can affect communications on Earth. While the atmosphere safely filters out the affects of these high energy phenomenon, astronauts are at risk. However, since these eruptions have a large distance to travel, we will have some warning.

At the speed of light, photons from the Sun take a little over 8 minutes to reach us. These ejections do not travel that fast. At best, these eruptions will allow for some fantastic appearing aurora at the North and South poles here on Earth.

A filament is a portion of the chromosphere that rises higher than normal. These areas are pulled by magnetic fields that are generated by sunspots. Filaments are generally cool.
This famous Skylab image of a very large prominence is quite striking. Unlike filaments, prominences are hotter but are also a product of magnetic influence.
This is a SOHO image of a Coronal Mass Ejection - the most violent event that can occur in our Solar System. Solar flares and coronal mass ejections are similar in mechanism, but differ in strength. While a solar flare can have 1030 Joules of energy (around 1014 one-megaton nuclear bombs), coronal mass ejections are much stronger and can last a few hours.

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