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Stars - Introduction

Stars are like our Sun, but there are many variations of them. One thing is true, they all begin there life by the spark of nuclear fusion at their cores.

Almost every dot in the night sky that we see are stars. All of those stars exist within our Milky Way Galaxy. Very rarely will a lone star actually exist in the spaces between galaxies, it is the norm for stars to only exist within galaxies.

There are two main groups of stars:

  • Population II Stars - old, metal poor stars
  • Population I stars - new, metal rich stars

In addition, there are two main endings of a stars life:

Lifetime of a normal star:

  • Dust cloud forms a Main Sequence star that burns for about 10 billion years
  • Star ends Main Sequence life and swells to a Red Giant (about the size of Earth's orbit) and burns for 100 million years
  • Star sheds is layers as a Planetary Nebula lasting 100,000 years
  • Only the core of the star remains as a White Dwarf

Lifetime of a large star:

  • Dust cloud forms a large star that burns on the Main Sequence for 50 million years
  • Star ends its Main Sequence life by swelling to a Red Supergiant (about the size of Mars' orbit) and burns for a million years
  • Core collapse can occur anytime after the million year Red Supergiant phase, and can go supernova
  • All that is left is a supernova remnant (a wispy looking nebula) and a compact object - Neutron Star or Black Hole

Stars can be classified as living in groups as there are no "stray" stars existing in the Universe. There are actually three types of stellar populations:

Open clusters reside mostly within the disk of a galaxy while globular clusters exist outside the galaxy filling a space called the halo. This halo is actually part of the galaxy and it surrounds the entire galaxy.

Open clusters and globular clusters will be discussed in greater detail in their own sections.

Of course, a star does not have to be in an open or globular cluster but almost always a star will be a part of a galaxy.

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But how does the life of a star begin?

(Science Cartoons Plus)

The most abundant material in the Universe is Hydrogen. Clumped together in cold clouds, hydrogen atoms can join together to form molecular hydrogen. This only occurs at extremely low temperatures.
These molecular clouds are very difficult to detect because no emission occurs. Much of the interstellar reddening (where a star or galaxy appears more red) occurs because of these molecular clouds. The image on the left is Barnard 68, a dark nebula. This is what a molecular cloud "looks" like. Notice the stars are barely visible behind this cloud. However, it is possible to view what's behind the cloud using an infrared filter.

A cloud has to be "just right" before it can host a future star system. For successful gravitational collapse of a cloud to form a proto-star (a star that has not yet initiated fusion), two criteria must be met:

  • Jeans Mass
  • Jeans Length

These two criteria, discovered by James Jean in the 1940's, places restrictions on a collapsing cloud:

Jeans length basically states that a molecular cloud of a particular size can become unstable and begin collapse.

The Radius in the above equation is Jeans Length, the minimum radius of the cloud before self-gravitation occurs.

The table on the left give some examples of the density of molecular clouds and the resulting size of the cloud. Within a molecular cloud, the distribution of debris is not always even. Fragmentation is suspected to occur in clouds exceeding

100 Solar masses. Smaller clouds within the large cloud can form stars. These molecular cloud fragments also fall under the Jeans criteria, and does affect the overall molecular clouds ability to continue self-gravitation, but that is an advanced topic.

So what can cause a molecular cloud to collapse?

  • Nearby stars that have ended their live in a supernova can send a shockwave stimulating collapse
  • Density waves within a galaxy propagate through the spiral structures that can stimulate collapse
  • Galaxy collisions can create huge gravitational forces to act of nearby clouds
  • A nearby Wolf-Rayet star can stimulate collapse
  • Sequential stellar formation - nearby stars forming close enough that their initial fusion can stimulate collapse

The cloud is collapsing, so now what?

As the molecular contracts under its own gravity, conservation of momentum forces the cloud to take on a disk shape, and it begins to spin. The very center of the cloud remains circular while the outlying gas forms a disk. Material from this disk is ejected perpendicular to to the disk as seen on the right.
(Image credit: Brooks/Cole Thomson Learning)

Once the proto-star reaches a certain temperature, the fusion of hydrogen atoms begins. The magic temperature is 10,000,000 Kelvin for our Sun, but for proto-stars of varying densities, the following formula applies:

It is important to know that there is a limit to stellar formation. The proto-star must fall within a:

  • lower limit mass of 0.8 Solar masses

  • an upper limit mass of 100 Solar Masses

A proto-star that is less than 0.8 Solar masses becomes a Brown Dwarf and a proto-star that exceeds 100 Solar masses becomes a Wolf-Rayet star - a very unstable star that cannot hold on to its outer layers.

As the Hydrogen atoms at the core of the proto-star are forced together by heat and pressure, the Coulomb Barrier is reached.

(Image credit: Brooks/Cole Thomson Learning)

The above image demonstrates that as the radius, r, or the space between protons is limited, a strong attractive force occurs.

(Image credit: Brooks/Cole Thomson Learning)

The Proton-Proton Chain begins with the fusing of hydrogen atoms into helium atoms - plus some gamma rays, neutrinos and photons.

Notice the times in the image above. It can take 1,000,000,000 years for the hydrogen atoms to exceed the Coulomb Barrier.

So what is the equation that demonstrates the energy produced by this reaction?

A T-Tauri star is a proto-star that has begun its fusion burning stage - with a bang and a shock wave that blows away any nearby debris close to the star.

I know this may seem like an overwhelming amount of information for an introduction, almost as complicated as trying to learn and understand the new system of POS software that came with the online store if there hadn't been some assistance available in learning the software along the way, but these pages are meant to aid you along as well. The following pages will revisit some of this material providing some insight with some real-world examples.

Stellar Astrophysics - the study of the stellar process - is not an easy subject, but hopefully this information will bring you one step closer to fully understanding the processes of not only our Sun, but all of the stars in the night sky.

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