From Stars to Galaxies and the Role of Black Holes

Stars are the most numerous objects in the sky. But, they are a part of galaxies. Galaxies are clusters of stars that usually have a definite geometrical shape with various forms of luminosity and spiral arms. A star is a bright spherical object, but a galaxy has geometry.

Stars are formed by nebula, which is a cloud of dust and gas out in space, though an extremely large one, but still just a dust and gas cloud. As the gas and dust coalesce, they form solid matter which continues to grow more massively. At this stage you have a protostar. The time that it takes for this event to occur can be in the range of several hundred thousand years.

Over time, however, as more matter begins to accumulate, you will have a bright object forming into a T Tauri star. This young star has variable luminosity and will have a mass from .5 to 2.5 solar masses. This is an unstable star because it has not reached critical mass where it can start it's own nuclear fusion. (Nuclear fusion is the process where two or more atoms merge together and create a different atom releasing tremendous energy. The hydrogen bomb is a fusion based nuclear explosion. The nuclear bombs used at Hiroshima and Nagasaki were fission based - like a cue ball hitting a collection of pool balls - they spread apart.)

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Stars have a life cycle. After they pass from the T Taurus formation period, the enter the main sequence.

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_{Image Source: http://en.wikipedia.org/wiki/File:TTauriStarDrawing.jpg}

The proto star enters the main sequence cycle.

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_{HR-Diagram: http://en.wikipedia.org/wiki/File:HRDiagram.png}

The main sequence as shown by the image above is a band of stars that is continuous and distinctive. On the Hertzsprung-Russell diagram it appears on plots of stellar color versus brightness.

After a star has formed, it continues to operate at a high energy level through the use of nuclear fusion where hydrogen atoms fuse into helium. During this stage of the star's lifetime, it is located along the main sequence primarily by its mass.

One of the main characteristics of main sequence stars is that they are in hydrostatic equilibrium where outward thermal pressure (heat from the hot core) is balanced by by gravitational pressure accumulating from the outer gaseous layers of the star.

Once a star begins its fusion reactions, the star may live for millions of years. It can take different directions depending on the amount of initial mass when it became a star.

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_{Star Life Cycle: www.ir.isas.jaxa.jp/ASTRO-F/Outreach/seika_e.html}

So, depending on its mass, a star could evolve into a Red Giant then either a SuperNova (explode), collapse into a black hole or dissipate into a nebula.

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Once a star is formed from the nebula neighborhood, chances are very good that other stars will likewise form. Once the fusion reactions kick in, then in a typical star the enormous gravitational effects also kick in. Simply put, stars are not isolated, they manage to get together with other stars. When you have millions of stars behaving that way, you will have a galaxy.

One may ask, if that were so, why is the Sun the only star in the viewable neighborhood? The answer is distance. The distances in the Universe are immense. For example, the nearest star is Proxima Centauri, and that is 4.22 light years from our Sun. A light year is the distance light travels in one year, and that is about six trillion miles. So Proxima Centauri is about 25 trillion miles from our Sun.