IX. ORIGIN OF STARS AND THEIR LIFE HISTORIES

All the Hydrogen and some of the Helium in our universe were condensed from the primordial radiation universe shortly after the Big Bang. How did all the other 90 some-odd elements come to be? How does a cloud of hydrogen and helium change into all the wondrous stars and galaxies we see today? 

The answer is that these other elements formed by fusion in stellar furnaces. It is supernovae (exploding stars) that both create elements heavier than Iron and release all elements back into space where they become mixed with the surrounding galactic material (mainly Hydrogen and Helium) and thereby enrich it in heavier elements. Our own solar system is but one of these hundreds of billions of systems that have formed in the Milky Way galaxy from materials enriched by neighboring supernova explosions. 

A. Formation of interstellar clouds (Nebulae). 

In our galaxy, there are large amounts of uncompressed gas and dust, nebulae, that form large clouds tens (10-50) of LY across of variable mass. Small nebulae may only have a fraction of a solar mass. One such, the Orion Nebula that can be seen with the naked eye in the Orion Constellation, includes some 10,000 solar masses. Others may have masses of millions of solar masses.  These are the clouds that eventually will form new star systems. So why would these clouds hang around for billions of years and then suddenly form star systems? 

There are several possibilities how these nebulae will compress to become star formation sites: 

  • when colliding galaxies’ gravitational fields disturb the gases 
  • when these clouds collide along the edge of a spiral arm, and 
  • when nearby supernovae (exploding stars) eject materials and the shock of these ejected materials colliding with the nebulae initiates contraction.
The first step in star formation is the formation of an opaque cloud of gas; in other words as the Hydrogen and Helium become denser, they increasingly block the passage of EM radiation and therefore appear dark. During this initial contraction, chemical reactions between the atoms of gases take place, and various molecules such as water, ammonia, silicon oxide, formaldehyde, methanol and many more are formed. All told, nearly one hundred of these molecules have been detected in nebulae, many of them organic molecules. 

As compression continues, the size of the nebula decreases, while gravity and density continue to increase. The contraction process however is not evenly spread throughout the nebula. In some parts of the nebula, there are areas where the gas and dust are more compressed than other areas, called clumps.