The Evolution of Stars

The universe is not static. A pretty obvious statement; but also a true one. Although, to my knowledge, no one has ever seen the birth of a star, we have seen Novas. The most famous one occurred in 1050, a little before the Norman invasion of England, and created the Crab Nebula. The following paragraphs outline the accepted theory about the evolution of stars.

Stars start as a cloud of dust. The cloud is large (many solar systems large), cold (~15 degrees Kelvin) and composed mainly of hydrogen. Initially the cloud is in equilibrium; then at some point, the equilibrium is disturbed and the cloud begins to collapse. The collapse continues until the internal temperature of the proto-star rises to a value sufficient to ignite fusion fires inside the core. This nuclear fusion generates an outward pressure which balances the inward pull of gravity and places the star back into equilibrium. During most of a star's remaining life, nuclear fusion maintains this equilibrium. The initial fuel for the stars "fusion reactor" is Hydrogen. As this fuel is used up, the star collapses some, the temperature rises a little and more exotic (heavier element) fuels are used to maintain the star. Eventually, enough nuclear fuel is used up and the core collapses. The object it collapses to is determined by the mass of the star in its final stage and the outward force available to stop the collapse. It may end as a White Dwarf, a Neutron Star, or a Black Hole.

At present, the only known "force" that is capable of stopping the collapse is provided by Quantum Mechanics; the Pauli Exclusion Principle. The Pauli principle was initially postulated for electrons. It is the "force" that accounts for atoms. While it is not a force in the classical sense, it does act like one. (Basically this means we don't understand it very well.) Simply stated, the Pauli Principle states that two electrons cannot occupy the same energy state in an atom. This "degeneracy pressure" opposes the collapsing star which is attempting to force all of the electrons down in the electrons lowest state. For stars whose mass in its final stage is up to (approximately) 1.4 times the size of our sun (solar masses), this force is strong enough to stop the collapse. The electrons in these star resist the collapse and the end result for these Stars is a White Dwarf (a star made that is essentially a large atom). A teaspoonful of white dwarf material would weigh five-and-a-half tons or more in the Earth's gravity!

For stars whose final mass is greater than 1.4 solar masses the Pauli Principle would require the electrons to travel faster than the speed of light to remain in orbit about the nucleus and stop the gravitational collapse. For these stars, the collapse continues and the electrons are forced into the nucleus. While in the nucleus, the electrons fuse with protons and create neutrons. Interestingly neutrons have their own (pseudo) Pauli Exclusion Principle. This is the "degeneracy pressure" which is thought to create Neutron stars (giant nuclei).

It may be infered from the previous paragraph that above a certain mass, even the Neutron degeneracy pressure will not be strong enough to stop the stars collapse. That occurs at approximately 3 solar masses. For stars meeting this criteria, there is no "known" force that is capable of stopping the collapse. Since the force of gravity is proportional to inverse R-squared, as the star collapses down to a point, the gravitational force increases to infinity, and the escape velocity

becomes greater than the speed of light. Under these conditions, nothing, even light, can escape the star; hence the name, Black Hole.

It was stated at the start of this page that this is a theory. Thats because none of it can be proven at the present time. However asside from some aspects of the Black Holes, there has been no absolute flaw found in the theory either. Nothing has been seen that says "this is wrong" or even, "perhaps, its not complete." In fact White Dwarfs and Neutron stars have been located in the universe. This is therefore not one of the present mysteries.