Evolution of Stars

Stars begin with the condensation of interstellar medium under the force of gravity until the core has sufficient mass and heat to start fusion reactions.  The primary elements in these young stars are hydrogen and helium, and the star uses hydrogen fusion reactions to produce energy.


Young stars are generally found on the Main Sequence of the H-R diagram, where they spend about 90% of their lifetime.  Once they begin to run out of fuel their cores begin to collapse and get hotter while their surface expands and cools.  Stars the size of our sun become red giants, and in this stage of their life begin to fuse helium as their primary energy source.  At the end of this period (about 8-10% of the star’s lifetime) the fuel for the star is spent, and its core rapidly collapses into a white dwarf, with the external surface material blowing away into space.  The white dwarf is sustained by heat generated from gravity until it fades away.


If a main sequence star has sufficient mass (about 10x’s that of our sun) the loss of hydrogen fuel results in the formation of a red super giant.  This star has enough heat and pressure to not only fuse helium, but carbon and oxygen, and it is here that elements up to iron are formed.  When this size star approaches the end of its fuel it rapidly collapses and explodes, resulting in a supernova.  The area left where the star was now contains a pulsar or black hole. 


In the shock wave generated by the supernova conditions exist that allow the formation of the other elements up to uranium.  These are dispersed through the interstellar medium, ready to begin the cycle of star (and planet) formation over again.  The presence of these other elements tells us that our solar system is at least a second generation star.

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