The main end points of stellar evolution are to produce white dwarf stars, neutr

Question

The main end points of stellar evolution are to produce white dwarf stars, neutron stars and black holes. Write a short account explaining which groups of main sequence stars (as specified by their spectral type and mass) will ultimately end as each of the previously mentioned types of remnant star; the upper limit on the mass of the remnant star; and what, if any, force supports the remnant star against further collapse. (approx. 75-100 words on each of: white dwarfs, neutron stars and black holes).

Explanation / Answer

White dwarf -

When the triple-alpha process in a red giant star is complete, those evolving from stars less than 4 solar masses do not have enough energy to ignite the carbon fusion process. They collapse, moving down and to the left of the main sequenceuntil their collapse is halted by the pressure arising from electron degeneracy. An interesting example of a white dwarf is Sirius-B, shown in comparison with the Earth's size below. The sun is expected to follow the indicated pattern to the white dwarf stage.

Neutron stars -

For a sufficiently massive star, an iron core is formed and still the gravitational collapse has enough energy to heat it up to a high enough temperature to either fuse or fission iron. Either in the aftermath of a supernova or in just a collapsing massive star, the energy gets high enough to break down the iron into alpha particles and other smaller units, and still the pressure continues to build. When it reaches the threshold of energy necessary to force the combining of electrons and protons to form neutrons, the electron degeneracy limit has been passed and the collapse continues until it is stopped by neutron degeneracy. At this point it appears that the collapse will stop for stars with mass less than two or three solar masses, and the resulting collection of neutrons is called a neutron star. The periodic emitters called pulsars are thought to be neutron stars.

Black holes -

Any mass can become a black hole if it collapses down to the Schwarzschild radius - but if a mass is over some critical value between 2 and 3 solar masses and has no fusion process to keep it from collapsing, then gravitational forces alone make the collapse to a black hole inevitable. Down past electron degeneracy, on past neutron degeneracy and then on past the Schwarzchild radius to collapse toward zero spatial extent - the singularity. The Schwarzschild radius (event horizon) just marks the radius of a sphere past which we can get no particles, no light, no information.

R ( Schwarzschild ) = (2MG)/c^2

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