The Fate of Massive Stars
The fate of massive stars (see Thorne 1994, chh. 4-9)
Massive stars begin to shrink if they run out of the "fuel" for burning. They become cooler and cooler, and smaller and smaller. In fact, astronomers found many cool and small stars called "white-dwarf". Their mean densities are astonishingly high.
Let us compare the mean densities of a white-dwarf, the sun, and the earth:
- White-dwarf Sirius B: 4,000,000 grams per cubic centimeter
- Sun: 1.4 grams per cubic centimeter
- Earth: 5.5 grams per cubic centimeter
Thus a white-dwarf has an extremely high density, and what sort of force is operating for sustaining such a high pressure? It is due to quantum mechanical laws: electron degenerancy produces such an enormous force counteracting the gravity; thus the star does not collapse. Chandrasekhar showed that there is a limit of the mass of the star if it is to remain as a white-dwarf, and this is Chandrasekhar Limit, approximately 1.46 times greater than the mass of the sun. This limit is not the value of initial mass of the star, and today it is known that a star whose initial mass is less than 4 times of the sun's mass is going to be a white-dwarf.
What if the initial mass is greater than this? Some will become a neutron star. Stars with an initial mass, about 20 times greater than the sun's mass, die by supernova explosion, thereby forming a neutron star at its core; its mean density is still higher than a white-dwarf, and it is approximately 100,000,000,000,000 grams per cubic centimeter! Its radius may be approximately 3 times as great as the Schwarzschild radius (horizon). Zwicky, Baade, Landau, and Oppenheimer contributed to clarifying the nature of a neutron star.
But what if a star has a still greater initial mass? We finally come to a black hole! Thus you may begin to understand why it was so difficult for many physicists to understand the possibility of a black hole.
Reference
Thorne, Kip (1994) Black Holes and Time Warps, Papermac, 1994.
Last modified July 4, 2002. (c) Soshichi Uchii