E7 Compact Objects

Topic

At the end of their lives, stars leave behind various types of compact objects, depending on the mass of the original star.

White dwarfs are supported by the degeneracy pressure of their own electrons—a pressure that holds firm up to a fixed mass limit of approximately one and a half solar masses; below this threshold, the white dwarf remains stable, but if its mass were to exceed it, that same degeneracy pressure would no longer suffice to support it.

Neutron stars, meanwhile, are the remnants of supernovae from stars far more massive than those that produce white dwarfs; they possess extreme densities and rotate rapidly—a rotation that is precisely what makes them observable as pulsars.

Black holes form when the mass of the stellar remnant exceeds a second threshold—one even higher than that of white dwarfs. Beyond this second threshold, no known pressure can halt the collapse, and all the remnant's matter concentrates within an event horizon—the Schwarzschild radius—whose size grows in direct proportion to the black hole's mass.

Hawking radiation is a quantum effect whereby black holes emit thermal radiation and lose mass very slowly; the temperature of this emission is inversely proportional to the black hole's mass, rendering it completely negligible for black holes of stellar mass or greater, and relevant only for hypothetical black holes much smaller than those. Thus, from the compact objects with which this document begins to the Hawking radiation with which it concludes—passing through the white dwarf mass threshold and the second threshold that gives rise to black holes—compact objects demonstrate that a star's ultimate fate is determined entirely by a single number: the mass remaining after its life has ended.

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