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A Black Hole Is Born

Black holes are thought to form from stars or other massive objects if and when they collapse from their own gravity to form an object whose density is infinite: in other words, a singularity. During most of a star's lifetime, nuclear fusion in the core generates electromagnetic radiation, including photons, the particles of light. This radiation exerts an outward pressure that exactly balances the inward pull of gravity caused by the star's mass.

As the nuclear fuel is exhausted, the outward forces of radiation diminish, allowing the gravitation to compress the star inward. The contraction of the core causes its temperature to rise and allows remaining nuclear material to be used as fuel. The star is saved from further collapse -- but only for a while.

Eventually, all possible nuclear fuel is used up and the core collapses. How far it collapses, into what kind of object, and at what rate, is determined by the star's final mass and the remaining outward pressure that the burnt-up nuclear residue (largely iron) can muster. If the star is sufficiently massive or compressible, it may collapse to a black hole. If it is less massive or made of stiffer material, its fate is different: it may become a white dwarf or a neutron star.

White Dwarf

When small stars (up to 8 times the size of the Sun) exhaust their nuclear fuel, they typically shed large amounts of matter, leaving a core that eventually cools and contracts gravitationally to about the size of the Earth. The result is a white dwarf: the more massive it is, the greater its inward gravitational pull, and the smaller it becomes.

A teaspoonful of white dwarf material would weigh five-and-a-half tons or more in the Earth's gravity! Yet a white dwarf can contract no further; its electrons resist further compression by exerting an outward pressure that counteracts gravity.

There are many white dwarfs in our galaxy, but most are too dim to be seen. One of the first to be discovered was Sirius B, the dense companion star to Sirius.

Sirius and its white dwarf companion
Sirius B was the first star shown to exhibit a gravitational redshift.

As such, Sirius B's redshift provided supportive evidence of an important prediction of Einstein's General Theory of Relativity. Until then (1924), gravitational redshift had been difficult to detect in lower mass/density stars such as the Sun.
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Sirius B has another claim to fame. This white dwarf star fueled a debate in the 1920s between leading astrophysicists Subrahmanyan Chandrasekhar and Sir Authur Eddington. At issue was the following question: How far can a star possibly collapse? And for a given mass, what will it collapse into?

Chandrasekhar derived a relation ship between the star's mass and its radius which sets an upper limit to the mass a white dwarf can have, beyond which it will collapse to a neutron star or, if sufficiently massive, to a black hole. Calculations put the "Chandrasekhar Limit" at 1.4 solar masses. Decades later Chandrasekhar's fundamental contributions were recognized when he won the 1983 Nobel Prize in Physics.

Neutron Stars

Supernova SN1987a
More massive stars tend to burn hotter and faster. Once all the nuclear fuel has been exhausted, such stars quickly collapse, shedding much of their mass in dramatic explosions called supernovae. The most recent event of this kind was observed in 1987 when a star weighing the equivalent of 20 suns blew up in a neighboring galaxy 160,000 light years away.
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If after such an explosion, the remaining material is greater than 1.4 solar masses, it will contract into an unimaginably dense core made solely of neutrons. Neutron stars are so dense a teaspoonful would weigh 100 million tons! Eventually astronomers may discover the telltale signs of a neutron star exactly where the old star met its doom, though as yet none has been detected.

As heavy as neutron stars are, if they're less than 2 solar masses, they too can only contract so far and no further. That's because, as crushed as they are, the neutrons also resist the inward pull of gravity, just as a white dwarf's electrons do. However, if after a star collapses, the remaining core exceeds approximately 2 solar masses, the outcome is thought to be very different. The precise mass limit is uncertain and depends on the nuclear physics going on within the core, a topic of much debate within the physics community.

Ed Seidel, NCSA/Univ. of Illinois, on-camera
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QuickTime Movie (783K); Sound File (515K); Text

If the star's final mass exceeds much beyond 2 solar masses, there is no outward force that can resist gravity. The core continues to collapse to a critical size or circumference beyond which there is only one fate: to form a black hole.

What is a black hole made of? Where does it begin? Where does it end?

Forward to Anatomy of a Black Hole
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Copyright 1995, The Board of Trustees of the University of Illinois

NCSA. Last modified 11/7/95.