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Chapter 4.5 More About White Dwarfs

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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.

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.

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