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What Is A White Dwarf Star?

A white dwarf is the final stage of the evolution of a star that is between .07 and 1.4 solar masses. White dwarfs are supported by electron degeneracy. They are found to the lower left of the main sequence of the Hertsprun Russel diagram. White dwarf stars got their name because the first to be discovered had a white color. They are characterized by a low luminosity, a mass close to that of our sun, and a radius only that of the earth. These stars are extremely dense because their large mass and small area White Dwarf stars are very dense. Their density is almost 1,000,000 time that of water. White dwarfs also have a low luminosity. This makes it so they have be within a few hundred parsecs away from earth to be observed (1 parsec = 3.26 light years).

Facts About White Dwarfs!

When a star stops burning the stars with less than 1.4 solar masses shrink greatly in size. While they shrink they start to become very faint. The value of 1.4 solar masses is referred to as the Chandrasekhar limit. Chandrasekhar reasoned that something must be holding up the White Dwarfs material against gravity. This was known as the electron degeneracy. When stars contract the electrons get close together there resistance keeps increasing and pushing closer together. This process is related to pressure. At great densities, pressure from the degenerate electrons is sufficiently high. It balances the force of gravity and the star stops contracting. So electron degeneracy stops the white dwarf form contracting and compresses the gas of the star. This means that the White Dwarf becomes incredibly dense. A mass the size of the sun is compressed into a volume only the size of the earth.

What Is The Chandrasekhar Limit?

The Chandrasekhar limit is the maximum possible mass for a stable White Dwarf star. The name was given after the Indian-born astrophysicist Subrahmanyan Chandrasekhar, who formulated it in 1930. Using Einstein’s special theory of relativity and the principles of quantum physics, Chandrasekhar showed that it is impossible for a white dwarf star, to be stable if its mass is greater than 1.4 times the mass of the Sun.

EXAMPLE: all direct mass determinations of actual white dwarf stars have resulted in masses slightly less than the Chandrasekhar limit.

A star that ends its nuclear-burning lifetime with a mass greater than the Chandrasekhar limit must form into either a neutron star or a black hole.

What Happens In Time?

Pressure from degenerate electrons doesn’t depend on temperature so stars are stable even though no more energy is ever generated within them. Because of electron degeneracy they are unable to contract any further. However, they still have stored energy that will radiate for a few billion years. Once the star burns out completely or stops radiating the white dwarf has reached the final stage of evolution in the cycle. It now becomes a cold and inert stellar remnant sometimes called a black dwarf. Our Sun is destined to die as a white dwarf. Before the sun transforms into a white dwarf it will turn into a red giant. When the sun becomes a red giant it will engulf Mercury and Venus in the process and at the same time it will blow away the earth’s atmosphere and boil the oceans. This will make earth uninhabitable. However this process will take billions of years to develop.

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