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 and they are found to the lower left of the main sequence of the HR (Hertsprung Russel) diagram. White dwarfs represent a stable phase in which stars of less than 1.4 solar masses live out the rest of their lives. White dwarf stars got their name because of the white color of the first few that were discovered. They are characterized by a low luminosity, a mass close to that of our sun, and a radius only that of the earth. Because of their large mass and small area these stars are extremely dense and compact objects with average densities approaching up to 1,000,000 times that of water. White dwarfs have low luminosities. Because of this they can be observed only within a few hundred parsecs from the earth ( 1 parsec = 3.26 light years).
Facts About White Dwarfs
All stars are burning at some point in their lives but eventually a star stops burning. When the stars stop burning the stars with less than 1.4 solar masses shrink in size. As they shrink they start to grow very faint. But regardless of their color they are called white dwarfs. The value of 1.4 solar masses is known as the Chandrasekhar limit. Chandrasekhar reasoned that something must be holding up material in white dwarfs against gravity, something known as electron degeneracy. When star contracts, electrons get close together and there is a continued increase in their resistance to being pushed even closer. This process is related to pressure. At great densities, pressure from the degenerate electrons is sufficiently great, 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. What this means is that a white dwarf is incredibly dense. A mass the size of the sun is compressed into a volume only the size of the earth. This is so dense that a teaspoon of white dwarf weighs ten tons.
What Is the Chandrasekhar Limit?
The Chandrasekhar limit is the maximum theoretically possible mass for a stable white dwarf star. The limiting value was named 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, which is supported solely by a degenerate gas of electrons, to be stable if its mass is greater than 1.4 times the mass of the Sun. If such a star does not completely exhaust its thermonuclear fuel, then this limiting mass may be slightly larger. For 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 Chandraskehar limit must become either a neutron star or a black hole.
What Happens In Time?
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 can’t contract 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 and it becomes a cold and inert stellar remnant sometimes called a black dwarf. Our Sun is destined to die as a white dwarf. But, before that happens, it will evolve 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 its oceans. This will make earth uninhabitable but this process will take billions of years to develop.
Pictures
http://www.britannica.com/bcom/eb/article/single_image/0,5716,30581+asmbly%5Fid,00.html
http://www.britannica.com/bcom/eb/article/single_image/0,5716,8308+asmbly%5Fid,00.html
http://www.britannica.com/bcom/eb/article/single_image/0,5716,4087+asmbly%5Fid,00.html
http://oposite.stsci.edu/pubinfo/stars.html
References
http://zebu.uoregon.edu/text.html
http://www.einet.net/galaxy/Science/Astronomy.html