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White Dwarf

Page history last edited by James McCarrick 15 years ago

White Dwarf in AE Aquarii System Credit: Casey Reed

White Dwarfs, sometimes known as Degenerate Dwarfs, are the remains of a start that has completed it's normal life cycle and has finished nuclear fusion. White dwarf are small stars made of electron-degenerate matter, they are about as massive as the sun but have the volume slightly bigger than the Earth. This makes white dwarfs one of the most dense forms of matter in the universe. White dwarfs have a very faint lumonosity because of their small size but the light comes from stored heat. 6% of all stars are thought to be white dwarfs.

 

White dwarfs are the final stage of about 97% of the stars in our galaxy with the remaining 3% becoming either Nuetron stars or black holes. Most stars live by fusing hydrogen within their cores into helium. Once all of the hydrogen is fused  the star starts to collapse but when the star compacts it heats up again and it fuses the very little hydrogen which is left in the shell wrapped around the core. The shell of hydrogen expands to the outer layer of the star and the star becomes a Red Giant. When the star expands it becomes cooler but the core keeps heating up until it can start fusing the helium made during the hydrogen fusing process. The Helium is fused into carbon and other heavier elements. The star won't be hot enough to burn the carbon so gravity will cause the core to contract causing a release of energy making the star an even larger giant. The star is very unstable and will begin to lose mass until it blows it's outer layer off the core remains intact which is known as a white dwarf. Meaning white dwarfs are ussually composed of Carbon and Oxygen but Helium white dwarfs are possible from mass loss in some binary systems. White dwarfs are surrounded by a expanding shell of gas called a planetary nebula.

 

White dwarfs no longer undergo fusion so the star has no source of forming energy and is not supported against collapse from gravity by the heat generated from fusion. It is only supported by Electron degeneracy pressure, this pressure sets a limit as to how much matter can be squeezed together without turning into a black hole, this causes the white dwarf to be very dense. If a carbon-oxygen white dwarf approached the mass limit it, ussually from mass traspher from a companion star, it may explose into a Type Ia Supernova from the process of carbon detonation.

 

When first created white dwarfs are very hot but since they have no source of energy the slowly begin to radiate away heat and begin to cool down. White dwarfs start with a high color temperature but will redden over time. As time passes a white dwarf will cool to a temperature to where it can no longer be seen and become a cold black dwarf but it is believed that there are no black dwarfs in existence yet.

 

Composition and Structure


 

Typically white dwarfs have a mass between .5 and .7 solar masses, but the radius of most are between .008 and .02 times the radius of the sun making it close to the size of Earth's radius. Thus white dwarfs have a mass the size of the sun's but a volume a million times smaller than the sun. Making a white dwarf 1,000,000 times more dense than the sun. White dwarfs are composed of one of the most dense types of matter in the galaxy with only Nuetron Stars and Black Holes being denser. The reason white dwarfs do not succumb to gravitational collapse is because of Electron degeneracy pressure. This is when a white dwarf becomes compressed, it will increase the number of electrons within the set volume this will make an increase of energy in the electrons causing pressure. Degenerate matter is easily compressed meaning that high mass white dwarfs density is much greater than a low mass white dwarf showing that as the radius of a white dwarf decreases its relative mass increases. White dwarfs have a mass limit of 1.4 solar masses. If a white dwarf approaches this limit the pressure of the electrons would not be able to equal the force of gravity and the white dwarf would collapse into an even denser object such as a black hole or neutron star. This would happen since white dwarfs do not have any nuclear reactions occuring in their cores. If a white dwarf were to be gaining mass from a neighboring star then the collapse would cause a Type Ia Supernova and the white dwarf would explode before reaching the mass limit.

 

The End of a White Dwarf


White dwarfs are stable from their creation and gradually begin to cool because they don't have any nuclear reactions in their core. Eventually white dwarfs will become black white dwarfs or known as black dwarfs which is the remmnant of a white dwarf that emits very little heat and light. In the future as the universe continues to expand it is thought that the galaxies will disintigrate and stars will begin to move into intergalactic space. White dwarfs should withstand this and though some may collide and create Type Ia supernova. The rest of a white dwarfs lifetime is thought to be the lifetime of a proton which varies depending upon different theories. If protons do decay according to theory then the mass of a white dwarf will deteriorate slowly until it becomes a chunk of nondegenerate matter and eventually disintergrates into nothing.

 

 

A type Ia Supernova in the galaxcy NGC 4526 www.astrophysics.com

References


 

http://hyperphysics.phy-astr.gsu.edu/hbase/astro/whdwar.html

 

http://csep10.phys.utk.edu/astr162/lect/novae/novae.html

 

http://www.astronomynotes.com/evolutn/s11.htm

 

http://imagine.gsfc.nasa.gov/docs/science/know_l1/dwarfs.html

 

http://www.windows.ucar.edu/tour/link=/the_universe/WD.html&edu=high

 

http://zebu.uoregon.edu/~js/ast122/lectures/lec17.html

 

 

 

By: James McCarrick and Mike DeLeo

Comments (1)

kevin kearney said

at 6:04 pm on Mar 22, 2009

This project is Obviously great! Kindergarten has been closed on James many times and that is why he made such a great project. it has alot of information and is easy to read. I like the seperation of sections, it makes it easy to follow. put more pictures to make it more visually addicting.

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