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[[Image:White dwarf stars browse.jpg|thumb|White Dwarf Stars photographed by the Hubble Space Telescope.]]

[[Image:White dwarf stars browse.jpg|thumb|White Dwarf Stars photographed by the Hubble Space Telescope.]]

A '''White dwarf''' is the electron-degenerate remnant of a [[star]] that has exhausted the nuclear fuel in its core and subsequently lost its outer layers. It represents the final stage of [[Stellar_Evolution|stellar evolution]] for most stars, including our own [[Sun]].<ref>http://adsabs.harvard.edu/abs/2001PASP..113..409F</ref>    White dwarfs are extremely dense - a white dwarf with a mass equivalent to our Sun would be compacted into a volume no greater than the Earth.  As there is no more thermonuclear fusion within white dwarfs, their only source of [[light]] and [[heat]] is leftover thermal energy.

A '''White dwarf''' is the electron-degenerate remnant of a [[star]] that has exhausted the nuclear fuel in its core and subsequently lost its outer layers. It represents the final stage of [[Stellar Evolution|stellar evolution]] for most stars, including our own [[Sun]].<ref>http://adsabs.harvard.edu/abs/2001PASP..113..409F</ref>    White dwarfs are extremely dense - a white dwarf with a mass equivalent to our Sun would be compacted into a volume no greater than the Earth.  As there is no more thermonuclear fusion within white dwarfs, their only source of [[light]] and [[heat]] is leftover thermal energy.

Typically any star between 0.5 and 8 solar masses will end its life as a white dwarf.  Most of these dwarfs are primarily made up of [[carbon]] and [[oxygen]], however stars of 4 solar masses or more will have the ability to fuse carbon, which will result in a white dwarf chiefly made up of oxygen, [[neon]], and [[magnesium]].<ref>http://adsabs.harvard.edu/abs/2005ASPC..334..165W</ref>  Any star greater then 8 solar masses will eventually develop a core that will be too massive to support by electron-degeneratcy (the [[Chandrasekhar limit]]) and collapse.  This collapse will result in a [[supernova]], and depending on the star's mass, the end result will be a [[neutron star]] or a [[black hole]].

Typically any star between 0.5 and 8 solar masses will end its life as a white dwarf.  Most of these dwarfs are primarily made up of [[carbon]] and [[oxygen]], however stars of 4 solar masses or more will have the ability to fuse carbon, which will result in a white dwarf chiefly made up of oxygen, [[neon]], and [[magnesium]].<ref>http://adsabs.harvard.edu/abs/2005ASPC..334..165W</ref>  Any star greater then 8 solar masses will eventually develop a core that will be too massive to support by electron-degeneratcy (the [[Chandrasekhar limit]]) and collapse.  This collapse will result in a [[supernova]], and depending on the star's mass, the end result will be a [[neutron star]] or a [[black hole]].

As a white dwarf is no longer powered by thermonuclear fusion, all the light and heat generated is from remaining thermal energy.  Thus, the star will very slowly cool over time into a hypothetical ''black dwarf'', where it can no longer emit light or heat in significant quantities.  As the time period required for this to occur is longer than 13.7 billion years, no black dwarf is expected to exist yet.  Even when the black dwarf state is reached, the stellar remnant itself is still expected to continue to exist indefinitely.

As a white dwarf is no longer powered by thermonuclear fusion, all the light and heat generated is from remaining thermal energy.  Thus, the star will very slowly cool over time into a hypothetical ''black dwarf'', where it can no longer emit light or heat in significant quantities.  As the time period required for this to occur is longer than 13.7 billion years, no black dwarf is expected to exist yet.  Even when the black dwarf state is reached, the stellar remnant itself is still expected to continue to exist indefinitely.

A white dwarf that is part of a binary system may gradually draw in matter from its companion star. If it accumulates enough mass to exceed the Chandrasekhar limit it will explode in a [[Supernova#Type_1a|type Ia supernova]]. What remains is an even denser object known as a [[neutron star]].

A white dwarf that is part of a binary system may gradually draw in matter from its companion star. If it accumulates enough mass to exceed the Chandrasekhar limit it will explode in a [[Supernova#Type 1a|type Ia supernova]]. What remains is an even denser object known as a [[neutron star]].

==Internal structure of a White Dwarf==

==Internal structure of a White Dwarf==

This density is brought about by [[gravity]] crushing the [[electrons]] of [[atoms]] together.  Normally electrons with the same spin cannot occupy the same energy level, therefore only two electrons can occupy a single energy level (as electrons can spin only one of two ways).  Because in a white dwarf the electrons are forced so closely, all the energy levels of the atoms are filled with electrons, and because no more than two electrons can fill the energy levels in the atoms within the white dwarf, the star becomes [[degenerate]].  At this point, there is insufficient gravity to compress the matter any more, as [[quantum mechanics]] will not allow for it, preventing the white dwarf from collapsing any further, stabilizing it.<ref name="nasa">http://imagine.gsfc.nasa.gov/docs/science/know_l2/dwarfs.html</ref>

This density is brought about by [[gravity]] crushing the [[electrons]] of [[atoms]] together.  Normally electrons with the same spin cannot occupy the same energy level, therefore only two electrons can occupy a single energy level (as electrons can spin only one of two ways).  Because in a white dwarf the electrons are forced so closely, all the energy levels of the atoms are filled with electrons, and because no more than two electrons can fill the energy levels in the atoms within the white dwarf, the star becomes [[degenerate]].  At this point, there is insufficient gravity to compress the matter any more, as [[quantum mechanics]] will not allow for it, preventing the white dwarf from collapsing any further, stabilizing it.<ref name="nasa">http://imagine.gsfc.nasa.gov/docs/science/know_l2/dwarfs.html</ref>

Gravity at the surface of a white dwarf is around 100,000 times that of the Earth, the result is that heavier atoms in the atmosphere will sink into the surface, leaving only the lightest elements behind.  Thus the thin atmosphere is made up mainly of [[hydrogen]] and [[helium]].  The temperature at this surface can range from 40,000 to 8,000 K, although white dwarfs as cool as 3,900 K have been found.  The cooler the white dwarf, the older it is likely to be as they lose thermal energy over time.<ref>http://adsabs.harvard.edu/abs/2004ApJ...612L.129G</ref> The surface itself is composed of a crust of [[carbon]] and [[oxygen]], the bottom of which is believed be a crystalline lattice.<ref name="nasa">http://imagine.gsfc.nasa.gov/docs/science/know_l2/dwarfs.html</ref>

Gravity at the surface of a white dwarf is around 100,000 times that of the Earth, the result is that heavier atoms in the atmosphere will sink into the surface, leaving only the lightest elements behind.  Thus the thin atmosphere is made up mainly of [[hydrogen]] and [[helium]].  The temperature at this surface can range from 40,000 to 8,000 K, although white dwarfs as cool as 3,900 K have been found.  The cooler the white dwarf, the older it is likely to be as they lose thermal energy over time.<ref>http://adsabs.harvard.edu/abs/2004ApJ...612L.129G</ref> The surface itself is composed of a crust of [[carbon]] and [[oxygen]], the bottom of which is believed be a crystalline lattice.<ref name="nasa"/>

==Observations of White Dwarfs==

==Observations of White Dwarfs==

[[Adriaan Van Maanen]] discovered the first lone white dwarf in 1917, now named after him as [[Van Maanen's Star]].  Over time, several other faint white stars were discovered to have high proper motion, indicating they were relatively near the [[Solar System]], confirming them as white dwarfs as well.  The term white dwarf itself was first coined by [[Willem Luyten]] in his observation of these stars in 1922.<ref>http://adsabs.harvard.edu/abs/2005AAS...20720501H</ref>  By 1939 18 such stars were known, increasing to over 100 by 1950, and over 9,000 known today.

[[Adriaan Van Maanen]] discovered the first lone white dwarf in 1917, now named after him as [[Van Maanen's Star]].  Over time, several other faint white stars were discovered to have high proper motion, indicating they were relatively near the [[Solar System]], confirming them as white dwarfs as well.  The term white dwarf itself was first coined by [[Willem Luyten]] in his observation of these stars in 1922.<ref>http://adsabs.harvard.edu/abs/2005AAS...20720501H</ref>  By 1939 18 such stars were known, increasing to over 100 by 1950, and over 9,000 known today.

The [[Hubble Space Telescope]], with its 2.4 meter mirror and advanced optics, has been successfully viewing white dwarfs with its Wide Field and Planetary Camera. In August of 1995, this camera observed more than 75 white dwarfs in the [[globular cluster]] M4 in the constellation [[Scorpius]]. These white dwarfs were so faint that the brightest of them was no more luminous than a 100 watt light bulb seen at the [[Moon|moon's]] distance.  

The [[Hubble Space Telescope]], with its 2.4 meter mirror and advanced optics, has been successfully viewing white dwarfs with its Wide Field and Planetary Camera. In August 1995, this camera observed more than 75 white dwarfs in the [[globular cluster]] M4 in the constellation [[Scorpius]]. These white dwarfs were so faint that the brightest of them was no more luminous than a 100 watt light bulb seen at the [[Moon|moon's]] distance.  

== Sources ==

== Sources ==