White dwarf

A white dwarf is a star with a color like most other stars, but withlow absolute brightness. Such stars were discovered in the 19th century; the first ones found were white. The color of a star isa measure of the surface temperature: white stars are like the Sun, blue stars hotter and red stars are cooler. White dwarfs areso dim because they are small and not because they are cool. Color and size explain the name white dwarf. A more appropriate namefor white dwarfs is degenerate dwarf (see below), an example of a degenerate star . Some white dwarfs are blue, rather than white. Whitedwarfs may have in principle any color and the term 'blue degenerate dwarf' is generally preferable to 'blue white dwarf'.

Formation

Most small and medium-size stars will end up as white dwarfs, after all the hydrogen they contain is fused into helium . Near the end of its nuclearburning stage, such a star goes through a red giant phase and then expels most ofits outer material (creating a planetary nebula ) until only the hot(T > 100,000 K) core remains, which then settles down to become a young white dwarf.

A typical white dwarf is half as massive as the Sun, yet only slightly bigger than Earth . This makes white dwarfs one of the densest forms of matter, surpassed only by neutron stars and quark stars . The higher the mass ofthe white dwarf, the smaller the size. There is an upper limit to the mass of a white dwarf, the Chandrasekhar limit (about 1.4 times the mass of the Sun ), after which the pressure of the electrons is nolonger able to balance gravity , and the star continues to contract, eventuallyforming a neutron star .

Despite this limit, most stars end their life as white dwarf, since they tend to eject most of their mass into space beforethe final collapse (often with spectacular results, see planetarynebula ). It is thought that even stars 8 times as massive as the Sun will in the end die as white dwarfs.

Characteristics

Many white dwarfs are approximately the size of the Earth , typically 100 times smallerthan the Sun . They may have the same mass as the Sun and so are very compact. A radius whichis 100 times smaller, implies that the same amount of matter is packed in a volume that is typically 100³=1,000,000 smaller thanthe Sun and so the average density of matter in white dwarfs is 1,000,000 times denser than the average density of the Sun. Suchmatter is called degenerate. In the 1930's the explanation is given as a quantum mechanical effect: the weightof the white dwarf is supported by the pressure of electrons ( electron degeneracy ), which only depends on density and not on temperature.

If, for all observed stars, one makes a diagram of (absolute) brightness versus color ( Hertzsprung-Russell diagram ), not all combinations ofbrightness and color occur. Few stars are in the low-brightness-hot-color region (the white dwarfs), but most stars follow astrip, called the main sequence . Low mass main sequence stars are smalland cool. They look red and are called red dwarfs or (even cooler) brown dwarfs . These form an entirely different class of heavenly bodies than whitedwarfs. In red dwarfs, as in all main-sequence stars, the pressure counterbalancing the weight is caused by the thermal motion ofthe hot gas. The pressure obeys the ideal gas law. Another class of stars is called giants: stars in the high-brightness part ofthe brightness-color diagram. These are stars blown up by radiation pressure and are very large.

White dwarf stars are extremely hot; hence the bright white light they emit. This heat is a remnant of that generated from thestar's collapse, and is not being replenished (unless they accrete matter from other close by stars), but since white dwarfs havean extremely small surface area from which to radiate this heat energy they remain hot for a long period of time.

Eventually, a white dwarf will cool into a black dwarf . Black dwarfs areambient temperature entities and radiate weakly in the radio spectrum, according to theory. However, the universe has not existed long enough for any white dwarfs to have cooled down this far yet, and so no blackdwarfs are thought to exist.

Many nearby, young white dwarfs have been detected as sources of soft X-rays (i.e. lower-energy X-rays); soft X-ray and extremeultraviolet observations enable astronomers to study the composition and structure of the thin atmospheres of thesestars.

White dwarfs cannot be over 1.4 solar masses, the Chandrasekhar limit , but there is a working method to get them over this limit. Like a nova , a white dwarf can accrete material from a companion. Unlike a nova, the material accretesslowly and remains stable. The mass of the white dwarf increases until it hits the 1.4 solar mass limit, at which degeneracypressure cannot support the star. This creates a type Ia supernova and is themost powerful of all the supernovae.

History of discoveries

In 1862 Alvan GrahamClark discovered a dark companion of the brightest star Sirius (Alpha CanisMajoris). The companion, called Sirius B or the Pup, had a surface temperature of about 25,000 kelvins, so it was classified as ahot star. However, Sirius B was about 10,000 times fainter than the primary, Sirius A. Since it was very bright per unit ofsurface area, the Pup had to be much smaller than Sirius A, with roughly the diameter of the Earth.

Analysis of the orbit of the Sirius star system showed that the mass of the Pup was almost the same as that of our own Sun.This implied that Sirius B was thousands of times more dense than lead. As more white dwarfs were found, astronomers began todiscover that white dwarfs are common in our Galaxy. In 1917 , Adriaan Van Maanen discovered Van Maanen's Star , the second known white dwarf and the closest one to the Sun other than SiriusB.

After the invention of quantum mechanics in the 1920's, anexplanation the density of a white dwarf was found in 1926 . R.H. Fowler explained the high densities in an article "Dense matter" (Monthly Notices R. Astron. Soc.87, 114-122) using the electron degenerate pressure a few monthsafter the formulation of the Fermi-Dirac statistics foran electron, on which the electron pressure is based.

S. Chandrasekhar discovered in 1930(Astroph. J. 74, 81-82) in an article called "The maximum mass of ideal white dwarfs" that no white dwarf can be more massivethan about 1.2 solar masses". This is now called the Chandrasekhar limit . Chandrasekhar received the Nobel prize in 1983.

The hot white dwarf Sirius B, at 8 light years the closest known white dwarf, seen in X-rays as an intense x-ray source(lines radiating from Sirius B are image artifacts). The white dwarf is part of a binary system. Its companion, in this image thefainter source, is at the position of Sirius A (Alpha Canis Majoris). In visible light, Sirius A is the brightest star in thenight sky. Sirius B is much dimmer and appears so close to the brilliant Sirius A that it was not actually sighted until 1862,during Alvan Clark's testing of a telescope. Both stars are at the same distance, the white dwarf being so much dimmer in opticallight because of its dwarf size. It has a radius just less than the Earth's. In x-rays Sirius B is much brighter. In the highgravity of this star, Hydrogen (transparent for x-ray but not in optical light) floats on top. In x-rays one sees the deeper,hotter layers at around 200,000 kelvins, in stead of the actual surface temperature in optical light of 25,000 kelvins. NASA / CXS

See also

• Brown dwarf
• Timeline of white dwarfs, neutron stars, and supernovae

White Dwarf is also the name of a magazine dedicated to Games Workshop 's role-playing games . See WhiteDwarf magazine