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White Dwarf

White dwarf

Words to Know

Black dwarf: Cooling remnants of a white dwarf that has ceased to glow.

Nebula: Cloud of interstellar gas and dust.

Nuclear fusion: Reaction involving the merging of two hydrogen nuclei into one helium nucleus, releasing a tremendous amount of energy in the process.

Red giant: Stage in which an average-sized star spends the final 10 percent of its lifetime; its surface temperature drops and its diameter expands to 10 to 1,000 times that of the Sun.

A white dwarf is the fate awaiting the Sun and other average-sized stars. It is the core of a dead star left to cool for eternity.

Nuclear fusion is the merging of two hydrogen nuclei into one helium nucleus, with the release of a tremendous amount of energy in the process. This occurs in the early stages of every star's life. It fuels the star and provides an outward pressure that acts as a balance to the star's tremendous gravity. In the absence of fusion, gravity takes over and causes a star to collapse upon itself. The larger the original star, the smaller a white dwarf it becomes. The reason for this pattern is that larger stars have stronger gravitational fields, which produce a more complete collapse.

An average-sized star like the Sun will spend the final 10 percent of its life as a red giant. In this phase of a star's evolution, the star's surface temperature drops to between 3,140 and 6,741°F (1,727 and 3,727°C) and its diameter expands to 10 to 1,000 times that of the Sun. The star takes on a reddish color, which is what gives it its name.

Buried deep inside the star is a hot, dense core, about the size of Earth. The core makes up about 1 percent of the star's diameter. The helium left burning at the core eventually ejects the star's atmosphere, which explodes off into space as a planetary nebula (gas and dust cloud). All that remains of the star is a glowing core, a white dwarf.

The term white dwarf is a bit misleading. The core starts out white, but as it cools it displays a range of colorsfrom yellow to red. When all heat within the core has escaped, the body ceases to glow and becomes a black dwarf. Billions of white dwarfs exist within our galaxy, many of them now in the form of black dwarfs. These cold, dark globes, however, are next to impossible to detect.

[See also Red giant; Star ]

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white dwarf

white dwarf, in astronomy, a type of star that is abnormally faint for its white-hot temperature (see mass-luminosity relation). Typically, a white dwarf star has the mass of the sun and the radius of the earth but does not emit enough light or other radiation to be easily detected. The existence of white dwarfs is intimately connected with stellar evolution. A white dwarf is the hot core of a star, left over after the star uses up its nuclear fuel and dies. It is made mostly of carbon and is coated by a thin layer of hydrogen and helium gases. The physical conditions inside the star are quite unusual; the central density is about 1 million times that of water.

Astronomers long believed this intense pressure could cause the carbon interiors of white dwarfs to crystallize. In 2004 the discovery of BPM-37093, a star that is located 50 light-years from the earth in the constellation Centaurus and is both pulsating and has sufficient mass to have a crystalline interior. By measuring the pulsations it was possible to study this white dwarf's interior and determine that it had crystallized to form an enormous diamond, some 950 mi (1,500 km) wide. Were it a diamond as we commonly know it, it would weigh some 10 billion trillion trillion carats.

The first white dwarf discovered (1844) was the faint companion in the binary star Sirius. Although invisible to the telescopes of the day, the white dwarf's mass was large enough to produce a noticeable wavy motion in its very bright partner as the two stars revolved around each other. It is believed that white dwarfs could represent as much as a third of the so-called dark matter in the universe.

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white dwarf

white dwarf High-density type of star about the size of the Earth, but with a mass about that of the Sun. White dwarfs are of low luminosity and gradually cool down to become cold, dark objects.

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White Dwarf

White Dwarf ★½ 1995

Confusing sci-fi adventure finds edgy Manhattan internist Driscoll Rampart (McDonough) serving his residency on Rusta, a wartorn planet located in a white-dwarf star system. The planet is divided into two hemispheres, one in perpetual darkness and the other in constant light. Rampart learns the ropes from mystical doc Akada (Winfield), who's also involved in a peace accord between Rusta's warring leaders. Visually striking but overly ambiguous—probably because it was intended as the pilot for a weekly series. 91m/C VHS . Paul Winfield, Neal McDonough, CCH Pounder, Ele Keats, Michael McGrady, Katy Boyer; D: Peter Markle; W: Bruce Wagner; M: Stewart Copeland. TV

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White Dwarf

White Dwarf

White dwarf stars, in astronomy, are low or medium mass stars that are not dense enough to generate core temperatures for the purpose of fusing the element carbon (C) in nucleosynthesis reactions. Consequently, it becomes a red giant, eventually expelling its outer layer to form a planetary nebula. It remains only a small core of carbon (C) and oxygen (O) in the last stage of its life.

In 1862, American astronomer Alvan Graham Clark (18321897) discovered that the second brightest star in the sky, Sirius (Alpha Canis Majoris), (the sun is the brightest star) was orbited by a much fainter companion (Sirius B). Analysis of the orbit yielded a mass for the companion similar to that of the sun while an analysis of its light suggested that its size was approximately the same as Earths. In 1917, Dutch-American astronomer Adriaan Van Maanen (18841946) discovered the second known white dwarf, what is now called Van Maanens star. It is 14.4 light-years (where one light-year is the distance that light travels in vacuum in one year) from the sun in the Pisces constellation.

Further observation revealed that white dwarfs are reasonably common in the solar system, and probably elsewhere in the universe. However, they had gone undetected because they are so faint. In the 1920s, the development of quantum mechanics helped to explain the density of white dwarfs. In 1930, Indian-American astrophysicist and physicist Subrahmanyan Chandrasekhar (19101995) reported that white dwarfs can be no more massive than about 1.4 the suns mass. This limit is called the Chandrasekhar limit. As of the 2000s, astronomers contend that white dwarfs comprise about 6% of all stars in the suns solar system. During this time, several space telescopes, such as NASAs Spitzer Space Telescope (which began operations in 2003) have discovered other white dwarf stars and helped astronomers learn more about their characteristics.

White dwarfs range in mass from perhaps one-third to just under one-and-one-half times the mass of the sun. Astronomers now know that these stars have exhausted their supply of nuclear fuel, which would enable them to shine like the sun and other ordinary stars. Under the weight of their own matter, they have collapsed to roughly one-hundredth the size of the sun. A fundamental law of quantum mechanics (i.e., the Pauli exclusion principle) limits the ability of the crushing gravity to pack electrons into an ever-decreasing volume. The pressure of these highly packed electrons balances the weight of the overlying stellar material, stopping any further collapse. Since the electron pressure results from a simple packing of the electrons in a particular volume, there is no need for them to be heated by nuclear fusion, as is the case in normal stars (the nuclear fusion produces the pressure to balance the weight of overlying material). Astronomers call such densely packed matter electron-degenerate. A teaspoon of such matter would weigh 40 tons (36.3 tonnes) on the Earth. Thus, white dwarfs are stable stars that slowly cool off, becoming dark stellar cinders.

Since two-thirds of all stars occur in binary star systems where the components orbit one another, it is not surprising that many white dwarfs are found in binary systems. As the companion of the white dwarf ages, it will expand and begin to lose matter to the gravitational pull of the white dwarf. This results in the injection of hydrogen-rich matter from the normal companion into the outer layers of the white dwarf. After a certain amount of this matter accumulates, it will undergo a nuclear fusion explosion, which will blow off the outer ten-thousandth of the white dwarf. The energy released during this explosion will cause the system to brighten perhaps a million times or more, forming what astronomers call a nova. The extent of the explosion critically depends on aspects of the donor star as well as the white dwarf. Smaller explosions may be called dwarf novae. This material will be blown clear of the system and the expanding shell will become visible to later observers.

There is a limit in mass to which the white dwarf can grow because there is a limit to the extent that electrons can resist the increased gravitational forces. Depending on the chemical composition of the white dwarf, this limit occurs when it reaches between 1.2 and 1.4 times the mass of the sun. In binary systems where donated mass from the companion forces the white dwarf beyond this limit, the white dwarf will collapse by a factor of one thousand to become a neutron star. The circumstances of the collapse may be so violent as to result in an explosion called a Type I supernova. Such an explosion is far more violent than a nova and may result in the disruption of the binary system.

See also Stellar evolution.

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White Dwarf

White dwarf

During the first quarter of this century astronomers found that the brightest star in the sky, Sirius, was orbited by a much fainter companion. Analysis of the orbit yielded a mass for the companion similar to that of the sun while an analysis of its light suggested that its size was approximately the same as Earth's. Further observation revealed that these small massive stars are reasonably common but had gone undetected because they are so faint. The stars of this unique class are called white dwarfs. They range in mass from perhaps a third to just under one and a half times the mass of the sun. We now know that these stars have exhausted their supply of nuclear fuel, which would enable them to shine like the sun and other ordinary stars. Under the weight of their own matter , they have collapsed to roughly a hundredth the size of the sun. A fundamental law of quantum mechanics (i.e., the Pauli exclusion principle) limits the ability of the crushing gravity to pack electrons into an ever decreasing volume . The pressure of these highly packed electrons balances the weight of the overlying stellar material, stopping any further collapse. Since the electron pressure results from a simple packing of the electrons in a particular volume, there is no need for them to be heated by nuclear fusion , as is the case in normal stars (the nuclear fusion produces the pressure to balance the weight of overlying material). We call such densely packed matter electron-degenerate. A teaspoon of such matter would weigh 40 tons (36.3 tonnes) on Earth . Thus, white dwarfs are stable stars which slowly cool off, becoming dark stellar cinders.

Since two thirds of all stars occur in binary star systems where the components orbit one another, it is not surprising that many white dwarfs are found in binary systems. As the companion of the white dwarf ages it will expand and begin to lose matter to the gravitational pull of the white dwarf. This results in the injection of hydrogen-rich matter from the normal companion into the outer layers of the white dwarf. After a certain amount of this matter accumulates, it will undergo a nuclear fusion explosion which will blow off the outer ten-thousandth of the white dwarf. The energy released during this explosion will cause the system to brighten perhaps a million times or more, forming what we call a nova . The extent of the explosion critically depends on aspects of the donor star as well as the white dwarf. Smaller explosions may be called dwarf novae. This material will be blown clear of the system and the expanding shell will become visible to later observers.

There is a limit in mass to which the white dwarf can grow because there is a limit to the extent that electrons can resist the increased gravitational forces. Depending on the chemical composition of the white dwarf, this limit occurs when it reaches between 1.2 and 1.4 times the mass of the sun. In binary systems where donated mass from the companion forces the white dwarf beyond this limit, the white dwarf will collapse by a factor of one thousand to become a neutron star . The circumstances of the collapse may be so violent as to result in an explosion called a Type I Supernova . Such an explosion is far more violent than a nova and may result in the disruption of the binary system.

See also Stellar evolution.

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