written by: Reeja Mathew•edited by: Laurie Patsalides•updated: 1/17/2010
Let's learn about the white dwarf star, its varieties, and its formation.
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White Dwarf Star
Every living and non-living object in the Universe follows a cycle of birth and death. Stars are not an exception. The analysis of the spectral lines of stars reveal that, major elements at the core of a star are hydrogen and helium, with a very small percentage of heavier elements. Stars produce energy through liberation of a huge amount of energy during fusion of hydrogen into helium.
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A star will keep shining until it runs out of hydrogen inside of its core. At this stage, a chain reaction occurs which will eventually lead to the death of a star. As soon as the nuclear force within the star ceases, the equilibrium between the outward pressure produced by the fusion and the inward gravitational force exerted by the stellar mass cease to exist. This will cause the star to contract. Due to this collapse of the star, the temperature of the core is again raised which in turn, will cause its outer layers to expand. As a result of this expansion, the outer layer gets cooled. Thus, the size of the star becomes very large and the temperature of outer layer lowers considerably giving it a red color. A star which reaches this stage is called a ‘Red Giant’. (The image below is the initial stage of a White Dwarf.)
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The core of a red giant continues to collapse, raising the core temperature higher. This temperature will become so high that, it will cause the fusion of helium to heavier elements like carbon. But, this fusion process won’t continue for long, as the size of the stars are not massive enough to produce the pressure and the temperature required for the fusion of heavier elements . At this juncture, the outer layer of the star is thrown away due to a violent explosion and forms a planetary nebula. This planetary nebula, which is gaseous in character, has the core of the star embedded in it. Due to the gravitational compression, the core exists like an electron gas. If the mass of the core is up to 1.4 times, the mass of the Sun, then the resulting electron gas can balance the inward force exerted by the gravitational force and an equilibrium is reached. This kind of star, which reaches an equilibrium is called a ‘White Dwarf’. This limit, at which a star become a white dwarf was first theoretically determined by S. Chandrasekhar and hence it is known as the Chandrasekhar Limit.(The image below is the hottest known white dwarf.)
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Here we detail the varieties of white dwarf stars and how are they classified. There are six varieties of white dwarfs. The classification of white dwarf stars is a topic of intense study, which astronomers are exploring with the help of new technologies.
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Due to the absence of nuclear fuel inside of the core, the white dwarf cools. This process continues for billions of years and finally it will become a black dwarf. They are so cool that they won’t emit any energy and hence will remain invisible. In some cases, as in the binary star system, in which two stars are orbiting around each other, if one of them becomes a white dwarf, its immense gravitational force will pull the neighboring star's stellar material into it and thus, will grow in size. When its mass crosses the Chandrasekhar limit, supernova explosion occurs.
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Varieties of White Dwarf
Sirius B, which is at a distance of 8.6 light years away from our galaxy, is the nearest white dwarf. White dwarfs are characterized by high density and low luminosity, so they are invisible with the naked eye. The Sun is supposed to become a white dwarf during its final stage.
There are six types of white dwarfs. White dwarfs are classified as DA, DB, DC, DO, DQ and DZ on the basis of the spectral lines present. This classification was first put forward by Edward M. Sion.
DA has abundance of Hydrogen
DB has neutral Helium.
DC has a continuous spectrum.
DO has single ionized Helium.
DQ has Carbon.
DZ has metal, like ionized Calcium.
Usually, a white dwarf is represented by the letter D followed by any of the letters A,B,C,O,Q, and Z which denotes the characteristic material present in it and finally its effective temperature. Scientists are studying white dwarfs to learn clues about the birth of the Universe. They are a rich source of information, which have not been explored fully. (The image below is Sirius B in binary system.)