The white dwarfs, stellar vestiges

The future of a star is determined by its mass. The most massive stars have a very violent death, exploding as supernovae and converting consequently into neutron stars, or, in some cases, even into dark holes. But, however, this isn’t the death that awaits most of the stars. The big majority of the stars that exist in our universe will convert into white dwarfs. This is the destiny that awaits the Sun.

More than 70% of the stars that form our universe are red dwarfs, the least massive and luminous stars. Following these ones, the most common are the orange and yellow dwarfs. These are the three star types that, in the future, will convert into white dwarfs. So, in many time, when the production of stars won’t be as frequent as in our days, the immense majority of stars will be white dwarfs.

How are they formed?

When producing energy thanks to nuclear fusion, the stars are able to surpass the gravitational force that tries to collapse them, and they expand at a more or less constant rhythm. In the case of yellow dwarfs, the stellar category the Sun belongs to, they expand to the point of reaching a diameter as big as the Earth’s or Mars’ orbit. The stars that are in this stellar phase are called red supergiants.

Once arrived at this point (in the case of yellow dwarfs it takes about 10 000 million years), the star contains elements that are so heavy that it can no longer extract energy, for which it collapses and converts into a white dwarf. The stars in this phase are enormously dense. They are approximately as big as the Earth, but normally have a mass of between 0.5 and 0.7 solar masses. The density of a white dwarf is 1 x 109 km/m3, while on the Earth, the average density is 5.4 x 103 km/m3. These stars are 200 000 times more dense than the Earth. A coffee teaspoon of a white dwarf would weigh about 5 tonnes on the Earth.

The age and destiny of white dwarfs

White dwarfs have no longer the capacity to produce energy. They are however visible due to the heat they emit, in a constant way, that has been stocked for billions of years in the nucleus of the star, the place where nuclear reactions are produced.

The destiny of most white dwarfs is to convert into black dwarfs. As I said, white dwarfs emit, in a constant rhythm, their heat out to space, for which, as time goes on, they cool down. This process is so long that it isn’t thought any black dwarf in the universe exists. It is calculated that to go from the category of the white dwarf to black dwarf some hundreds of billions of years would need to pass. As our universe has an approximate age of 13.8 billion years, it is very improbable for any black dwarf to exist.

Their composition

White dwarfs are what remains of the nucleus of the initial star, which was a red, orange or yellow dwarf. Before the star converted into a white dwarf, that to say, when it was in the phase of a red supergiant, its nucleus was at the point of fusion of carbon and/or oxygen. Therefore, the stars in this stellar phase (white dwarf) are mainly composed of carbon, oxygen, helium, and hydrogen. These two last elements are the ones that form more than 90% of the Sun, for which it is supposed that they are also present in the white dwarfs, even if, being lighter, they are found at the surface of these bodies.

All these elements swim in a sea of electrons with high energy. It is precisely the combined pressure of the electrons, that stops this type of star to collapse into even stranger bodies, like neutron stars or black holes. This pressure that the electrons exert is provoked by one of the properties of quantum mechanics: the Pauli exclusion principle.

Chandrasekhar limit and type Ia supernovae

Many white dwarfs end their lives by emitting all their heat into space, until converting into black dwarfs. But not all white dwarfs finish their days in this way.

Many white dwarfs exist that orbit around another star. White dwarfs in these conditions lead to enormous explosions. Under these conditions, the white dwarf steals material from its companion. As it accumulates more mass, by strange as it may seem, it gets each time smaller. This is due to that, the more mass a white dwarf has, the more its electrons have to be compressed to maintain sufficient pressure towards the outside to support the extra mass.

However, there is a limit to the quantity of mass a white dwarf can have. This limit is known as the Chandrasekhar limit, and it corresponds to 1.4 solar masses. At this point, and if the white dwarf steals matter sufficiently quick from its companion, the whole stellar nucleus is destroyed in one of the most powerful events in the universe: type Ia supernovae. In a second, the white dwarf releases as much energy as the Sun does during 10 billion years. During weeks it can be more luminous than a whole galaxy.

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