What is a white dwarf?

Not the name of a star, that's the name of a star. Like humans, the planets' life is divided into three phases: young, middle-aged, old. White dwarfs are a kind of star in their old age.

What is a white dwarf?

White dwarfs are celestial bodies created when stars of low and medium mass "die" (consuming up nuclear fuel in stars).

These stars are not heavy enough to generate the core temperature needed to melt carbon in nuclear fusion after they turn into red giant stars during the helium burning phase. At the end of this period, the outer half of the red star will be ejected into the nebula to form a nebula, leaving behind an inert core containing mainly carbon and oxygen, which is the white dwarf.

Sirius A and Sirius B, taken by Hubble telescope.Sirius B, a white dwarf, can be seen as a blur on the bottom left next to Sirius A much brighter.

The process of forming white dwarfs

Most small and medium-sized stars will end up as white dwarfs, after all of them are converted into helium. Near the end of their fusion phase, such stars will expand and turn into red giant stars and then gradually lose most of the material in the outer layers (creating nebulae) while still very hot core (T> 100,000 K), this core then becomes a young white dwarf.

A white dwarf has about the same mass as the Sun , only slightly larger than the Earth. This makes white dwarfs one of the most dense forms of matter, only neutron stars, strange stars and quantum (hypothetical) stars that are much denser than it. The larger the mass, the smaller the size of the white dwarf. There is an upper limit on the mass of white dwarfs, which is the Chandrasekhar limit (about 1.4 solar masses), but beyond it, the pressure of electrons cannot be balanced with gravity, and throne. stars continue to shrink, eventually forming neutron stars.

Despite this limitation, most stars end their lives as white dwarfs, because they tend to spread most of their mass into the universe before it completely collapses (usually with spectacular results - see nebulae. It is thought that even stars with 8 times the mass of the Sun will eventually be turned into white dwarfs.

Characteristics of white dwarfs

Many white dwarfs are roughly the size of Mars about 100 times smaller than the Sun. They may have approximately the Sun's mass so they are very dense.

White dwarfs are very hot, so they emit white light. This heat is the remainder of the heat generated by star collapse and it is not added (unless they obtain material from nearby stars), but due to the very small radiation surface they maintaining heat for a long time.

Eventually, the white dwarf will cool down and become a black dwarf. Black dwarfs, in theory, are low-temperature entities and weak radiation in the radio spectrum. However, the universe did not exist long enough for any white dwarf to cool enough to become a black dwarf.

Comparing IK Pegasi B (middle), IK Pegasi A (left) and Sun (right).This white dwarf has a surface temperature of about 35,500 K.

Many nearby white dwarfs have been discovered as sources of soft X-ray radiation (low-energy X-rays); X-ray and ultraviolet observations allow astronomers to study the composition and structure of the thin atmosphere of these stars.

White dwarfs cannot have masses exceeding 1.4 solar masses, Chandrasekhar limits, but there is a way for them to overcome this limit. If dwarfs fly in pairs with another normal star, it can draw matter from the companion star. The suction material is very slow and stable. The mass of a white dwarf increases until it exceeds the Chandrasekhar limit, from which the degradation pressure cannot be maintained. It forms the type Ia supernovae and is the strongest in supernovae.

In some cases, matter absorbs a companion star that contains a lot of hydrogen, causing a nuclear reaction to explode in the form of a weaker supernova, known as white dwarf explosions. These explosions occur only in the shell containing newly absorbed materials, do not affect the inner core of the white dwarf, and can be repeated if there is still a flow of high-flow hydrogen materials.

1. What is a Neutron Star?