Once a star reaches the end of its life, several scenarios can unfold depending on its mass. If the most massive ones generally end their life in spectacular explosions to give way to neutron stars or black holes, the less massive ones usually end up in the white dwarf stage. But according to the theoretical models, there could still exist a stage beyond the latter:the black dwarf.
A black dwarf is a white dwarf that has cooled so much that it no longer gives off any visible light or measurable heat. The physical process leading to this temperature is of longer duration than the current age of the Universe. No black dwarf has therefore been able to form yet, which is why it is impossible to observe any at present. In addition, the coldest white dwarfs are used as observational constraints to define the age of the Universe.
Initially, black dwarfs referred to objects that are now called brown dwarfs, that is, substellar objects whose mass is insufficient to initiate thermonuclear fusion reactions.
However, the term black dwarf was already used to characterize cold white dwarfs. So much so that in 1975 the American astronomer Jill Tarter proposed the term brown dwarf in order to avoid confusion in the future, the two objects being radically different from each other.
For stars of low to medium mass (below 10 solar masses), the expected end of life is the white dwarf stage. That is, a sphere of electrons and degenerate matter slowly cooling by thermal radiation.
In view of the many physical unknowns that still remain (possible disintegration of the proton, nature of dark matter and energy, etc.), it is complicated to be able to define a duration from which a white dwarf passes to the dwarf stage. black.
Related topic:The region near our Solar System is said to be filled with brown dwarfs
According to astrophysicists John Barrow and Frank Tipler, it would take about 10 15 years for the temperature of a white dwarf to reach 5 K (-268 ° C). In the event that WIMPs — candidate particles for dark matter — exist, interactions of these with the white dwarf would maintain a certain amount of heat, which would extend the cooling time to at least 10 25 years.
If black dwarfs did exist, they would be extremely difficult, if not impossible, to observe directly. Their very low temperature would be the cause of too weak radiation, drowned in that of the cosmic microwave background. The only way to detect them would therefore be through their gravitational influence.
White dwarfs of spectral type M0 (below 3600°C) and estimated to be between 11 and 12 billion years old were detected in 2012 using the MDM Observatory telescope .
