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A white dwarf star has been discovered that is slightly larger than the Moon.
It is only 134 light years distant and could be the result of the merger of two smaller white dwarfs.
One million tons per tablespoon
A white dwarf is the stellar corpse that remains when a star with a mass less than 10 solar masses dies.
Indeed, when one of these stars exhausts the nuclear fuel inside, it expels most of its mass to form a planetary nebula.
At the center of this nebula there is only one cold stellar object: the white dwarf, which maintains its stability thanks to the properties of the material of which it is composed.
White dwarfs do not have an internal source of energy, therefore, they tend to collapse due to the effect of their own weight and compress on themselves.
The atoms of the material of which they are made come closer to each other, thus creating a highly compressed plasma.
The typical density of one of these stars is so high that a tablespoon of its material has a mass in excess of one million tons.
The electrons in this ultra-dense material (due to a quantum-mechanical effect known as the 'Pauli exclusion principle') are forced to move at very high speeds, thus generating a very high pressure that slows down gravitational collapse.
In this way a state of stability is reached in which the stellar corpse can remain for hundreds or billions of years.
97% of adult stars have masses less than 10 times the mass of the Sun, that is, 97% of stars (including the Sun) will end their lives as white dwarfs.
Along with red dwarfs, these are the most abundant stars in the universe.
Recreation of a pair of white dwarfs by Giuseppe Parisi
One stellar spin every 7 minutes
Astronomer Ilaria Caiazzo (Caltech, USA) has led a study during which they have discovered a white dwarf with extreme properties.
It is the star named ZTF J190132.9 + 145808.7, where 'ZTF' indicates that it was found in observations made with the Zwicky Transient Facility telescope, located on Mount Palomar, in California.
This panoramic telescope scans the celestial vault every two nights, which allows it to detect stars that vary very quickly, on time scales of minutes.
Such rapid variations always reveal interesting and often violent phenomena.
After detecting this star that showed very rapid variations in brightness, follow-up observations were made that confirmed that it was a rotating star with a rotation period of only 7 minutes.
Typically, white dwarfs rotate with periods of several hours (although the record is held by the star EPIC 228939929, which makes one revolution every 5.3 minutes).
The most dwarf white
A star rotating at such a speed must create a very strong magnetic field. To measure it, Caiazzo and collaborators went to the large Keck telescope in Hawaii, where they found that the magnetic field of this white dwarf is one billion times more intense than the Earth's. From photometric measurements with different filters they were also able to estimate the radius of the star, which turned out to be only 2,140 kilometers, that is, slightly higher than the radius of the Moon (1,737 kilometers). It is therefore the smallest known white dwarf star.
Paradoxically, the smaller a white dwarf is, the higher its mass.
This is because the force of gravity, greater in the most massive ones, causes a greater contraction.
Observations indicate that ZTF J190132.9 + 145808.7 has a mass between 1.327 and 1.365 solar masses.
It is, therefore, a mass very close to the limit of 1.37 solar masses, known as the Chandrasekhar limit, above which stars lose their stability and collapse to become black dwarfs.
From its intense magnetic field, the high mass (in the context of white dwarfs) and its small size, Caiazzo and his team deduce that this star must have been formed by the fusion of a pair of white dwarfs that were held together. by its gravity, orbiting one with respect to the other, until they came into contact and merged.
Crystallization
The authors also speculate on the future evolution of such a peculiar star.
In all white dwarfs, the nuclei of the heaviest atoms are believed to settle in the most central region.
Their nuclear reactions produce neutrinos, ghostly particles that easily escape, taking more and more energy, thus cooling the central stellar zone.
This area freezes and solidifies, forming a kind of crystalline lattice that again slows down the collapse.
Illustration of the crystallization of a white dwarf Univ.
by Warwick / M.
Garlick
Caiazzo and collaborators estimate that the complete crystallization of this white dwarf could occur in a period of between 10 and 100 million years. This is a much shorter time than it takes for a less massive white dwarf to crystallize, estimated at billions of years.
This white dwarf is only 134 light years distant from Earth.
Being so close is an indication that such objects could be abundant in the Milky Way.
And in fact, at least one other very fast-spinning white dwarf is already known.
But to be sure if they are really abundant, more observations will have to be made.
When the Vera Rubin Observatory, which has a large panoramic telescope, starts up, many more rapidly variable stars will be able to be detected.
In this way, the abundance and properties of these tiny stars will continue to be studied.
The article by Caiazzo et al. Entitled
A highly magnetized and rapidly rotating white dwarf as small as the Moon
has been published in a recent issue of the journal
Nature
.
____________________________________________________________________
Rafael Bachiller is director of
the National Astronomical Observatory
(National Geographic Institute) and academic of the
Royal Academy of Doctors of Spain
.
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