The smallest white dwarf ever seen before

The smallest white dwarf ever seen before

It is a long dead star and the smallest of its kind. Its size is comparable to that of our Moon, but its mass is greater than that of the Sun

(Image: Giuseppe Parisi) A record-breaking white dwarf, as never before seen. It contains one and a half times the mass of the Sun, but is "packed" into a sphere whose diameter is slightly larger than that of our Moon. It has been out for some time, generates a very strong magnetic field, rotates very fast on itself and is still shrinking, which raises concerns that it may become unstable and explode in a supernova. The details of this incredible star are described in an article just published in Nature by a team of Caltech scientists, including the Italian Ilaria Caiazzo.

The white dwarf is officially called ZTF J1901 + 1458 and is located about 130 light years from Earth. Its mass and dimensions place it right next to the so-called Chandrasekhar limit, the maximum value allowed for the mass of a white dwarf so that it does not become so dense and compact as to explode. "The observation of an object like this, massive almost to the point of exploding, made us very curious - explained Caiazzo -. We are about to really understand how massive a white dwarf can be ”. White dwarfs are the smallest of the stars: they are the remains of the nuclei of other much larger stars, up to eight times our Sun, which, once the fuel for the primary fusion reactions is exhausted, expel the outermost gases. At that point their nucleus collapses into super-dense objects: the white dwarfs, to be precise.

When the mass of white dwarfs does not exceed the Chandrasekhar limit, equal to about 1.4 solar masses, a phenomenon called electron degeneration pressure prevents the star from collapsing under the force of its own gravity: basically , the electrons are "torn" from the nuclei of the atoms and provide an outward pressure that balances that due to gravity. Moreover, many white dwarfs exist in binary systems, that is, they orbit in pairs, one around the other: sometimes this process causes the gravitational attraction of one of the two stars of the pair to subtract matter from the other and go beyond the limit. of Chandrasekhar, triggering, as we have seen, the formation of a supernova. This is not the case with ZTF J1901 + 1458, which instead seems to have been generated by the fusion of two even smaller white dwarfs.

Other peculiarities are its very intense magnetic field, about a billion times more powerful than that of the Sun, and its strong rotation speed, which leads it to make a revolution about itself every seven minutes. The understanding of the behavior of this white dwarf and all similar stars, however, is far from complete: "There are still many questions to be answered," says Caiazzo. And he adds: “We don't know, for example, how often white dwarfs merge in galaxies, and whether the frequency of these events is high enough to justify all the supernovae we observe, or how such a strong magnetic field is generated. To solve these puzzles we have to look for other stars of this type and observe their behavior in detail ".

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