It is on the verge of collapse, the star that has the double distinction of being the smallest and at the same time the densest ever observed: it has a diameter of only 4,300 kilometers, slightly larger than that of the Moon, and of greater mass to that of the Sun. His discovery, published in the journal Nature, is due to the Italian theoretical astrophysicist Ilaria Caiazzo, who studied in Italy, between Genoa, Pisa and Milan, and who now works in the United States, at the California Institute of Technology (Caltech).
Nothing like it had been seen so far. The star is a white dwarf, that is, a star that has now reached the end of its life cycle, and is the smallest of its kind ever observed. “It might seem counterintuitive, but the smallest white dwarf is also the most massive,” notes Caiazzo. This is due to the fact, unlike normal stars, white dwarfs are not powered by nuclear combustion which allows them to resist their own gravity. Instead, the rules of quantum mechanics dictate the law in these stars.
The discovery was made possible by the Zwicky Transient Facility (ZTF), active at the Palomar Observatory of Caltech, in collaboration with a choir of instruments, based on Earth and in space: from the Hale telescope to Palomar to Keck, up to the telescope European space Gaia and that of Nasa Swift, up to the Pan-STARRS (Panoramic Survey Telescope and Rapid Response System) program of the University of Hawaii.
Artist’s impression of the smallest and densest white dwarf ever discovered, in comparison with the Moon (source: Giuseppe Parisi)
White dwarfs are the remnants of stars that once had a mass up to eight times larger than the Sun and the discovery of this extremely dense little star, called ZTF J1901 + 1458, allows for the first time to understand what could be happen to stars that reach white dwarf stage.
ZTF J1901 + 1458, for example, was formed when a pair of white dwarfs collided, merging and releasing energy in the form of gravitational waves. But this is only one possibility, the other is that in the collision the stars could explode in a type 1A supernova. The fate of the small star could instead be the transformation into a neutron star