It is on the verge of collapse, the star that has the distinction of being both the smallest and the most dense at the same time: its diameter is only 4,300 km, it is slightly larger than the diameter of the Moon, and of greater mass. His discovery, published in Nature, is attributed to Italian theoretical astrophysicist Ilaria Caiazzo, who studied in Italy between Genoa, Pisa and Milan, and now works in the United States, at the California Institute of Technology (Caltech).
Nothing like it has been seen yet. The star is a white dwarf, i.e. a star that has now reached the end of its life cycle, and is the youngest of its kind ever. “It may seem counterintuitive, but the smallest white dwarf is also the largest,” Kaizu notes. This is due to the fact that, unlike regular 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 on these stars.
The discovery was made possible by the Zwicky Transit Facility (ZTF), active at Caltech’s Palomar Observatory, in collaboration with a chorus of instruments, based on Earth and in space: from the Hill telescope to Palomar to Keck, to the European space telescope Gaia and NASA Swift, even the European Space Telescope Gaia and NASA Swift Program Pan-STARRS (Panoramic Survey Telescope and Rapid Response System) of the University of Hawaii.
Artist’s impression of the smallest and densest white dwarf ever discovered, compared to the Moon (Source: Giuseppe Baresi)
White dwarfs are the remnants of stars that were eight times more massive than the Sun and the discovery of this tiny, extremely dense star, called ZTF J1901 + 1458, allows for the first time to understand what might happen to stars that reach the white dwarf stage.
ZTF J1901 + 1458, for example, formed when a pair of white dwarfs collided, merging and releasing energy in the form of gravitational waves. But that’s only one possibility, and the other is that in the event of a collision the stars could explode into a Type 1A supernova. The young star could instead be doomed to transform into a neutron star
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