The discovery of the smallest and massive white dwarf

A few days ago, the discovery of the smallest and most massive white dwarf observed so far was announced. The study, published in Nature, sees as its first author the Italian researcher Ilaria Caiazzo.

White dwarfs represent the last evolutionary phase of stars with up to eight solar masses. After finishing the red giant phase the star loses its superficial layers in space, while its core contracts and cools becoming a white dwarf. 

White dwarfs, unlike normal stars, are not sustained by nuclear combustion reactions, but by the pressure of degenerate electrons, regulated by the laws of quantum mechanics. For this reason, the more massive a white dwarf is, the smaller its size.

The white dwarf discovered by the Caiazzo research group is just 130 light-years away from Earth. It has a mass 1.35 times that of the Sun and a diameter of just 4300 km, slightly higher than that of the Moon. It also completes a spin on itself in just 7 minutes.

To explain the values found, it has been suggested that this white dwarf is the result of the fusion of two white dwarfs located in a binary system. In fact, two white dwarfs orbiting each other would continuously lose energy in the form of gravitational waves and their orbital radius would be reduced more and more, until a possible collision. If the total mass of the system is less than a certain limit mass (as in this case) another white dwarf is formed, otherwise a type Ia supernova is produced.

This discovery makes it possible to study the maximum mass that a white dwarf can have: exceeding a certain limit the white dwarf would in fact undergo a further collapse, which would lead to the formation of a neutron star.

Credit: Giuseppe Parisi.