Moon-sized star

A smaller and more massive white dwarf star than any other known has been discovered. The dormant star formed when two less massive white dwarfs merged, concentrating a mass greater than that of our Sun in a body the size of our Moon.

It may seem counterintuitive, but the smaller white dwarfs turn out to be more massive. This is because white dwarfs lack the nuclear combustion that keeps normal stars “bloated” against their own gravity, and that is why the mass tends to compress more the larger it is.

The discovery was made by the team of astrophysicist Ilaria Caiazzo, from the California Institute of Technology (Caltech) in the United States. Several astronomical observatories have been used for the observations, among them the WM Keck in Maunakea, Hawaii, United States.

White dwarfs are “retired” stars, in which their natural nuclear fusion reactor has shut down. Although their stellar activity has ceased, they continue to emit light and heat as they take a long time to cool. Its mass has been greatly compressed, as the force of gravity cannot be counteracted by the expansive force derived from stellar activity (nuclear fusion). The stars that end up as white dwarfs are those that once had no more than about eight times the mass of the Sun.

Our Sun, for example, although it will swell more than normal to become a red giant in about 5 billion years, it will eventually shed its outer layers and shrink until it becomes a compact white dwarf. About 97% of stars become white dwarfs.

While our Sun is alone in space without being a member of a stellar pair, many stars orbit each other in pairs. Stars age together, and if they both have less than eight solar masses, they will both evolve into white dwarfs.

The new discovery offers an example of what can happen after this phase. The pair of white dwarfs, which revolve closer and closer to each other, lose energy in the form of gravitational waves and end up colliding and merging. If both stars are massive enough, they explode in what is called a Type Ia supernova. But if they are below a certain mass threshold, they combine into a new white dwarf that is more massive than either of the two parent stars. This fusion process enhances the magnetic field of that star and accelerates its rotation compared to that of the progenitors.

The newly discovered little white dwarf, named ZTF J1901 + 1458 and located about 130 light-years distant from Earth, in the constellation of the Eagle (Aquila), took that last path of evolution; its parents merged and produced a white dwarf 1.35 times the mass of our Sun. The white dwarf has an extreme magnetic field, almost a billion times stronger than that of our Sun, and rotates on its axis at a rate frenzy of a revolution every seven minutes.

At about 4,300 kilometers in diameter, the discovered white dwarf is only slightly larger than the Moon, whose diameter is about 3,500 kilometers. In the image, the white dwarf is shown next to the Moon and a portion of the Earth to clarify the size relationship. (Image: Giuseppe Parisi)

Caiazzo and his collaborators believe that the merged white dwarf may be massive enough to evolve into a neutron star, which usually forms when a star much more massive than our Sun explodes in a supernova.

Neutron stars are much smaller and denser than white dwarfs. They are corpses of stars, compacted in such a way that their diameter is similar to the distance between two extremes of a large city. The compression that reigns in a neutron star is so brutal that in the atoms it forces the electrons to “embed” themselves against the protons, giving rise to neutrons. Hence, these kinds of objects are called neutron stars.

Caiazzo’s team has published the technical details of the find in the academic journal Nature, under the title “A highly magnetised and rapidly rotating white dwarf as small as the Moon.” (Source: NCYT from Amazings)

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