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Tiny, fast-spinning white dwarf is the most massive ever discovered

The white dwarf ZTF J1901+1458 is about 2,670 miles across, while the moon is 2,174 miles across. It is depicted above the Moon in this artistic representation.
The white dwarf ZTF J1901+1458 is about 2,670 miles across, while the moon is 2,174 miles across. The white dwarf is depicted above the Moon in this artistic representation; in reality, the white dwarf lies 130 light-years away in the constellation of Aquila. Giuseppe Parisi

When our sun finally runs out of fuel and nears the end of its life, it will puff up to become a red giant before it throws off its outer layers and shrinks down to become a white dwarf. This is the eventual fate of the vast majority of stars in the universe, which will come to an evolutionary end as white dwarfs.

Now, researchers from the W. M. Keck Observatory have found an unusual white dwarf which is both the smallest and the most massive ever observed. It is named J1901+1458 and is located relatively nearby, at 130 light-years from Earth.

This remarkable object is “packing a mass greater than that of our sun into a body about the size of our moon,” said lead author Ilaria Caiazzo of Caltech. “It may seem counterintuitive, but smaller white dwarfs happen to be more massive. This is due to the fact that white dwarfs lack the nuclear burning that keep up normal stars against their own self gravity, and their size is instead regulate­­­d by quantum mechanics.”

The researchers believe this particular white dwarf became so massive because it was once part of a pair of white dwarfs orbiting each other. These two stars eventually crashed together and merged into one heavier star. This process also increases the magnetic field around the star, which makes it spin faster. This white dwarf spins at a head-turning rate, completing a rotation every seven minutes.

Due to its mass, this white dwarf might evolve further into a neutron star, which is almost as dense as a black hole and usually forms from a supernova explosion.

“This is highly speculative, but it’s possible that the white dwarf is massive enough to further collapse into a neutron star,” said Caiazzo. “It is so massive and dense that, in its core, electrons are being captured by protons in nuclei to form neutrons. Because the pressure from electrons pushes against the force of gravity, keeping the star intact, the core collapses when a large enough number of electrons are removed.”

“We caught this very interesting object that wasn’t quite massive enough to explode,” said Caiazzo. “We are truly probing how massive a white dwarf can be.”

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