White dwarf star is the size of the moon but more massive than the sun

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star and moon

The newly discovered white dwarf shown next to the moon as a size comparison

Caltech

The smallest white dwarf star ever found is about the same size as Earth’s moon, but more massive than the sun. It appears to be shrinking, which could lead to a colossal explosion.

Ilaria Caiazzo at the California Institute of Technology and her colleagues discovered this star, called ZTF J190132.9+145808.7, using the Zwicky Transient Facility in California. They then performed additional observations with other telescopes to confirm its properties.

The researchers found that it rotates extraordinarily quickly, spinning once every 6.9 minutes. The magnetic field strength at its surface ranges between 600 and 900 megagauss – more than 1 billion times stronger than Earth’s magnetic field.

The star has a radius of about 2140 kilometres, only 400 kilometres bigger than the moon. But it also has a mass about 1.3 times that of the sun, close to the limit for how massive white dwarfs can become. While most white dwarfs form from single stars at the ends of their lives, this one’s extreme mass indicates that it may be the end result of a merger of two progenitor white dwarfs.

“If [the two progenitor white dwarfs are] both massive, then the final outcome is an explosion, a supernova… but if they are two not particularly massive white dwarfs, then the system can survive and create another white dwarf, a very peculiar one,” said Caiazzo in a press conference. “This one is really at that limit – it just avoided explosion.”

Because of the extreme pressure at its core, the researchers say that a strange atomic process may take place there that turns protons into neutrons, allowing it to become even more compact.

“It’s not only the smallest white dwarf ever found, it’s also shrinking,” said Caiazzo. “If this continues, it might collapse.” That collapse could result in either an extraordinarily dense neutron star or an explosion in the next few hundred million years. So the smallest white dwarf may not have permanently avoided exploding after all.

Journal reference: Nature, DOI: 10.1038/s41586-021-03615-y

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