This moon-sized white dwarf is the smallest ever found

Only a smidge bigger than the moon, a newfound white dwarf is the smallest of its kind known. 

The white dwarf, a type of remnant left behind when certain stars peter out, has a radius of about 2,100 kilometers, researchers report June 30 in Nature. That’s remarkably close to the moon’s approximately 1,700-kilometer radius. Most white dwarfs are closer to the size of Earth, which has a radius of about 6,300 kilometers.

The white dwarf’s small girth means, counterintuitively, that it is also one of the most massive known objects of its kind, at about 1.3 times the sun’s mass. That’s because white dwarfs shrink as they gain mass (SN: 8/12/20).

“That’s not the only very amazing characteristic of this white dwarf,” astrophysicist Ilaria Caiazzo of Caltech said June 28 in an online news conference. “It is also rapidly rotating.”

The white dwarf spins around approximately once every seven minutes. And it has a powerful magnetic field, more than a billion times the strength of Earth’s. Caiazzo and colleagues discovered the unusual stellar remnant, dubbed ZTF J1901+1458 and located about 130 light-years from Earth, using the Zwicky Transient Facility at Palomar Observatory in California, which searches for objects in the sky that change in brightness.

The white dwarf probably formed when two white dwarfs orbited one another and merged to create a single white dwarf with an extra-large mass and extra-small size, the team says. That convergence would also have spun up the white dwarf and given it a strong magnetic field.

This white dwarf is living on the edge: If it were much more massive, it wouldn’t be able to support its own weight, causing it to explode. Studying such objects can help scientists understand the limits of what’s possible for these dead stars.