Incredibly Dense White Dwarf Star Packs the Mass of the Sun Into the Size of the Moon

Researchers say if the star was any more massive it would likely collapse under its own weight and explode

white dwarf star and the moon
A newly discovered white dwarf star (right) is only slightly larger than the moon (left). Giuseppe Parisi

Astronomers have discovered the smallest white dwarf star ever documented around 130 lightyears from Earth, reports Leah Crane for New Scientist. The star, officially given the catchy designation of ZTF J190132.9+145808.7, is roughly the same size as our moon, but what this white dwarf lacks in diameter it makes up for in density with a mass about 1.3 times that of the sun.

The white dwarf was first spotted by Kevin Burdge, a postdoctoral scholar at Caltech, who was looking over all-sky images captured by the Zwicky Transient Facility at Caltech's Palomar Observatory, according to a statement.

The little star is so dense that researchers think it is the progeny of a merger between two formerly separate white dwarfs, they report in a study published this week in the journal Nature.

A white dwarf emerges when certain stars begin to “peter out,” writes Emily Conover for Science News. More commonly, these pint-sized stars are about the size of Earth, which has a radius of 3,958 miles; this white dwarf, by contrast, tacks just 248 miles onto the moon’s roughly 1,000-mile radius.

In the statement, study author Ilaria Caiazzo, a Caltech astrophysicist, explains that the star’s huge mass paired with its petite size isn’t so strange in the world of white dwarfs.

“It may seem counterintuitive, but smaller white dwarfs happen to be more massive," Caiazzo says. "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."

Apart from being one of the most massive white dwarfs on record, the star has two other unique characteristics: it’s spinning very fast and it has an extremely powerful magnetic field. Per the study, the star does a full rotation about every seven minutes, and the strength of its magnetic field ranges between 600 and 900 megagauss, which makes it nearly one billion times stronger than the sun’s magnetic field.

According to Science News, this particular star is right on the edge of the possible parameters for a white dwarf. If the star were any more massive it would collapse under its own weight and explode in a “type 1a” supernova.

"We caught this very interesting object that wasn't quite massive enough to explode," says Caiazzo. "We are truly probing how massive a white dwarf can be."

In the statement, Caiazzo further speculates that “it's possible that the white dwarf is massive enough to further collapse into a neutron star." According to Caiazzo, the white dwarf is so 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."

If this theory is proven, it would give astronomers an intriguing window into what may be a common path for the formation of neutron stars.