This Blasted Star Is Getting the Hell Out of the Milky Way

A field of stars in the Milky Way imaged by the Hubble Space Telescope in 2017.

Careening through the Milky Way at nearly 2 million miles per hour, the star LP 40–365 shows no signs of stopping. A team of astronomers recently figured out that the star was propelled into its current speedrun by a supernova explosion millions of years ago.

LP 40–365 is unusual. It’s a white dwarf, a small, compact star at the end of its life, and it’s very rich in metals. LP 40–365 also has own atmosphere, which is mostly composed of oxygen and neon. But most important to this story is that the star is a runaway from a huge stellar explosion, which set in motion its dash out of the galaxy.

When a white dwarf is orbiting another (in what’s called a white dwarf binary), one star gives up mass to the other, which gobbles it up steadily. The binaries can also emit gravitational waves—perturbations in spacetime—as they orbit one another, with the hungry star (the accretor) in the duo detonating in a huge thermonuclear explosion.

The team behind the new research isn’t sure whether stars like LP 40–365 are typically the donors or the accretors in their white dwarf binary systems, but they believe this particular hot metal ball is basically stellar shrapnel from the accreting star, which eventually exploded in fantastic fashion. Their findings were published this week in The Astrophysical Journal Letters.

“To have gone through partial detonation and still survive is very cool and unique, and it’s only in the last few years that we’ve started to think this kind of star could exist,” Odelia Putterman, a researcher now at Occidental College and a co-author of the paper, told The Brink, a publication of Boston University.

A NASA illustration of one white dwarf in a binary system, bleeding off mass to its partner.

The team found the star using observations from the Transiting Exoplanet Survey Satellite (TESS) and the Hubble Space Telescope, which turned up a fast-moving object with a regular pattern of dimming and brightening. That suggested the star was slowly rotating—completing its rotation every nine hours—as it hurtled through space. That’s a pretty slow rotation rate, and weird to think about in conjunction with how fast the star is moving through space. It’s from that rotation rate that the team figures the white dwarf is the remnant of one star in a binary system collapsing in on itself, blasting its partner and all else in the area outwards at extraordinary speed. Based on the team’s calculations, they believe LP 40–365 has been traveling from its origin galaxy for a little over 5 million years.

“The star is basically being slingshotted from the explosion, and we’re [observing] its rotation on its way out,” Putterman told The Brink .

More: Astronomers Think They’ve Spotted a Rare Kind of Supernova Only Predicted to Exist