Scientists find chunk of blown-apart star hurtling through Milky Way at breakneck speed

A chunk of stellar shrapnel is careering toward the edge of our Milky Way galaxy at almost 2 million mph (3.2 million kph), a new study reports. "The star is moving so fast that it's almost certainly leaving the galaxy," study co-lead author J.J. Hermes, an associate professor of astronomy at Boston University, said in a statement. The star, known as LP 40-365, currently lies about 2,000 light-years from Earth. And calling it a star may be a bit generous, actually; Hermes and his colleagues think it's a hunk of a superdense stellar corpse called a white dwarf that was blown apart in a violent supernova explosion after gobbling up too much mass from a companion. "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," study co-author Odelia Putterman, a former Boston University student who has worked in Hermes' lab, said in the same statement. The speedy star was spotted during an analysis of survey data gathered by NASA's Hubble Space Telescope and Transiting Exoplanet Survey Satellite (TESS). The researchers noticed that LP 40-365 is not only racing along but is also rotating once every nine hours as it goes.The rotation in itself is nothing unusual, for all stars rotate; our own sun spins on its axis every 27 Earth days. However, according to researchers, a nine-hour rotational period is considered to be relatively slow for an object that went through something as catastrophic as a supernova. 

It's this sluggish rotation that implies LP 40-365 was once part of a two-star system with an unhealthy feeding habit.

According to the researchers, stars commonly orbit each other in close pairs, including highly dense white dwarfs. In such binary systems, if one white dwarf transfers too much mass to the other, the result can be a supernova — the largest explosion that takes place in space, according to NASA.

It's usually hard to determine which star was the "donor" and which was the "eater." But because LP 40-365's rotation is relatively slow, the research team feels confident that the object is cosmic shrapnel from the exploded star. As the two stars orbited each other at high speeds and in close proximity, the resulting supernova likely catapulted both stars out at breakneck speed, but we've only been able to spot LP 40-365, according to the statement.

"This [paper] adds one more layer of knowledge into what role these stars played when the supernova occurred," and what can happen after the explosion, Putterman said. "By understanding what's happening with this particular star, we can start to understand what's happening with many other similar stars that came from a similar situation."

These supernova survivors are even more intriguing as they are metal-rich, unlike our sun, which is primarily composed of hydrogen and helium. (Astronomers consider any element heavier than hydrogen and helium a metal.)

"These are very weird stars," Hermes said. "What we're seeing are the byproducts of violent nuclear reactions that happen when a star blows itself up." Strange stars like LP 40-365 are therefore fascinating targets to study, the researchers said.

The research is described in a study published June 10 in The Astrophysical Journal Letters.