A 'Runaway' Star Is Hurtling Around the Milky Way. And It's Going the Wrong Way

It was launched by a tremendous explosion.
Brad Bergan
An illustration of a tremendous cosmic explosion.FlashMovie / iStock

Sometimes, you just want to get out.

And recently, astronomers noticed a star careening out of our galaxy at nearly 2 million mph (3.2 million km/h), which is roughly four times the speed of the sun's galactic orbit, according to a recent study published in The Astrophysical Journal Letters.

And, worryingly, it's going the wrong way and barreling against the flow of orbiting stars like it has a death wish.

A star rich in metals is careening out of the Milky Way

"It is exceptionally weird in a lot of different ways," said Astronomer J.J. Hermes of Boston University, who was lead study author, in an initial Live Science report. The unconscionably fast star has already achieved escape velocity, and will eventually depart the Milky Way. From this uncommonly high rate of escape, astronomers think this metal-rich star was launched into its wild trajectory by a stellar catastrophe: a supernova. But the specifics of how this happened remain elusive. Called LP 40-365, the star could be a remnant of the exploded star, but it could also be a partner star that was flung away by a shockwave when the other star exploded in a violent ball of apocalyptic light?

The recent study executed another analysis of the old data, and found that the star, which is a white dwarf, is spinning on its axis at a relatively calm rate, leading the astronomers to initially think it was likely a fragment of stellar debris, and not an ejected partner star, that somehow survived one of the most destructive events in the galaxy. "We can now connect this star to the shrapnel from an exploded white dwarf with a lot more confidence," said Hermes, in the report. Not long after it was discovered, astronomers came to grasp that the distinct features of the white dwarf implied it has survived a supernova from the most dangerous close-quarters distance. Most stars are forged mainly of helium and hydrogen, but LP 40-365 contains only heavy elements, like neon, magnesium, and oxygen. 

The careening star fragment offers a glimpse into the supernova furnace of life

These heavier elements are forged in the furnace of a supernova, which is where this zipping star got its "metals", as astronomers call every element heavier than helium. The researchers concluded that this star was once one of two binary white dwarfs, which are the postmortem remnants of sun-like stars. But white dwarfs explode when they suck up enough mass from their partner star. These tremendous explosions are so forceful that they fuse the star's nuclei into wholly new elements. The astronomers then considered two scenarios to explain the rapid-moving star. It may have experienced a clean explosion that blasted its partner star with heavy metals, and sent it veering out into interstellar space.

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However, it might also be a remnant of the exploding star that was ejected from the supernova reaction, brimming with heavy metals formed during the explosion. "One big question is: Which part of the binary star are we observing," said Astronomer Saurabh Jha of Rutgers University, New Jersey, who didn't participate in the recent study, in the Live Science report. "Both are exciting, giving us a new way to understand exploding white dwarfs." Hermes and their colleagues used data from NASA's TESS and Hubble, the astronomers deduced from the star's spin that it probably survived a supernova roughly 5 million years ago, and probably formed from a diffuse fragment of an exploded star. This is especially significant, because it means studying L 40-365 offers us a glimpse into the black box environment of a supernova, where the elements required for life as we know it, like carbon, oxygen, and more, are born.