Millions of years ago, tens of thousands of light-years away, two giant white stars brushed against each other.
Then they exploded.
That’s not unusual. Star explosions—aka supernovas—occur every century or so in our galaxy, the Milky Way. There are trillions of galaxies. Do the math. Across the incalculable breadth of the universe, supernovas probably occur at a rate of many millions per day.
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But the supernova that the two stars triggered in our own celestial neighborhood, all those millions of years ago, was special. The blast ejected a giant chunk of metal and sent it flying across the Milky Way at a speed of 2 million miles per hour.
LP 40-365, as astronomers have named the object, is weird. Really, really weird. And it’s compelling scientists to rethink a whole lot of old assumptions.
For one, they’ve started calling LP 40-365 a star. But the speedy stellar shrapnel doesn’t look like most stars, or behave like them. So what makes stars stars? How are they born? How do they die? Can they come back to life, so to speak? And what do they get up to in their long, sometimes strange, lives?
A June paper in The Astrophysical Journal Letters—the final product of a survey led by Boston University astronomer J.J. Hermes and his co-author Odelia Putterman—added a new wrinkle to LP 40-365’s odd story.
Not only is the star exiting our galaxy at a high rate of speed, it’s spinning like a top as it goes. “LP 40-365 is one of the weirdest stars in the sky, and probably will be for a long time,” Hermes told The Daily Beast.
A team of astronomers led by Stephane Vennes at the Astronomical Institute in the Czech Republic, combing through data from telescopes at the Kitt Peak National Observatory in Arizona, first detected LP 40-365 six years ago.
The object’s rapid sprint across the sky—combined with its blue color and the absence of the telltale streaks indicating the presence of hydrogen or helium—told Vennes and his colleagues they had found something special.
“Its motion in the Milky Way and its surface composition indicate a violent past,” Vennes told The Daily Beast. Working backward, the scientists reconstructed LP 40-365’s history in order to understand its possible future.
They concluded that LP 40-365 was born when a pair of neighboring stars got too cozy and gravity started playing odd tricks. One star began stealing mass from the other—until it stole too much, and exploded.
It was a fairly weak explosion by supernova standards, however. Instead of obliterating itself, the bursting star kicked out at least one giant chunk of neon and magnesium and sent it flying toward the edge of the galaxy.
Vennes and his team published their findings in Science in 2017. Two years later, scientists including Mark Hollands—an astronomer at the University of Warwick in the United Kingdom—found three more stars similar in composition to LP 40-365. “A distinct class of chemically peculiar runaway stars that are the survivors of thermonuclear explosions,” is how they described the objects in Monthly Notices of the Royal Astronomical Society.
Others use a catchier name. “Zombie stars.” Strange, half-alive galactic travelers sharing the cold dark of space with free-floating “rogue” planets.
It got weirder. A closer look revealed that LP 40-365 has an uneven surface, which could be evidence that the star has a magnetic field—although Hollands stressed no one has detected one yet. Once Hermes was able to analyze LP 40-365’s strobe-like pulsing and conclude the star is spinning, a more complete picture took shape.
LP 40-365 is metallic. Unusual in shape. Rotating fast. And destined to break free of the Milky Way in a few hundred million years. What happens after that… is hard to describe. There are trillions of galaxies, but they’re really far apart. What lies between is very dark, very cold and very lonely.
That’s where LP 40-365 will live, flying fast and slowly cooling, until it passes close enough to a new galaxy for the gravity of nearby stars to pull it in. That could take billions of years, Vennes noted. Our own star, and our whole species, might be dead by then.
“Discovering weird stars like LP 40-365 helps us better understand the range of catastrophic fates for some stars in the sky,” Hermes said. It also helps us to appreciate our own star, which compared to LP 40-365 is gassy, immobile and very common.
Our humdrum sun is exactly what we and all other known life needed in order to evolve, however. Sure, faraway exploding stars, passing asteroids and the occasional acid-soaked meteorite stir the chemical stew in our own corner of the universe and probably helped juice evolution on Earth.
But our steady, predictable, not-too-hot, not-too-cold sun kept single-cell organisms warm as they adapted, mutated, grew more complex and eventually turned into us. “We are incredibly lucky that our own star is boring!” Hermes said. “We certainly wouldn’t want to orbit a star that ends its life as violently as LP 40-365.”
The runaway zombie star and its three known cousins are surely just the tip of the cosmic iceberg. The harder we look, through increasingly sophisticated telescopes, the more strange stuff we find—and the more sense we can make of our own seemingly unlikely existence in the vast, colorful cataclysm of space.
“We should definitely be able to trace back the origin of LP40-365 and its siblings to the original events that created them,” Vennes said.
If we can pinpoint two stars that look like they’re about to initiate an unhealthy exchange of stellar material, we might get lucky and witness the kind of supernova that produces a zombie star. That, in turn, could lead us along other lines of inquiry.
Where do these stars come from? How do they get so close together? Do strange supernovas tell us anything about the most important explosion of all, the Big Bang? “I think we can absolutely expect many exciting discoveries in this area over the coming years,” Hollands said.
