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Most stars orbit the center of the galaxy. Some stars don't. Learn what scientists think is going on, with Reid Reimers!
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The Sun is great at what it does. No one’s gonna argue otherwise. But... it’s not the best at everything. For one thing, the Sun orbits the center of the galaxy at about 220 kilometers every second. Which seems pretty fast. I mean, the fastest objects humans ever sent into space -- the Helios probes, launched in the 1970s -- eventually reached a top speed of around 70 kilometers a second.

But there’s a star out there in our galaxy, known only as US 708, that’s moving 1200 kilometers per second, and it isn’t orbiting the center of the galaxy at all. Instead, it’s speeding toward intergalactic space. And it’s the fastest rogue star we’ve ever seen. 708 is what’s known as a hypervelocity star -- a star that’s going so fast it’s escaped the gravitational pull of the galaxy.

Hypervelocity stars were first discovered in 2005, when a group of astronomers from the Harvard-Smithsonian Center for Astrophysics were analyzing blue stars and noticed that one of them was going, like, ridiculously fast. When they tried to calculate its orbit, they realized it had come from somewhere near the center of our galaxy... and was headed straight out of it. That’s why at first, astronomers thought these rogue stars came from binary systems -- two stars orbiting each other -- that got too close to the black hole at the center of the galaxy. They figured one star got sucked in, and once it was gone, the second star was thrown completely off balance and rocketed away.

Problem was, later studies found some hypervelocity stars that couldn’t have come from the center of the galaxy -- some were too young, others weren’t going in the right direction. There’s even one hypervelocity star that seems to have come all the way from another galaxy -- its composition doesn’t line up with stars from our own galaxy, but it does match the stars from the Large Magellanic Cloud, one of our galactic neighbors. And, we’re pretty sure the Large Magellanic Cloud doesn’t actually have a black hole in the middle.

Which means that there’s probably some other way for stars to go rogue like this -- though it likely still has to do with multiple star systems that got messed up somehow. One possibility involves triple systems, with two stars that closely orbiting each other but are also bound to a third star a little farther away. If the third star is swallowed up by a black hole, the other two could go zooming off as hypervelocity stars.

But 708 probably escaped in a completely different way, according to a new study just published in March 2015 in the journal Science. The new theory has to do with the type of star 708 is: it’s what’s known as a hot subdwarf. Hot subdwarfs form from red giants -- huge older stars with helium cores and a hydrogen-and-helium shell, which is where the nuclear fusion happens. If something makes a red giant lose that shell, it becomes a hot subdwarf, which is mostly just the helium core.

Picture a very big, very hot helium balloon. Now, it’s possible that 708 was once a red giant, locked in a binary dance with a white dwarf, another type of very dense star. And white dwarf have gravitation that tends to suck in anything that gets close enough -- even material from other stars. Which turns out to be important, because if you dump enough helium into a white dwarf, it can go supernova.

So, here’s what the researchers think happened: US 708 was once a red giant that was paired up with a white dwarf, which slowly drew its helium away, turned 708 into a hot subdwarf. And when the white dwarf absorbed enough material, it exploded, and the supernova sent 708 speeding away through the galaxy at 1200 kilometers per second. That supernova would’ve been the incredibly bright Type Ia, which makes 708’s story incredibly useful for researchers. Because: not only can it explain why there are rogue, ridiculously fast stars flying all over the galaxy, it might also help us understand the explosions that made them what they are.

See, Type Ia supernovas are used by astronomers for all kinds of things, like calculating how far away other galaxies are, and how fast they’re moving. And scientists are pretty sure they’re set off by white dwarfs that pull in too much other stuff. But it’s hard to know for sure. So even though 708 will eventually leave the Milky Way entirely, it did more than just break a galactic record while it was here -- it also taught us a little more about some of the most useful explosions in the universe.

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