The Exploding Runaway Star

The Exploding Runaway Star

Most stars in the Milky Way have humdrum lives, tracing slow predictable orbits around the galactic center. But a few have a different destiny. Instead of orbiting, the hypervelocity stars are leaving the galaxy at high speeds, kicked out by the powerful black hole at the Milky Way’s center.

These stars are rare: astronomers have only found about 20 of them so far, with the latest announced just last week. However, hypervelocity stars can tell us about two very different parts of the galaxy: the crowded galactic center, and the dark outer regions. Thanks to their origins and trajectories, they could reveal much about the Milky Way’s central black hole, but also tell us something about the mysterious dark matter that surrounds the bright portions of the galaxy.

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The most recent hypervelocity star discovery is known as LAMOST-HVS1. In typical fashion, it’s named for the observatory where it was discovered: the Large-sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST), located near Beijing in China. (Astronomers love convoluted acronyms.) The “HVS1” means it’s the first hypervelocity star (HVS) found there.

LAMOST-HVS1 is approximately 42,000 light-years from Earth (A light-year is the distance light travels in one year, approximately 6 trillion miles or 9 trillion kilometers.), and moving 477 kilometers per second (1.1 million mph) away from the galactic center. That makes it the closest hypervelocity star yet discovered. It’s also the second brightest, though you’d still need a big telescope to see it. In technical terms, it’s 13th magnitude. That makes it appear about six times brighter than Pluto, but still far fainter than any star we can see with the unaided eye.

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Based on calculations, astronomers predict that one hypervelocity star is kicked out of the galaxy every 100,000 years, on average. That’s a blink of an eye in galactic terms, so a fair number should be around, at various distances based on when they were launched. However, identifying individual stars at large distances is tough. We’re lucky with LAMOST-HVS1: it’s a bright blue-white star, roughly 9 times the mass of the Sun, so it emits a lot of light. Other fainter stars would require more careful observation, but that’s part of the LAMOST observatory’s goal: identifying enough hypervelocity stars to extrapolate to those we can’t see as easily.

Our Sun is, in many respects, a completely ordinary star, located about 26,000 light-years from the galactic center. That places us a little more than halfway to the edge of the galactic disk, the part of the Milky Way that contains its spiral arms.

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The Solar System moves fairly quickly around the center of the galaxy, completing one orbit roughly every 250 million years. We don’t move much toward or away from the center: almost all of the Sun’s motion is in a circular path. (Despite the large mass of the central black hole, the motion of the Sun is mostly from the gravity of other stars, which outweigh it by a huge amount.) By contrast, LAMOST-HVS1 is more than twice as far from the center of the galaxy, and moving outward at three times the Sun’s speed.

So why are hypervelocity stars so fast? Many stars are in binary systems, locked in mutual orbit with another star. If a binary strays too close to the galaxy’s massive central black hole, the complex gravitational interaction between the two stars and the black hole can split the system apart. In that case, one star ends up in a closer orbit, while the other gets a big boost of speed—maybe even fast enough to escape from the galaxy.

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LAMOST-HVS1 definitely is moving fast enough to escape, but that’s not its primary interest for us. As the star moves from the brightly lit parts of the galaxy into the dark halo surrounding it, the star will act as a space probe for us. The halo is a huge region, roughly spherical in shape, containing a few stars, but full of dark matter: the invisible stuff making up 80% of the mass of the galaxy. We haven’t detected dark matter directly, but we can see their gravitational effect on stars and gas inside the Milky Way.

As hypervelocity stars enter the halo, they are mostly affected by the gravitational pull of dark matter. While a single star won’t tell us too much we don’t already know about the outer galaxy, as astronomers discover more of them, their motion could help provide more data on the halo’s shape and extent—something very difficult to do.

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There’s also a chance that LAMOST-HVS1 didn’t come from the galactic center. Despite the star’s fast speed, it’s a big galaxy, so we didn’t see its full trajectory, only where it is now. Extrapolating from that, the researchers estimated that while it most likely originated at the Milky Way’s core, it might have been kicked out of the disk instead. In that case, there might be a hidden middleweight black hole somewhere else, much more massive than a star, but less monstrous than the central black hole.

But that’s the excitement of research: we don’t know all the answers before we start. Whenever a new kind of object is discovered—like a hypervelocity star—there’s a good chance it can tell us something new about the cosmos. A runaway star could bring light to both the outer and inner darkness in the galaxy.

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