The stillness of the night sky is deceiving. Because of the sheer vastness of space, stars appear unmoving like celestial fixtures. In actuality, though, they're zipping through the cosmos - some at ridiculously high speeds: thousands, and even tens of thousands of kilometres per second.
That's roughly 100,000 times faster than the speediest train and 1,000 times faster than the fastest spacecraft that's ever flown. That's fast enough for a few spins around Earth in the time it takes to put on your socks. The point is, that's fast.
These individual stars basically travel from one side of the universe to the other
Some astrophysicists have suggested that, in principle, stars could go even faster - even as fast as light. Such stars may even harbour planets, prompting speculation that they could serve as intergalactic transport for alien life.
But you don't need to speculate to find stars rocketing out of our own Milky Way Galaxy. A speed of a thousand or so kilometres per second is already fast enough to send a star hurtling toward the lonesome expanse. These hypervelocity stars, as they're called, were only discovered about 10 years ago. So far, astronomers have found a total of about two dozen leaving the Milky Way. And they're trying to find more.
Despite their name, however, hypervelocity stars aren't the fastest known stars. That title belongs to the handful of stars whirling around the supermassive black hole at the galactic centre. One of the fastest reaches 12,000 km/s. But these stars are so close to the behemoth, which weighs as much as four million suns, that such speeds aren't enough to escape its gravitational grip. These stars, however, may have played an integral role in kicking hypervelocity stars out of the galaxy.
In 1988, astrophysicist Jack Hills of Los Alamos National Laboratory in the US described a hypothetical encounter between a supermassive black hole and a binary star system, which consists of two stars orbiting each other.
These stars would be one way for alien life to spread from galaxy to galaxy. No fancy spaceships needed
He realised that if the binary got too close, the gravitational dance with the black hole would fling one of the stars out at thousands of kilometres per second. He dubbed these exiled stars hypervelocity stars. Meanwhile, the black hole pulls the other star into a tight orbit.
But for years, no one paid much attention to this idea. After all, no one had ever seen a star escaping the galaxy.
Then, in 2005, an astronomer named Warren Brown was searching for a certain type of bright, blue star in the Milky Way. By tracking their motions, and thus the galaxy's gravitational influence on them, he was trying to measure the mass of the galaxy. But what he found instead was a star moving really fast. Too fast. It was leaving the galaxy at 853 km/s - more than 3 million km/h. "The speed was unlike anything I'd ever seen before," says Brown, who's at the Harvard-Smithsonian Center for Astrophysics in the US.
Then he came across Hills's paper, which seemed to explain the discovery perfectly. "If you have a supermassive black hole at the very centre of the galaxy, every so often it should slingshot a star out of the galaxy," Brown says. This mechanism would also leave a lot of stars in tight orbits around the central black hole, which is exactly what astronomers observe.
Buoyed by this discovery, Brown and other astronomers set out to find more fast stars. Today, they've found about two dozen of them - a number that's about right considering how often the galaxy's black hole should be tossing out stars. "The numbers add up," Brown says. "It's pretty likely that even though they're now hundreds of thousands of light years away from the Milky Way proper, these stars were indeed formed right in the heart of the Milky Way."
But Brown wants to find more. The ones he detected were big, blue, and bright - a hundred times more luminous than the sun - simply because those were the ones that stand out amidst the hundreds of billions of stars in the galaxy. According to estimates, Brown says, about a thousand hypervelocity stars might be in the galaxy's vicinity, and chances are that many of them are smaller and dimmer, making them hard to find.
To know for sure if a star is escaping the galaxy, astronomers need to pinpoint its speed. As a star moves away, its light turns redder, stretching to longer wavelengths. So by measuring how much a star's spectrum - its light broken up into its constituent wavelengths - is shifted toward redder colours, astronomers can determine its speed.
Above: Breath-taking time-lapse of the stars seen in the Southern sky
But that technique only reveals how fast it's moving away along the line of sight. To know the true speed, you need to know its trajectory - and thus how fast it's moving across the sky, which requires incredibly precise measurements beyond the capability of most current techniques. Pinpointing a star's trajectory will also show whether it's indeed coming from the centre of the galaxy.
Fortunately, that's exactly what ESA's Gaia spacecraft will do. Launched in 2013, Gaia is measuring the velocities and positions of about a billion of the galaxy's stars. When it's done, astronomers expect it will identify yet more hypervelocity stars. And that will help them better understand the galaxy.
These stars are born and launched from the galactic centre, and their speeds and properties will offer a unique glimpse as to what it's like in the bustling and crowded environment near the central black hole.
The stars can also help astronomers map out all the mass in the galaxy. "Any deviation of their trajectory betrays the influence of the underlying mass pulling on them," Brown explains. Most of the galaxy's mass is composed of the mysterious, invisible stuff known as dark matter. To figure out what it is, astronomers want to know exactly how much there is and how it's distributed across the galaxy.
The weird one
While most hypervelocity stars seem to have come from the galactic centre, that's not necessarily the case for all of them. In fact, the fastest known hypervelocity star - an object dubbed US 708 hurtling outward at 1,200 km/s (more than four million km/h) - has a completely different origin. "This thing," Brown says, "is weird."
When a team of astronomers discovered the star in 2005, they clocked it at 750 km/s. It wasn't until this year that a team led by Stephan Geier of the European Southern Observatory in Germany realised that the star was going much faster than that.
Comparing new observations from the Pan-STARRS survey with archival images dating back to the 1950s, the astronomers did what Gaia is now doing for other stars: determine the star's motion across the sky. They revealed not only its faster speed, but also its trajectory. And apparently, US 708 did not come from the galactic centre, ruling out a black hole origin.
While no one's sure yet, astronomers think it was a huge explosion that launched the star. The first clue is the fact that US 708 is a rare type of star called a hot subdwarf.
In the past, however, it was once a normal star. According to the hypothesis, it was part of a binary system with a white dwarf - a hot, dense object that's the remnant of a star such as the sun. The two were in a tight orbit, and during its normal aging process, US 708 expanded into a red giant and engulfed the white dwarf. Meanwhile, the white dwarf continued orbiting, and as it did so, it plowed away US 708's outer layers. With only its hot, helium-burning core remaining, US 708 became a subdwarf.
Then, the two objects spiraled toward each other, losing energy by emitting gravitational waves, ripples in the space-time fabric of the universe. Eventually, they got so close that the subdwarf started spilling helium over onto the white dwarf. So much helium accumulated that it ignited nuclear fusion, causing the core to explode and destroy the white dwarf.
"Nuclear fusion of helium is much more violent than nuclear fusion of hydrogen in our sun," Geier explains. "This does not go slowly. This happens in a flash."
Before the blast, though, the two stars had been orbiting each other extremely fast - about once every 10 minutes, according to calculations. So when the white dwarf blew up, and there was no longer anything holding onto US 708, the subdwarf was promptly thrown out. Think of two figure skaters spinning in each other's arms. If one lets go, the other flies away.
Above: Breath-taking time-lapse of the stars seen in the Northern sky
US 708 is the fastest star seen dashing out of the galaxy, but it couldn't have gone much faster. Because it was orbiting its partner so closely, it was already going as fast is it could. Which raises the question: If not via stellar explosions, how could you accelerate stars even faster?
The answer might be with supermassive black holes, according to astrophysicists Avi Loeb and James Guillochon, of the Harvard-Smithsonian Center for Astrophysics. But unlike with the other hypervelocity stars, you don't just need one black hole. You need two.
If you got two supermassive black holes - millions or even billions of times as massive as the sun - together with a star, their interactions could kick that star out at a speed ten times greater than any of the hypervelocity stars known.
These high-speed rendezvous can happen relatively often in the universe. Almost every galaxy, such as the Milky Way, has a supermassive black hole at its centre. And galaxies tend to gravitate toward one another, making collisions somewhat commonplace. When they do, the two central black holes spiral in toward each other and eventually merge. Stars that get in the way either fall into the black holes, are tossed aside but remain in the galaxy, or are completely ejected.
Most of those ejected stars will be about as fast as the conventional hypervelocity stars. But about one percent of them could surpass 10,000 km/s, reaching up to 100,000 km/s, or one-third the speed of light. "Beyond 10,000 km/s - this is really the only game in town," Guillochon says. "There's really no other way to accelerate stars up to that speed."
While the observable universe could have a trillion of these speedsters zooming around at 10 percent the speed of light, only a few thousand would reach the Milky Way's neighborhood. That may sound like a lot, but they would account for only one out of every hundred million stars in the galaxy. They wouldn't be easy to find.
But it's possible, Guillochon says. The next generation of telescopes, such as the James Webb Space Telescope or the Large Synoptic Survey Telescope now being built in Chile, could detect one of these stars. While the normal hypervelocity stars are moving too slowly to get very far, these super speedsters can cover lots of ground. "These individual stars basically travel from one side of the universe to the other," Guillochon says.
And that makes them useful for science. By combining the ages of the stars with their speeds, astronomers could estimate the distance the stars have traveled, providing a new way to measure cosmic distances.
These superfast stars would also act as beacons that herald the merging of two supermassive black holes. Astronomers can then follow up with ESA's eLISA satellite, slated for launch in 2028, which will detect the gravitational waves produced from these violent collisions.
A tiny fraction of the stars could conceivably be even faster. If a supermassive black hole were spinning rapidly, and a star were orbiting in the same direction as the spin, an incoming secondary black hole could expel the star to speeds approaching that of light. But, Guillochon says, that would require such a rare configuration that even in a universe of possibilities, it would be practically impossible to detect such a star.
Still, even sub-light-speed stars would be the ultimate spacefarers, fast enough to have crossed large swaths of intergalactic space. A planet could orbit one of these stars, and if the orbit were tight enough - comparable to the distance between Earth and the sun - the planet would survive the expulsion from its galaxy.
But given the harsh environment around a black hole, it would be difficult for life to evolve, Guillochon says. If it could, however, these stars would be one way for alien life to spread from galaxy to galaxy. No fancy spaceships needed.
Of course, that scenario is more science fiction than anything. But it's something to think about the next time you look up at those stars, sparkling and seemingly still.