Earth - Speedy Stars Fly Across Universe

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Bizarre Cosmic Objects Universe

Speedy stars fly acrossUniverse

Some stars may travel across the cosmos, perhaps with

aliens in tow

By Marcus Woo7 September 2015

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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 trainand 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

Stars moving across the Nambian sky (Credit: Steve Bloom Images/Alamy)
http://ichef.bbci.co.uk/wwfeatures/976_549/images/live/p0/31/lc/p031lc5g.jpghttp://gizmodo.com/how-did-we-get-to-pluto-so-fast-1716533356http://www.slashgear.com/japans-new-maglev-bullet-train-is-now-the-fastest-in-the-world-22380159/


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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

Can aliens hitchhike alongside shooting stars? (Credit: Royal ObservatoryEdinburgh/SPL)
http://ichef.bbci.co.uk/wwfeatures/976_549/images/live/p0/31/ld/p031ldtq.jpghttp://adsabs.harvard.edu/abs/2005ApJ...620..744G


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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 encounterbetween 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 fancyspaceships 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.
http://iopscience.iop.org/1538-4357/622/1/L33/fulltext/http://www.nature.com/nature/journal/v331/n6158/abs/331687a0.html


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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 - anumber 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."

Some stars move through the Universe (Credit: NASA/ESA/STSCI/A.Schaller/SPL)
http://ichef.bbci.co.uk/wwfeatures/976_549/images/live/p0/31/lc/p031lck6.jpg


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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 topinpoint 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.

Zeta Ophiuchi, a runaway star moving through space (Credit:NASA/JPL/Caltech/UCLA)
http://ichef.bbci.co.uk/wwfeatures/976_549/images/live/p0/31/lf/p031lfdp.jpg


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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

Black holes can attract stars (Credit: NASA/CXC/M.Weiss)
http://ichef.bbci.co.uk/wwfeatures/976_549/images/live/p0/31/lc/p031lclx.jpghttp://www.sciencemag.org/content/347/6226/1126http://www.bbc.com/earth/story/20150824-what-is-the-universe-made-of


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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

Two stars can orbit each other, spinning fast like skaters (Credit: ChrisButler/SPL)
http://ichef.bbci.co.uk/wwfeatures/976_549/images/live/p0/31/lc/p031lct7.jpghttp://www.sciencemag.org/content/347/6226/1126


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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.

A single star can then be flung deeper into space (Credit: CVI Textures/Alamy)
http://ichef.bbci.co.uk/wwfeatures/976_549/images/live/p0/31/lc/p031lcxq.jpg


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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 andeventually 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."

Do some stars wander the Universe at super speed? (Credit: JG

Photography/Alamy)
http://ichef.bbci.co.uk/wwfeatures/976_549/images/live/p0/31/lc/p031lczr.jpg


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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 Telescopeor the Large Synoptic

Survey Telescopenow 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.

A black hole is at the centre of the Milky Way (Credit: NASA/JPL-Caltech)
http://ichef.bbci.co.uk/wwfeatures/976_549/images/live/p0/31/ld/p031ldhq.jpghttp://arxiv.org/abs/1411.5030http://www.lsst.org/http://www.jwst.nasa.gov/


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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 eLISAsatellite, 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.

The Indian Night Sky (credit: Navaneeth Unnikrishnan).
http://ichef.bbci.co.uk/wwfeatures/976_549/images/live/p0/30/jt/p030jtgf.jpghttps://www.elisascience.org/


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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.

Pulsars spin so fast they are capable of warping atoms

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http://www.bbc.com/earth/columns/earth-capturehttp://www.bbc.com/earth/story/20150902-cute-animalshttp://www.bbc.com/earth/columns/extreme-lifehttp://www.bbc.com/earth/story/20150904-the-bizarre-beasts-living-in-romanias-poison-cavehttp://www.bbc.com/earth/story/20150902-cute-animalshttp://www.bbc.com/earth/story/20150904-the-bizarre-beasts-living-in-romanias-poison-cavehttp://www.bbc.com/earth/tags/cave


23/27

The internet is arguing over which are the cutest. Should these win?

We're obese, smell, don't hang with penguins and have a thing for brown bears

The worlds cutest animals

The Truth About Animals

The truth about polar bears

Polar bear
http://www.bbc.com/earth/columns/the-truth-about-animalshttp://www.bbc.com/earth/story/20150902-the-truth-about-polar-bearshttp://www.bbc.com/earth/story/20150902-the-truth-about-polar-bearshttp://www.bbc.com/earth/tags/polar-bearhttp://www.bbc.com/earth/story/20150902-cute-animals


24/27

This is what the New Horizons probe saw as it whizzed past

Video

Astonishing flyby of Pluto

Pluto

Around the BBC

Future

The lost tunnels of Liverpool
http://www.bbc.com/http://www.bbc.com/future/story/20150902-the-lost-tunnels-of-liverpoolhttp://www.bbc.com/future/story/20150902-the-lost-tunnels-of-liverpoolhttp://www.bbc.com/earth/columns/videohttp://www.bbc.com/earth/story/20150901-pluto-as-seen-by-new-horizonshttp://www.bbc.com/earth/story/20150901-pluto-as-seen-by-new-horizonshttp://www.bbc.com/earth/tags/pluto


25/27

Future

Is alcohol actually bad for you?

Culture

The mystery of punctuations origin
http://www.bbc.com/http://www.bbc.com/culture/story/20150902-the-mysterious-origins-of-punctuationhttp://www.bbc.com/http://www.bbc.com/future/story/20150901-is-alcohol-really-bad-for-youhttp://www.bbc.com/culture/story/20150902-the-mysterious-origins-of-punctuationhttp://www.bbc.com/future/story/20150901-is-alcohol-really-bad-for-you


26/27

Culture

They policed the police

Capital

The best beach city for expats?
http://www.bbc.com/http://www.bbc.com/capital/story/20150902-is-this-the-best-beach-city-for-expats-in-finance-and-techhttp://www.bbc.com/http://www.bbc.com/culture/story/20150903-the-black-panthers-they-policed-the-policehttp://www.bbc.com/capital/story/20150902-is-this-the-best-beach-city-for-expats-in-finance-and-techhttp://www.bbc.com/culture/story/20150903-the-black-panthers-they-policed-the-police


27/27

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Capital

Why strategy beers are important
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Earth - Speedy Stars Fly Across Universe