Hypervelocity stars wander cosmos: Some stars may travel across the Universe, perhaps with aliens in tow #Beekhaybee

i

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?
Super speedsters
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.

Advertisements