Well hi! Unless you’re an astronomer, you probably
don’t think of stars as moving all that quickly. You might not even know they move at all. If that’s the case, you’d be surprised
to learn that a star called S5-HVS1 is zipping through our galaxy at over ten thousand miles
per second. [siren] Alright buddy, what’s your hurry? License and registration. Ow -- you’re awfully bright…) As with many of the best discoveries, astronomers
found this rogue star entirely by accident. Researchers were conducting a survey of what
they called stellar streams, remnants of smaller galaxies, torn apart and absorbed into the
much larger Milky Way. An astronomer by the name of Sergey Koposov
was going through some of the survey data when he came across some unusual readings. He was looking for stars with unusually rapid
orbits, but was surprised to find one fleeing the galaxy at record speeds. Now, it’s hard to tell by looking at them,
but stars are in constant motion, orbiting the center of the galaxy just like the Earth
orbits the sun. Astronomers call this normal movement proper
motion. Stars move at varying rates, but due to the
distances involved, it can take centuries for these changes in position to become apparent. However, S5-HVS1 (you know, let’s just him
“Speedy”) is no ordinary star, and its movement isn’t at all normal. This recently discovered star is racing toward
the outer edge of our galaxy, having been ejected from its orbit at a velocity never
before seen in recorded history. Astronomers classify this white-hot ball of
plasma as an A-type main-sequence star and have estimated it to be more than two times
the size of the Sun. That alone wouldn’t make it unusual, but
it’s very odd to find a star of that size moving at such high speeds. Scientists call these astral speedsters hypervelocity
stars, and they tend to be either much larger, or substantially smaller than Speedy. The latter aren’t technically true stars
at all. Known as neutron stars, these extremely dense,
but comparatively tiny, balls of slowly cooling plasma are what’s left over after a star
completes its life cycle and goes supernova. And when I say small but dense, I mean it. These former stars are usually only about
twelve or so miles in diameter, but contain more matter than the sun. In those cases, you can probably guess how
these cosmic leftovers find themselves hurled through space at breakneck speeds. When a star goes supernova, it doesn’t always
explode evenly. These lopsided detonations not only expel
a few dozen Suns worth of gas and plasma, but also hurl what remains of the star through
the cosmos at almost half the speed of light. Considering humans have only been able to
propel an object at a fraction of that, it looks like mother nature has us beat. But Speedy isn’t a neutron star. So why is it getting expelled from the galaxy
like a spare tire improperly secured to the back of a truck? The simple answer is gravity. The slightly less simple answer is gravity
from black holes, and the vastly more complicated answer is what scientists call a three-body
exchange. They work a bit like this. You start with two stars orbiting each other
in what’s known as a binary star system. Although the immense distance between star
systems makes it impossible to tell with the naked eye, multi-star arrangements like this
aren’t that uncommon. Alpha Centauri, the closest star system to
the sun, is actually made up of three stars of varying sizes. While these systems might seem chaotic and
unstable, as with any set of roommates, order can be maintained as long as everyone agrees
to follow clear boundaries. Oh, and if anyone was wondering, the gravity
of these two stars does some weird things to the planets orbiting them. While some scientists believe life could still
develop on these hostile worlds, I wouldn’t hold my breath. ([Inhale] Nope, can’t do it.) As I was saying, things can get pretty wild
when black holes are added to the mix. In this analogy, they’d be somebody’s
loud friend who’s always around, leaving dishes in the sink and drinking the last Coke
without asking. When these binary systems drift too close
to a black hole, its mass disrupts the delicate balance of gravity and momentum that keeps
the system stable. What follows is a chaotic dance, [adding sounds
of the dancers] as the stars loop wildly around their new anchor, the lighter objects building
speed as they slingshot around it. Each star’s gravity influences the other’s
movement, sending them careening through space on crazy trajectories. Eventually one of the stars gains enough velocity
to break away from the system, flying off in a random direction at a high rate of speed. If you ever swung your keychain around your
finger only to have it soar out of reach, you’ve seen a version of this in action. What differentiates Speedy from the other
black hole ejections is the star’s comparatively low mass. Despite being more massive than our own Sun,
it’s a shrimp compared to the much larger blue and ultraviolet stars more commonly seen
in such events. Less mass means less gravity, which, in turn,
means less energy propelling the star. And it took a lot of energy to expel Speedy
at its current speed. Enough force in fact, that if the Earth were
to be somehow subject to it, our planet would accelerate to 99.7 percent of the speed of
light. Um, while that almost definitely won’t happen,
feel free to hold on to something solid just in case. To generate that much energy with a star that
small, the black hole that disrupted Speedy’s orbit would need to be extremely massive. Supermassive might be the more accurate term,
since astronomers have determined that based on the star’s trajectory, it was sent flying
by none other than the supermassive black hole at the center of our galaxy. Known to astronomers as Sagittarius A, this
enormous collapsed star is what gives the Milky Way its shape. It’s also the only object with a strong
enough gravitational field to play driver to our favorite cosmic golf ball. [driver hits ball]
That makes Speedy the first confirmed object to be ejected from our galaxy’s core. This is an important discovery, as it fills
in a crucial missing piece towards understanding how our galaxy works. Before being batted across the universe, Speedy
would’ve been paired with a second, much smaller star. This star would’ve been significantly less
massive than the Sun, and was most likely absorbed into the black hole during the catastrophic
disruption of its orbit. As for how exactly Speedy found itself swatted
across the galaxy by the universe’s largest bat, astronomers have two competing theories. The first is that both stars originated in
the supermassive black hole’s accretion disk. This is the astronomical term for the broad
ring of gasses and dust that orbits just beyond the event horizon, the point of no return
from which nothing, not even light, can escape the black hole’s intense gravitational pull. Conditions inside the disk would be perfect
for the formation of stars, and the black hole’s enormous size guarantees there would
be enough matter for that to happen. This unusual method of creation would explain
how two stars of such radically different mass could form a stable binary system. Given their proximity to the black hole, the
stars wouldn’t have far to drift to find themselves close enough for their orbits to
be disrupted. The other possibility is that a smaller black
hole could have been pulled into its more massive cousin. If this happened, their combined gravity could’ve
been enough to wrench nearby stars out of orbit. While astronomers consider this the less likely
of the two possibilities, a definitive answer either way, would give researchers a lot to
work with. Astronomers may not have a perfect answer
yet, but Speedy is still under observation and there’s no doubt more to uncover. Researchers hope that information gained from
the Gaia star mapping project, which is set to complete in 2021, will shed new light on
this cosmic mystery. For now, astronomers are eager to find more
hypervelocity stars in hopes of gaining a better understanding of what’s going on
in the Galactic Center, and how mass is distributed across the vast collection of stars we call
the Milky Way. These discoveries would radically change our
understanding of the cosmos, so it’s no surprise that astronomers are chomping at
the bit for more information. (Hmm, astronomers chomping at the bit. Hey I’d watch that. )
How about you? Are there any other scientific mysteries you’re
eager to see uncovered? Let me know down below. Hey, if you learned something new today, then
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enjoy! Stay on the Bright Side of life!
