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.
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