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If you’ve ever looked up at the night sky from a really dark place,
you might’ve seen the narrow band of the Milky Way.
Our galaxy is a hundred thousand light-years across,
but it looks so thin to you because you’re seeing its disk edge-on,
and there’s not much above or below it to light up the sky.
Still, if you happened to have superhuman vision, you might see a totally different picture:
two enormous plumes of light, bulging out of the Milky Way’s center.
Scientists call these Fermi bubbles.
They were discovered less than a decade ago,
and they could have a lot to teach us about the center of our galaxy.
That is, if we can figure out where they came from.
No matter how hard you try, you can’t see Fermi bubbles normally
because they shine not with visible light, but with gamma rays.
These rays are the most energetic form of light, and they’re really, really tiny:
Powerful ones can have a wavelength a million times smaller than what your eyes can detect.
So unless you happen to have a gamma-ray telescope, there’s no hope for you skywatchers.
Fortunately, though, NASA has just the right equipment.
Gamma rays are really common throughout the universe, and NASA studies them using the
Fermi Gamma-ray Space Telescope, which has been orbiting the Earth since 2008.
Still, even once the telescope was launched,
it took another two years for scientists to notice the Fermi bubbles.
That’s because, in space, gamma rays are so common that they create a sort of “fog”.
It comes from sources like supernovas and black holes all over the universe,
and it’s so permeating that it concealed the bubbles for a while.
They were only found in 2010, when researchers working on another investigation
just happened to be filtering out noise from the fog.
But now that we know where to look, they’re kinda hard to miss.
Each stretches more than 25,000 light-years across space,
blossoming out of the galaxy’s very center.
For comparison, the Milky Way’s disk is only about 1,000 light-years tall.
So these things are enormous.
Even though they’re often described as “gamma ray bubbles,”
it’s worth noting that the bubbles aren’t actually made of light.
After all, light normally travels in straight lines.
Instead, they’re made of super hot plasma, which emits the gamma rays.
Right now, there are two basic hypotheses about where all that plasma comes from.
One option, called the black hole hypothesis,
suggests that it came from the black hole at the middle of our galaxy.
After all, the Fermi bubbles seem to come from the galaxy’s center,
right where our enormous supermassive black hole lurks.
In other galaxies, it’s pretty common to find huge jets of gas blasting out of
the top and bottom of a central black hole.
So it makes sense that our galaxy wouldn’t be much different.
As gas, dust, stars, and, well, anything falls toward a black hole,
friction and pressure heat it up until it vaporizes into a plasma.
But sometimes, that plasma doesn’t actually fall into the black hole.
Sometimes it just whips around and shoots out the top or bottom, instead.
And maybe that’s where the Fermi bubbles come from.
Now, these days, our supermassive black hole is pretty quiet.
But according to data from the Hubble Space Telescope, it might’ve pulled in enough
material around six to nine million years ago to form these glowing blobs.
So maybe we’re just seeing the leftovers of the black hole having a huge snack.
Now, according to another idea, Fermi bubbles might not have come from a black hole at all.
Instead, the star formation hypothesis points out that there’s something else lurking
at the center of the Milky Way: what astronomers call a star-forming region.
These regions are capable of rapidly churning out new stars and, along with them,
big outflows of particles like electrons.
So many, the electrons are causing the Fermi bubbles, instead.
So far, there’s some evidence for this hypothesis, too.
But to find it, you’ve got to look all the way on the
other end of the electromagnetic spectrum at radio waves.
By studying the Fermi bubbles in these wavelengths, astronomers discovered they were emitting
a special kind of radio emission, called synchrotron radiation.
It’s produced when electrons move in the presence of a powerful magnetic field,
and strong fields like this are a common signature of star-forming regions.
Of course, these fields are also found around supermassive black holes, too,
so that’s not the only evidence.
The radio wave data also revealed ridges or ripples in the Fermi bubbles,
which could also be signals of a star-forming region.
The region could imprint that spiral, ripple-like pattern in the bubbles
as it circles our galaxy’s black hole.
As a bonus, if rapid star forming has been using up all the gas and dust
at our galaxy’s center, that could also explain why our black hole has been so quiet!
If this is the right answer, those ripples could help us track the history of star formation
over the last ten million years.
We’ll definitely need some more detective work to figure it out.
But whatever their cause, black hole or star formation, Fermi bubbles are a good reminder
that there’s a lot more to the universe than meets the eye!
Thanks for watching this episode of SciShow Space!
If you’d like to learn more about the supermassive black hole at the center of the Milky Way,
you can watch our episode all about it.
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