Blowing Bubbles in the Gamma-ray Sky

Did you know our Milky Way galaxy is blowing

bubbles? Two of them, each 25,000 light-years tall! They extend above and below

the disk of the galaxy, like the two halves of an hourglass. We can’t see them

with our own eyes because they’re only apparent in gamma-ray light, the highest-energy light in the

universe.

We didn’t even know these humongous structures were smack in the middle of

our galaxy until 2010. Scientists found them when they

analyzed the first two years of data from NASA’s Fermi Gamma-ray Space Telescope.

They dubbed them the “Fermi bubbles” and found that in addition to being really

big and spread out, they seem to have well-defined edges. The bubbles’ shape

and the light they give off led scientists to think they were created by a

rapid release of energy. But by what? And when?

One possible explanation is that they could be

leftovers from the last big meal eaten by the supermassive black hole at the

center of our galaxy. This monster is more than 4 million times the mass of our

own Sun. Scientists think it may have slurped up a big cloud of hydrogen

between 6

and 9 million years ago and then burped jets of hot gas

that we see in gamma rays and X-rays.

Another possible explanation is that the bubbles

could be the remains of star formation. There are massive clusters of stars at

very the center of the Milky Way — sometimes the stars are so closely packed they’re a million times more dense than in the outer

suburb of the galaxy where we live. If there was a burst

of star formation in this area a few million years ago, it could have created

the surge of gas needed to in turn create the Fermi bubbles.

It took us until 2010 to see these Fermi bubbles

because the sky is filled with a fog of other gamma rays that can obscure our

view. This fog is created when particles

moving near light speed bump into gas, dust, and light in the Milky Way. These

collisions produce gamma rays, and scientists had to factor out the fog to

unveil the bubbles.

Scientists continue to study the possible causes

of these massive bubbles using the 10 years of data Fermi has collected so far.

Fermi has also made many other exciting discoveries — like the the collision of superdense neutron stars

and the nature

of space-time. Learn more

about Fermi and how we’ve been celebrating its first decade in space.

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