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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 rays, the highest-energy light in the universe.

In this animated GIF, the camera pans back and forth as the Milky Way’s spiral arms appear to dance. At the center of the spiral structure are two magenta bubbles, one each extending above and below the plane of the Milky Ways’ arms.
Using data from NASA's Fermi Gamma-ray Space Telescope, scientists discovered a gigantic, mysterious structure in our galaxy. This feature looks like a pair of bubbles extending above and below our galaxy's center, as illustrated here.
NASA's Goddard Space Flight Center

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?

Animation of the Fermi Gamma-ray Space Telescope. The satellite features a large black box structure with white instruments underneath. Two long solar arrays extend from opposite sides, just under the black box.
NASA’s Fermi Gamma-ray Space Telescope, illustrated here, scans the entire sky every three hours as it orbits Earth.
NASA’s Goddard Space Flight Center/Conceptual Image Lab

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.

In this illustration, a thick accretion disk has formed around a supermassive black hole following the tidal disruption of a star that wandered too close. Stellar debris has fallen toward the black hole and collected into a thick chaotic disk of hot gas. Flashes of X-ray light near the center of the disk result in light echoes.
In this illustration, a thick accretion disk has formed around a supermassive black hole following the tidal disruption of a star that wandered too close.
NASA/Swift/Aurore Simonnet (Sonoma State Univ.)

Another possible explanation is that the bubbles could be the remains of star formation. There are massive clusters of stars at 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.

This infrared image of Milky Way galaxy looks like a splatter of gold paint and burgundy clouds on a black, star-studded canvass.
This near-infrared image of the Milky Way Center from the Hubble Space Telescope reveals knots of cloud edges and emission that mark the plane of our galaxy.
NASA, ESA, and G. Bacon (STScI)

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.

An orange squiggle representing a lower-energy photon, perhaps microwave or ultraviolet, collides with a yellow electron. The result is a purple squiggle representing a gamma ray, which bounces away from the electron in the direction of the original photon.
An electron travelling at close the speed of light has a head-on collision with a low-energy photon (from microwave to ultraviolet) in this animation. The photon picks up energy from the electron and becomes a gamma ray, a process called inverse Compton scattering.
NASA’s Goddard Space Flight Center

Scientists continue to study the possible causes of these massive bubbles using data collected by Fermi, which has made many other exciting discoveries — like the collision of superdense neutron stars and the nature of space-time.

Magenta bubbles extend above and below a flat, clumpy Milky Way. The Milky Way is seen as a flat disk, with clouds of material lit from behind by bright stars and gas. This plane stretches almost the width of the image. Two magenta circles dominate the image, each resting at the center of the Milky Way’s plane, one above and one below.
From end to end, the gamma-ray bubbles extend 50,000 light-years, or roughly half of the Milky Way's diameter, as shown in this illustration. Hints of the bubbles' edges were first observed in X-rays (blue) by ROSAT (Röntgen Satellite), a German-led mission operating in the 1990s. The gamma rays mapped by Fermi (magenta) extend much farther from the galaxy's plane.
NASA's Goddard Space Flight Center