Above and below the center of our galaxy sit two colossal bubbles of energy, each about 25,000 light-years tall and invisible to the human eye. Together, the Fermi Bubbles span roughly 50,000 light-years, about half the width of the Milky Way itself. Nobody knew they existed until 2010. Today they’re one of the strongest clues that the black hole at the heart of our galaxy was once far more violent than it is now.
In this article we’ll cover what the Fermi Bubbles are, how astronomers found them hiding in plain sight, and the two leading theories for what blew them into existence.
Key Facts About the Fermi Bubbles
| Discovered | 2010, by Meng Su, Tracy Slatyer and Douglas Finkbeiner (Harvard-Smithsonian Center for Astrophysics) |
|---|---|
| Telescope | NASA’s Fermi Gamma-ray Space Telescope |
| Size | About 25,000 light-years per bubble, roughly 50,000 light-years end to end |
| Seen in | Gamma rays (also traced in X-rays, microwaves and radio) |
| Age | A few million years, very young by cosmic standards |
| Likely source | A past outburst from Sagittarius A*, the Milky Way’s central black hole |
What Are the Fermi Bubbles?
The Fermi Bubbles are two enormous lobes of hot gas and high-energy particles that extend straight out of the plane of the Milky Way, one heading “north” and one heading “south.” They are centered on the galactic core, where a supermassive black hole called Sagittarius A* sits about 26,000 light-years from Earth.
What makes the bubbles stand out is how they glow. They give off gamma rays, the most energetic form of light. Gamma rays are produced when particles moving at nearly the speed of light collide with lower-energy light or with gas. Something had to accelerate an enormous number of particles to those speeds, and it had to happen fairly recently.
The bubbles also have surprisingly sharp edges. A slow, gentle process would leave a fuzzy, faded glow. Crisp boundaries point to a single, sudden burst of energy that is still pushing outward.
How Astronomers Discovered Them
NASA launched the Fermi Gamma-ray Space Telescope in 2008 to map the high-energy sky. Its main instrument, the Large Area Telescope (LAT), scans the entire sky every three hours.
In 2010, Harvard astronomers Meng Su, Tracy Slatyer and Douglas Finkbeiner were working through public Fermi data. After they subtracted the known sources of gamma rays (the glow of the galactic disk, nearby pulsars and distant galaxies), two giant shapes were left over above and below the galactic center. They published the discovery that year, and the structures became known as the Fermi Bubbles.
Looking back, there had already been hints. NASA’s WMAP satellite had picked up a strange microwave “haze” around the galactic center years earlier, and the Fermi Bubbles turned out to be the high-energy counterpart of that haze.
What Created the Fermi Bubbles?
Astronomers agree that the bubbles came from a huge release of energy at the center of the Milky Way. They still debate what that release was. There are two main candidates.
Theory 1: Sagittarius A* Woke Up
Today Sagittarius A* is a quiet black hole. It has about 4 million times the mass of the Sun, but it swallows very little material. The leading theory says it wasn’t always this calm.
A few million years ago, a large cloud of gas may have fallen onto the black hole. As it fed, Sagittarius A* would have flared into an active galactic nucleus, briefly shining like a small quasar and firing powerful jets or winds out of the galactic plane. Those outflows would have inflated the bubbles we see today.
There’s supporting evidence beyond the bubbles. The Magellanic Stream, a long ribbon of gas trailing our neighboring dwarf galaxies, shows signs of having been lit up by an intense flash of radiation from the galactic center around 3.5 million years ago. That timing lines up well with the age of the Fermi Bubbles.
Theory 2: A Starburst at the Galactic Core
The other explanation is a burst of star formation near the center of the galaxy. If many massive stars formed at once, their winds and supernova explosions could have driven hot gas outward over millions of years.
The very sharp edges of the bubbles are harder to explain this way, because a starburst would release its energy more gradually. That’s why many astronomers lean toward the black hole explanation. The two ideas can also work together, since galactic centers often feed their black holes and form stars at the same time.
Even Bigger: The eROSITA Bubbles
In 2020, the German-Russian X-ray telescope eROSITA found another pair of bubbles that dwarfs the first. The eROSITA bubbles glow in X-rays and stretch roughly 45,000 light-years above and below the galactic plane, and they appear to surround the Fermi Bubbles.
Many astronomers think both pairs came from the same kind of activity at the galactic center, possibly the same event, with the X-ray bubbles marking the outer shock wave. Either way, our galaxy’s core has clearly gone through powerful outbursts in its recent past.
Why Can’t We See the Fermi Bubbles?
The Fermi Bubbles cover a huge part of the sky. Seen from Earth, they stretch more than 50 degrees above and below the galactic center, which is wider than 100 full Moons lined up. You still can’t see them, for two reasons:
- Our eyes can’t detect gamma rays or X-rays. The bubbles barely shine in visible light.
- Earth’s atmosphere blocks gamma rays. They can only be seen from space, which is why it took the Fermi telescope to reveal them.
Why the Fermi Bubbles Matter
The Fermi Bubbles give us a close-up record of something we usually only see in distant galaxies: a supermassive black hole shaping its home galaxy. Outflows like these can heat and push out gas, which slows the birth of new stars. Studying the bubbles helps astronomers understand how black holes and galaxies grow together.
They also show that the Milky Way isn’t a calm, unchanging place. Only a few million years ago, around the time early human ancestors walked in Africa, the center of our galaxy flared with enough energy to blow bubbles 50,000 light-years tall. Those bubbles are still expanding.
Keep exploring: Sagittarius A* doesn’t only blow bubbles, it throws stars too. Read S5-HVS1: The Star Our Galaxy Threw Away at 6 Million km/h, or see how far our nearest big neighbor is in You’re Looking 2.5 Million Years Into the Past When You See Andromeda.
Frequently Asked Questions
What are the Fermi Bubbles?
The Fermi Bubbles are two giant lobes of hot gas and high-energy particles that rise about 25,000 light-years above and below the center of the Milky Way. They glow in gamma rays and were discovered in 2010 using NASA’s Fermi Gamma-ray Space Telescope.
How big are the Fermi Bubbles?
Each bubble is roughly 25,000 light-years tall, so the whole structure spans about 50,000 light-years, around half the diameter of the Milky Way’s disk.
What caused the Fermi Bubbles?
The most widely supported explanation is an outburst from Sagittarius A*, the Milky Way’s central supermassive black hole, a few million years ago. A burst of star formation at the galactic center is the main alternative.
Can you see the Fermi Bubbles with a telescope?
No. The bubbles mainly shine in gamma rays, which Earth’s atmosphere blocks. They can only be detected by space telescopes such as Fermi.