For years, astronomers have been puzzled by the presence of two gigantic, high-energy “bubbles” towering over and beneath the Milky Way’s center, extending 50,000 light-years into space. First detected by NASA’s Fermi Gamma-ray Space Telescope in 2010, and later by the eRosita X-ray telescope in 2020, these structures—now known as the Fermi and eRosita Bubbles—suggest that something catastrophic happened in our galaxy’s core millions of years ago.
Now, a new study from the University of Michigan provides the strongest evidence yet that these enormous structures were created by a violent outburst from the Milky Way’s supermassive black hole, Sagittarius A*. This discovery not only rewrites our understanding of our galaxy’s past but also sheds light on how black holes shape their surrounding environments.
What caused this explosive event? And what can these massive energy bubbles tell us about the hidden forces at work in the heart of the Milky Way? Scientists are finally getting answers.
The Fermi and eRosita Bubbles—What Are They?
Astronomers first identified the Fermi Bubbles in 2010 when the Fermi Gamma-ray Space Telescope detected two symmetrical, balloon-like structures extending tens of thousands of light-years above and below the Milky Way’s core. These enormous lobes are composed of high-energy gamma rays, hinting at an incredibly powerful source of radiation at their origin.
A decade later, the eRosita X-ray telescope discovered another set of even larger bubbles, which exist in X-ray wavelengths rather than gamma rays. While the Fermi Bubbles measure around 50,000 light-years across, the eRosita Bubbles stretch even farther—nearly twice the size of the Fermi structures. This raised a fundamental question:
- Were these two sets of bubbles caused by separate events, or are they two different layers of the same explosion?
Scientists now believe that both the Fermi and eRosita Bubbles were created by a single, powerful eruption from Sagittarius A*—a black hole-driven jet that shot out massive amounts of cosmic rays and X-ray energy.
Illustration Credit: NASA, ESA, Gerald Cecil (UNC-Chapel Hill), Dani Player (STScI)
“We Need to Understand How Black Holes Interact with Galaxies”
Black holes are often thought of as cosmic vacuum cleaners, devouring anything that gets too close to their event horizon. But in reality, they can also release vast amounts of energy back into space. This process is known as an Active Galactic Nucleus (AGN) outburst, and researchers now believe this is exactly what happened in the Milky Way 2.6 million years ago.
Astronomers at the University of Michigan ran detailed simulations to test whether a black hole-driven jet could have formed these structures—and the results were astonishing.
One of the lead researchers, Mateusz Ruszkowski, emphasized why this discovery is so significant:
“Our findings are important in the sense that we need to understand how black holes interact with the galaxies that they are inside, because this interaction allows these black holes to grow in a controlled fashion as opposed to grow uncontrollably.”
This means that Sagittarius A* may not just be a dormant black hole quietly sitting at the center of our galaxy—it may have undergone a massive energy eruption that shaped the very structure of the Milky Way.
Simulating the Event That Created the Bubbles
To better understand how the Fermi and eRosita Bubbles formed, researchers used advanced computational models that simulated the interaction between cosmic rays, interstellar gas, and black hole-driven jets.
These simulations allowed scientists to rule out previous theories, such as the starburst model, which suggested that a sudden burst of star formation caused the bubbles. Instead, they were able to confirm that the black hole jet model perfectly reproduces the size, shape, and energy of the observed bubbles.
“We not only can rule out the starburst model, but we can also fine-tune the parameters that are needed to produce the same images, or something very similar to what’s in the sky, within that supermassive black hole model,” said Ruszkowski.
This research also pinpoints the exact duration of the event. Astronomers estimate that the outburst from Sagittarius A* lasted around 100,000 years, which is an incredibly short timespan in cosmic terms.
“On the other hand, our active black hole model accurately predicts the relative sizes of the eRosita X-ray bubbles and the Fermi gamma-ray bubbles, provided the energy injection time is about one percent of that, or one tenth of a million years,” explained astrophysicist Ellen Zweibel.
In other words, this was not a slow-burning process—it was a rapid, high-energy explosion that radically reshaped the space surrounding the Milky Way’s core.
Key Findings from the New Study
| Discovery | Significance |
|---|---|
| Fermi and eRosita Bubbles originate from a single event | Confirms a black hole-driven explosion as the cause. |
| Sagittarius A released energy 2.6 million years ago* | Indicates our galaxy’s core was much more active in the past. |
| Outburst lasted ~100,000 years | Proves this was a short-lived but powerful event. |
| Cosmic rays expanded and formed the gamma-ray bubbles | Explains how high-energy radiation spread through space. |
What This Means for the Future of the Milky Way
These findings don’t just tell us about the past—they also provide insight into what might happen in the future. If Sagittarius A* erupted once, it could do so again.
While the black hole at the heart of the Milky Way is currently quiet, scientists believe that future events—such as a gas cloud or star falling into its gravitational grasp—could trigger another massive outburst. If that happens, new bubbles could emerge, altering the shape of our galaxy once more.
Additionally, this research has implications beyond our own galaxy. Other galaxies with similar bubble-like structures have been observed throughout the universe, suggesting that this phenomenon may be common among supermassive black holes.
As astronomers continue to study the Fermi and eRosita Bubbles, they are unlocking new clues about the life cycle of galaxies, the behavior of black holes, and the unseen forces shaping our universe.
