Two Bizarre Bubbles Sandwich The Milky Way Galaxy. Here's What They Could Be

Despite being the place we call home, much about the Milky Way galaxy remains a mystery. That's partly due to its size — at 100,000 light years across, it's unfathomably massive, containing hundreds of billions of stars. So it's no surprise that we're learning fascinating and sometimes unnerving things about it all the time, like the fact that the Milky Way is colliding with another galaxy. Yet it was still a surprise in 2010 when NASA's Fermi Gamma-ray Space Telescope detected two structures spanning 50,000 light years above and below the center of our galaxy. Dubbed the Fermi bubbles, they're balloon-like, feature a symmetrical appearance, and are evidence of an unknown violent event in the Milky Way's past.

But the mystery only became more intriguing in 2020 when the eROSITA X-ray telescope — a part of the joint German-Russian space mission known as Spektr-RG — revealed new bubbles in nearly the exact same location as the Fermi bubbles. Researchers observed that these eROSITA bubbles were being emitted in the X-ray spectrum, however, and were slightly larger and contained more energy than the Fermi bubbles.

What is the nature of these bubbles, and where do they come from? Scientists still aren't exactly sure, with theories ranging from the involvement of our galaxy's black hole to the results of a mass star-formation event. Let's take a look at the possible culprits to get a better idea of how these bubbles changed our understanding of the galaxy we call home. 

Towering tens of thousands of light-years above and below the Milky Way's galactic plane, the Fermi and eROSITA bubbles are among the most enigmatic structures in our galaxy, and scientists have proposed several compelling theories for them, each offering a different glimpse into the Milky Way's dynamic history.

One hypothesis posits the bubbles formed from an enormous eruption of material from Sagittarius A*, the supermassive black hole at the center of our galaxy. If true, this suggests that our quiet-looking galactic core was not always so dormant. Some researchers believe that around 2.6 million years ago, a dramatic feeding event where the black hole rapidly consumed large amounts of matter triggered two massive, high-energy jets that carved out the structures we see today.

In a 2022 paper in the journal Nature Astronomy, scientists carried out mathematical simulations of the kinds of energy released involving gravity, galactic gasses, and cosmic rays. "The cosmic rays, injected with the jets of the black hole, expand and form the Fermi bubbles that shine in gamma rays," Hsiang-Yi Karen Yang, an astronomer and lead author of the study, told Sci News. "The same explosion pushes gas away from the galactic center and forms a shock wave that is observed as the eROSITA bubbles." This process is known as a "black hole jet," when matter surges into a black hole before being shot out at high speeds. This theory has a lot going for it, as similar phenomena have been observed in other galaxies before. This is significant, as it would indicate that the Milky Way isn't as calm as it is today, but could have undergone several outburst episodes in its history that shaped what the galaxy looks like today.

While the idea of a black hole eruption is a strong contender, it's not a guarantee. After all, there are dozens of mysteries about black holes that scientists still can't explain, so it's good to remember that humanity is operating in an area of relative unknown here. However, scientists can do a lot with model-based predictions, and another explanation for the Fermi and eROSITA bubbles paints a different picture — one driven not by the Milky Way's central black hole, but by a burst of furious star formation. This alternative theory suggests that millions of years ago, a starburst event at the heart of our galaxy unleashed an enormous outflow of energy.

Starbursts are periods of intense stellar birth, during which vast clouds of gas collapse to form new stars at an accelerated rate. As these young, massive stars burn through their fuel, they unleash powerful stellar winds from collective supernovae. Put enough of them in a concentrated space, like at the center of a galaxy, and you might have bubbles like what astronomers see in the Fermi and eROSITA structures. If correct, it would mean that the Milky Way's recent history witnessed an extreme period of star formation.

However, the black-hole models that researchers have put to the test show that they align more closely with what we've actually observed in the galaxy so far. As scientists work to better align their models and gather more data, the true origins behind our galaxy's strange bubble bookends will become ever clearer. If you want to learn more about the kinds of dynamics behind this mystery, check out our breakdown of what happened when physicists simulated a black hole in a lab.