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Rick Robinson

Apr 30th 2021

Fermi Bubbles Reveal an Exploding Galaxy: Our Own

A decade ago, NASA’s Fermi Gamma-ray Space Telescope detected two enormous plumes of highly energetic gas extending above and below the disk of the Milky Way galaxy. The combined plumes, dubbed Fermi bubbles, extend a total of some 50,000 light years.

These “bubbles” remain largely mysterious, and continued research has turned up several surprises. According to Science News, recent findings from the eROSITA X-ray telescope has shown that the plumes produce a powerful flux of X-rays and energetic gamma rays.

While much remains mysterious about the Fermi bubbles, one thing about them seems clear: They are the aftermath of a stupendous explosion, a few million years ago, in the heart of our galaxy.

The Power of a Million Suns

How big was the explosion? Cosmos magazine reports that the blast released about 1057 ergs of energy. By itself, this number — equivalent to a 1 followed by 57 zeroes — is pretty abstract in human terms, but it amounts to about a million times as much energy as the Sun will release in its entire estimated lifetime of 10 billion years.

This stupendous blast was powerful enough to devastate any habitable planets within 3,000 light years of the explosion center. From Earth’s perspective, being as far as nine times that distance, the explosion would still have been spectacular. And because explosions that big take a while to happen, it would have gone on being spectacular for perhaps a few hundred thousand years.

So, what happened?

When Galaxies Explode

The twin-lobed Fermi bubbles are oriented along the polar axis of the Milky Way galaxy’s rotating disk. This provided the first crucial hint to how they might have developed.

Astronomers have long observed signs of powerful explosions or eruptions in the centers of distant galaxies. As Astronomy notes, these explosions often take the form of jets of hot gas, in pairs, streaming from the north and south poles of a disk galaxy — a distinctive pattern resulting from energy release inside a spinning magnetic field.

The Fermi bubbles look very much like what you would expect if the Milky Way experienced a similar explosion in the recent past — at least on the cosmic scale.

Observing pattern similarities is often a key step in scientific discoveries, and this is likely to be no exception. But before we can really understand the Fermi bubbles, we have to answer a basic question: Why? What makes a galaxy explode?

Cosmic Crescendo

It’s probably no coincidence that most large galaxies, including our own, have supermassive black holes at their center. But the one at the center of the Milky Way, designated Sagittarius A* (the asterisk is part of the name, pronounced “A-star”), is currently quiescent.

Typically, the center of a galaxy is a pretty crowded region, and interstellar gas will often continue to fall into the central black hole’s intense gravity field. Entire stars can also get caught in the gravitational maw and pulled down.

Most of the mass and energy of the star falls through the black hole’s event horizon and vanishes from our universe. But some of the matter is caught in turbulence and deflected, much like mist rising above a waterfall. This produces the explosions and jets we observe in distant galaxies.

A similar process, culminating a few million years ago, is a leading suspect in the creation of the Fermi bubbles. But as Space.com notes, there is more than one way for a galaxy to explode. Massive dying stars are known to undergo supernova explosions, and in the densely packed core of a galaxy, large numbers of supernovae going off together could build to a cosmic crescendo, leaving such bubbles in its aftermath.

Whatever the cause, we can confidently say that the center of the Milky Way galaxy has not always been a placid place. That means that astronomers have a lot of debris to examine as they piece together the remnants of a colossal blast in our galactic past.

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