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Did the Milky Way's huge black hole kill all the red giants around it?

This illustration shows a black hole emitting jets of fast-moving plasma above and below it, as matter swirls around in an orbiting disk.
This illustration shows a black hole emitting jets of fast-moving plasma above and below it, as matter swirls around in an orbiting disk. (Image credit: NASA's Goddard Space Flight Center)

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of How to Die in Space. He contributed this article to Space.com's Expert Voices: Opinions and Insights.

Beginning in the 1990s, astronomers noticed a disturbing lack of red giant stars in the Milky Way's center. 

Theories abounded to explain the absence, and now a new theory proposes something truly frightening: a massive jet launched from our galaxy's supermassive black hole destroyed any red giants that wandered into its path.

Our Milky Way galaxy: A traveler's guide

Not seeing red

Back in 1990, astronomer Kris Skellgren observed a significant lack of carbon monoxide (CO) in the light from stars near the galactic center. CO is most commonly found in the upper atmospheres of red giant stars. Since then, more detailed observations have started examining individual stars, and the same problem remained: there are about 1,000 missing red giant stars in the galactic center.

The red giant stage is the very last phase of a sunlike star's life. When such stars stop fusing hydrogen in their cores, they swell and turn red, coming into their name as red giants. These giants are found all throughout the disk of the Milky Way. We even have a bunch as nearby neighbors, including Betelgeuse, which is a mere 646 light-years away.

So why is the center of our galaxy missing so many red giants? Over the years, astronomers have put forward a bunch of clever ideas. Most of those clever ideas center on Sagittarius A*, the supermassive black hole sitting at our galactic center. That black hole is massive, weighing over 4.5 million times the mass of the sun, and naturally is the big bully in that part of the galactic neighborhood.

With that mass, Sagittarius A* can wreak whatever kind of havoc it wants, including disrupting the normal population of red giants. For example, with its extreme gravity, Sagittarius A* can rip apart stars. So if a red giant wanders too close, it just gets torn to shreds. 

Or maybe red giant stars collide with any other remnants near the galactic center. The lair of Sagittarius A* is littered with the bones of countless stars, all packed together in a relatively small volume, so red giants have to navigate carefully. 

In another scenario to explain their absence, smaller black holes (or other massive objects) can get caught in the gravitational snare of Sagittarius A*. As they fall inward, they can kick smaller objects (and in comparison to black holes, red giants are indeed "smaller") out of the core region.

Images: Black holes of the universe

The blast zone

But none of these scenarios are entirely satisfactory. Why should these processes tend to remove red giants, and not smaller stars? In response, a team of astronomers has proposed something new: a killer death ray.

I'm not making this up, as you can see in their paper, which recently appeared in the preprint journal arXiv.

Supermassive black holes like Sagittarius A* can launch massive jets of radiation and high-energy particles. The jets aren't thrown off by the black holes themselves — black holes aren't capable of emitting even a single photon, let alone gigantic jet — but by the gas and dust that swirls around them in the form of an accretion disk. These disks host intense electric and magnetic fields (they're also some of the brightest objects in the entire universe), and those fields can force particles to flow around the black hole and up into jets.

Those jets are truly stupendous. When a giant black hole is actively feeding on new material, it can launch jets that reach for tens of thousands of light-years, clear out of their host galaxies altogether.

Our own Sagittarius A* may have had such an episode in the past few million years, as evidenced by the so-called Fermi Bubbles. These bubbles are thin but vast regions of high-energy particles glowing in gamma rays (hence their name, since they were first detected by NASA's Fermi Gamma-ray Space Telescope). Astronomers think that these bubbles were formed by an intense jet outflow event coming from Sagittarius A*.

And if you were a red giant star that happened to be in the path of that jet, it would be bad news for you.

A new hope

The physics here is relatively straightforward. Red giants are large, but their atmospheres are relatively loosely attached to the stars. The jets launched by supermassive black holes contain streams of particles traveling at nearly the speed of light.

When a red giant crosses the path of a jet, the jet is capable of stripping away the outer layers of the star's atmosphere, leaving behind a burnt husk — a relative cinder compared to the glorious star that it once was.

Repeated again and again, the jet could remove red giants near Sagittarius A*. Combined with other processes, like the occasional red giant-black hole collision (which would be spectacular to watch but kill the red giant), the tidal disruption of red giants (where the star gets torn up by the extreme gravity of the supermassive black hole), and red giants plunging through the accretion disk (which is not at all comfortable), this could explain the lack of red giant stars in the center of the galaxy.

But currently, Sagittarius A* isn't feeding. It doesn't have a strong jet. It's not actively forming bubbles. The event that led to the Fermi Bubbles is safely in the past. It could be that the last great jet-forming episode killed any nearby red giants, but that might not stop new giants from forming.

Perhaps if we wait a few million years — and Sagittarius A* stays slumbering — we might recover the population of giants in the center.

Read more: "Missing bright red giants in the Galactic center: A fingerprint of its once active state?"

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Paul Sutter

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe, to the earliest moments of the Big Bang, to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space," and frequently appears on TV.

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