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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 ideas.
Beginning in the 1990s, astronomers noticed a disturbing lack of red giant stars at the center of the Milky Way.
Theories abounded to explain the absence, and now a new theory proposes something really frightening: a massive jet launched from the supermassive of our galaxy. black hole destroy all the red giants that have wandered in its path.
Our Milky Way Galaxy: A traveller’s guide
See no red
In 1990, astronomer Kris Skellgren observed a significant lack of carbon monoxide (CO) in starlight near the galactic center. CO is most often found in the upper atmospheres of the red giant stars. Since then, more detailed observations have begun to examine individual stars, and the same problem has remained: there are around 1,000 missing red giant stars in the galactic center.
The red giant stage is the very last phase in the life of a sun-like star. When these stars stop fusing hydrogen in their nucleus, they swell and turn red, becoming red giants. These giants are found all along the Milky Way’s disk. We even have a bunch of close neighbors including Betelgeuse, which is only 646 light years away.
So why is the center of our galaxy missing so many red giants? Over the years, astronomers have come up with a bunch of clever ideas. Most of these smart ideas focus on Sagittarius A *, the supermassive black hole sitting at our galactic center. This black hole is massive, weighing more than 4.5 million times the mass of the sun, and is understandably the great tyrant in this part of the galactic quarter.
With this mass, Sagittarius A * can wreak any havoc they want, including disrupting the normal population of red giants. For example, with its extreme gravity, Sagittarius A * can tear stars. So if a red giant wanders too close, it will tear itself apart.
Or maybe giant red stars collide with other remains 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 the red giants must navigate with care.
In another scenario to explain their absence, smaller black holes (or other massive objects) may be caught in the gravitational trap of Sagittarius A *. When they fall inward, they can eject smaller objects (and compared to black holes, red giants are indeed “smaller”) out of the central region.
Images: Black holes of the universe
The blast zone
But neither of these scenarios is entirely satisfactory. Why should these processes tend to eliminate red giants and not little stars? In response, a team of astronomers came up with something new: a killer deadly ray.
I’m not making this up, as you can see in their paper, which was recently published in the preprint journal arXiv.
Supermassive black holes like Sagittarius A * can launch massive jets of radiation and high energy particles. The jets are not thrown by the black holes themselves – black holes are not capable of emitting even a single photon, let alone a gigantic jet – but by the gas and dust swirling around. of them in the form of an accretion disk. These discs harbor intense electric and magnetic fields (they are also some of the brightest objects in the entire universe), and these fields can force particles to circulate around the black hole and rise in jets.
These jets are really great. When a giant black hole actively feeds on new material, it can launch jets that reach tens of thousands of light years, completely evacuating their host galaxies.
Our own Sagittarius A * may have had such an episode in the last millions of years, as evidenced by the so-called Fermi bubbles. These bubbles are thin but large regions of high-energy particles that glow in gamma rays (hence their name, since they were first detected by NASA’s Fermi Gamma Space Telescope). Astronomers believe these bubbles were formed by an intense jet exit event from Sagittarius A *.
And if you were a red giant star that happened to be in the path of this jet, that would be bad news for you.
A new hope
The physics here are relatively straightforward. The red giants are large, but their atmospheres are relatively loosely attached to the stars. Jets launched by supermassive black holes contain streams of particles moving near the speed of light.
When a red giant crosses the path of a jet, the jet is able to strip the outer layers of the star’s atmosphere, leaving behind a scorched envelope – relative ash to the glorious star it once was.
Repeated over and over again, the jet could take out 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 red giant tidal disturbance (where the star is torn apart 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 * is not feeding. It doesn’t have a powerful jet. It does not actively form bubbles. The event that led to the Fermi Bubbles is safely in the past. The last big jet-forming episode may have killed some nearby red giants, but that may not stop new giants from forming.
Maybe if we wait a few million years – and Sagittarius A * stays asleep – we could get the population of central giants back.
Read more: “Lack of bright red giants in the galactic center: a fingerprint of its once active state?“
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