Paul M. Sutter is an astrophysicist at SONK Stony Brook and the Flatiron Institute, host of Ask an astronaut 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 notice a disturbing lack of red giant stars in the center of the Milky Way.
Theories abounded to explain the absence, and now a new theory proposes something frightening: a massive ray launched from our galaxy’s supermassive black hole destroy any red giants that have strayed in his path.
Our Milky Way: A Travel Guide
Do not see red
In 1990, astronomer Kris Skellgren observed a significant lack of carbon monoxide (CO) in the light of stars near the galactic center. CO is mostly found in the upper atmosphere of the red giant stars. Since then, more detailed observations have begun to examine individual stars, and the same problem has remained: there are about 1,000 missing red giant stars in the galactic center.
The red giant stage is the last phase of a sunny star’s life. When such stars stop melting hydrogen in their core, they swell up and turn red and come in their name as red giants. These giants are found everywhere on the disk of the Milky Way. We even have a bunch of nearby 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 the smart ideas go on Sagittarius A *, the supermassive black hole that sits at the center of our galactic. That black hole is massive and weighs more than 4.5 million times the mass of the sun, and is obviously the big bully in that part of the galactic environment.
With the mass, Sagittarius A * can wreak havoc, including the normal population of red giants. With its extreme gravity, Sagittarius A * can, for example, tear stars apart. So if a red giant wanders too close, it’s just torn to pieces.
Or maybe red giant stars collide with any other remnants near the galactic center. The army of Sagittarius A * is littered with the legs of countless stars, all packed 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 be trapped in the gravity of Sagittarius A *. If they fall inwards, they can kick smaller objects (and compared to black holes, red giants are indeed ‘smaller’) from the core region.
Images: Black holes of the universe
The explosion zone
But none of these scenarios are entirely satisfactory. Why would these processes tend to remove red giants, and not smaller stars? In response, a team of astronomers proposed something new: a death ray.
I do not make it up, as you can see in their paper, which recently appeared in the preprint journal arXiv.
Supermassive black holes like Sagittarius A * can start massive radiations of radiation and high-energy particles. The jets are not ejected by the black holes themselves – black holes can not emit even a single photon, let alone a giant ray, but by the gas and dust swirling around them in the form of a growth disk. These slides offer intense electric and magnetic fields (these are also some of the brightest objects in the entire universe), and the fields can force particles to flow up around the black hole and into rays.
Those jets are really awesome. If a giant black hole actively feeds on new material, it can launch jets that reach thousands of light-years, which can be completely removed from their host systems.
Our own Sagittarius A * may have had such an episode in the last few years, as can be seen from the so-called Fermi Bubbles. These bubbles are thin but expansive areas with high-energy particles that glow in gamma rays (hence their name, as they were first detected by NASA’s Fermi gamma-ray space telescope). Astronomers think that these bubbles were formed by an intense radiation outflow of Sagittarius A *.
And if you were a red giant star who happened to be in the way of that jet, that would be bad news for you.
A new hope
The physics here is relatively simple. Red giants are large, but their atmosphere is relatively loosely attached to the stars. The jets, which are launched through supermassive black holes, contain streams of particles that move near the river. speed of light.
When a red giant crosses the path of a ray, the ray is able to strip the outer layers of the star’s atmosphere and leave a burnt canvas behind – a relative glow compared to the glorious star that it used to be.
The jet can be repeated over and over again and can remove red giants near Sagittarius A *. Combined with other processes, such as the occasional collision with a black giant-black hole (which would be spectacular to see but kill the red giant), the tidal disruption of red giants (where the star torn by the extreme gravity of the supermassive black hole), and red giants descending through the growth disk (which is not comfortable at all), this may explain the lack of red giant stars in the center of the galaxy.
But at present the Sagittarius does not feed A *. It does not have a strong radiator. It does not actively form bubbles. The opportunity that led to the Fermi Bubbles has been safe in the past. It may be that the last major ray-forming episode killed any nearby red giants, but that could not stop new giants from forming.
Maybe if we wait a few million years – and Sagittarius A * slumbers – we can 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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