Scientists have discovered the gruesome secret behind the apparent youth of some stars at the heart of the Milky Way — stars that are participating in a kind of cosmic demolition derby around our Milky Way’s supermassive black hole, Sagittarius A* or Sgr A*.
Like a cosmic version of 17th-century serial killer Elizabeth Bathory, who supposedly tried to maintain her youthful glow by bathing in the blood of her victims, some of these stars seem to stay looking young by bumping into their neighboring stars and cover themselves with the stolen stellar material. This cannibalistic process leaves the stellar victim as a strange, stripped-down “zombie” star and condemns the cannibal star to an early death.
This is just a taste of the strange findings that emerged from a simulation of a thousand tightly packed stars orbiting the Milky Way’s central supermassive black hole, a study conducted by scientists at Northwestern University.
Related: New view of the supermassive black hole at the heart of the Milky Way hints at an exciting hidden feature (image)
“Observing the centers of other galaxies is very difficult because they are so far away,” Sanaea C. Rose, a research leader and scientist at Northwestern University, told Space.com. “Studying our own galactic center can tell us what is happening at the centers of all galaxies.”
The heart of the Milky Way is one of the most extreme environments that astronomers can observe from Earth. This region is home to Sgr A*, which is not only a black hole with a mass of 4.5 million suns, but also a cosmic monster with more than a million stars orbiting it.
These stars are packed into an area about four light-years wide, about the distance between the Sun and its nearest star, Proxima Centauri. That means events like stellar collisions, which are relatively rare in our sparsely populated region of the Milky Way, are almost commonplace around Sgr A*.
“The supermassive black hole at the heart of the Milky Way is surrounded by a very dense cluster of stars, and many of these stars spin in their orbits at speeds between hundreds and thousands of kilometers per second,” Rose said.
Studying the violent heart of the Milky Way could reveal how stars behave, evolve and interact under the influence of the extreme gravity of a supermassive black hole.
The part about the cosmic demolition derby
Although the team’s simulations took into account many factors and characteristics, such as the mass of stars and the density of star clusters, one was particularly important in determining the fate of a star near the heart of the Milky Way.
Rose explained that a star’s distance from Sgr A* is generally a good indication of whether it will collide with another star and what type of collision that would be.
“The cluster becomes more and more densely populated the closer you get to the supermassive black hole, so the probability of a collision increases,” she said. “It’s a bit like running through an incredibly busy New York City subway station during rush hour. If you don’t encounter other people, you pass them very close.”
The closer a star is to Sgr A*, the faster the black hole’s immense gravity spins it. As a result, stars close to the supermassive black hole can move at a speed of about 18 million miles per hour, making the heart of the Milky Way more like a demolition derby than a crowded New York subway.
That means that collisions in the innermost region of Sgr A*, referring to an area within about 2,000 times the distance between Earth and the Sun, or 0.01 parsecs, tend to be destructive in nature.
Such stars within about 0.01 parsec of Sgr A* collide continuously, but this is rarely a head-on collision. That means that, like a demolition derby car that has its bumper ripped off before speeding away, the grazing effects cause a star to shed its outer layers and then race on a collision course next to another neighbor.
‘They bump into each other and keep going. They just graze each other like they’re exchanging a very violent high-five,” Rose said.
However, how much material a star undergoing this chaos loses depends on how fast it is moving and how much it overlaps with the star it collides with. One result of these destructive collisions is a strange population of stripped stars and stars that appear youthful thanks to being bathed in hydrogen-rich ejected stellar matter emitted by the former.
However, achieving this youthful appearance comes at a price. The more massive a star is, the faster it depletes the fuel supply it holds for intrinsic nuclear fusion, a process that prevents it from collapsing under its own gravity. So by piling up this stolen material, these massive stars are shortening their own lives.
Further away from Sgr A*, at about 0.1 parsec, or about 21,000 times the distance between the Sun and Earth, stars collide less frequently and at more relaxed speeds. When these slower collisions occur, Rose and colleagues’ simulation showed, a full merger where two stars become one big star is likely.
“For collisions beyond 0.01 parsec, the colliding stars are more likely to merge,” she added. “Stars within 0.1 parsec of Sgr A* have a very high probability of experiencing at least one collision during their lifetime.”
related stories
– How do some black holes get so big? The James Webb Space Telescope may have an answer
– The brightest quasar ever seen is powered by a black hole that eats ‘a sun a day’
—Black hole-like “gravastars” can be stacked like Russian tea dolls
Rose explained that one of the most satisfying aspects of using this model to address some unexplained observations of stars in the heart of the Milky Way is the fact that it is based on a relatively simple calculation.
“Something I personally found very special about my research is that it is based on calculating a collision time scale, something that is taught relatively early in physics education,” she said. “It was great to use a relatively simple calculation to learn about an extremely complex environment that is unlike anything we encounter in our solar environment.”
The team has conducted two studies using the model so far, one published in The Astrophysical Journal Letters this month and another in September 2023, but they’re not done with it yet.
“The next steps are to expand the physics currently in the model,” Rose concluded. “The Galactic Center is an extremely complex environment, so there are always things we can add, and our work is never done!”
Rose presented this research at the April meeting of the American Physical Society, held in Sacramento, California, on Thursday (April 4), as part of the session “Particle Astrophysics and the Galactic Center.”