In 2022, astronomers made history when they unveiled the first image of the supermassive black hole at the center of our Milky Way galaxy. But you wouldn’t tell from this benign image just how strange and chaotic things can be deep in the heart of the galaxy. This is a mad place where stars slingshot at an appreciable fraction of the speed of light, propelled by the stupendous gravity of Sagittarius A*, a black hole six million times more massive than the Sun.
It’s really crowded there, too. Alpha Centauri, the closest star to our Sun, is a little over four light-years away. Meanwhile, within four light-years of Sagittarius A*, there could be more than a million stars. Things are bound to get messy.
Indeed, a new study that sounds more like science fiction than fact (except it is), astronomers from Northwestern University found that some stars in the chaotic heart of the Milky Way consume one another, in a sense. In the process, they emerge rejuvenated, appearing as much younger versions of themselves to our telescopes. This bizarre phenomenon is not a result of cosmic beauty treatments but a stark reminder of the brutal reality of existence near a supermassive black hole.
A stellar Hunger Games
“The galactic center is an extreme and unique environment. The centers of other galaxies are too far away for us to observe in detail, but we can learn about these environments in general by studying the center of the Milky Way. Observations have shown that a dense cluster of stars surrounds our central supermassive black hole. How, where, and when these stars formed and how they behave and interact under the influence of the supermassive black hole remain active areas of research.
“Additionally, observational studies have presented many astrophysical puzzles and unexplained phenomena. These include young, massive stars very near the supermassive black hole and mysterious puffy balls of dust and gas. I work on theoretical models of this region, and what we try to do is explain these unusual findings with stellar interactions, which are common in the dense star cluster. In particular, we focus on collisions between stars,” Sanaea C. Rose, a postdoc at Northwestern who led the research, told ZME Science.
“Through many collisions and mergers, stars like our Sun can go on to form more massive stars ten times the initial mass. The massive collision products can masquerade as young-looking stars, even though they formed from an older population. Collisions may therefore explain the presence of young-seeming, massive stars very near the supermassive black hole.”
The study focuses on the violent dynamics around Sagittarius A* (Sgr A*), the supermassive black hole at the galaxy’s core. Here, space is at a premium, with stars packed so densely that collisions are not just likely; they’re inevitable. The gravitational forces at play accelerate stars to phenomenal speeds, setting the stage for a high-stakes game. Here, stellar encounters can lead to either the stripping away of a star’s mass or a merger forming a larger, seemingly younger star.
The research simulated 1,000 stars orbiting around Sgr A* to investigate the outcomes of these high-velocity encounters. The simulation considered various factors, including the density of the stellar cluster and the mass and velocity of the stars, to trace the fate of stars in this tumultuous region.
“In order to model the cluster, we needed to start by making some simplifying assumptions. We then slowly added additional physics and began considering deviations from those assumptions. This approach allowed us to understand the results we were seeing and build intuition about an environment that is alien to us in almost every way,” Rose said.
Stellar survival of the fittest
The closer you are to the black hole, the more crowded it gets. The simulation accounts for this fact, so the likelihood of collision increases. The most chaotic region is about a hundredth of a parsec (equal to about 3.26 light-years) from the central black hole, where the stars are not just many but extremely fast too at thousands of kilometers per second. But, despite the huge kinetic energy involved, impacts are not strong enough to obliterate the colliding stars. Instead, they graze each other as though “they are exchanging a very violent high five,” Rose says, causing the stars to lose their outer layers. As such, you end up with a strange subset of stripped down, low-mass stars.
Outside this exceptionally crowded area, things are more relaxed. There, stars travel “only” at about a few hundreds of kilometers per second. When these stars collide, they don’t have enough energy to escape so they merge, becoming more massive. In some cases, stars can merge multiple times, becoming up to ten times more massive than our sun.
“Through many collisions and mergers, stars like our Sun can go on to form more massive stars ten times the initial mass. The massive collision products can masquerade as young-looking stars, even though they formed from an older population. Collisions may therefore explain the presence of young-seeming, massive stars very near the supermassive black hole,” Rose said.
“I wouldn’t say I was surprised by the results, exactly. At the start of studies like these, I try to remain open-minded and temper any hopes or expectations I have of the results. But once we started seeing some initial results, we were very excited by the potential of stellar collisions to explain a variety of phenomena at the Galactic center. Much remains to be done, of course, even beyond these studies. The environment is complex, and there is always more to be added and explored in our models.”
These “zombie” stars that eat their neighbors may look young and bright, but their life is short. It doesn’t take very long (on a cosmic timescale) for them to burn through their hydrogen fuel.
Modeling the core of our galaxy
Given the sheer violence at the galactic center, one can only wonder what would happen to the planets, moons, and other cosmic bodies along for the ride. It’s anyone’s guess at this point, but it’s not pretty that’s for sure.
“I think there could be some planets floating around the galactic center, but it would be bad news for anyone living on those planets! I would also expect some planets to get destroyed during these collisions. It’s hard to imagine a planetary system staying intact around a star that’s collided, but maybe a lucky few survive,” Rose said.
For now, the researchers were only focused on modeling star movements and collisions around the galactic center. And that’s the point of models like these. They don’t reflect all the bits and pieces that make up the complex soup of reality — these models are content to approximate reality. Sometimes, even simple simulations are enough, and that’s quite powerful.
Rose says that some of the modeling physics she used was taught to her in undergraduate school during physics classes that started with a simple question: how long before a particle collides with another particle in gas?
“The answer depends on how dense the collection of particles is and how quickly they are moving. I will experience a collision faster if I run through a crowded train station in Manhattan than if I walk through a rural station during off-peak hours. The physics in our models begins from the same collision timescale calculation. It was wonderful to use a relatively simple calculation, one which can be intuitively understood as an undergraduate, to learn about an environment that is unlike anything we encounter in our neighborhood of the galaxy,” she added.
The findings were presented at this week’s American Physical Society’s (APS) April meeting in Sacramento, California. You read more about it in two papers published in ArXiv here and here.
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