Stellar collisions produce strange, zombie-like survivors

They ate their neighbors.

That is just one among the more peculiar findings from recent Northwestern University research. Using a brand new model, astrophysicists traced the violent journeys of 1,000 simulated stars orbiting our galaxy’s central supermassive black hole, Sagittarius A* (Sgr A*).

So densely filled with stars, the region commonly experiences brutal stellar collisions. By simulating the consequences of those intense collisions, the brand new work finds that collision survivors can lose mass to turn into stripped down, low-mass stars or can merge with other stars to turn into massive and rejuvenated in appearance.

“The region across the central black hole is dense with stars moving at extremely high speeds,” said Northwestern’s Sanaea C. Rose, who led the research. “It is a bit like running through an incredibly crowded subway station in Recent York City during rush hour. In case you aren’t colliding into other people, you then are passing very closely by them. For stars, these near collisions still cause them to interact gravitationally. We desired to explore what these collisions and interactions mean for the stellar population and characterize their outcomes.”

Rose will present this research on the American Physical Society’s (APS) April meeting in Sacramento, California. “Stellar Collisions within the Galactic Center” will happen on Thursday (April 4) as a part of the session “Particle Astrophysics and the Galactic Center.”

Rose is the Lindheimer Postdoctoral Fellow at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). She began this work as a Ph.D. candidate at UCLA.

Destined to collide

The middle of our Milky Way is an odd and wild place. The gravitational pull of Sgr A* accelerates stars to whip around their orbits at terrifying speeds. And the sheer variety of stars packed into the galaxy’s center is upwards of one million. The densely packed cluster plus the lightning-fast speeds equal a high-speed demolition derby. Within the innermost region — inside 0.1 parsecs of the black hole — few stars escape unscathed.

“The closest star to our sun is about 4 light-years away,” Rose explained. “Inside that very same distance near the supermassive black hole, there are greater than one million stars. It’s an incredibly crowded neighborhood. On top of that, the supermassive black hole has a extremely strong gravitational pull. As they orbit the black hole, stars can move at hundreds of kilometers per second.”

Inside this tight, hectic neighborhood, stars can collide with other stars. And the closer stars live to the supermassive black hole, the likelihood of collision increases. Curious of the outcomes of those collisions, Rose and her collaborators developed a simulation to trace the fates of stellar populations within the galactic center. The simulation takes several aspects into consideration: density of the stellar cluster, mass of the celebrities, orbit speed, gravity and distances from the Sgr A*.

From ‘violent high fives’ to total mergers

In her research, Rose pinpointed one factor that’s probably to find out a star’s fate: its distance from the supermassive black hole.

Inside 0.01 parsecs from the black hole, stars — moving at speeds reaching hundreds of kilometers per second — consistently bump into each other. It’s rarely a head-on collision and more like a “violent high five,” as Rose describes it. The impacts aren’t strong enough to smash the celebrities completely. As an alternative, they shed their outer layers and proceed speeding along the collision course.

“They whack into one another and keep going,” Rose said. “They only graze one another as if they’re exchanging a really violent high five. This causes the celebrities to eject some material and lose their outer layers. Depending on how briskly they’re moving and the way much they overlap once they collide, they could lose quite a little bit of their outer layers. These destructive collisions end in a population of strange, stripped down, low-mass stars.”

Outside of 0.01 parsecs, stars move at a more relaxed pace — a whole bunch of kilometers per second versus hundreds. Due to slower speeds, these stars collide with each other but then haven’t got enough energy to flee. As an alternative, they merge to turn into more massive. In some cases, they could even merge multiple times to turn into 10 times more massive than our sun.

“A number of stars win the collision lottery,” Rose said. “Through collisions and mergers, these stars collect more hydrogen. Although they were formed from an older population, they masquerade as rejuvenated, young-looking stars. They’re like zombie stars; they eat their neighbors.”

However the youthful appearance comes at the associated fee of a shorter life expectancy.

“They die in a short time,” Rose said. “Massive stars are type of like giant, gas-guzzling cars. They begin with a variety of hydrogen, but they burn through it very, very fast.”

Extreme environment ‘unlike some other’

Although Rose finds easy joy in studying the bizarre, extreme region near our galactic center, her work can also reveal information in regards to the history of the Milky Way. And since the central cluster is amazingly difficult to watch, her team’s simulations can illuminate otherwise hidden processes.

“It’s an environment unlike some other,” Rose said. “Stars, that are under the influence of a supermassive black hole in a really crowded region, are unlike anything we’ll ever see in our own solar neighborhood. But when we will study these stellar populations, then we would give you the chance to learn something recent about how the galactic center was assembled. On the very least, it actually provides some extent of contrast for the neighborhood where we live.”

Rose’s APS presentation will include research published by The Astrophysical Journal Letters in March 2024and by The Astrophysical Journal in September 2023.

This work was supported by the National Science Foundation (grant number AST 2206428) and NASA (grant number 80NSSC20K050) in addition to by the Charles E. Young Fellowship, the Dissertation Yr Fellowship at UCLA, the Thacher Fellowship, the Bhaumik Institute and the CIERA Lindheimer Fellowship.