New research from Northwestern University using simulations of 1,000 stars around the Milky Way’s supermassive black hole, Sagittarius A*, shows that high-velocity stellar collisions result in the formation of youthful-looking stars. These stars either reduce and have low mass or merge into massive structures that have a tapered appearance despite their ancient origins. Photo credit: SciTechDaily.com
New research tracks the fate of stars that live nearby Milky Wayis central black hole.
Despite their age, some stars orbiting the Milky Way’s central supermassive black hole appear deceptively youthful. But unlike people who might appear rejuvenated after a new round of collagen injections, these stars look young for a much darker reason.
They ate their neighbors.
This is just one of the more peculiar findings from the new research Northwestern University Research. Using a new model, astrophysicists have tracked the massive journeys of 1,000 simulated stars orbiting our galaxy’s central supermassive black hole, Sagittarius A* (Sgr A*).
The region is so densely packed with stars that brutal stellar collisions often occur. By simulating the effects of these intense collisions, the new work concludes that collision survivors can lose mass and become stripped-down, low-mass stars or merge with other stars to become massive and take on a tapered appearance.
“The region around the central black hole is dense with stars moving at extremely high speeds,” said Sanaea C. Rose of Northwestern, who led the research. “It’s a bit like walking through an incredibly crowded subway station in New York City during rush hour. Unless you collide with other people, drive very close to them. For stars, these near collisions still cause them to interact gravitationally. We wanted to explore what these collisions and interactions mean for the stellar population and characterize their consequences.”
This image shows the orbits of stars very close to Sagittarius A*, a supermassive black hole at the heart of the Milky Way. Image credit: ESO / L. Calçada / Spaceengine.org
Rose presented this research today (April 4) at the April meeting of the American Physical Society (APS) in Sacramento, California. “Stellar Collisions in the Galactic Center” was part of the session “Particle Astrophysics and the Galactic Center.”
Rose is a Lindheimer Postdoctoral Fellow at the Northwestern Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). She began this work as a doctoral student. candidate at UCLAwhere she was advised by astrophysicist and former Northwestern postdoctoral fellow Smadar Naoz.
Destined to clash
The center of our Milky Way is a strange and wild place. The gravitational pull of Sgr A* accelerates stars, causing them to race around their orbits at frightening speeds. And the sheer number of stars packed into the center of the galaxy is over a million. The dense crowding and rapid speeds result in a fast-paced demolition derby. In the innermost region – within 0.1 parsec of the black hole – few stars escape unscathed.
“The closest star to our sun is about four light-years away,” Rose explained. “Near the supermassive black hole, there are more than a million stars at the same distance. It’s an incredibly crowded area. In addition, the supermassive black hole has a really strong gravitational pull. While orbiting the black hole, stars can move at thousands of kilometers per second.”
In this close, hectic neighborhood, stars can collide with other stars. And the closer stars live to the supermassive black hole, the greater the chance of a collision. Rose and her collaborators were curious about the results of these collisions and developed a simulation to track the fate of star populations in the galactic center. The simulation takes into account several factors: density of the star cluster, mass of the stars, orbital speed, gravity and distances from Sgr A*.
From “violent high fives” to total mergers
In her research, Rose has identified one factor that most likely determines a star’s fate: its distance from the supermassive black hole.
Within 0.01 parsec of the black hole, stars moving at speeds of thousands of kilometers per second constantly collide with each other. It’s rarely a head-on collision, but more like a “fierce high five,” as Rose describes it. The impacts are not strong enough to completely shatter the stars. Instead, they shed their outer layers and continue racing on a collision course.
“They hit each other and keep going,” Rose said. “They’re just brushing each other off like they’re exchanging a very strong high-five. This causes the stars to eject some material and lose their outer layers. Depending on how fast they are moving and how much they overlap in the collision, they may lose some of their outer layers. These destructive collisions result in a population of strange, stripped-down, low-mass stars.”
Outside 0.01 parsec, stars move at a more relaxed pace – hundreds of kilometers per second as opposed to thousands. Due to their lower speed, these stars collide with each other but then do not have enough energy to escape. Instead, they merge and become more and more massive. In some cases, they could even merge multiple times, reaching a mass ten times greater than our Sun.
“A few stars win the collision lottery,” Rose said. “Through collisions and mergers, these stars collect more hydrogen. Although they arose from an older population, they pose as rejuvenated, young-looking stars. They are like zombie stars; They eat their neighbors.”
But the youthful appearance goes hand in hand with a shorter life expectancy.
“They die very quickly,” Rose said. “Massive stars are like huge, gas-guzzling cars. You start with a lot of hydrogen, but you burn it very, very quickly.”
Extreme environment “like no other”
Although Rose simply enjoys exploring the bizarre, extreme region near our galactic center, her work can also provide information about the history of the Milky Way. And because the central cluster is extremely difficult to observe, her team’s simulations can shed light on otherwise hidden processes.
“It’s an environment like no other,” Rose said. “Stars under the influence of a supermassive black hole in a very crowded region are unlike anything we will ever see in our own solar environment. But if we can learn something about these stellar populations, we might learn something new about how the galactic center was built. At least it provides a point of contrast for the neighborhood we live in.”
Rose’s APS presentation included research published by The Astrophysical diary letters in March 2024 and until The Astrophysical Journal in September 2023.
References:
“Collisional Shaping of Nuclear Star Cluster Density Profiles” by Sanaea C. Rose and Morgan MacLeod, February 22, 2024, The astrophysical diary letters.
DOI: 10.3847/2041-8213/ad251f
“Stellar Collisions in the Galactic Center: Massive Stars, Collision Remnants, and Missing Red Giants” by Sanaea C. Rose, Smadar Naoz, Re’em Sari and Itai Linial, September 14, 2023, The Astrophysical Journal.
DOI: 10.3847/1538-4357/acee75
This work was supported by the National Science Foundation (grant number AST 2206428). NASA (Grant number 80NSSC20K050) as well as the Charles E. Young Fellowship, the Dissertation Year Fellowship at UCLA, the Thacher Fellowship, the Bhaumik Institute, and the CIERA Lindheimer Fellowship.