Zombie stars in the heart of the Milky Way defy time

New research traces the fate of stars living near the planet milky wayCentrality Black hole.

Despite their ancient age, some stars orbiting the Milky Way's central supermassive black hole look deceptively young. But unlike humans, who may appear rejuvenated after a fresh round of collagen injections, these stars look youthful for a darker reason.

They ate their neighbors.

This is just one of the more surprising results from the new Northwestern University research. Using a new model, astrophysicists tracked the violent journeys of 1,000 simulated stars orbiting our galaxy's central supermassive black hole, Sagittarius A* (Sgr A*).

Because it's so crowded with stars, the region often experiences brutal star collisions. By simulating the effects of these intense collisions, the new work finds that collision survivors can lose mass to become bare, low-mass stars or can merge with other stars to become massive and rejuvenated.

“The region around the central black hole is dense with stars moving at very high speeds,” said Sania C. Rose of Northwestern University, who led the research. “It's a bit like running through an incredibly crowded subway station in New York City during rush hour. If you're not bumping into other people, you're passing by them. For stars, these close collisions still cause them to interact with gravity. We wanted to explore What these collisions and interactions mean for stars and a description of their results.

This illustration shows the orbits of stars very close to Sagittarius A*, a supermassive black hole at the heart of the Milky Way. Source: 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 at the Center of the Galaxy” was part of the session “Astro-Particle Physics and the Center of the Galaxy.”

Rose is a Lindheimer Postdoctoral Fellow at Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). I started this work as a Ph.D. Candidate in University of CaliforniaShe was advised by astrophysicist and former postdoctoral fellow at Northwestern University Smadar Naoz.

Destined to collide

The center of our Milky Way Galaxy is a strange and wild place. The gravitational force of Sagittarius A* accelerates the rotation of stars around their orbits at terrifying speeds. The huge number of stars crowded in the center of the galaxy reaches one million stars. A densely packed set plus breakneck speeds equals high-speed demolition derby. In the innermost region – within 0.1 parsec of the black hole – a few stars escape unharmed.

“The closest star to our sun is about four light-years away,” Rose explained. “And within the same distance near the supermassive black hole, there are over a million stars. It's an incredibly crowded neighborhood. On top of that, the supermassive black hole has really strong gravity. As they orbit the black hole, the stars can move at thousands of kilometers per second.” .

And in this narrow, hectic neighborhood, stars can collide with other stars. The closer the stars get to the supermassive black hole, the more likely they are to collide. Curious about the outcomes of these collisions, Rose and her collaborators developed simulations to track the fates of star clusters at the galactic center. The simulation takes into account several factors: stellar mass density, stellar mass, orbital velocity, gravity, and distances from Sagittarius A*.

From “violent high fives” to total mergers

In her research, Rose has identified one factor that is likely to determine the fate of the star: its distance from the supermassive black hole.

At a distance of 0.01 parsecs from the black hole, stars – moving at speeds of thousands of kilometers per second – are constantly colliding with each other. This is rarely a head-on collision, but more like a “brutal knockout,” as Rose describes it. The impacts are not strong enough to completely shatter the stars. Instead, they shed their outer layers and continue speeding along the collision path.

“They bump into each other and keep going,” Rose said. “They graze each other as if they were exchanging very violent blows. This causes the stars to eject some material and lose their outer layers. Depending on how fast they move and how much they overlap when they collide, they may lose a significant portion of their outer layers. These destructive collisions result in A group of strange, low-mass stars.

Beyond 0.01 parsec, stars move at a more relaxed pace – hundreds of kilometers per second rather than thousands. Because of the slow velocities, these stars collide with each other but then do not have enough energy to escape. Instead, they merge to become more massive. In some cases, they may merge several times to become ten times larger than our Sun.

“Some stars win the collision lottery,” Rose said. “Through collisions and mergers, these stars accumulate more hydrogen. Even though they were formed from older people, they masquerade as rejuvenated stars. They are like zombie stars. They eat their neighbors.”

But a youthful appearance comes at the cost of a shorter life expectancy.

“They die very quickly,” Rose said. “Massive stars are like giant cars that consume a lot of fuel. They start out with a lot of hydrogen, but they burn through it very quickly.”

Harsh environment 'unparalleled'

Although Rose finds simple pleasure in studying the extreme and strange region near the center of our galaxy, her work can also reveal information about the history of the Milky Way. Because the central cluster is difficult to monitor, her team's simulations could shed light on hidden processes.

“It's an environment like no other,” Rose said. “The stars, which are under the influence of a supermassive black hole in a very crowded region, are unlike anything we will ever see in our solar neighborhood. But if we can identify these star clusters, we may be able to learn something new about how the center of the galaxy is put together. At least Appreciating, it certainly provides a point of contrast to the neighborhood in which we live.

Rose's presentation to APS included research she had published the Astrophysical Journal Letters In March 2024 and by the Astrophysical Journal In September 2023.

References:

“Collision modulation of nuclear star cluster density profiles” by Sanya C. Rose and Morgan MacLeod, February 22, 2024, Astrophysical Journal Letters.
doi: 10.3847/2041-8213/ad251f

“Stellar Collisions at the Galactic Center: Massive Stars, Collision Remnants, and Missing Red Giants” by Sanaa C. Rose, Smadar Naoz, Reem Sari, and Itay Linial, September 14, 2023, Astrophysical Journal.
doi: 10.3847/1538-4357/acee75

This work was supported by the National Science Foundation (Grant No. AST 2206428) and NASA (Grant No. 80NSSC20K050) as well as from a Charles E. Young Fellowship, a UCLA Dissertation Year Fellowship, a Thacher Fellowship, a Bhowmick Institute, and a CIERA Lindheimer Fellowship.