A new study tracks the fate of stars living near the black hole at the center of the Milky Way. Some stars orbiting the supermassive black hole at the center of the Milky Way look deceptively young despite their ancient age. But unlike people who can look younger after new collagen injections, these stars look young for a much darker reason.

They ate the neighbors.

This is just one of the most surprising findings of a new study from Northwestern University. Using a new model, astrophysicists tracked the turbulent journeys of 1,000 simulated stars orbiting our galaxy’s central supermassive black hole, Sagittarius A* (Sgr A*).

This region is so densely packed with stars that violent interstellar collisions are common. By modeling the effects of these intense collisions, the new study found that those that survive the collision can either lose mass and become low-mass stars, or merge with other stars and become larger and younger.

“The region around the central black hole is full of stars moving at extremely high speeds,” said Northwestern’s Sanaea S. Rose, who led the study. “It’s like a crowded subway station during rush hour in New York. If you don’t encounter others, you will pass them too closely. For stars, these close collisions still cause gravitational interactions. “We wanted to investigate what these collisions and interactions mean for the stellar population and characterize their consequences.”

Rose presented the 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 “Astrophysics of Elementary Particles and the Center of the Galaxy”.

Rose is a Lindheimer Fellow at the Northwest Center for Interdisciplinary Research and Research in Astrophysics (CIERA). She began this work as a Ph.D. candidate at the University of California, Los Angeles, where he is mentored by astrophysicist and former doctoral student Smadar Naoz.

Doomed to a collision

The center of our Milky Way is a strange and wild place. Sgr A*’s gravitational pull accelerates stars into their orbits at very high speeds. The number of stars gathered at the center of the galaxy exceeds one million. A dense cluster plus lightning-fast speeds equals a high-speed demolition derby. Very few stars remain intact in the deepest 0.1 parsec region of the black hole.

“The closest star to our Sun is about four light-years away,” Rose said. “At the same distance, there are more than a million stars near a supermassive black hole. This is an incredibly crowded region. Additionally, a supermassive black hole has a very strong gravitational force. Stars can travel at thousands of kilometers per second while orbiting a black hole.”

Stars can collide with other stars in this close, turbulent neighborhood. The closer the stars are to the supermassive black hole, the more likely they are to collide. Interested in the consequences of these collisions, Rose and his colleagues developed simulations to track the fate of stellar populations in the galactic center. The simulation takes into account several factors: the density of the star cluster, the mass of the stars, the orbital speed, the gravitational force, and the distance to Sgr A*.

From “brutal rises” to full mergers

In his research, Rose identified one factor most likely to determine a star’s fate: its distance from a supermassive black hole.

Stars moving at thousands of kilometers per second within 0.01 parsec of a black hole are constantly colliding with each other. This is rarely a head-on collision and is more like a “brutal crash” as Rose describes it. The effects are not strong enough to completely shatter the stars. Instead, they shed their outer layers and continue hurtling toward a collision course.

“They crash into each other and keep going,” Rose said. “They’re grazing each other, like they’re giving each other a very violent high-five. This causes the stars to expel some material and lose their outer layers. Depending on how fast they’re moving and how much they overlap when they collide, they can lose a lot of their outer layers. This is devastating.” “The collisions give rise to a population of strange, truncated, low-mass stars.”

Beyond 0.01 parsecs, stars move more slowly; not thousands but hundreds of kilometers per second. Due to the low speed, these stars collide with each other but then do not have enough energy to escape. Instead, they merge and become larger. In some cases, they can even merge multiple times, reaching a mass 10 times greater than the Sun.

“A few stars win the crash lottery,” Rose said. “Through collisions and mergers, these stars accumulate more hydrogen. Even though they are made up of an older population, they masquerade as young, young stars. They are like zombie stars; they eat their neighbors.”

But the youthful appearance comes at the cost of shortening one’s lifespan.

“They die very quickly,” Rose said. “Massive stars are like giant cars consuming gas. “They start with a lot of hydrogen, but they burn it very, very quickly.”

Extreme environment “unlike any other”

While Rose gets the simple pleasure of studying the strange extreme region near our galactic center, his work could also reveal information about the history of the Milky Way. Because the central cluster is extremely difficult to observe, his team’s simulations could shed light on hidden processes.

“This is a one-of-a-kind environment,” Rose said. “Stars under the influence of a supermassive black hole in a very crowded region are unlike anything we would see in our own solar neighborhood. But if we can learn about these stellar populations, we can learn something new about how the galactic center comes together. “It definitely creates a point of contrast, at least for the neighborhood we live in.”