Stars Residing Close to the Center Black Hole of the Milky Way are Being Studied for Their Futures

Some stars around the center supermassive black hole of the Milky Way appear surprisingly young for their age. But these celebrities have a far darker explanation for their youthful appearance than do people, who could seem refreshed after receiving new collagen injections.

They consumed their neighbors.

This is but one of the most unusual discoveries made by recent study from Northwestern University. Astrophysicists used a novel model to track the ferocious orbits of 1,000 simulated stars around Sagittarius A* (Sgr A*), the core supermassive black hole in our galaxy.

Because the region is so densely populated with stars, violent star collisions frequently occur there. The new research, which simulates the impact of these violent collisions, discovers that stars that survive collisions can either merge with other stars to become enormous and renewed in appearance, or they can lose mass and become stripped-down, low-mass stars.

The research’s principal investigator, Sanaea C. Rose of Northwestern University, stated, “The region around the central black hole is dense with stars moving at extremely high speeds.”

“It’s a bit like running through an incredibly crowded subway station in New York City during rush hour. If you aren’t colliding into other people, then you are passing very closely by 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 outcomes.”

Rose will present this work at the April conference of the American Physical Society (APS) in Sacramento, California. Thursday, April 4, is designated as “Stellar Collisions in the Galactic Center” in the context of the session “Particle Astrophysics and the Galactic Center.”

Rose works at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) as a Lindheimer Postdoctoral Fellow. She started this work while pursuing her doctorate at UCLA.

Destined to Clash

Our Milky Way’s center is an odd and untamed location. Stars are accelerated by Sgr A*’s gravitational pull, causing them to thrash around their orbits at terrible speeds. Furthermore, there are upwards of a million stars crammed into the center of the galaxy. Lightning-fast speeds combined with a closely packed cluster make for a high-speed demolition derby. Few stars survive in the innermost zone, which is located within 0.1 parsec of the black hole.

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

Stars can crash into each other in this crowded, close neighborhood. And the chance of a collision rises with the proximity of the stars to the supermassive black hole. Rose and her colleagues created a simulation to track the destinies of star populations in the galactic core because they were curious about what would happen after these encounters. The simulation considers a number of variables, including distances from Sgr A*, orbit speed, mass of the stars, density of the stellar cluster, and gravity.

‘Violent high fives’ to Complete Mergers

Rose’s research has identified the distance of a star from the supermassive black hole as the most likely element determining that star’s fate.

Stars continuously collide with one other within 0.01 parsecs of the black hole, despite their fast motion that can reach thousands of kilometers per second. It’s more of a “violent high five,” as Rose puts it, and rarely a head-on collision. The hits aren’t powerful enough to totally shatter the stars. Rather, they shed their outer layers and keep racing toward the point of impact.

“They whack into each other and keep going,” Rose explained. “They just graze each other as though they are exchanging a very violent 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 when they collide, they might lose quite a bit of their outer layers. These destructive collisions result in a population of strange, stripped down, low-mass stars.”

Stars travel at a slower speed outside of 0.01 parsecs, hundreds of kilometers per second instead of thousands. These stars crash with one another due to their slower velocities, but their collisions are then too intense for them to break free. Rather, they combine to increase in mass. They may even combine more than once in certain circumstances to have ten times the mass of our sun.

Rose remarked, “A few stars win the collision lottery.” “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 are like zombie stars; they eat their neighbors.”

However, a reduced life expectancy is a price for the young appearance.

Rose remarked, “They die very quickly.” “Massive stars are sort of like giant, gas-guzzling cars. They start with a lot of hydrogen, but they burn through it very, very fast.”

‘Unlike Any Other’ Extreme Surroundings

Rose enjoys investigating the strange, extreme area close to our galactic center, but her research can also provide light on the Milky Way’s past. Furthermore, her team’s simulations can reveal otherwise hidden processes because it is very challenging to see the central cluster.

Rose remarked, “It’s an environment unlike any other.” “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 own solar neighborhood. But if we can learn about these stellar populations, then we might be able to learn something new about how the galactic center was assembled. At the very least, it certainly provides a point of contrast for the neighborhood where we live.”

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