Elimination race taking place in the center of the galaxy

This illustration shows stars orbiting near the Milky Way's central supermassive black hole. The black hole accelerates nearby stars and causes them to collide with each other.

The center of the Milky Way is dominated by a supermassive black hole. The overarching gravity of Sagittarius A* creates a chaotic region where tightly packed, high-speed stars crash into each other like cars in a race.

These collisions and impacts change the stars forever. Some turn into strange low-mass stars, while others take on new life.

The supermassive black hole (SMBH) of the Milky Way is called Sagittarius A* (Sgr. A*). Sgr. A* is about four million times more massive than the Sun. With such mass, much smaller stars nearby are easily caught by the black hole's powerful gravity and accelerated to enormous speeds.

Within 0.1 parsec of it, or a third of a light year, stars are zipping by at thousands of kilometers per second. Outside this area the speed is much slower. Stars beyond 0.1 parsec move at speeds of hundreds of km/s.

But speed is not the only reason for collisions. Region also tightly packed stars, forming what astronomers call a nuclear star cluster (NSC). The combination of high speed and high stellar density creates an area where stars are sure to collide.

A new study led by Northwestern University simulated the stars orbiting Sgr. A* to understand interactions and collisions and their results. The study is called “Stellar collisions at the galactic center: Massive stars, collision remnants and missing red giants“Lead author is Sanea S. Rose from the Department of Physics and Astronomy at the University of California. The research was also recently presented at the April meeting of the American Physical Society.

The researchers simulated a population of 1,000 stars embedded in the NSC. Stellar masses ranged from 0.5 to 100 solar masses, but in practice the upper limit was about 30 solar masses due to the initial mass function. Other characteristics, such as orbital eccentricities, were varied to ensure that the sample included stars at different distances from Sgr. A*. This is necessary in order to get a complete picture of stellar collisions.

“The region around the central black hole is densely packed with stars moving at extremely high speeds,” said lead author Rose. “It's like running through an incredibly crowded subway station in New York during rush hour. If you don't collide with other people, you pass very close to them. For stars, these close encounters still result in gravitational interactions. We wanted to study what what these collisions and interactions mean for the stellar population, and characterize their results.”

The stellar density within 0.1 parsec is not at all similar to the neighborhood of our solar system. The closest star to our Sun is low-mass Proxima Centauri. It is located just over four light years away. Compared to the center of the Galaxy, we have practically no neighbors.

But in YaZS everything is completely different.

At the center of the Milky Way Galaxy is a supermassive black hole (Sgr A*, shown in the inset on the right), embedded in a nuclear star cluster (NSC) at the center, highlighted and enlarged in the middle panel. The image on the right shows stellar density in the NSC.

“The closest star to our Sun is about four light years away,” Rose explained. “At the same distance, there are more than a million stars next to a supermassive black hole. This is an incredibly close neighborhood. In addition, a supermassive black hole has a very strong gravitational attraction. Stars orbiting a black hole can move at speeds of thousands of kilometers per second.”

With such a high density of stars, collisions are inevitable. The closer the stars are to the SMBH, the higher the collision rate. In their study, Rose and her colleagues modeled the region to determine the impact of collisions on individual stars and the stellar population.
Simulations have shown that head-on collisions are rare. Therefore, stars are not destroyed. Instead, they are more like glancing blows, where stars can shed their outer layers before continuing on their trajectories.

“They crash into each other and fly further,” Rose says. “They just scratch at each other, as if exchanging very stormy greetings.” As a result, stars eject some material and lose their outer layers. Depending on how fast they move and how much they intersect when they collide, they can lose quite a lot of their outer layers. These destructive collisions lead to the emergence of a population of strange, stripped, low-mass stars.”

As a result, these stars migrate away from the SMBH. The authors say there is likely a population of such low-mass stars scattered throughout the galactic center (GC). They also argue that the ejected mass from these infalling collisions could lead to the formation of gas and dust objects that other researchers have observed in GCs, such as X7, and G objects such as G3 and G2.

X7 is an elongated structure of gas and dust at the galactic center. Researchers speculate that it may consist of matter torn from stars during collisions between fast-moving stars near Sgr. A*. G3 and G2 are objects that resemble clouds of gas and dust, but have the properties of stellar objects.

Outside the 0.1 parsec region, stars move more slowly. As a result, collisions between stars are not as energetic and destructive. Instead of creating a population of weakened stars, collisions allow stars to merge, creating more massive stars. Multiple mergers are possible, resulting in the formation of stars more massive than our Sun.

“A few stars win the collision lottery,” says Rose. “Through collisions and mergers, these stars accumulate more hydrogen. Although they formed from an older population, they masquerade as rejuvenated, young stars. They are like zombie stars; they eat their own neighbors.”

But after they gain mass, they accelerate their death. They become like young massive stars that quickly use up their fuel.

This artist's illustration shows a massive star orbiting Sagittarius A*. After a collision, some stars gain mass and eventually shorten their lives.

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

Another puzzling thing about this inland region is the absence of red giants. “GC observations indicate a scarcity of red giants within 0.3 pc of the SMBH,” the authors write, citing other studies. Their results may explain this. “We are considering whether collisions between main sequence stars could help explain this observational mystery,” they write. “We find that within ~0.01 pc of the SMBH, stellar collisions destroy most low-mass stars before they have time to evolve from the main sequence. Thus, we expect the absence of RG in this region.”

The region around the Milky Way SMBH is chaotic. Even if you ignore the black hole itself, its spinning accretion disk and tortured magnetic fields, the stars that dance to its tune are chaotic. Simulations indicate that most stars in the GC will experience direct collisions with other stars. But their chaotic lives may shed light on how the entire region developed. And since the region has resisted astronomers' efforts to observe it, simulations like these are their next best tool.

“This is an environment like no other,” says Rose. “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 neighborhood. But if we “If we can learn about these stellar populations, we may be able to learn something new about how the galactic center formed. At the very least, it certainly looks very different from the neighborhood we live in.”