The Stellar Demolition Derby in the Centre of the Galaxy

The region near the Milky Way’s centre is dominated by the supermassive black hole that resides there. Sagittarius A*’s overwhelming gravity creates a chaotic region where tightly packed, high-speed stars crash into one another like cars in a demolition derby.

These collisions and glancing blows change the stars forever. Some become strange, stripped-down, low-mass stars, while others gain new life.

The Milky Way’s supermassive black hole (SMBH) is called Sagittarius A* (Sgr. A*). Sgr. A* is about four million times more massive than the Sun. With that much mass, the much smaller stars nearby are easily affected by the black hole’s powerful gravity and are accelerated to rapid velocities.

In the inner 0.1 parsec, or about one-third of a light-year, stars travel thousands of kilometres per second. Outside that region, the pace is much more sedate. Stars beyond 0.1 parsec travel at hundreds of km/s.

But it’s not only the speed that drives the collisions. The region is also tightly packed with stars into what astronomers call a nuclear star cluster (NSC.) The combination of high speed and high stellar density creates a region where stars are bound to collide.

“They whack into each other and keep going.”

Sanaea Rose, Department of Physics and Astronomy, UCLA

New research led by Northwestern University simulated stars orbiting Sgr. A* to understand the interactions and collisions and their results. It’s titled “Stellar Collisions in the Galactic Center: Massive Stars, Collision Remnants, and Missing Red Giants.” The lead author is Sanaea C. Rose from UCLA’s Department of Physics and Astronomy. The research was also recently presented at the American Physical Society’s April meeting.

The researchers simulated a population of 1,000 stars embedded in the NSC. The stars 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, like orbital eccentricities, were varied to ensure that the sample caught stars at different distances from Sgr. A*. That’s necessary to build a solid understanding of the stellar collisions.

“The region around the central black hole is dense with stars moving at extremely high speeds,” said lead author Rose. “It’s a bit like running through an incredibly crowded subway station in New York City during rush hour. If you aren’t colliding with 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.”

“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 neighbourhood.”

Sanaea Rose, Department of Physics and Astronomy, UCLA

The stellar density in the inner 0.1 parsecs is nothing like our Solar System’s neighbourhood. The nearest star to our Sun is the low-mass Proxima Centauri. It’s just over four light-years away. It’s like having no neighbours at all.

But in the NSC, things are way different.

“The closest star to our sun is about four light-years away,” Rose explained. “Within that same distance near the supermassive black hole, there are more than a million stars. It’s an incredibly crowded neighbourhood. 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 kilometres per second.”

In a stellar density that high, collisions are inevitable. The rate of collisions is more severe the closer stars are to the SMBH. In their research, Rose and her colleagues simulated the region to determine the collisions’ effect on individual stars and the stellar population.

The simulations showed that head-on collisions are rare. So stars aren’t destroyed. Instead, they’re more like glancing blows, where stars can be stripped of their outer layers before continuing their trajectories.

“They whack into each other and keep going,” Rose said. “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.”

These stars end up migrating away from the SMBH. The authors say that there is likely a population of these low-mass stars spread throughout the galactic centre (GC.) They also say that the ejected mass from these grazing collisions could produce the gas and dust features other researchers have observed in the GC, like X7, and G objects like G3 and G2.

Outside of the 0.1 parsecs region, the stars are slower. As a result, collisions between stars aren’t as energetic or destructive. Instead of creating a population of stripped-down stars, collisions allow the stars to merge, creating more massive stars. Multiple mergers are possible, creating stars more massive than our Sun.

“A few stars win the collision lottery,” Rose said. “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 neighbours.”

But after they gain that mass, they hasten their own demise. They become like young, massive stars that consume their fuel quickly.

“They die very quickly,” Rose said. “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.”

Another puzzling thing about this inner region is the lack of red giants. “Observations of the GC indicate a deficit of RGs within about 0.3 pc of the SMBH,” the authors write, referencing other research. Their results could explain it. “We consider whether main-sequence stellar collisions may help explain this observational puzzle,” they write. “We find that within ~ 0.01 pc of the SMBH, stellar collisions destroy most low-mass stars before they can evolve off the main sequence. Thus, we expect a lack of RGs in this region.”

The region around the Milky Way’s SMBH is chaotic. Even disregarding the black hole itself and its swirling accretion disk and tortured magnetic fields, the stars that dance to its tune live chaotic lives. The simulations show that most stars in the GC will experience direct collisions with other stars. But their chaotic lives could shed light on how the entire region evolved. And since the region resists astronomers’ attempts to observe it, simulations like this are their next best tool.

“It’s an environment unlike any other,” Rose said. “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 neighbourhood. 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 neighbourhood where we live.”

Note: these results are based on a pair of published papers:

• Collisional Shaping of Nuclear Star Cluster Density Profiles
• Stellar Collisions in the Galactic Center: Massive Stars, Collision Remnants, and Missing Red Giants