Globular Star Clusters are Constantly Kicking Stars out of the Galaxy

All the stars we can see with the naked eye are part of the Milky Way. The gravitational power of the galaxy’s combined mass binds the stars to the galaxy. But sometimes stars are evicted from the galaxy.

These stars are called hypervelocity stars, and some of them are born from powerful gravitational interactions in globular clusters.

Globular clusters (GCs) are tightly-bound groupings of stars found in the outer halo of the Milky Way. They can contain millions of stars that are older and of lower metallicity than the general population. Their origins are unclear, but the Milky Way (MW) contains at least 150 globular clusters and probably many more.

Their cores are densely packed with stars, and the dense environment can lead to powerful gravitational interactions. Those interactions can propel individual stars to extremely high velocities. Their high velocities can break the MW’s gravitational hold on them, and they can escape into intergalactic space.

Astronomers, armed with more and more powerful observational technologies, are beginning to spot more of these stars and trace them back to their origins. But exactly what’s behind these stars and how many of them there are is still being determined. A new study used observations and modelling to determine the population of runaway stars in the MW and their sources.

The study is “Runaway and Hypervelocity Stars from Compact Object Encounters in Globular Clusters.” The authors are Tomas Cabrera and Carl L. Rodriguez, from Carnegie Mellon University and the University of North Carolina, respectively. The paper hasn’t been peer-reviewed yet.

Most stars are born in embedded clusters. Eventually, these clusters dissipate, and the stars, like our Sun, orbit around the center of the galaxy, bound by the Milky Way’s powerful gravity. Other stars are parts of either open clusters or globular clusters. When a star escapes from its cluster, astronomers call it a runaway star. If it’s travelling fast enough to escape the Milky Way, astronomers call it a hypervelocity star (HVS.)

Astronomers have spotted many runaway and hypervelocity stars using telescopes like the Hubble.

These Hubble images show four runaway stars creating bright patterns in gas as they plow through interstellar space. Image Credit: By NASA – Hubble’s Advanced Camera for Surveys – http://hubblesite.org/newscenter/archive/releases/2009/03/image/a/, Public Domain, https://commons.wikimedia.org/w/index.php?curid=5668638

The classical definition of a HVS comes from a paper published in Nature in 1988 by Jack Hills. At that time, supermassive black holes at the center of galaxies were theorized about but not proven. Hills explained that when a binary star interacted with an SMBH, the SMBH would capture one of the stars and eject the other at high speed. This is called the “Hills Mechanism,” and astronomers think it can accelerate stars up to speeds of 4,000 km per second. Naturally, this means that HSVs are evidence of SMBHs and can be used to probe the center of the Milky Way, where the SMBH resides.

Astronomers discovered the first HSV in 2005, and it originated in the galactic center. Astronomers discovered more HSVs over the years that also came from the galactic center and interactions with the SMBH. But they’ve also discovered HSVs that didn’t originate from the galactic center. Two different mechanisms can create these HSVs: a binary supernova star (BSS,) where one star explodes and propels its companion at high velocity, and dynamical ejection scenarios (DES,) strong interactions between three or four stellar objects. This is where GCs come in.

“Globular clusters (GCs) are obvious candidate matrices for both of these events due to their high stellar
densities, but the DES might be exceptionally amplified due to the presence of BH subsystems in the centers of GCs,” the authors write. Here they’re alluding to the fact that astronomers have found stellar-mass black holes in some GCs.

47 Tucanae is the second-brightest globular cluster in the night sky and contains millions of stars. It might also contain a stellar-mass black hole. Globular Clusters’ high densities make them likely sources of hypervelocity stars. Image Credit: By ESA/Hubble, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=40242016

In their paper, the authors focus on binary star and single-star encounters with compact objects (BSCO=Binary-Single Compact Object.) Compact objects are stellar remnants like white dwarfs, neutron stars, or black holes. These objects exert powerful gravitational force on stars that stray too close and can drive them out of the GC. Supernova explosions can account for some runaway and hypervelocity stars, but this study focused only on gravitational interactions, which are far more likely in GCs due to the tight grouping of objects in their cores.

“We study high-speed stellar ejecta originating from GCs by using Monte Carlo N-body models,” the authors write. Monte Carlo simulations are widely used in astronomy. Astronomers use them to estimate possible outcomes of an event for which the outcomes aren’t certain. N-body simulations are also common tools in astronomy. They model interactions between bodies and treat each body as a particle.

The pair of researchers matched observations of Milky Way GCs with N-body models to come up with a synthetic population of stellar ejecta in the MW. They found that GCs can eject stars at higher velocities than thought, up to 2,000 km per second. Prior to these results, astronomers didn’t think that star-only encounters could propel stars to such high velocities and that only the Hills Mechanism could do it.

Since discovering the first one in 2005, astronomers have found hundreds of stars travelling fast enough that they could escape the Milky Way as HVSs. Image Credit: NAOC/Kong Xiao

These results show that there’s more going on in GCs than thought and that SMBHs aren’t the only thing that can propel stars to galactic escape velocities. That likely means that there are more HSVs than thought. And since astronomers can’t determine the origins of all HSVs, it complicates the identification of their ejection mechanisms.

Globular clusters can suffer core collapses, especially when a stellar-mass black hole is present. These collapses can bring objects closer to one another, creating more dynamic interactions that can propel individual stars out of the GC at high velocities.

GCs also dissipate, expand, and lose mass over time. As the density in their cores decreases, there are fewer binary-single compact object (BSCO) encounters and fewer HSVs ejected. There’s a link between BSCO ejections and the age of the GC’s core, where close encounters occur. As the GC ages and loses mass, it ejects fewer stars. So the researchers’ models showed that the majority of the ejections, and the fastest ones, occurred early in a GC’s evolution.

This is the globular cluster Messier 15, located some 35 000 light-years away in the constellation of Pegasus (The Winged Horse). It is one of the oldest globular clusters known, with an age of around 12 billion years. Both very hot blue stars and cooler golden stars can be seen swarming together in the image, becoming more concentrated towards the cluster’s bright centre. Messier 15 is one of the densest globular clusters known, with most of its mass concentrated at its core. As well as stars, Messier 15 was the first cluster known to host a planetary nebula, and astronomers found a rare type of black hole at its centre. Image Credit: NASA/ESA

“Our study concludes that while the GC BSCO runaway rate might have been a few 10% of the overall rate in the first few Gyr of the MW, in the present day, it is likely no more a few 1% of the same,” the authors conclude.

In some recent research, astronomers have leaned heavily on velocity to show that HVSs must come from the galactic center due to interactions with the SMBH. But this paper shows that it might be time for a re-think. The picture might not be as clear as thought, and velocity might not always be the over-arching factor.

There’s still more work to be done before astronomers can untangle HSVs and their origins. Models and simulations are powerful tools and are important in astronomy. But observations always bring clarity, and a more complete survey of HSVs in the Milky Way will no doubt clarify their origins.

After the ESA’s Gaia mission released its second data set in 2018, astronomers found 20 stars out of 7 million that are fast enough to escape from the Milky Way. Gaia released its third data set in June 2022, and it contains data from 1.8 billion different sources. Maybe there are more HSVs in all that data, and it’ll bring more clarity.

But whatever the final picture might be, it looks like globular clusters are still ejecting stars out of the galaxy.