Hyperspeed stars train us about black holes and supernovae

Hypervelocity Stars (HVS) live up to their name, traveling at thousands of kilometers per second or a fraction of the speed of light (relativistic speeds). These speed demons are thought to be the result of galactic or black hole mergers, globular star clusters ejecting members, or binary pairs where one star is ejected when the other goes supernova. Occasionally, these stars are fast enough to escape our galaxy and (in some cases) take their planetary systems with them. This could have drastic implications for our theories of how life might be distributed in the cosmos (also known as the panspermia theory).

There are thousands of these stars in our galaxy, and tracking them has become the task of cutting-edge astrometric missions (such as ESA’s Gaia Observatory). In previous research, astronomers suggested that these stars could be used to determine the mass of the Milky Way. In a recent study from Leiden University in the Netherlands, Ph.D. Candidate Fraser Evans showed how data from HVS could be used to probe the mysteries of the most extreme objects in our universe – supermassive black holes (SMBHs) and the massive supernovae of massive stars.

The study, titled Far From Home: The Science Exploitation of the Fast Milky Way Stars, was conducted by Evans as part of his PhD. This consisted of using data from the Gaia Observatory, which has mapped over two billion stars in the Milky Way, to create the largest 3D catalog of celestial objects ever created. Evans used this to run computer simulations of how millions of stars were flung through the Milky Way to better understand where they are forming (and where their speed is coming from).

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As a reminder, all stars move around the center of the Milky Way at an average speed of 100 km/s (62 mps). What sets HVS apart is the fact that its speed is far greater than that of other stars, sometimes to the point where they reach escape velocity. The existence of HVS was first predicted in 1988 by astronomer and Los Alamos National Laboratory collaborator Jack G. Hills, but the first discovery didn’t come until 2005. Thanks to observatories like Gaia and major sky survey telescopes, astronomers have identified more than a thousand HVS since then.

Still, there are many unanswered questions about where HVS are most likely to emerge and what mechanisms give them incredible speed. Although Evans had no particular aspirations to become an astronomer as a child, he was fascinated by his studies and research of high-speed stars. “These are such cool objects. A thousand kilometers per second is extremely fast. You could fly around the world in less than a minute,” he said in a recent interview with Leiden University. “They also have a story to tell about processes in the universe about which we know little and still have much to discover.”

Most of the HVS studied so far are believed to have formed near the center of the Milky Way, where there is a larger and more closely bound stellar population. Additionally, many of these stars are gravitationally bound to the supermassive black hole at the center of our galaxy, Sagittarius A*. But astronomers have also discovered fast-moving stars originating in globular clusters and Magellanic Clouds, suggesting different mechanisms may be responsible. As Evans explained:

“We can safely assume that some of the now-discovered hypervelocity stars were ejected after a gravitational encounter with the massive black hole at the center of the Milky Way: Sagittarius A*. We see a similar effect in the Large Magellanic Cloud, another galaxy that we have reason to believe also contains a black hole.”

Artist’s rendering of hypervelocity stars ejected from their galaxy by interaction
with a black hole (far left). Photo credit: ESA

The possibility of a black hole in LMC was confirmed in 2021 by astronomers using the European Southern Observatory’s Very Large Telescope (VLT). Found in the Tarantula Nebula based on the motion of stars within it, this dormant black hole (VFTS 243) was the first of its kind to be discovered beyond the Milky Way. Based on his simulations, Evans also concluded that given the right conditions, supernovae could also eject hypervelocity stars from our galaxy. From this, Evans realized that HVS could represent a way to study objects in our galaxy that are difficult to observe.

“The stars that go into supernovae are incredibly rare in our Milky Way, and the event is so short-lived that it’s difficult to measure. Also, so many stars and dust are flying around Sagittarius A* that we can’t really see what’s going on there. Some hyperspeed stars fly in more visible parts of space and can tell us more about where they’re coming from. For example about the gravity of black holes or the amount of energy that a supernova produces.”

In this regard, the study of HVS will build on the long history of studying black holes by observing their effects on their surroundings. In addition, they could offer insights into transient phenomena that are extremely strong but short-lived (e.g. supernovae). Given the amazing rate at which HVS are being discovered, a larger sample to study could mean robust scientific results in the not-too-distant future.

Further reading: Leiden University

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