Key takeaways
- A UCLA-led study shows that in the early universe, stellar systems were extremely dense, with stars frequently colliding, destroying them or causing them to merge. Repeated mergers could produce a single extraordinarily massive star, some of which eventually collapse into a black hole.
- The researchers' models suggest that intriguing objects known as "little red dots" may be dense star clusters filled with stellar collisions that are forming a very large star, which may eventually become a black hole.
- The work helps explain how supermassive black holes formed so early in time by drawing on the science of our own galactic center, which is a supermassive black hole embedded within a dense star cluster.
In our solar system's corner of the galaxy, stars rarely collide. It makes sense: Space is vast, and stars have room to roam. Even in most of the present-day universe, stellar collisions are rare. The James Webb Space Telescope, however, peers deep enough into space to see the universe as it was not too long after the Big Bang. Its observations have revealed extraordinarily compact stellar systems, raising the possibility that collisions between stars were once much more common.
A new study led by UCLA astrophysicists and published in AAS Journals finds that in the early universe, stellar systems were extremely dense, with stars frequently hitting each other. These collisions could happen at either very rapid speeds, which destroyed the stars, or more slowly, in which case they merged into one. Repeated mergers could produce a single extraordinarily massive star, which might eventually collapse into a black hole. Some of these gigantic stars eventually became black holes. The finding could help explain how the seeds of supermassive black holes formed early enough to grow into the enormous black holes already present in the young universe.
"Perhaps the strangest outcome of our simulations is that stars can merge repeatedly within the cluster until they form one gigantic star with a mass equal to that of an entire cluster," said UCLA doctoral student and corresponding author Claire Williams. "When a star this huge reaches the end of its life, models suggest that it may have no option but to collapse into itself and form a massive black hole."
The center of the Milky Way provides a nearby example of the same basic ingredients: a massive black hole embedded within a dense nuclear star cluster. The model suggests that once a black hole is present, its gravity accelerates nearby stars, causing collisions close to the black hole to destroy them rather than merge them and release gas into the surrounding region.
"One place in the nearby universe where collisions between stars are common is the galactic center," Williams said. "In our model, once these very massive stars turn into intermediate-mass black holes, the system resembles our own galactic center, which is a black hole surrounded by a dense star cluster."
The origin of supermassive black holes has been a mystery. Astronomers expected they would grow gradually, but the James Webb Space Telescope has found them forming quickly after the Big Bang. The new research is not the first to propose stellar collisions as the mechanism, but the modeling ties together observations, such as intriguing "little red dots" (LRDs), and theoretical models to identify clear pathways to collisions for stars within dense clusters and the formation of supermassive black holes.
LRDs — extremely compact objects in the early universe that look like red dots — are believed to contain a rapidly growing black hole at the center. The new simulations suggest that LRDs may be dense star clusters filled with stellar collisions that are forming a very large star, which may eventually become a black hole.
"Little red dots are a hot topic in astronomy. You can compare studies of people trying to understand LRDs to the tale of blind people who are trying to describe an elephant by touching it, and each person describes it differently based on what part they're touching," UCLA astrophysicist and co-author Smadar Naoz said. "Our research connects several of those perspectives together and may help us see more of the elephant."
Intermediate-mass black holes may need a cluster like this in order to exist. These are an elusive type of black hole — larger than those that form when a star dies, but smaller than the supermassive black holes that dominate the centers of galaxies. They are almost never observed. The clusters originate from conditions unique to the early universe, which was very different from what we see today. There were no big spiral galaxies like the Milky Way, just regions with dense clusters of star formation. Within these clusters, stars were colliding and forming black holes, around which galaxies would eventually form. Eventually, the universe expanded, became less extreme and moved toward the equilibrium state they're in now.
"Dense star clusters provide a natural birthplace for massive black-hole seeds in our model," Williams said. "The same environment that forms the black hole can also provide stars and gas that influence its subsequent growth."
