The Orion Nebula Cluster might be home to a black hole more than 100 times the mass of the Sun, according to a recent simulation. At 1,300 light-years away, that would make it the closest known black hole to Earth.
Most of space is empty. Even when galaxies collide, individual stars are far more likely to exchange passing glances than meet head-on.
But space is a little less empty in the Orion Nebula Cluster, one of the many star clusters hanging from the mythic warrior’s belt. In the cluster’s early years, stars might have been packed so tightly that physical run-ins would be unavoidable. According to a new simulation conducted by Ladislav Šubr (Charles University, Czech Republic) and his colleagues in the September 20th Astrophysical Journal, these multiple mergers could have created a massive black hole that dominates the core of Orion Nebula Cluster.
The odds that a star will run directly into another star increase with the star’s mass. When two normal stars do collide, the result is a single, more massive star. Because this larger star is more likely to have yet another star run into it (because it’s bigger than it was before), collisions can lead to a runaway effect, as this star continues to grow via stellar cannibalism.
The computer simulation performed by Šubr and his colleagues shows that this runaway string of collisions could grow a central black hole up to 150 times more massive than the Sun.
The idea of runaway black hole growth in stellar clusters isn’t new, but previous studies have focused on high-mass star clusters, ideal breeding grounds for black holes thousands of times the mass of the Sun. The new simulation is the first to show that even low-mass star clusters could grow their own, more modest black holes, says Jan Pflamm-Altenburg (Bonn Astronomy Institute), who was not involved in this study.
The team’s simulation is simple in principle — the computer code calculates primarily the gravitational interactions between thousands of stars. If two stars get too close together, the code merges them into one. Stars can also escape the cluster if they gain enough speed through interactions with other stars. Stellar mergers happen often in the simulated cluster’s early years, when the newborn stars huddle closer together. Multiple runs of the simulation make clear that, assuming the Orion Nebula Cluster was more compact in its past, a black hole with a mass of at least 100 Suns is inevitable.
If it exists, that black hole probably isn’t growing much today. Astronomers think that after only a few hundred thousand years, the Orion cluster’s hottest, most massive stars began emitting intense radiation that drove away the star-forming gas enshrouding the young cluster. The cluster swells in size as the gas escapes because there is less mass holding the cluster together, and some weakly bound stars escape. The simulation mimics this expulsion, producing a cluster that loses about two-thirds of its mass and expands to five times its initial size. In the expanded cluster stars collide less frequently, and the runaway growth of the black hole slows down.
In the simulation, many of the heaviest stars either merge or escape, which might explain why only 10 OB stars are currently observed in the cluster, much fewer than the 40 OB stars expected.
Even if a black hole is inevitable in the Orion cluster, the trick will be to observe it. In general, black holes become visible when they feed on gas from their surroundings, because the gas heats up and emits light. But there’s not enough gas in the Orion Nebula Cluster to feed the black hole in sufficient quantities. And even though the simulation indicates that the black hole probably forms a binary system with a star, the two will likely be too far apart for the black hole to feed off its companion.
A black hole could also be observed indirectly, by measuring its effect on the stars whirling in the cluster core. Šubr and his colleagues suggest that a 150-solar-mass black hole could explain the high speeds of the Trapezium stars, which lie at the Orion Nebula’s heart. But thorough observations of the cluster’s innermost 0.2 light-year are still needed to reveal whether the black hole exists, the authors conclude.
The Orion Nebula Cluster is one of the closest young star clusters, about 1300 light-years away, making it easier to study than the massive clusters, which lie at greater distances. “If a black hole really exists in the ONC, then it might be the closest black hole known to Earth,” Pflamm-Altenburg adds.