"Runaway Stars May Impact the Cosmos Beyond Their Home Galaxies"

Dozens of fugitive stars were caught fleeing a dense star cluster in a satellite galaxy of the Milky Way. The swarm of speeding stars could mean that such runaways had a bigger influence on cosmic evolution than previously thought, astronomers report October 9 in Nature.

Massive stars are born in young clusters, packed so close together that they can jostle each other out of place. Sometimes, encounters between pairs of massive stars or neighboring supernova explosions can send a star zipping out of the cluster altogether, to seek its fortune in the wider galaxy and beyond.

Astronomer Mitchel Stoop and his colleagues searched for runaway stars around a huge cluster of massive stars called Radcliffe 136 using data from the Gaia spacecraft on the speeds and positions of billions of stars (SN: 6/13/22). R136 is located about 170,000 light-years from Earth in the Large Magellanic Cloud, a dwarf galaxy that orbits the Milky Way.

The cluster “is an iconic object,” says astrophysicist Sally Oey of the University of Michigan in Ann Arbor, who was not involved in the new work. The view from Earth’s neighborhood is so clear, “we can really look at things up close and personal.”

Previous studies had found a few stars fleeing the cluster (SN: 5/7/10). But in a wider search, Stoop found an astonishing 55 stars had fled at speeds faster than roughly 100,000 kilometers per hour in the past 3 million years.

“That is an incredible number to think about,” Stoop says. The observation suggests that as many as a third of the brightest, most massive stars born in the cluster have left home.

That means runaway stars could be an underappreciated force in the universe. These massive stars, about five to 140 times the mass of the sun, emit ultraviolet radiation and supersonic stellar winds that can sculpt the gas and dust around them (SN: 7/11/22). At the end of their lives, the heavyweight stars explode as supernovas, spreading heavy elements around the galaxy (SN: 7/7/21).

“Before, we’d expect maybe there are a handful of runaways,” Stoop says. But because of their presumed low numbers, he says, they would be left out of studies and simulations. If each cluster instead loses about a third of its stars to the surrounding galaxy, or even the space between galaxies, “they can maybe have a major contribution to dumping all these ultraviolet photons into the intergalactic medium.”

Such escapees could also have had a profound influence on the evolution of the early universe. Within a few hundred million years of the Big Bang, more than 13 billion years ago, some source of ultraviolet radiation stripped electrons from a pervasive fog of hydrogen atoms, a phenomenon called reionization (SN: 11/7/19).

Astronomers think most of the photons, or particles of light, that cleared the cosmic fog came from dwarf galaxies (SN: 2/6/17). But simulations have found that only a fraction of the photons needed can escape the environments of those galaxies. Runaway stars could help account for the difference, Stoop says.

“Maybe this happened in [early universe] galaxies as well, during the epoch of reionization,” he says.

Oey says, “There’s no doubt that runaway stars are really important and have been underappreciated.” But, she says, there are other ways to get ionizing radiation out of galaxies, and it’s not clear how much of a difference including runaway stars would make.

The timing of the stars’ escape from R136 might also throw a wrench in the broader relevance of runaway stars to reionization.

Surprisingly, the stars didn’t all migrate in one wave. The scientists know this because they have the stars’ speeds and distances and can calculate when they started their escape. Most of the runaways fled R136 in all directions about 1.8 million years ago, when the cluster was forming. That’s what you’d expect if they were booted out by encounters with other massive stars.

But 16 of the escapees left the cluster more recently, just 200,000 or so years ago. And they were all fleeing in the same direction. Stoop and his colleagues think those stars’ escape might have been triggered by a merger with another cluster.

“That seems like a fairly unique occurrence,” says astrophysicist Kaitlin Kratter of the University of Arizona in Tucson. If R136’s double ejection is unusual, then it might be hard to extrapolate how many stars other clusters lose to their cosmic surroundings. Finding evidence of similar waves in other clusters would help resolve the question.