Dozens of runaway stars were caught escaping from a dense star cluster in a satellite galaxy of the Milky Way. The cluster of fast-moving stars may mean that such runaways had a greater influence on cosmic evolution than previously thought, astronomers report Oct. 9 in Nature.
Massive stars are born in new clusters, packed so close together that they can shake each other out of place. Sometimes, encounters between pairs of massive stars or neighboring supernova explosions can send a star crawling out of the cluster altogether, to seek its fate in the wider galaxy and beyond.
Astronomer Mitchel Stoop of the University of Amsterdam and his colleagues searched for runaway stars around a large cluster of massive stars called Radcliffe 136 using data from the Gaia spacecraft on the velocities 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 orbiting 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 see things up close and personal.”
Previous studies had found some stars escaping the cluster (SN: 5/7/10). But in a broader search, Stoop found that an astonishing 55 stars had fled at speeds greater than roughly 100,000 kilometers per hour in the past 3 million years.
“That’s an incredible number to think about,” says Stoop. The observation suggests that a third of the brightest and most massive stars born in the cluster have left home.
This means that runaway stars may be an underestimated 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, heavy stars explode as supernovae, spreading heavy elements around the galaxy. (SN: 7/7/21).
“Before, we would expect that there would probably be some escapees,” says Stoop. But because of their presumed low numbers, he says, they would be left out of studies and simulations. If each cluster loses about a third of its stars to the surrounding galaxy, or even the space between galaxies, “they could probably make a big contribution to dumping all these ultraviolet photons into the intergalactic medium.”
Such escapees may also have had a profound impact on the evolution of the early universe. Within a few hundred million years of the Big Bang, more than 13 billion years ago, a source of ultraviolet radiation stripped electrons from a diffuse cloud of hydrogen atoms, a phenomenon called reionization (SN: 11/7/19).
Astronomers think that most of the photons, or particles of light, that cleared the cosmic haze came from dwarf galaxies. (SN: 2/6/17). But simulations have revealed that only a fraction of the necessary photons can escape the environments of those galaxies. Runaway stars can help account for the change, Stoop says.
“Maybe this happened in [early universe] galaxies too, during the reionization epoch,” he says.
Oey says, “There’s no doubt that runaway stars are really important and underrated.” But, she says, there are other ways to remove ionizing radiation from galaxies, and it’s not clear how much of a difference including runaway stars would make.
The timing of the runaway stars from R136 may also throw a wrench into the broader connection of runaway stars to reionization.
Surprisingly, the stars did not all migrate in one wave. Scientists know this because they have the velocities and distances of the stars and can calculate when they began their escape. Most escapees left R136 in all directions about 1.8 million years ago, when the group was forming. That’s what you’d expect if they were pulled from dating other massive stars.
But 16 of the escapees left the group more recently, just 200,000 or so years ago. And everyone was running in the same direction. Stoop and his colleagues think that the escape of these stars may have been caused by a merger with another cluster.
“This seems like a pretty unique phenomenon,” says astrophysicist Kaitlin Kratter of the University of Arizona in Tucson. If the double ejection of R136 is unusual, then it may be difficult to extrapolate how many stars other clusters lose to their cosmic environment. Finding evidence of similar waves in other clusters would help resolve the question.
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