Astronomers Find Stars Cast Away from Galactic Neighbors

Our galaxy is so big that astronomers are still exploring its stellar backwaters. Now, new observations have enabled them to map a previously uncharted structure in the Milky Way.

Scientists have found 13 stars that they believe are associated with the Magellanic Stream — a giant ribbon of gas stretching over three-quarters of the way across the sky. The researchers presented their findings on the arXiv astronomy preprint server in June.

The data used in making this discovery is part of an ambitious survey to examine over 200 stars at the outskirts of our galaxy. As part of the survey, astronomers collect spectra of these stars, revealing the unique chemical fingerprints that tell us what the stars are made of and how they move through space.

"It's a very uncharted region of the galaxy, because it's been difficult to find bright stars in those regions that are bright enough that you can go get a good spectrum," says Vedant Chandra (Center for Astrophysics, Harvard & Smithsonian), the study’s lead author.

The Magellanic Stream is a gaseous trail drawn out from the Large and Small Magellanic Clouds, a pair of dwarf galaxies currently falling into the Milky Way’s gravitational well. Since its discovery more than five decades ago, astronomers have struggled to find any stars associated with this gas. They’d expect, though, that as the Milky Way’s gravity tugs on the Clouds during their infall, stars should be pulled out along with the gas.

After a year of data collection at the 6.5-meter Baade Telescope at Las Campanas Observatory in Chile, Chandra and his colleagues had a “eureka” moment.

When they looked at the motion of the faint stars captured in their survey, they realized there was a group of 13 following the same motion as the gas in the Magellanic Stream. These stars are moving faster than other stars at similar distances, and in the same direction as the Magellanic Clouds. They also had chemical signatures consistent with having an ancestral home in the Clouds. 

The astronomers used their observations to calculate the stars’ past trajectory, and their simulations showed that they were likely once bound to the Clouds. Even more surprisingly, the observations are consistent with models of what stellar debris from the Clouds should look like. All these clues make for compelling evidence that the group has finally found faint members of the elusive Magellanic Stellar Stream.

"We've been looking for so long, like since the 1980s, to try and find these stars," says Gurtina Besla (University of Arizona), who was not involved in the study. "So that there's now pretty convincing evidence that the stars exist is just exciting."

Magellanic Stream: Past and Future

With stars in hand, the team measured the distance to the stream, which is difficult to do using gas measurements alone. Their calculations put it at over 300,000 light-years away, several times farther than some astronomers have speculated in the past. This distance measurement is crucial, because it provides a tight constraint on the past motions of the Clouds, such as the direction they orbit each other and the overall path they’re coming in along, Chandra explains. 

The discovery also validates theories about the stream’s creation. The observations confirm that gravity indeed ripped the stars from the Clouds, as predicted by certain simulations, Besla says. 

Not only does the new data reveal the history of our prominent neighbors, it also paints a map of our own galaxy’s present and future. Both Chandra and Besla emphasize that the gas the stream supplies to the Milky Way will fuel future star formation. The astronomers were able to pin down a more accurate distance to the 13 stars — and therefore the accompanying gas — which in turn enables a better measure of the gas mass. Their data put the distance at twice what had been estimated before, which in turn suggests that the gas mass, too, is twice previous estimates. That means double the supply for our galaxy’s future stars. 

The stream’s stars will also shed light on the Milky Way “shakes and wakes,” as Chandra puts it, in response to gravitational interactions with the Clouds. In addition, they will help astronomers to chart the stream’s structure and composition.

The future for astronomers, however, lies not only in the stars, but also in simulations — seeing if new simulations with the latest, cutting-edge physics can reproduce both the gas and the stars in the stream, Chandra says. 

Harnessing the power of both observations and simulations enables astronomers to turn back the clock, find the building blocks that formed the galaxy, and understand the impact that our backyard denizens will have on their own evolution and on that of the Milky Way.

“It would provide a revolution in our understanding of how our home Galaxy formed and evolved over cosmic time, leading to the [Milky Way] galaxy that we observe today,” says Pratik Gandhi (University of California, Davis) who was not involved in the study.

The survey is only half complete, and the stream likely hosts hundreds of stars that are fainter than the team can currently detect. But with the next generation of surveys dedicated to observing unexplored regions of our galaxy, astronomers are finding themselves in a “Golden Age” of Milky Way science, Gandhi says.

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Katya Gozman is a PhD candidate in Astronomy & Astrophysics at the University of Michigan. Her research involves using telescopes in remote Chilean deserts and in space to do extragalactic archaeology — digging up extremely faint stars in the outskirts of nearby galaxies to learn about their merger histories and to discover the faintest dwarf galaxies we can find. She enjoys telling stories about the universe as part of the Astrobites collaboration and as a docent at the Detroit Observatory.