The Pleiades is part of an enormous stellar complex birthed by the same star-forming event

Pasadena, CA—New work from a research team including Carnegie’s Luke Bouma demonstrates that the Pleiades star cluster—also known as the Seven Sisters—is part of an enormous stellar complex spread over nearly 2,000 light-years. Their work uses one of the most historically significant stellar clusters to demonstrate a new approach for tracing stellar origins—which have posed long-standing challenges for astronomers.

Stars are born in clouds of dust and gas. Pockets of this material clump together, eventually collapsing in on themselves to create what becomes a star’s hot core. Star formation often happens in bursts, with many stars forming in close proximity and succession. 

Groups of stars that formed in the same molecular cloud are called a cluster. They remain gravitationally bound to each other for many millennia. Eventually—tens to hundreds of millions of years after their formation—the star-forming material from which they emerged is ejected from their vicinity by cosmic winds, radiation, and other astrophysical phenomena. 

When this occurs, individual stars dissolve into their host galaxy and it can be extremely challenging to identify their relationships and trace the chronology of their origin story, especially after 100 million or more years have passed. 

Bouma, along with first author Andrew Boyle and co-author Andrew Mann, both of University of North Carolina Chapel Hill, combined data from NASA's TESS mission, ESA's Gaia spacecraft, and the Sloan Digital Sky Survey (SDSS) to show that the Pleiades cluster constitutes the core of a much larger structure of related stars that are distributed over more than 1,950 light-years. 

“We are calling this the Greater Pleiades Complex,” Bouma said. “It contains at least three previously known groups of stars, and likely two more. We were able to determine that most of the members of this structure originated in the same giant stellar nursery.” 

The key to their approach is the fact that the speed of a star’s rotation slows as it ages. Their work leveraged a combination of stellar rotation observations from TESS—which was designed to identify exoplanets that transit in front of their host stars—and observations of stellar motion from Gaia—which was designed to map our Milky Way galaxy. Using this information, they developed a new rotation-based way to single out and identify stars that share an origin story.  

“It was only by combining data from Gaia, TESS, and SDSS that we were able to confidently identify new members of the Pleiades. On their own, the data from each mission were insufficient to reveal the full extent of the structure. But when we integrated them—linking stellar motions from Gaia, rotations from TESS, and chemistry from SDSS—a coherent picture emerged,” Boyle explained. “It was like assembling a jigsaw puzzle, where each dataset provided a different piece of the larger puzzle.”

Beyond having similar ages, the team demonstrated that the stars in the Greater Pleiades Complex have similar chemical compositions, and that the stars used to be closer together. Data from the Sloan Digital Sky Survey’s fifth generation, directed by Carnegie’s Juna Kollmeier, was used for chemical abundance analysis. 

“The Pleiades has played a central role in human observations of the stars since antiquity,” Bouma concluded. “This work marks a big step toward understanding how the Pleiades has changed since it was born one hundred million years ago.” 

Looking ahead their methodology can be used to age-date hundreds of thousands of stars in our neighborhood of the galaxy.