Also known as “Seven Sisters” and “Messier 45” Pleiades – open star cluster near 440 light years from Earth in the constellation Taurus – is the connected core of a much larger structure that contains several known clusters distributed over 600 parsecs (1,950 light-years). This structure, called the Great Pleiades Complex, contains at least 3,091 stars.
Stars are born in clouds of dust and gas. Pockets of this material clump together, eventually collapsing in on themselves, forming what becomes the hot core of the star.
Star formation often occurs in bursts, with many stars forming in close proximity and sequentially.
Groups of stars formed in one molecular cloud are called a cluster.
They remain gravitationally bound to each other for many millennia.
Eventually – tens or hundreds of millions of years after their formation – the star-forming material from which they arose is ejected from their surroundings by cosmic winds, radiation and other astrophysical phenomena.
When this happens, individual stars dissolve into the parent galaxy, and it can be extremely difficult to determine their relationships and trace the chronology of their origin stories, especially after 100 million years or more have passed.
In a new study, Dr. Luke Bouma of the Carnegie Institution for Science Observatory and his colleagues focused on the Pleiades.
Using data collected by NASA's TESS mission, ESA's Gaia spacecraft and the Sloan Digital Sky Survey (SDSS), they found that this star cluster represents the core of a much larger structure of sister stars that are distributed over 1,950 light-years away.
“We call it the Greater Pleiades Complex,” Dr. 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 from the same giant stellar nursery.”
Key to their approach is the fact that the star's rotation rate slows down as it ages.
Their work used a combination of observations of stellar rotation with TESS (which was designed to identify exoplanets passing in front of their stars) and observations of stellar motion with Gaia (which was developed to map our Milky Way Galaxy).
Using this information, they developed a new spin-based way to isolate and identify stars that share a common origin story.
“Only by combining data from Gaia, TESS and SDSS were we able to confidently identify new members of the Pleiades,” Dr. Bouma said.
“The data from each mission alone was not enough to reveal the full scale of the structure.”
“But when we put them together, linking the motion of the stars from Gaia, the rotation from TESS, and the chemistry from SDSS, a consistent picture emerged.”
“It was like putting together a jigsaw puzzle, with each data set representing a different piece of a larger puzzle.”
In addition to being similar in age, the authors demonstrated that the stars in the Greater Pleiades Complex have similar chemical compositions and that the stars used to be closer together.
Fifth generation SDSS data were used for chemical composition analysis.
“The Pleiades have played a central role in human observations of the stars since ancient times,” Dr. Bouma said.
“This work marks a big step toward understanding how the Pleiades have changed since their birth a hundred million years ago.”
According to the team, their new findings have broad implications.
The Pleiades are not only an astrophysical landmark for young stars and exoplanets, but also a cultural benchmark, represented throughout the world in the Old Testament and Talmud, celebrated as Matariki in New Zealand, and even represented by the Subaru logo in Japan.
“We understand that many stars near the Sun are part of huge extended stellar families with complex structures,” said University of North Carolina at Chapel Hill professor Andrew Mann.
“Our work provides a new way to uncover these hidden connections.”
A paper the findings were published this week in the journal Astrophysical Journal.
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Andrew W. Boyle etc.. 2025. Lost Sisters Found: TESS and Gaia discover the dissolving Pleiades complex. APJ 994, 24; two: 10.3847/1538-4357/ae0724
