Pleiades May Be About 20× Larger Than Thought — Thousands of Hidden Stellar Siblings Found

Pleiades may be far larger than the familiar 'Seven Sisters'

A new study suggests the Pleiades star cluster — long recognized by its brightest seven stars, the "Seven Sisters" — contains roughly 20 times more members than previously known. By combining precise rotation measurements from NASA's Transiting Exoplanet Survey Satellite (TESS) with astrometric data from ESA's Gaia mission, researchers identified thousands of widely scattered stars that share the Pleiades' youth and origin.

The discovery, led by Andrew Boyle, a graduate student in physics and astronomy at UNC–Chapel Hill, shifts how scientists view the cluster: "This study changes how we see the Pleiades — not just seven bright stars, but thousands of long-lost siblings scattered across the whole sky," Boyle said.

The team used stellar rotation rates as an age indicator — a method related to gyrochronology. Young stars spin faster and gradually slow with age, so matching rotation periods helps link stars that are now widely separated on the sky but were likely born together. TESS provided rotation measurements from light curves, while Gaia supplied precise positions and motions to confirm common origins.

"We're realizing that many stars near the Sun are part of massive extended stellar families with complex structures," said Andrew Mann, professor of physics and astronomy at UNC–Chapel Hill. "Our work provides a new way to uncover these hidden relationships."

Beyond revising the Pleiades' known membership, this rotation-based mapping technique could reveal that other clusters thought to be distinct are actually part of more extended stellar families. It may also aid efforts to trace the dispersed stellar group that produced the Sun — a discovery that would sharpen our understanding of the solar system's origins and the Milky Way's structure.

Publication: The team's results were published on Nov. 12 in The Astrophysical Journal.