For thousands of years, the Pleiades star cluster—also known as Messier 45 or the Seven Sisters—has been a celestial beacon, easily recognizable in the constellation Taurus. Now, groundbreaking research combining data from NASA’s TESS (Transiting Exoplanet Survey Satellite) and the European Space Agency’s (ESA) Gaia mission has revealed that this familiar cluster is dramatically larger than previously believed, effectively tripling its known size.
The discovery redefines the cluster as the Greater Pleiades Complex, a vast, dispersed collection of stars that formed together roughly 100 million years ago. This finding not only sheds new light on the life of this iconic star group but also offers a powerful new method for tracking down other hidden, dispersed clusters across the galaxy.
The Unseen Members: How TESS Redefined a Star Cluster.
Historically, the Pleiades was counted as a loose grouping of about 1,000 members. The new complex, however, encompasses a massive stream of stars arcing across the sky, revealing that the cluster’s original boundaries were far too conservative.
“When I started this research, I didn’t expect the cluster to balloon to the size that it did,” said Andrew Boyle, a graduate student at the University of North Carolina at Chapel Hill and lead author of the paper published in The Astrophysical Journal. “It really touches on a human note… there’s so much more to them than we knew.”
The Three-Pronged Discovery Method.
The research team, led by Boyle, utilized a rigorous three-step process, leveraging distinct data sets from different missions to confirm the shared ancestry of these newfound stars:
1. Tracing the Orbital Path (Gaia).
The first step utilized ESA’s Gaia satellite data, which precisely maps the positions and motions of billions of stars in our Milky Way galaxy. Researchers initially identified over 10,000 potential candidates that were orbiting the galaxy at a velocity similar to the known members of the Pleiades.
2. Measuring Age with Stellar Rotation (TESS).
This is where TESS proved crucial. TESS is best known for hunting exoplanets by watching for dips in starlight, but it is also a master at measuring stellar rotation. By monitoring tiny, regular fluctuations in a star’s light caused by dark surface features called star spots, TESS can determine how fast a star is spinning. Crucially, a star’s rotation speed slows down as it ages. By filtering for stars rotating at the same rate—meaning they shared the same approximate age of 100 million years—the team successfully picked out the true Pleiades siblings.
3. Confirming Chemistry (Sloan Digital Sky Survey).
Finally, data from ground-based missions, such as the Sloan Digital Sky Survey, were used to verify the chemical composition. “The core of the Pleiades is chemically distinct from the average star in a few elements like magnesium and silicon,” noted co-author Luke Bouma. The new stars shared this exact chemical signature, providing the definitive evidence that they all originated from the same molecular cloud.
The Legacy of a Dispersing Cluster.
The existence of the Greater Pleiades Complex confirms that star clusters, even after their energetic formation, are not static. The team believes that all these stars began in a much tighter group before gradually dispersing. This stellar exodus was driven by two main forces: the explosive impact of internal supernovae and the powerful tidal forces of the Milky Way’s gravity, which stretches the cluster into a long, arcing stream.
This discovery is a major triumph for data synthesis in astrophysics, showcasing TESS’s value far beyond its primary exoplanet mission. Researchers are now working on the TESS All-Sky Rotation Survey, a massive database that will give scientists the tools to uncover even more hidden stellar connections and scattered star complexes across our galaxy.