The Pleiades ranks among humanity's most culturally significant celestial object, appearing in the Old Testament, celebrated as Matariki in New Zealand, and even inspiring Subaru's corporate logo. But astronomers have long suspected this tight cluster of bright stars represents only a fragment of something larger. The challenge lay in proving it.
Most stars, including our Sun, form in groups within giant molecular clouds. These stellar siblings remain gravitationally bound for millions of years before gradually drifting apart, spreading across hundreds of light years. After roughly 100 million years (the Pleiades' current age) the original cluster becomes so diffuse that traditional methods struggle to identify which scattered stars share a common birthplace.
On the Nebra sky disc, dated circa 1600 BC, the cluster of seven dots in the upper right portion of the disk is believed to be the Pleiades showing the significance of this prominent cluster (Credit : Dbachmann)
Andrew Boyle, a graduate student at the University of North Carolina at Chapel Hill, led a team that developed an ingenious solution using stellar rotation as a clock. Young stars spin rapidly, while older stars rotate more slowly as magnetic braking gradually slows their rotation over time. Stars born together should spin at similar rates, providing a powerful age diagnostic that works even when the stars have drifted far apart.
The team combined rotation measurements from NASA's Transiting Exoplanet Survey Satellite with precise positional and motion data from the European Space Agency's Gaia spacecraft. They also incorporated chemical abundance measurements from the Sloan Digital Sky Survey. Each dataset alone proved insufficient, but together they revealed a coherent picture.
The fully integrated Transiting Exoplanet Survey Satellite (TESS), which launched in 2018 to find thousands of new planets orbiting other stars (Credit : Orbital ATK / NASA)
The results identified 3,091 member stars forming what the researchers call the Greater Pleiades Complex, distributed across more than 600 parsecs (1 parsec is equal to 3.26 light years.) This structure contains at least three previously known stellar groups, possibly five, all sharing remarkably similar properties. The stars exhibit uniform ages based on rotation rates, travel along coherent paths through the galaxy, and display matching chemical compositions, exactly what you'd expect from siblings born in the same stellar nursery.
To confirm their findings, the team ran kinematic traceback simulations, essentially rewinding the stars' motions through space. About 100 million years ago, these now dispersed stars occupied a much smaller volume, strongly suggesting they originated from a single giant molecular cloud.
"This study changes how we see the Pleiades, not just seven bright stars, but thousands of long lost siblings scattered across the whole sky," Andrew Boyle of the University of North Carolina.
The methodology employed by the team can be applied to other stellar associations, potentially revealing that many seemingly independent stars near our Sun actually belong to vast, previously unrecognized families. The approach might even help trace the Sun's own origins. Our star likely formed within a cluster that has long since dissolved, scattering its members across the galaxy. By applying this rotation based technique to nearby stars, astronomers might eventually identify the Sun's long lost siblings and reconstruct the stellar nursery where our Solar System was born.
Source : The “Seven Sisters” Just Found Thousands of Long-Lost Siblings
