Astronomers have found something incredible hiding behind the Pleiades

Astronomers have long treated the Pleiades – the glittering “Seven Sisters” in the constellation Taurus – as a compact, well-studied star cluster. 

A new study led by researchers from the University of North Carolina at Chapel Hill reveals that the Pleiades are actually the dense core of a sprawling stellar complex that stretches almost 2,000 light-years across. 

By stitching together space-telescope measurements and ground-based spectra, the team has mapped out a much larger, related population they dub the “Greater Pleiades Complex.”

Stars drift away from their family

Stars are born in cold clouds of gas and dust. When parts of these clouds collapse, they ignite clusters of new stars, often in bursts that produce many siblings at once. 

For tens to hundreds of millions of years, gravity keeps these siblings loosely bound while stellar winds and radiation blow away their natal gas. 

Over time, the group disperses into the galaxy. Once the stars drift and mix, tracing who’s related to whom becomes notoriously difficult – especially a hundred million years or more after birth.

Rotation as a cosmic timekeeper

The key to reconnecting these stellar families is age, and this team leaned on a powerful, physics-driven clue: stars spin down as they grow older. 

NASA’s TESS satellite, designed to catch tiny dips in starlight from transiting exoplanets, also measures periodic brightness variations caused by starspots, which reveal rotation periods.

Combine those “stellar clocks” with ESA’s Gaia measurements of precise positions and motions, and you can pick out coeval stars moving together through space even after their cluster has smeared across the sky.

Three missions, one picture

Study co-author Dr. Luke Bouma is a Carnegie postdoctoral fellow at the Observatories in Pasadena.

Dr. Bouma and colleagues blended rotation periods from TESS, kinematics from Gaia, and chemical fingerprints from the Sloan Digital Sky Survey (SDSS).

The team flagged stars with shared motion, shared age, and shared chemistry. These three independent lines of evidence pointed to a common origin. 

The research shows that the classic Pleiades cluster sits at the heart of a much larger structure spanning roughly 1,950 light-years.

“We are calling this the Greater Pleiades Complex,” said Dr. Bouma. “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.”

Chemistry seals the family resemblance

Stars born together inherit the same elemental recipe from their natal cloud. Using SDSS’s latest spectroscopic data, the team showed that candidate members of the Greater Pleiades Complex share similar abundances. 

That chemical kinship, paired with rotation-based ages and Gaia kinematics, strengthens the case that these stars were once packed far closer together before galactic tides and random motions pulled them apart.

Assembling a jigsaw puzzle

Individually, none of the datasets could have told the full story. Gaia’s sky map is exquisite but agnostic about age.

TESS’s rotation periods date stars but don’t reveal their orbits. Spectra provide chemistry but not motion. Yet fused, they manage to unlock ancestry. 

Study first author Andrew Boyle noted that it was only by combining data from Gaia, TESS, and SDSS that his team could 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,” said Boyle.

“But when we integrated them – linking stellar motions from Gaia, rotations from TESS, and chemistry from SDSS – a coherent picture emerged. It was like assembling a jigsaw puzzle, where each dataset provided a different piece of the larger puzzle.”

A new look at the Pleiades

The Pleiades have anchored sky lore for millennia and served as a benchmark in modern astronomy for distances, ages, and stellar physics. 

Reframing them as the luminous hub of a much larger, coeval complex changes how astronomers reconstruct the cluster’s birth environment and subsequent evolution.

The research also provides a robust template for finding other dispersed stellar families hiding in plain sight across the Milky Way.

Reconnecting countless star families

Because rotation slows predictably with age for Sun-like stars, the team’s approach can, in principle, date vast numbers of stars and group them by shared origin. 

With TESS continuing to revisit large swaths of sky, Gaia refining motions in future data releases, and SDSS and expanding their chemical atlases, the rotation-kinematics-chemistry trifecta should uncover many more larger star complexes. 

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

Looking ahead, the same methodology could age-date hundreds of thousands of stars in our galactic neighborhood, while reconnecting countless stellar families long thought lost to time.

The study is published in The Astrophysical Journal.

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