Extract from ABC News
The bright stars of the Pleiades star cluster (as they appear in the Southern Hemisphere) can be seen in the night sky around the world. (Supplied: NASA, ESA and AURA/Caltech)
In short:
The Pleiades star cluster or Seven Sisters is part of many Indigenous groups' songlines, as well as being important in cultures around the world.
A new study has uncovered more than 3,000 stars that researchers say are part of the larger Pleiades cluster.
What's next?
The researchers are using the same method to look at other nearby stars to see if they can discover other similarly large clusters.
The Pleiades, or Seven Sisters star cluster, is one of the most iconic features in the night sky.
While the cluster contains six to seven bright blue stars that can be seen by the naked eye, it's actually surrounded by a huge, hidden stellar family.
That's according to new research out of the US, which used data from two large telescopes to find thousands of "siblings" of the Sisters.
The study, which was published in The Astrophysical Journal, means the cluster is 20 times larger than previously thought.
For astronomers, the Pleiades is one of the best-studied clusters of stars.
But it's also important in many cultures around the world — including Indigenous Australian and Māori cultures — and has been depicted in books and art since the Bronze Age.
"The Pleiades has been recognised by human cultures for thousands of years — it appears in mythology and folklore from around the world," Andrew Boyle, an astronomer at the University of North Carolina, said.
"It's remarkable that this same group of stars is also one of the most important clusters for modern astrophysics."
'Dissolving' star clusters
While many stars — including our Sun — are born into lightly gravitationally bound groups of stars called clusters, most don't stay together.
Eventually, stars start to drift apart, which is known as "dissolving" these groups.
"As time passes, the gravity of the galaxy and passing objects in space push and pull on the group of stars and break them apart," Mr Boyle said.
But once clusters have begun to dissolve, it's extremely hard to work out what they originally looked like, or which stars belonged with which cluster.
Past research tried to work out which stars may have been part of the Pleiades cluster by using the location and the direction the stars were moving in, and trace them back to where they started.
"The Pleiades is one of, if not the most, well-studied group of stars that exists," Mr Boyle said.
"[But] because the Pleiades is surrounded by thousands of unrelated stars, it's difficult to isolate true cluster members using position and motion alone."
So instead, the team combined data from two space telescopes — the European Space Agency's GAIA telescope with NASA's TESS telescope — to look at the Pleiades cluster and surrounding stars in more detail.
New rotation data 'very clever'
The team first used the GAIA telescope to gather precise information about where stars were and how fast they were moving to identify potential cluster "siblings" that had drifted away from the Seven Sisters.
Then, the TESS telescope provided data on how quickly each star rotated, which allowed the researchers to drop any stars that were too old to be part of the Pleiades cluster.
Stars can start their life spinning quickly but slow down as they age. Because Pleiades is a relatively young cluster of stars that spin fairly fast — completing one rotation in under 12 days — any surrounding stars that took longer to rotate could be ruled out of the cluster.
Tim Bedding, a University of Sydney astronomer who was not involved in the study, called this technique "very clever".
"If a star has a rotation period of faster than 12 days, it must be young," Professor Bedding said.
"Therefore it's more likely to be a member of this Pleiades complex."
The Seven Sisters cluster is shown in green, with the Greater Pleiades Complex shown in white. (Supplied: Andrew Boyle/University of North Carolina)
The researchers discovered 3,091 stars that fit the bill, spread across the sky, and termed this larger group the "Greater Pleiades Complex".
"It covers the whole sky … you can see it spread across from the horizon to horizon," Professor Bedding said.
"[The research] is really a beautiful piece of work.
"It shows that, once again, things are more complicated than we thought."
Mr Boyle said the research team was now looking at whether they could expand this new method of finding clusters to other groups of nearby stars.
"Astronomers have been able to identify hundreds of new groups of stars, but I think it remains to be seen how many of these groups of stars are actually truly independent."
"The Pleiades structure, we found, links together at least six of these 'independent' groups of stars."
Seven Sisters form a global songline
The brightest and most prominent stars in the Pleiades don't just have siblings in space.
Wirangu woman Susie Betts, from Port Lincoln in South Australia, said First Nations people around the world shared a kinship with the Seven Sisters.
Ms Betts travelled North and South America on a Churchill Fellowship to learn about how different cultures connected to the star cluster.
"These stars were their star kin, their star brothers and sisters, they were a part of their family," she said.
"These stars were part of their whole being, a part of their whole culture.
"They weren't separate at all and that's what I found I connected to, that I found so comforting that wherever I went I was home."
Wirangu woman Susie Betts has travelled the world collecting knowledge on the Seven Sisters. (Supplied: Waldya Spirit Immersions)
Ms Betts said it was like the Seven Sisters, known as Yugarilya by the Warangu, were part of a global songline.
Songlines in Australia are cultural connections between different First Nations groups that can traverse the land, sea and sky.
Ms Betts said the Seven Sisters Songline was one of the most significant in Australia.
"Anywhere you go in Australia you'll find a connection to the Seven Sisters," she said.
"It's connected to different significant sites on Country."
The Pleiades cluster is known as "Subaru" in Japanese. (Wikimedia Commons: The Pleiades Star Cluster/Stephen Rahn/Public Domain)
Stories around the Sisters in Australia can vary but Ms Betts said a common one was about an old or young man chasing seven girls or women across the landscape.
"As that happens, the landscape is created by these seven girls," she said.
"There's many layers to it, because it's all a part of our law. Our connection to country. Our sky law as well.
"It's about those two dimensions, the sky and the earth, connecting those two."
When can you see the Pleiades?
Summer is the best time to see the Pleiades in Australia with the naked eye.
In November, they are visible in the early hours of the morning.
"If you go out around 5am, 4am, they're up there, you'll see them in the north-west sky," Ms Betts said.
"That's a beautiful time, that time in the morning … the stars are really bright.
Then they are prominent in the evening sky from mid-December until March.
You'll find them in the constellation of Taurus, to the left of Orion aka the Saucepan, above the northern horizon.
"I've always had a special connection to the star cluster ever since I remember as a little girl … on summer nights looking up at these little stars in the sky," Ms Betts said.
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