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Are you familiar with the group  of stars known as the Pleiades? Maybe you’ve heard them referred  to as the Seven Sisters.

They’re bright and easy to spot, and as such,  they’re well known to people all over the world. Cultures from the ancient Greeks to the  Aboriginal Australians tell stories about them. Often a story of seven women.

Quick problem, though. Have a quick peek at the star cluster. In fact, if it’s a clear night where you are,  pause the video and pop outside – I’ll wait.

How many stars do you see? …Six, huh? So how did so many people around  the world make up a seventh? There may be an answer to that question.

An answer that, with a little astrophysics,   could lead us to one of the oldest  stories humans have ever told. [♪ INTRO] The story of the Pleiades as seven women  is remarkably widespread around the world. In Greek mythology, the seven  stars of the Pleiades are the   seven daughters of the Titan Atlas,  who was chased by the hunter Orion. The god Zeus scooped up the seven sisters  and placed them in the sky as stars,   out of the reach of the nefarious Orion.

Hindu epics call them seven wives of wise men,   some Aboriginal Australian cultures  call them the seven water girls, and legends from the Western Mono  of North America call them seven   wives who were thrown out of their  homes for eating too many onions. Which is harsh, but… Onion breath. I get it.

The stories aren’t ubiquitous. In Japan, for example, the cluster  is referred to as having six stars. You can even see this in the logo of Japanese car   maker Subaru, which might as well be the  unofficial state car here in Montana.

Still, with so many cultures with such  similar stories about such a specific,   incorrect number of stars, one starts to wonder: is this a coincidence, or  some remarkable common thread throughout humanity? Aboriginal Australians and ancient  Greeks, for example, had not been in   contact since their ancestors left Africa  a minimum of sixty thousand years ago. Yet they tell similar stories.

Does that mean they brought the  same story out of Africa with them? Could the story of the seven sisters  be tens of thousands of years old? That would make it one of the  oldest stories we know about.

To find out, let’s look at the stars themselves. With modern astronomy, we now know the  Pleiades are part of an open star cluster,   that is, a grouping of stars all born around  the same time from the same huge dust cloud. With a telescope, we can see that the  Pleiades actually have over a thousand stars.

But our ancestors in ancient  times didn’t have telescopes,   so let’s focus on what we  can see with our own eyes. Unless you’ve got spectacular eyesight,  you can probably see six stars. And even if you can see more than six,  it’s probably way more than seven!

Some of the Pleiades myths  actually acknowledge this mystery,   with an explanation of how the  seventh sister went missing. The Greeks, for example, said that one of the   divine sisters went into hiding for  the shame of marrying a mortal man. And there are more cultures than just the Greeks  that acknowledge the missing seventh sister.

The Nez Perce, a nation of Indigenous people  from the region around Idaho and Oregon,   told that one sister fell  in love with a mortal man. When he died and she was grieving,  she felt her sisters were ashamed,   so she hid herself behind a veil, leaving  only six sisters visible in the sky. I mean, basically the same story!

This brings us back to the beginning: what the  heck is going on with that missing seventh star? Why do all these people across time and  space on Earth agree there’s a seventh star,   where did it go, and when did it disappear? One group of researchers thinks  they may have figured it out.

According to their calculations, if  you track the motions of the stars,   you can see that a seventh bright star  may have actually been visible 100,000   years ago — we’ll come back to  that time frame in a minute. Stars move across the night  sky in a couple of ways. They appear to us to be moving  because Earth is spinning around   its axis and moving in its orbit around the Sun.

At the same time, stars are  actually moving through space. Astronomers record how much stars really move  on the night sky, not including the motion   that we only see because of the spinny  little vantage point on our homeworld. This is known as a star’s proper motion, usually  measured in tiny fractions of a degree per year.

The researchers thinking about the Pleiades  used these measured proper motions to predict   where the stars of the Pleiades may have  been hundreds of thousands of years ago. With this method, the scientists found that two  of the Pleiades — Atlas and Pleione — might have   been far enough apart that our eyes could see  them as two separate stars back in the day. They claim that now we only see six  because they’re too close together   for our eyes to tell them apart without  help from binoculars or a telescope.

One small problem. This finding was presented in a chapter  in a book, where you’re not required to   show your work in as much detail as  you would be in a paper in a journal. So we had to check it ourselves.

Yes, really. We asked an astrophysicist to  check this and she did the math. Here it is.

What this shows is that yes, indeed, Atlas  and Pleione used to be farther apart. Maybe far enough to see them as two  separate objects with the naked eye. It’s an amazing idea, but there are a few  issues that we’ll get to after this quick break.

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We have the claim from these authors  that our ancestors would have been   able to see seven stars around 100,000 years ago. That supports the idea that stories  about the Pleiades have been around   since ancient humans were all  bopping around together in Africa. It’s a really cool idea, but we  have to note a couple of possible   snags in the narrative these researchers present.

The stars in the Pleiades are quite young,   and young stars surrounded by dust can  sometimes be variable in their brightness. The “Atlas and Pleione on the move” idea  doesn’t account for the fact that maybe   one of these could have been way  brighter or fainter in the past. And when we talk about hundred thousand  year timescales there’s a lot more to   consider with how stars move  that we didn’t really talk about.

For example, Earth’s orbit is  even more complicated than the   spin around our axis and our orbit around the Sun. The circularity of Earth’s orbit and the tilt and   wobble of Earth’s axis are subject to  change over tens of thousands of years. All of these things could contribute  to Atlas and Pleione appearing in   slightly different places than our simple  calculation from proper motion predicts.

Plus, even a tiny margin of error  could matter a lot here — as far   as humans leaving Africa compared  to the time scales of the universe. And, quick note from when we  were fact checking this script. The authors of the piece say that the ancestors  of the Aboriginal Australians left Africa    a hundred thousand years ago, but we aren’t  completely sure where they got that number.

The scholarly consensus seems to be that  although some humans left Africa much earlier,   the bulk of the migration was  only around 60,000 years ago. It still works if you imagine that the stars  were visible a hundred thousand years ago and   people were still telling that story  by the time the migration happened,   but still, just pointing that out. The independent scholar Jon White also notes  that some versions of the stories of the seven   sisters are actually more different from  each other than they seem at first glance.

Like, the Aboriginal Australian stories focus a  lot more on water and ritual than the Greek myth. Plus, there are a whole bunch of other  Pleiades stories we haven’t even mentioned! Maōri mythology called them Matariki, and they  represented the eyes of a god lost in battle.

Hawaiian mo’olelo tales say that a greedy  nobleman hoarded food and stored it in the sky,   out of reach, until a little mouse  came along and gnawed the ropes of   the net until the food fell back  down to the people on the islands. Those are only two examples, and  there are plenty more – and I’d say   they’re both pretty different from seven sisters! So, after considering all this, did this story  of the Pleiades travel out of Africa with us?

Did people notice a star disappearing  over time, and adjust the story to it? It’s really hard to be sure, but this is a neat  and at least somewhat scientifically backed idea. Plus, it’s pretty incredible to think about  humans telling stories about stars for so   many thousands and thousands of years that a star  was able to move through the sky and disappear.

What an amazing way to preserve the history of the  sky itself – and connect us all at the same time. [♪ OUTRO]