Built on Facts

The Pleiades

Before we figured out that the universe was a very large open space with lots of stuff scattered about, the ancients had some interesting ideas on how the sky worked. Roughly speaking, celestial phenomena came in three kinds. There are the stars, which remain a constant, fixed background. There are the planets, which move about the sky in complicated but predictable patterns. And finally there were other transient phenomena – nova, supernova, comets, meteors, and the rest. These last were often seen as particularly significant by virtue of their rarity.

If you go outside tonight and look toward the western sky, you’ll see tonight’s Earth and Sky’s “Sky Tonight” object of interest: the Pleiades. It’s an open cluster of stars that many people initially mistake for the Little Dipper. It’s little, and it’s in about the same shape as the dippers. There’s six prominent stars visible, and many more dimmer stars visible in binoculars or a telescope. They are not a coincidental alignment of unrelated stars, but are in fact a gravitationally bound cluster with associated nebulosity that can be photographed with a good telescope. Though they won’t look so dramatic to the naked eye, photographed with a long exposure they look like this:

i-34d0c525d74f32f991137e20c268056a-pleiades.png
Fig. 1: The Pleiades (image credit Wikipedia)

It’s more than a little odd that there’s only six bright stars. The Pleiades are the seven sisters in Greek mythology. Where’s the other one?

Nobody knows. Maybe one of the dimmer stars used to be brighter, or maybe a brighter star was obscured by some other astronomical phenomenon in the cluster. Or maybe the Greeks were just using creative license and figured that six bright stars plus some dim ones were close enough to seven to work. But if it really did change at some point, it may well be the earliest recorded instance of one of the “permanent” features of the starry sky changing. It would have been a watershed moment in the history of physics, astronomy, and cosmology. They just wouldn’t have recognized it at the time.

Comments

  1. #1 Kobra
    March 30, 2009

    If one star did in fact disappear, wouldn’t there be evidence? Either of a black hole or supernova?

  2. #2 Eric Lund
    March 30, 2009

    The car maker Subaru uses a depiction of the Pleiades as their logo. “Subaru” is the Japanese name for the Pleiades.

  3. #3 Emory Kimbrough
    March 30, 2009

    The “associated nebulosity” is kind of half-associated. The stars in the cluster just by coincidence happen to be passing through a particularly dusty part of space. Thus, there’s a reflection nebula, but the dust that’s reflecting doesn’t share an origin with the stars providing the light.

  4. #4 Lassi Hippeläinen
    March 30, 2009

    Maybe the seventh star was Pleione. It is variable, and might have been visible at some time, if you looked hard. The Greeks stared a lot at the Pleiades, because they were used as an agricultural calendar. Besides, the athmosphere was less dirty in those times.

    On the other hand, in Indian mythology there are only six stars in Krittika. Maybe the Greeks just made up the seventh. Somebody had forgotten how to mark the number six with the ancient letter system and rounded it up…

  5. #5 Peter
    March 30, 2009

    Back in the day when the sky up here was dark and I had decent vision, the Pleiades appeared to have seven stars, the close pair of the seventh and eigth brightest could be imagined as a seventh star.

    We used that “seventh” star, and the double star in the big dipper handle as a criteria for clear viewing and good seeing conditions.

    I have not seen a truely dark sky now for several decades. Even in Northern Ontario the horizon has the glow of artificial light now. Many people alive today will never see a dark sky, (unless the economy gets REALLY bad!)

  6. #6 dreikin
    March 30, 2009

    From a naive view (seriously – I don’t even know what the constellation’s supposed to be composed of), and using

  7. #7 dreikin
    March 30, 2009

    Apparently forgot to close a tag – let me try again:
    From a naive view (seriously – I [didn't] even know what the constellation’s supposed to be composed of), and using this as a reference for names, I thought initially the Pleiades were composed of Pleione, Atlas, Alcyone, Maia, Taygeta, Electra, and Merope. Pleione might be more visually significant because of its position and brightness, moreso than either alone would make it, anyway.

    If the resolution’s not enough to tell them apart, then could Sterope and neighbor blend together to make the seventh?

  8. #8 KAS
    March 30, 2009

    I seem to remember from HS Astronomy that the seven sisters in a Greek myth were the names of the seven stars of Pleiades; as I believe drelkin goes into more detail with in the comment above. I think that you are just not able to see all seven anymore due to light pollution and air pollution…?

  9. #9 Carl Brannen
    March 30, 2009

    Wikipedia claims that there are nine named stars in the cluster, and that they correspond to the seven sisters, plus Atlas and Pleione, the parents of the seven. I found it rather convincing.

  10. #10 Astronomy Link List
    March 31, 2009

    This article has been added to the Astronomy Link List.

  11. #11 Jos Verhulst
    April 2, 2009

    The answer can already be found in the cave of Lascaux. There the Pleiades are depicted, and seemingly there are six stars here too.
    However, one star might be double. It’s the uppermost one in this picture:
    http://www.spacetoday.org/images/SolSys/Earth/LascauxCavesStarMap.jpg

    That pair of stars corresponds to Atlas and Pleïone, the parents of the daughters.

    Note also:
    => Aldebaran (eye of the bull) with the Hyades
    => The girdle of Orion, curiously with 4 stars
    => the (somewhat misplaced) cross before the eye of the bull, corresponding to the autumnal equinox point, that permits to date this painting (16.700 BC, give or take a couple of centuries).

    Paper is forthcoming in ‘Natuurwetenschappen en Techniek’ (in Dutch).

  12. #12 defender
    April 4, 2009

    I ama conservative creationist, but you got me on this one. Nice post! great photo.

  13. #13 starman
    April 19, 2009

    This ancient Japanese star chart which is believed to date from 65 BC shows 7 stars.

    http://www2.gol.com/users/stever/kitora.htm

  14. #14 paluskar vinayak
    October 29, 2010

    in rigveda third mandala 3-1-7 by vishwamitra gathin,it is noted that ‘the seven hevanly buties are holding the agni’he
    also said that ‘antarikshia agni as vaiswanar agni’.in next
    some suktas he said that the above agni was seen with surya
    in the day also.after some period it was disappiered.but the team of rishis found it after some months in the west sky at night.thease richha’s are the origine of kartikeya
    storyof the puranas.

  15. #15 Anonymous
    December 2, 2011

    You stink

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