Wednesday 11 June 2014

Seven sisters

I fear I'm on an astronomy jag this month.  Writing about Mina Fleming and the Henry Draper star catalogue set me thinking about the concept of stellar magnitude.  One of the 'facts' that I could dredge up from my expensive education (I used to know the names of all the bright stars in Ursa Major (aka The Plough, the Big Dipper), was that the constellation Pleiades (M45) served, in ancient times, as a test for how good your eyesight was.  If you could clearly see seven sisters in the cluster, then you could apply for the post of lookout on the Argo and get to touch the Golden Fleece.  I could only count 6 and my eyesight was really good up until I was about 40. That disconnect was explained by somebody saying that, in the 2500 years since Socrates last sat down to dinner, the brightness of some of those stars had undergone a perceptible change. But it could be that I was just too lazy-arsed to actually look with care and attention at the cluster to see what I could resolve. Michael M√§stlin (1550-1631), Kepler's mentor, for one example, had accurately drawn 11 stars in the cluster before telescopes were available to enhance the eye.
Then I found a nice Pleiades page put together by Steven Gibson from the amazing Arecibo radio telescope (pictured) in Puerto Rico.  Turns out that I could only see five of the Pleiades because the cluster includes their parents Pleione and Atlas.  Atlas was too faint for me, as were Asterope and Celaene. On the other hand, Alcyone, Maia, Taygeta, Electra and Merope (and their mum Pleione) were and are up there off the right shoulder of Orion.  In mythology he was/is pursuing them across the sky. Other (minority) versions of the story gives different names: Maia, Coccymo, Glaucia, Protis, Parthenia, Stonychia, and Lampado.  But that's highly parochial detail; most cultures had a name for this distinctive cluster of stars.  In Japan they were known as Subaru and six of them appear in the logo of the car company.

As long ago as 1767, John Mitchell in "An Inquiry into the probable Parallax, and Magnitude, of the Fixed Stars, from the Quantity of Light which they afford us, and the particular Circumstances of their Situation" calculated that it was vanishingly unlikely that so many bright stars should be apparently in the same section of the sky without being actually in the same section of the three-dimensional Universe.  And modern astronomers agree that the Pleiades are physically close (radius 8 light years) to each other, in the sense that we are physically close to Alpha Centauri which is 4 light years away. And also close to us at about 440 lty distance. In other words, distance is relative.

The Pleiades are nothing to do with the Seven Sisters district in North Central London - they were seven elm Ulmus procera trees planted in a circle on a patch of green in what is now part of Haringey.  The elms, along with all the mature elms of Europe are long dead and have been recently replaced with a little ring of hornbeams Carpinus betulus. Long may they flourish.

1 comment:

  1. Actually with an inexpensive education I used to know the stars in the plough myself. Must have been my trade. We also called the pleides the seven fishermen