Seven starry sisters

Ken Tapping, October 31, 2012

In the sky this week…

  • Mars is very low in the Southwest after sunset.
  • Jupiter rises around 8 pm and Venus around 5 am.
  • The Moon will reach Last Quarter on the 6th.

These evenings, if it is clear, you will see a small sickle-shaped group of stars in the eastern sky. Depending upon your eyes and the clarity and darkness of the sky, you will see between six and ten stars in the group. Binoculars will show it to contain hundreds of stars. It is a very good example of what is known as an "open star cluster".

In ancient times, when skies were clearer and darker, and light pollution had not yet been invented, the cluster was more conspicuous than it is for most of us now. The Greeks called it the Pleiades. The nine brightest stars are named after the daughters of Atlas: Alcyone, Calaeno, Electra, Maia, Merope, Sterope and Taygete, and their parents, Atlas and the sea nymph Pleione. Obviously Atlas was not holding up the sky all the time. Our Mediaeval ancestors called the cluster the "Seven Sisters". The Japanese call it Subaru.

This star cluster was known by the North American Aboriginal nations. In one of their legends, Tio Pepe, or the Devil's Tower, the volcanic neck in Wyoming featured in the movie "Close Encounters of the Third Kind", was a refuge for seven maidens pursued by a bear. The vertical striations on the tower were said to be scratches from the bear's claws. The less romantic explanation is that the scratches are columns formed from cooling lava.

You don't have to be an astronomer to enjoy the beauty of the Pleiades, but star clusters like the Pleiades are also jewel boxes from a purely scientific point of view.

All the sibling stars in a cluster were born at about the same time. However, some of them managed to capture more material from their birth cloud than the others did. This is important because a star with twice the mass of the Sun radiates energy at more than ten times the rate. It will therefore age five times faster. Grabbing more stuff is not really that good an idea if you are a star.

By measuring the relative brightnesses of the stars in the cluster we can estimate their relative masses. By measuring their temperatures as well we can estimate what stage they have reached in their lives, although they were all born at the same time. This makes the star cluster a laboratory for studying the lives of stars and how they work.

The stars of the Pleiades are all blue-white in colour. Through binoculars they look like a rich sprinkling of diamonds on black velvet. The movement of our atmosphere makes them twinkle, which makes the sight even more spectacular. The stars are embedded in a silvery cloud. We used to think that was the final remnant of the stars' birth cloud. However, now we know it is just a cloud of material the cluster happens to be drifting through.

Old astronomy books often started chapters with a bit of poetry. One on my bookshelf quotes a bit of Tennyson, "Many a night I saw the Pleiads, rising through the mellow shade, glitter like a swarm of fireflies, tangled in a silver braid". The poetry might have vanished from astronomy books, but it has not vanished from astronomy. Grab the binoculars, look at the Pleiades and see for yourself.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

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