14 February 2007

The Pleiades

The winter sky is dominated by the constellation Orion which fills the southern sky during the evening hours. Orion is a guide to a few other gems in the cold winter sky such as the star Sirius, the brightest star in the skies, and the constellation Taurus which features the bright orange star Aldebaran. The three distinctive belt stars of Orion serve as the guide to these two items, Sirius by following the line of the belt stars down to the lower left of Orion, and Taurus by following the line of the belt stars up to the upper right of Orion. But even more interesting is the very distinctive star cluster just past Taurus, the Pleiades.

The Pleiades are a fascinating group of very young stars which are, relatively speaking, quite close to Earth compared to most of what we see in the night sky. The Pleiades, also known as the Seven Sisters, is an "open cluster" of stars which actually are all related to each other and are about the same age and size. The cluster formed about 100 million years ago, which in astronomical terms is recent history. For comparison, this represents about 1/40th of the time since the formation of our Sun and the Solar System. They are quite hot stars and as such are a distinctive blue color. They are all just over 400 light years away which is relatively close, a distance which is less than 0.1% of the size of the Milky Way galaxy.

During the winter you can find this cluster high in the southern sky, moving westward during the evening. As mentioned, you can use Orion's belt as a guide to find Taurus and a bit further away, the Pleiades. The brightest 9 stars have names (see diagram) which are derived from Greek mythology. The Pleiades are the seven sisters Sterope, Maia, Alcyone, Merope, Electra, Celaeno and Taygeta. Pleione and Atlas are their parents.