The Pleiades, also called Open cluster M45, visible from both the northern and southern hemispheres, consists of many bright, hot stars that were all formed at the same time within a large cloud of interstellar dust and gas. The blue haze that accompanies them is due to very fine dust which still remains and preferentially reflects the blue light from the stars. The picture shows the cluster with south at the top, as seen in an astronomical telescope.

The Pleiades (also known as M45, or the Seven Sisters) is an open cluster in the constellation of Taurus. It is among the nearest to earth of all open clusters, probably the best known and certainly the most striking to the naked eye.

General information

The Pleiades are about 450 light years away from Earth and were formed approximately 100 million years ago. There are approximately 500 stars in total in the cluster, but it is dominated by young hot blue stars, up to 14 of which can be seen with the naked eye, depending on local observing conditions. The arrangement of the brightest stars is somewhat similar to the Plough and Ursa Minor (the Little Dipper).

Under ideal observing conditions, some hint of nebulosity may be seen around the cluster, and this shows up in long exposure photographs. It is a reflection nebula, caused by dust left over from star formation reflecting the blue light of the hot young stars.

Like most open clusters, the Pleiades will not stay gravitationally bound forever, as the component stars are moving faster than the escape velocity of the cluster. Calculations suggest that the cluster will take about 250 million years to disperse.

Names and technical information

The 9 brightest stars of the Pleiades are named for the Seven Sisters of Greek mythology: Asterope, Merope, Electra, Maia, Taygete, Celaeno and Alcyone, along with their parents Atlas and Pleione. The Hyades were sisters of the Pleiades. The following table gives details of these stars:

m: visual magnitude

Catalog: catalog number of associated nebulosity

The name of the cluster itself has uncertain etymology, with suggested origins being from plein, to sail, making the Pleiades the 'sailing ones'; from pleos, full or many; or from pelaiades, flock of doves (in the Greek legend, the sisters are transformed into birds).

The Pleiades in folklore

The Pleiades' high visibility in the night sky has guaranteed it a special place in many cultures, antique and modern:

• To the Vikings, they were Freya's hens.
• To the Maori of New Zealand, the Pleiades are called Mataariki and their heliacal rising signifies the beginning of the new year (around June).
• The Australian Aborigines believed they were a woman who had been nearly raped by Kidili, the man in the moon. Alternatively, they were seven sisters called the Makara.
• It is said that the Native Americans measured keenness of vision by the number of stars the viewer could see in the Pleiades. Even in historical Europe, especially in Greek circles, the Pleiades were a common vision test.
• The Sioux had a legend that linked the origin of the Pleiades to Devils Tower.
• In Japan, the Pleiades are known as Subaru. The Subaru car company uses the Pleiades as their logo.

External links and references

• Ovid XIII, 293.
• A photo and information on the Pleiades (http://www.ras.ucalgary.ca/~gibson/pleiades/)
• Messier 45 (http://www.seds.org/messier/m/m045.html), SEDS Messier pages
• Subaru