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The Seven Sisters, M45

Proper NamesThe Pleiades, The Seven Sisters
Messier NumberM45
NGC/IC NumberN/A
Other Designations(SC 2000) 75
Right Ascension3h 47m 10s
Declination+24° 7' 32"
Distance440 light years
135 parsecs
MagnitudeApparent: +1.2
Absolute: -4.3
DiameterApparent: 1° 50'
Actual: 15 light years
Number of Starsc. 1,000
Optimum VisibilityNovember/December
Map of the Pleiades

One of the most prominent and famous open clusters in the entire sky. The Pleiades is a rich and nebulous cluster of stars about four hundred light years from the Solar System.

Location of the Pleiades

Though the sky immediately around the Pleiades contains no bright stars, the cluster is easy to locate. It lies directly to the northwest of the V-shaped Hyades, the 'face' of Taurus the Bull.

Relative Galactic Position of the Pleiades

The constellation of Taurus lies in the direction of our Galaxy's rim. All the objects it contains, including the Pleiades, are further from the Galaxy's core than our own Solar System.

A view from inside the Pleiades

Inside the cluster: A view from Alcyone of several of its fellow stars within the Pleiades. From bottom left to top right: Atlas, Pleione, Asterope, Taygeta and Electra.

Mythological Origins

Mythologically, the Pleiades represents the seven daughters of Atlas and Pleione: Maia, Electra, Taygeta, Alcyone, Celaeno, Asterope and Merope. Each of the Seven Sisters (as the cluster is often called) has a named star, and two stars in the eastern part of the cluster are named for their parents Atlas and Pleione.

The Pleiades were mythologically connected with Orion, though various versions of the myth explain their relationship in different ways. A common story has Orion giving chase to the Seven Sisters, with Zeus saving them by transforming them into a flock of doves, and then into a cluster of stars. The cluster is still chased across the sky by Orion, whose own constellation lies close by to the south and east, rising and setting shortly after the Pleiades.

There is some uncertainty about the connection of the cluster and its myth, because the etymology of the name Pleiades is unclear (the interpretaion 'daughters of Pleione' appears to be a later development). It may be ultimately derived from a nautical term, suggesting that the cluster was first named by Greek navigators, with the myth of its origins appearing later. Alternatively, it may come from the Greek for 'flock of doves', with obvious connections to the story of the sisters' escape from Orion.

The Pleiades in the Sky

The Pleiades are perhaps the most easily distinguished star cluster in the entire sky, and one of the easiest to locate. Especially prominent during winter (in the northern hemisphere), the cluster falls northwestward of Orion, beyond the Hyades that form the head of Taurus the Bull. They cover an area of the sky some 110 arcminutes in diameter (nearly four times that of the disc of the Moon).

The structure of the Pleiades in the sky is a small 'dipper' shape, with Atlas forming the handle attached to a quadrangle made up of Alcyone (the brightest of the Pleiades) along with Electra, Merope and Maia. The other sisters are clustered around the western end of this 'dipper', while the star of their mother Pleione lies close to her brighter companion Atlas in the east.

Origins and Structure

The core regions of the Pleiades occupy a region of space some 15 light years across, though the fringes of the cluster extend out to a reach a diameter of some eighty light years. The centre of the cluster falls about 440 light years from the Solar System, making it the third closest open cluster (after the Hyades and the Coma Star Cluster). Though the cluster is defined by a handful of brilliant blue stars, it is actually far more extensive than these few bright stars, with perhaps as many as a thousand less prominent stars, and also a significant population of brown dwarfs.

The Pleiades is a young cluster in stellar terms, probably about 100 million years old, but perhaps somewhat younger or older. The young blue stars that make up the cluster are surrounded by a mesh of nebulous material that was at one time thought to represent the remnants of the original nebula from which the cluster formed. This is now thought to be unlikely, and rather the Pleiades are passing through a cloud of material unconnected with their formation as they follow their course through the Galaxy.


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