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The Pleiades is an open cluster in the constellation of Taurus also known as Messier 45, or just M 45. This relatively nearby cluster (about 380 light years) is also known as the Seven Sisters although from cities only the five or six brightest stars are visible. From a dark site, ten or more are visible. The stars are surrounded by nebulosity, observable in photographs taken using long period exposures through telescopes with significant apertures. Due to the particular arrangement of the stars in the cluster and its compact appearance, it is often mistaken for the Little Dipper by those with little knowledge of astronomy.

The Pleiades star cluster M45 (visible from the northern hemisphere) 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.

Visible members of the Pleiades are luminous blue or white stars. The cluster contains hundreds of other stars too faint to be visible to the naked eye. This is a young cluster, with an estimated age of about 100 million years, and a projected life of only 250 more million years.

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

Each of the names is assigned to a single star in the cluster. They were mountain-nymphs (Oreads), the daughters of Atlas and Pleione, who are also represented by stars in the cluster; the granddaughters of Iapetus and Clymene, and the sisters of the Hyades, Calypso, and Dione. They committed suicide after the deaths of their sisters, the Hyades.

Alternatively, they were known as the Atlantides.

See also: La Pléiade, a group of 16th-century French poets

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