M45

M45, commonly known as the Pleiades, is a nearby open star cluster in the constellation Taurus. It is one of the most conspicuous star clusters in the night sky and is traditionally identified with the Seven Sisters of Greek mythology. In the Messier catalog it is listed as Messier 45 (M45).

Distance and age: The cluster lies at about 444 light-years (136 parsecs) from Earth. It is relatively young, with an estimated age of around 100 million years. It spans roughly 2 degrees on the sky and measures about 8–9 light-years in diameter. It contains several hundred bright stars and likely more than a thousand members when fainter stars are counted.

Composition and features: The brightest members are hot blue B-type stars; the cluster’s most prominent stars form the traditional Seven Sisters: Maia, Electra, Alcyone, Taygeta, Celaeno, Sterope (Asterope), and Merope. The cluster is embedded in a faint reflection nebula that gives it a characteristic blue hue; dust in the region reflects light from the cluster’s young stars, producing visible nebulosity, especially around Merope.

Observational and scientific significance: M45 is one of the closest open clusters to Earth, making it a standard object for studying stellar evolution and early cluster dynamics. Its proximity allows astronomers to resolve its stars and to study stellar atmospheres and the initial mass function.

History and observing: The Pleiades have been known since antiquity and appear in many cultures. Galileo described it through a telescope; it was cataloged by Charles Messier in 1771. It is a popular target for amateur observers and is easily visible to the naked eye in dark skies; binoculars reveal a richer stellar population, and telescopes resolve many of its members.