The Pleiades, also known as the Seven Sisters, Messier 45, and other names by different cultures, is an asterism and an open star cluster containing middle-aged, hot B-type stars in the north-west of the constellation Taurus. At a distance of about 444 light years, it is among the nearest star clusters to Earth. This also makes it the nearest Messier object to Earth, and is the most obvious cluster to the naked eye in the night sky. It is also observed to house the reflection nebula NGC 1432.

The cluster is dominated by hot blue luminous stars that have formed within the last 100 million years. Reflection nebulae around the brightest stars were once thought to be left over material from their formation, but are now considered likely to be an unrelated dust cloud in the interstellar medium through which the stars are currently passing. Computer simulations have shown that the Pleiades were probably formed from a compact configuration that resembled the Orion Nebula. Astronomers estimate that the cluster will survive for about another 250 million years, after which it will disperse due to gravitational interactions with its galactic neighborhood.

The M45 group played an important role in ancient times for the establishment of calendars thanks to the combination of two remarkable elements. The first, which is still valid, is its unique and perfectly identifiable aspect on the celestial vault near the ecliptic. The second, essential for the Ancients, is that in the middle of the third millennium BC., this asterism marked the vernal point. The earliest-known depiction of the Pleiades is likely a Northern German Bronze Age artifact known as the Nebra sky disk, dated to approximately 1600 BC. The Ancient Egyptians may have used the names "Followers" and "Ennead" in the prognosis texts of the Calendar of Lucky and Unlucky Days of papyrus Cairo 86637. On numerous cylinder seals from the beginning of the 1st millennium BC., M 45 is represented by seven points, while the Seven Gods appear, on low-reliefs of Neo-Assyrian royal palaces, wearing long open robes and large cylindrical headdresses surmounted by short feathers and adorned of three frontal rows of horns and a crown of feathers, while carrying both an ax and a knife, as well as a bow and a quiver.