On 4th of March 1769, Charles Messier included the Pleiades as No. 45 in his first list of nebulae and star clusters, published 1771. The Pleiades, or M(essier)45 cluster, are an open star cluster that can be seen from both hemispheres in the constellation Taurus. The accuracy of the cluster’s distance to Earth is still debated, however and according to the various technical instruments used, it is estimated to be around 444 light years.

The origin of the name “Pleiades” comes from Greek mythology: the Pleiades are seven sisters, daughters of Atlas and Pleioné: Asterope, Mérope (or Dryope, or Aero), Electra, Maïa, Taygète, Célaéno (or Selene) and Alcyone. Today there are around 3,000 stars in this cluster, a dozen of which are visible to the naked eye. It extends over 2 °, or the equivalent of four times the full Moon diameter. Its density is therefore relatively low compared to other open clusters.

The age of the cluster is estimated at 100 million years, but it should not live long since it should separate in 250 million years, in part due to its low density (here it is the life of the cluster and not that of the stars that compose it). The 9 brightest stars in the cluster are the names of the 7 sisters and their parents. Their magnitude is between 2.86 and 5.44, therefore accessible to the naked eye. Asterope has the particularity of being a double star.

The Pleiades are not only a open cluster but also considered as nebulae, blue-colored, which indicates that they are reflection nebulae, reflecting the light of the bright stars situated near (or within) them. The brightest of these nebulae is around Merope. Physically, the reflection nebula is probably part of the dust in a molecular cloud, unrelated to the Pleiades cluster, which happens to cross the cluster’s way. It is not a remnant of the nebula from which the cluster once formed, as can be seen from the fact that the nebula and cluster have different radial velocities, crossing each other with a relative velocity of 11 km/sec.(sources Wikipedia, obspm).