Serendipity has created a magnificent astronomical spectacle in the Pleiades as a moving cluster of stars encountered an isolated molecular cloud. The rich blue clouds and delicate tendrils of reflection nebulosity surrounding the brightest members have their origin in an unrelated molecular cloud and do not represent the ancestral cloud which gave rise to the Pleiades. Recent data suggests that the collision may have been more complex than suspected, possibly involving separate molecular clouds having different paths, velocities and origins. The trajectory of the principle gas cloud forming the reflection nebulosity can be traced back in time to an elliptical concentration of gas clouds and star formation known as "Gould's belt". In 1879 the astronomer Benjamin Gould reported his survey on the distribution of bright stars in the local Milky Way. Gould's work showed that a true local subsystem of young stars and gas exists in a rotating flat disk inclined some 20 degrees to the proper disk of the Milky Way. The disk extends some 2000 light years across and contains some of the most famous astronomical objects including the Pleiades. The origin of the aberrant system of stars and gas is still a mystery but may have been triggered by a supernova blast or density wave some 30 million years ago. The Pleiades is truly a galactic vagrant which has wandered into the interior of a local supershell whose walls represent the interstellar clouds of Gould's Belt.
The proximity of this great cluster allows a close up view of a young open cluster and the fascinating interplay of a moving star cluster with the interstellar medium. The Pleiades is moving through space presently at about 40 kilometers per second. The relative tightness of the cluster is indicative of its young age as the member stars were formed some 100 million years ago and will probably travel together through space as a bound cluster for another 250 million years before the gravity of the Milky Way breaks up the cluster into individual field stars. Although the core of the cluster contains some 100 bright stars, mostly hot A and B types, the total number of stars may be closer to 400 as many lower mass members have been identified recently. The central core radius of the cluster is only about 4.5 light years but the remote outer regions of the cluster may extend out as far as 52 light years from the center. The brighter members of the cluster are blue stars with surface temperatures of about 20,000 degrees.
Aside from the bright high mass stars we typically associate with the Pleiades, there exists several populations of highly evolved low mass stars which are difficult to explain. Recently white dwarfs have been identified within the Pleiades which raises the question of how a white dwarf, a very late phase of stellar evolution, can exist in a young cluster. One theory is that the white dwarfs were formally very massive young stars which evolved rapidly by losing mass via rapid rotation or to neighboring stars in binary systems. The mechanism for their existence remains controversial. More recently a population of very low mass brown dwarfs was found within M45. Their formation may have occurred in the collapse of isolated molecular cores within the Pleiades cloud.