File:Messier 45 - The Pleiades - Flickr - gjdonatiello (1).jpg

Messier 45 - The Pleiades (mosaic) Credit: Giuseppe Donatiello

After decades of investigations, there are still those who claim that the cirrus clouds in the Pleiades are remnants of their formation. How true will it be? About 135 pc distant and with an estimated age of 130 ±20 Myr (Barrado y Navascués et al. 2004), M45 is certainly a young-than-average open cluster, but not as young as believed. The higher mass members are: Alcyone, Electra, Atlas, Maia, Merope, Taygeta, Pleione and Celeno. Some of them are stars of class Be, however under 8 solar masses. It must be said, however, that some are binary and multiple systems, therefore the real masses of the individual components could be lower [A&A 425, L45-L48 (2004)]. The group mainly contains stars of class A and F, therefore between about 1 and 2.5 solar masses. Clearly there are red dwarfs and substellar objects such as numerous brown dwarfs. Taking into account the estimated age, all stars with masses greater than 8 times the sun have already exploded as supernovae millions of years ago. Some of the brightest stars are now in the process of evolving into red giants and some have already done so, as some massive white dwarfs have been discovered. If we had observed the Pleiades a few million years ago, we would have seen a nice mix of white stars and red giants, as in some younger OCs. Regardless, M45 could not have remnants of the original cloud since they usually last only a few million years before being swept away by the radiation pressure exerted by the most massive stars. Those clouds, which many believe to be remnants of the original hydrogen cloud, are instead cirrus clouds belonging to the larger Taurus Molecular Cloud (TMC-1) that the cluster is now passing through. That cluster and cloud are not correlated can be seen from their reciprocal, almost opposing motions (Steven J. Gibson and Kenneth H. Nordsieck 2003 ApJ 589 362) The Pleiades are moving towards the constellation of Orion and will still resist another 250 million years before completely disintegrating under the effects of the tidal forces exerted by the Milky Way.