|AAPOD page link 28/May/2011: Each day a different image of our fascinating universe is featured as Amateur Astronomy Picture of Day.
The cluster, the most obvious to the naked eye in the night sky, is dominated by hot blue stars that have formed within the last 100 million years, are among the nearest star clusters to Earth.. Dust that forms a faint reflection nebulosity around the brightest stars was thought at first to be left over from the formation of the cluster (hence the alternate name Maia Nebula after the star Maia), but is now known to be an unrelated dust cloud in the interstellar medium that the stars are currently passing through.
The Pleiades have been known since antiquity to cultures all around the world.
As the cluster is so close to the Earth, its distance is relatively easy to measure. Accurate knowledge of the distance allows astronomers to plot a Hertzsprung-Russell diagram for the cluster which, when compared to those plotted for clusters whose distance is not known, allows their distances to be estimated. The distance to the Pleiades is currently thought to be the higher value of about 135 parsecs (roughly 440 light years)
The cluster core radius is about 8 light years and tidal radius is about 43 light years. The cluster contains over 1,000 statistically confirmed members. It is dominated by young, hot blue stars, up to 14 of which can be seen with the naked eye depending on local observing conditions.
Under ideal observing conditions, some hint of nebulosity may be seen around the cluster, and this shows up in long-exposure photographs. It is a reflection nebula, caused by dust reflecting the blue light of the hot, young stars.
It was formerly thought that the dust was left over from the formation of the cluster, but at the age of about 100 million years generally accepted for the cluster, almost all the dust originally present would have been dispersed by radiation pressure. Instead, it seems that the cluster is simply passing through a particularly dusty region of the interstellar medium.