The ubiquitous M45, a.k.a the Pleiades/Subaru/Mao/Makalii, etc, an open cluster of around 1000 stars, lies some 444.2ly away from the Earth, in the constellation of Taurus. Also known as the Seven Sisters, due to the prominence of its largest and brightest stars, the cluster is believed to be around 115Ma in age and probably formed within the Taurus Molecular Cloud Complex, a star forming region which lies some 470ly from Earth.

The seven famous stars, Alcyone, Taygeta, Merope, Asterope, Electra, Maia and Celaeno, along with Atlas and Pleione, make up the core of the cluster. All the stars are B class main sequence stars and giants. The brightest of the grouping, Alcyone, is a B7IIIe class star shining at 2.86mag. The dimmest of the group, Asterope, is a double star of B8Ve/B9V class and shines with an apparent magnitude of 5.64 and 6.41, respectively.

The core of the cluster has a radius of around 8ly and a tidal radius of 43ly, which denotes the outer boundary of the cluster. Most of the stars with in the cluster are of M and K class main sequence stars, with some of the smallest stars still contracting to the main sequence. However, the most numerous objects within the cluster are brown dwarfs, making up some 25% of all objects within the cluster. The largest of the dwarfs have burnt all the lithium they were born with and shine brightly in the IR. The fact they have burnt off the lithium is used to date the age of the cluster and gives the accepted age for the cluster of 115Ma.

At present, the cluster is moving through a cloud of dust and gas that is incidental to the cluster. It's this dust and gas that shines bright blue from reflected light of the stars. The true space motion of the cluster is towards the foot region of Orion, however, the constituents of the cluster will most likely be different by the time they reach the area. This is because the cluster is gradually breaking apart due to the influence of the galactic environment they're moving through and the effects of gravitational forces from passing stars and the galaxy as a whole. It's believed that the cluster will disperse, by and large, within the next 250Ma.

Open clusters such as the Pleiades are sites of planets formation and planets appear to be present within the cluster. The cluster star, HD23514, an F6 class star, appears to be surrounded by an extraordinary amount of dust and has a large IR excess. This excess points towards the possible formation of planets around this star. Many of the other late stars within the cluster are similarly surrounded by extensive disks of material, so the cluster appears to be a promising site for the formation of planets.

Pic was taken with t68 (iTelescope, NMSO) 10x3min subs