Messier 45, the cluster known as The Pleiades was the last object in the first edition of Messier's catalogue published in 1774. Messier wrote the following brief description about the object: "Cluster of stars, known by the name Pleiades. Reported position is of the star Alcyone". The cluster is only 444 light-years from us, therefore it is quite noticeable - even with naked eye - in the autumn-winter sky. No wonder it was known thousand years prior to Messier. The seven brightest stars of it can be seen separately by the unaided eye, hence the name: Seven Sisters. It was Galileo Galilei who directed a telescope on the cluster for the first time. He noticed it consisted of several tiny stars not visible without using a telescope. In his treatise Sidereus Nuncius published in 1610 he included a drawing of the cluster containing 36 stars. Actually Pleiades has more than a thousand stars, but this fact does not mitigate the merit of Galilei.
The cluster consisting of hot blue stars of spectral class B is situated in the direction of the constellation of Taurus. The age of its stars have been determined using various methods, fortunately all of them gave very similar results, confirming each other: The members of the cluster were born about 100 million years ago.
The stars are surrounded by a remarkable blue nebulosity, which is quite usual for young star clusters, where the remains of the original cloud the cluster was formed of reflect the blue light of the stars. However Messier 45 is not young at all. Radiation pressure of the stars must have dispersed the original cloud of dust and gases by now. The fact the cluster is yet being surrounded by dust is because the stars are passing through a region rich of interstellar material.
We should not waste time if we want to admire the view, because as nothing lasts forever in the Universe, the cluster will dissolve in the surrounding field of stars due to gravitational interactions with its neighbourhood in the coming 250 million years...
