Open Clusters

Open star clusters are accumulations of between a hundred and up to thousand stars that were formed within the same molecular cloud. Even though their concentration towards the cluster centre is not very pronounced they are conspicuous star assemblies against the general stellar background.

h + χ Persei
Maksutov-Newton, Canon 1100Da, 66 minutes

Stars are frequently found in clusters. The underlying cause for this is their formation from large molecular clouds, which consist of gas and dust. Since the stars in such clusters have their own individual motions they tend to drift apart, with the cluster dissolving. Hence such open clusters are typically only a couple of hundred million years old, so relatively young. Open clusters do not become very old, as the stars run apart with time. The total gravitational force of such stellar clusters is not sufficient to bind the stars permanently. The above image shows the wonderful double star cluster h + χ Persei. It is recognisable with the naked eye in the constellation of Perseus. Located at a distance of 6800 ly, it presents an amazing view in the binocular!

Plejades, M 45
Maksutov-Newton, Canon 1100Da, 6.1 hours

A prime example of young open cluster are the Plejades. This open stellar cluster is only about 440 ly away from us. Since the stars are very young they have high surface temperatures, which makes them appear very blue. One can easily make out the gaseous remnants surrounding them, which were left behind after their formation. They are arranged in filaments, which is most likely caused by magnetic fields aligning them. The blue colour of these reflection nebulae is caused by the stronger refraction of the blue light as compared to the red one, which also causes the blue colour of the earth's atmosphere and of the sea. The Plejades can be used for an eye test: very good eyes recognize eight stars, good eyes see seven. If you see fewer, then you will have to see your oculist - you are near-sighted. In the old days, the Plejades when appearing on the sky in late fall reminded the farmers to take the cattle off the meadows: it was getting time to take them back into the cow barn.

Plejades, M 45
Televue Pronto, Canon 1100Da, 2.5 hours