An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still gravitationally bound to each other. Open clusters are found only in spiral and irregular galaxies, in which active star formation is occurring. They are usually less than a few hundred million years old: they become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic centre, as well as losing cluster members through internal close encounters. Young open clusters may still be contained within the molecular cloud from which they formed, illuminating it to create an H II region. Over time, radiation pressure from the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest away. Open clusters are very important objects in the study of stellar evolution. Because the stars are all of very similar age and chemical composition, the effects of other more subtle variables on the properties of stars are much more easily studied than they are for isolated stars.

Read the rest of this article: http://en.wikipedia.org/wiki/Open_cluster

_______________________________ Today's selected anniversaries:

1030: – King Olaf II fought and died in the Battle of Stiklestad, trying to regain his Norwegian throne from the Danes. (http://en.wikipedia.org/wiki/Battle_of_Stiklestad)

1907: – The Scouting movement began with the first scout camp at Brownsea Island in Dorset, England. (http://en.wikipedia.org/wiki/Scouting)

1947: – ENIAC, the world's first all-electronic digital computer, was turned on; it remained in continuous operation until October 2, 1955. (http://en.wikipedia.org/wiki/ENIAC)

1954: – The Fellowship of the Ring, the first part of The Lord of the Rings, was published in the United Kingdom.

1957: –- The International Atomic Energy Agency was established. (http://en.wikipedia.org/wiki/International_Atomic_Energy_Agency)

_____________________ Wikiquote of the day:

"All parts should go together without forcing. You must remember that the parts you are reassembling were disassembled by you. Therefore, if you can't get them together again, there must be a reason. By all means, do not use a hammer." -- IBM maintenance manual (1925)