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open cluster (218 words) |
 | Open clusters are only weakly held together by gravity and are fated to disperse over a period of several hundred million years. |
| The stars in open clusters have formed together within the same interstellar cloud; indeed, in many diffuse nebulae, the birth of new open clusters can be seen taking place. |
| As open clusters drift along, some of their members escape due to velocity changes in mutual closer encounters, tidal forces in the galactic gravitational field, and encounters with field stars and interstellar clouds passing their way. |
| Messier Object 39 (434 words) |
| M39 is a very large but very loose open cluster, situated some 9 degrees east and a bit north of Deneb (Alpha Cygni). |
| M39 is approaching us at 28 km/s; its proper motion was given as 0.024" per year in the direction of position angle 222 deg (by E.G. Ebbighausen 1940, according to Burnham). |
| M39 is 3 deg N and 1/4 deg W. It is also 2 1/2 deg W and 1 deg S of 4.5-mag Pi2 Cygni. |
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