Star clusters are just some of the non-galaxy objects to be found in the SDSS survey. There are 17 Globular Clusters and many Open Clusters in the SDSS Data Release 7 currently used by Galaxy Zoo. They have been discovered and re-discovered several times! Identifying them visually can be a problem as one cluster – especially a Globular Cluster – looks much like another! NED often helps but not always. Because most of them do not have their own ObjID one of many nearby ObjIDs has to be used for identification which is why searching the forum does not always produce a result. So to help out with the identification I have catalogued them below. If you find one – now you’ll know which one it is! If you find one which is not on the list please post it in either the
Starclusters or
Globular Clusters thread and I will add it here.
Globular clusters are stable, gravitationally bound concentrations of stars which live within the halo of the Milky Way and orbit the galactic centre in an elliptical orbit. Each Globular contains approximately ten thousand to one million stars. Most are spherical with a diameter varying from 20 to 200 light years. They are thought to be some of the first objects to form after the formation of the Milky Way and contain old, metal poor stars though there are some exceptions. On a clear dark night several Globular Clusters can be seen through binoculars as large fuzzy “stars” or through a telescope as bright balls of tiny stars.
The 13 Palomar Globular Clusters were discovered in the 1950s on the survey plates of the first Palomar Observatory Sky Survey. Not as tight as the more usual Globulars, many appear faint and dim because they are obscured by dust in our line of sight.
Open clusters are physically related groups of stars held together by mutual gravitational attraction. They are all roughly the same distance from us in the same region of space and are thought to have originated from the same cosmic dust cloud. They reside in the spiral arms of the Milky Way and there are estimated to be between 50,000 to 100,000 of them. They are looser collections of stars than Globulars containing anything from 5 to 500 stars. Because of their apparent size many are best viewed through binoculars.
Enjoy!
GLOBULAR CLUSTERS
OPEN CLUSTERS
