|Right Ascension||12 : 25: (h:m)
|Declination||+26 : (deg:m)
|Visual Brightness||1.8 (mag)
|Apparent Dimension||275 (arc min)|
Cataloged by Ptolemy about 138 AD.
This large and conspicuous cluster was first cataloged by Ptolemy; it is scattered over an area of about 4.5 degrees diameter. Although conspicuous, it was neither included in Messier's nor in the NGC catalog, because its nature as a true, physical cluster was proven only in 1938 by R.J. Trumpler who identified 37 stars as true cluster members. Prior to this, P.J. Melotte had cataloged it in his 1915 catalog as No. 111.
The Coma star cluster is currently neither approaching nor receding from us, i.e. it moves tangentially to us with a velocity directed to position angle PA=218 deg, visible as a proper motion of 0.02 arc seconds per year. So member stars are easily identified by having this proper motion and no Doppler shift in their spectra. Its distance has recently been refined by data of ESA's astrometric satellite Hipparcos, and is now estimated at 288 light years (previous estimate was 260). This makes it one of the nearest open star clusters, only the Ursa Major Cluster and the Hyades are even closer.
The Sky Catalog 2000 gives an age of 400 million years, and the brightest star as of apparent magnitude 4.35, the hottest spectral type of A0.
It is a bit surprising that while the cluster contains stars down to magnitude 10.5, no fainter member stars have been found so far. Perhaps this is due to the cluster's low total mass which couldn't prevent them from escaping into the surrounding space long ago.
Our image was obtained by amateur
More images from Jack Schmidling
In John Caldwell's observing list.
Last Modification: November 9, 2005