The Hercules Clusters of Nebulae.
Abstract
The northern of two clusters of nebulae in Hercules, first listed by Shapley in 1933, is an irregular group of about 75 bright nebulae and a larger number of faint ones, distributed over an area about 1" X 40'. A set of plates of parts of this cluster, taken by Dr. Walter Baade with the 200-inch Hale reflector, is shown and described. More than three-quarters of the bright nebulae have been classified, and, of these, 69 per cent are spirals or irregulars and 31 per cent elliptical or SO. Radial velocities for 7 nebulae were obtained by Humason, and 10 have been obtained by us with the 82-inch reflector. The mean red shift is 10775 km/sec. From this sample, the total kinetic energy of the nebulae has been estimated. By measuring the distances between all pairs on a 48-inch Schmidt enlargement, the total potential energy has been estimated. From these results it is concluded that, if the cluster is to be in a stationary state, the average galactic mass must be 1052Mo. Three possibilities are discussed: that the masses are indeed as large as this, that there is a large amount of intergalactic matter, and that the cluster is expanding. The data for the Coma and Virgo clusters are also reviewed. It is concluded that both the Hercules and the Virgo clusters are probably expanding, but the situation is uncertain in the case of the Coma cluster. Some discussion of the implication of these results is given.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- September 1959
- DOI:
- 10.1086/146752
- Bibcode:
- 1959ApJ...130..629B