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On the Physical Characteristics of the Hydra Cluster of Nebulae

Zwicky, F. (1942) On the Physical Characteristics of the Hydra Cluster of Nebulae. Proceedings of the National Academy of Sciences of the United States of America, 28 (4). pp. 150-155. ISSN 0027-8424. http://resolver.caltech.edu/CaltechAUTHORS:ZWIpnas42c

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Abstract

A. Observational Data. - In a previous paper [1] counts of the brighter nebulae in the Hydra cluster were communicated. The distance of the Hydra cluster was estimated as 7.3 X 10^6 parsecs and the average apparent velocity of recession to be expected was given as v = 4100 km./sec. No observations of the red shift for nebulae in the Hydra cluster were available at the time. Dr. Hubble subsequently obtained a spectrum plate for NGC 3309 which nebula is a member of the Hydra cluster. He kindly informs me that the apparent velocity of recession for NGC 3309 is of the order of 3950 km./sec., a value which is in good agreement with our original estimate. B. The Radial Distribution of Nebulae in the Hydra Cluster. - As emphasized previously [1] the Hydra cluster exhibits spherical symmetry and is therefore suspected to have reached a statistically stationary state. In order to check this conclusion two additional tests are available. In the first place we may compare the observed radial distribution of nebulae in the Hydra cluster with the distribution derived by Emden [2] for the bounded isothermal gravitational gas sphere and secondly we can verify whether the ratio wr^2/p0 of the square of the dispersion in radial velocities of the cluster nebulae to the central density po of the cluster can be correctly determined from the observed structural length or structural index a of the cluster [3].


Item Type:Article
Additional Information:Copyright © 1942 by the National Academy of Sciences Communicated March 12, 1942
Record Number:CaltechAUTHORS:ZWIpnas42c
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:ZWIpnas42c
Alternative URL:http://www.pnas.org/content/vol28/issue4/
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:5683
Collection:CaltechAUTHORS
Deposited By: Archive Administrator
Deposited On:27 Oct 2006
Last Modified:14 Nov 2014 19:19

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