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OBSERVED CHANGES IN THE STRUCTURE OF THE "CRAB" NEBULA (N. G. C. 1952)

1921; Institute of Physics; Volume: 33; Issue: 192 Linguagem: Inglês

10.1086/123039

1538-3873

C. O. Lampland,

History and Developments in Astronomy

Variations or fluctuations in brightness and changes in form (and possibly changes of position) of the structure in nebulae have so far been observed with certainty in only three objects, N. G. C. 1555 (Hind's variable nebula in Taurus), N. G. C. 2261 (Hubble's variable nebula in Monoceros) and N. G. C. 6729 (Schmidt's and limes' variable nebula in Corona Australis). In all these objects the fanor comet-shaped nebulosity is associated with prominent variable stars situated at the apex: T Tauri, R Monocerotis and R Coronae Australis. Changes in the structure of the Crab nebula, which belongs to an entirely different class than the nebulae just mentioned, was definitely made out at this Observatory only a short time ago. The announcement of discovery was telegraphed to the Harvard College Observatory March 7th. But a series of photographs of it has been in the making for a number of years and the negatives have been examined from time to time during this interval. In giving an account of the discovery of these changes in the nebula the writer wishes to explain the introduction of digressions that may not appear to bear directly on the subject. The other observational work mentioned hereafter thruout the paper is, however, rather closely related to that on the Crab nebula, for it is only one of the many objects which have been examined repeatedly for the purpose of detecting possible changes, for a period extending over many years. The observed changes in the Crab nebula were established wholly on material obtained with the 40-inch Lowell reflector. The published reproductions of photographs of the nebula are unsuitable for delicate observations of this kind, as the structure is partially obscured or obliterated, or of such exaggerated contrasts in parts that trustworthy intercomparisons would be out of the question. I might mention here that all the photographs of nebulae of the series intended for accurate examinations to ascertain if changes may have occurred were exposed to show as well as possible the detail in the brighter parts. In these observations I have assumed to begin with that these brighter parts of the nebulae would probably be most likely to undergo changes, and exposures suitable to show structure to the best advantage in these regions have been made. In the case of the Crab nebula observations were