The spiral galaxy, NGC 472,5, photographed by the 100-inch telescope at Mt. Wikon. The picture on the left shou>,sthe galaxy before the supernova explosion. At right the arrow iri(lic(tte,sthe supernova. Search for Supernovae

by Fritz Zwicky

Supernovae are cosmic explosions which. at maxi- important ones were discovered by a collaborator in mum brightness, radiate away as much energy ever! rny group, Dr. \I. L, Humason (who retired in June day (in the form of light and corpuscular radiation) 1957 as a staff member of the Mount Wilson and as the sun does in a hundred million years. Palornar Observatories ) . Studies of these phenomena may be of great im- Existence of the supernova phenomenon was first portance for views on the evolution of stars and clearly pro%ed from observ ations at the Palomar stellar systems- the nature of neutron stars, and the Observatory in 1937. with the aid of the 18-inch origin of cosmic ravs. ~4lso.there is the possibility Schmidt telescope. With tin's instrument. specificallv that data on superriovae may be used to calibrate built for the supernova search Dr. J. J. Johnson distances to the \ er> pen'phec of the visible nrmrrse. (then rcsearc-h fellow in aqtrophyics) arid I, discov- The origir1al Wrnoxa search began at Caltech ~rfd10 qiqx2rnoTat> in tk pt riofl from Sept~miwr in 1933 and continued until 1942 çlierthe war - 1936 to [simian 1912. This iri~estimation sliov rd that and the work load - forced its abandonment. In 1956 on the average, one supernova flares up in a normal it was decided to renew the search for supernovae gala';? about once in 360 years. through the cooperative effort of several observa- It was also found, from the analysis of the light tories. such as Palomar Lick. and Steward in this curves and of the spectra, that there are several types country, and Berne in Switzerland. This international of supernovae. The two most prominent are desig- enterprise, of which I am director. is largely financed nated as types I and I[. by funds from the National Science Foundation and Perhaps the most important result to come from from the Swiss National Science Fund. this study was the conclusion, drawn first in 1937 From this combined effort, a dozen supernovae (long before the 11-bomb e~periments)that the sn- have been f~xiridin the last fe- >ears. (Only about pprnovii phenomenon is caused by a stupendous 60 have been recorded in histon. ) Two of the most miclmr fusion chain reaction.

Engineering and Science Data on supernovae may be used to calibrate distances to the very periphery of the visible universe

Type I supemovae are the brightest known so far. found. Its apparent magnitude was -1-12.5 and its Their spectra, which consist of ill-defined bands, have absolute photographic magnitude was estimated as completely defied interpretation in spite of con- about -17 (or about 600 millions times brighter than certed efforts by the world's best spectroscopists. the sun). This failure to understand the origin of even one Fortunately, Dr. 1. L. Greenstein (Caltech prn- single feature in the spectrum of the brightest super- fessor of astrophysics and staff member of the Mount novae is one of the reasons the search for supemovae Wilson and Palomar Observatories) and Dr. Minkow- was resumed a few years ago. ski were at hand to repeatedly photograph the spec- In some cases the decline in luminosity (as seen in trum of this explosion. As a result of their efforts, it blue light only) of type I supernovae is approximately was clearly established that the spectrum is actually exponential for periods of several hundred days, similar to that of some common novae - as Min- starting from 50 to 100 days after maximum. This kowski's spectra had indicated 20 years ago. The gas means that the photographic magnitude increases clouds, which are ejected at a velocity of about 6000 linearly with time. km/sec instead of only 1000 km/secà as in common During the past few years, some observers have novae - seem to be of unexpectedly large mass. published data indicating that the photographic brightness of supernovae of type I, regardless of Contrary evidence absolute brightness, declines by one magnitude every 52 days. This supposition induced a number of in- Also, not only were the emission lines of the vestigators to propose that the light curves of super- Balmer series of hydrogen photographed, but emis- novae of type I can be explained by assuming that sion lines of other elements such as helium and the decay of some radioactive isotope (Californium highly ionized carbon could be identified as well. 254, for instance) is responsible for the emission of There is no indication, however, that heavier ele- light by the gas clouds expelled by the supernova. ments were ejected in any great abundance, which is This theory has proved to be incorrect because the clearly contrary to the idea advocated by Professor decay rates of supernovae of type I, found so far, are Fred Hoyle (now visiting professor of astronomy at not all the same; they lie in the range from 28 to 52 Caltech, from Cambridge University) and others, that days. supernovae populate interstellar space w i t h t h e From our extended search with the 18-inch Schmidt heavier elements. telescope on Palomar Mountain. between 1936 and The second bright supernova discovered by Dr. 1941, it was found that supernovae of type I1 are Humason made its appearance in an open barred intrinsically fainter than those of type I. In contra- spiral galaxy which may be a member of the large distinction to the spectra of those of type I, the Virgo cluster of galaxies. This supernova was of type spectra of type I1 supernovae seem to show con- I and, spectacularly enough, at maximum it was siderable similarity with the spectra of some of the several times as bright as the entire galaxy of stars common novae. In fact, Dr. R. L. Minkowski (staff in which it occurred. Its apparent photographic mag- member of the Mount Wilson and Palomar Observa- nitude at maximum was about +13.5. tories) obtained strong evidence, from the widths of The two newly discovered supernovae have tem- the emission lines of hydrogen, that gas clouds are porarily disappeared from sight. Early in 1960, when being ejected with velocities between 5000 and 7000 the respective constellations come into reach of our km/sec. telescopes again, we hope to be able to photograph them at later stages of their development. Then we Two new supernovae may be able to determine the physical characteristics of the tail ends of their light curves by photographins The first of the two important new supernovae dis- them in several colors. This should enable us to estah- covered by Dr. Humason last June appeared next to lish the necessary data for the use of supernovae as a spiral arm, and slightly within an absorbing lane, distance indicators to the most remote parts of the of the giant spiral galaxy, NGC 7331. This object is universe which can be reached with present telescopic probably the brightest supernovae of type I1 ever equipment. November 1959