140 ANNUAL REPORT SMITHSONIAN INSTITUTION, 194 2 



own galactic system. One hypothesis of the origin of the cosmic rays 

 ties them up with the violence of the supernova. 



The most distant individual stars yet photographed are some of the 

 supernovae that are in galaxies tens of millions of light-years distant. 

 When more complete records are obtained we shall be able to see if, 

 at maximum brightness, they are sufficiently alike so that their appar- 

 ent magnitudes at maximum can be used as a practical criterion of 

 distance. At present there seems to be too large a dis])ersion in the 

 intrinsic luminosities to make supernovae useful criteria in distance 

 measurement in the metagalaxy. 



Long before supernovae were recognized, and long before there was 

 the faintest notion of their enormous size as celestial phenomena, they 

 played a very important part in astronomical development and in 

 knowledge of the universe. For it happens that new stars suddenly 

 appearing in 1572 and 1604 were important in the inspiration of two 

 of the great astronomers of that time and of all time — Tycho Brahe, 

 the Dane, and Johannes Kepler. Now we know that these stellar out- 

 bursts in Cassiopeia (Tycho's star) and in Ophiuchus (Kepler's steUa 

 nova) were most probably supernovae. Both stars rose to a brightness 

 comparable with that of the brightest planet ; both stars changed ex- 

 plosively 15 magnitudes or more, an increase of brightness of more 

 than a million times. 



A third supernova of our galactic system was recorded by the Japa- 

 nese and Chinese astronomers in 1054. The phenomenon was the 

 parent of the present well-known Crab nebula which is still rapidly 

 expanding as a result of the eleventh-century disaster — eleventh cen- 

 tury in our records, but 5,000 years earlier on the cosmic clock. 



2. The supergiant S Doradus. — The distinction of holding top place 

 as a luminous star has been the lot of an object at the edge of one 

 of the open clusters in the Large Magellanic Cloud. It is a variable 

 star with average luminosity half a million times that of the sun. 

 It is somewhat exceeded in radiation output by supernovae, but they 

 do not last, whereas S Doradus has been continuously radiating for 

 the past half century, pouring out more than 100 trillion tons of light 

 per minute. It must have enormous resources to persist at such high 

 luminosity. Is it perhaps some very slow type of supernova? The 

 variations we now observe are irregular in character, but of no great 

 moment in alleviating the expenditure of radiant energy. The spec- 

 trum of the star is of the rare P-Cygni type, which is indicative of 

 unusually hot surface conditions. The Harvard photographic records 

 of this star do not extend much bef<n-e 1890, but of course there is 

 little reason to suspect that S Doradus has been the supreme super- 

 giant for only the past brief 50 years. Recent photographs with the 

 large southern reflector have shown that stars nearly as bright as 



