ASTRONOMICAL PHOTOGRAPHY — ^MEES 215 



to erupt radially from the sun in marked contrast to the general be- 

 havior of other prominences. 



Now let us turn from the photography of the planets and the sun 

 and celestial objects which are near us in space to the work that has 

 been done on the very distant parts of the universe. Over a hundred 

 years ago, Sir John Herschel called attention to the presence among 

 the stars of what he called nebulae — little clouds — and as telescopes 

 have grown in size and photographic materials have increased in 

 sensitivity these nebulae have attracted more and more attention. 



As soon as the spectroscope was used to analyze the stars, it became 

 evident that the word "nebula" was being used for two classes of 

 objects entirely different in structure. If with a pair of fieldglasses 

 you look at the sword of Orion, you will see a misty patch surrounding 

 a star in the middle of the sword. This is the great nebula of 

 Orion, a mass of gas of enormous extent. If, on the other hand, 

 you turn your fieldglasses on the constellation of Andromeda, you 

 will find another misty patch, but this is not a mass of gas like the 

 nebula of Orion; it is a vast agglomeration of stars at a distance so 

 great that in an ordinary telescope the individual stars cannot be 

 seen. The distinction becomes clear when we look at the spectra. 

 The Orion nebula gives us a spectrum consisting of bright lines, as 

 would be expected from a mass of gas, whereas the spectrum of the 

 Andromeda nebula is essentially that of a star though it is really 

 a composite of all the stars of the nebula — a sort of average spectrum. 



The gaseous nebulae that we can investigate are situated in our 

 own galaxy, and in some cases they are probably connected with the 

 explosion of stars in the form of novae and supernovae. Wlien a 

 star explodes in a nova, it produces an expanding shell of gas and 

 can be photographed for many years. By the use of suitable plates 

 and filters, photographs can be taken by monochromatic light, show- 

 ing differences in the structure of the envelope as a result of the 

 distribution of different gases. Thus in photographs which have been 

 taken of the expanding gas shell produced by Nova Herculis, which 

 exploded in 1934, the emission of ionized oxygen at A3727 A. causes 

 a nearly homogeneous cloud around the star, whereas emission of the 

 same element (in the same state of ionization) at A4986 A. and A5006 

 A. causes a more clearly defined ring. Emission of H-alpha and 

 nitrogen in the red causes a ring with a strongly accentuated crossbar. 

 When the gas shells produced by the explosion of supernovae expand 

 sufiiciently, we get a permanent nebula, usually referred to by astron- 

 omers as a planetary nebula, such as the Crab Nebula. 



A recent photograph of the Crab Nebula is shown in plate 3, figure 

 2. The different pliotographs were taken by blue light, yellow light, 



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