216 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



red light, and infrared light, and they show differences in the struc- 

 ture of the nebula. As is well known, this object is all that remains 

 of a supernova which flared up in A. D. 1054. It is recorded in 

 Chinese history as having been seen in full daylight. 



Turning to the other type of nebulae— those that are like the An- 

 dromeda Nebula, those that are agglomerations of stars— in the last 

 30 years the nature of these great spiral nebulae, as they are called, 

 has been elucidated. It has been found that they are no less than 

 stellar universes and that if we could observe the Milky Way, in 

 which our sun is situated, from the Andromeda Nebula, our galaxy 

 would appear very much as the Andromeda Nebula does to us, the 

 two galaxies being of approximately the same size, having in each 

 of them about a hundred million stars, and according to some very 

 recent work, being about 1,500,000 light-years apart. 



Long ago E. E. Barnard called attention to the existence in our 

 galaxy of great clouds of obscuring matter as well as of widely dis- 

 tributed nebulosities. The whole galaxy, in fact, when viewed with 

 telescopes of low magnification, shows streaks, which may be either 

 bright or dark against the backgi-ounds of suns. It is, indeed, prob- 

 able that a very substantial proportion of the matter of the universe 

 is not agglomerated into stars, but is dispersed through intergalactic 

 space in particles and in the molecular form. This dispersed matter 

 is, of course, greatly concentrated in the galaxies so that perhaps only 

 half the mass of a galaxy is in the form of stars. The effect on the 

 calculated dynamics of the galaxies is, of course, enormous, and it gives 

 a very much simpler pattern of the disklike structure of a rotating 

 galaxy than if it is assumed that the mass of such a galaxy consists 

 entirely of discrete stars. One of the most remarkable of these masses 

 of gas appearing dark against the skies is shown in plate 4. It was 

 obtained with a 2-hour exposure on a red-sensitive plate with a red 

 filter and was taken with the Hale telescope. The formation occurs 

 along the edge of a large cloud of opaque dust and gas in the con- 

 stellation of Orion. The edge of the cloud is illuminated by nearby 

 bright stars. The cause of the streamei-s running outward nearly 

 perpendicular to the cloud front has never been explained nor has 

 the cause of the large extension of the cloud front known as the Horse- 

 head Nebula, so-called because of its shape. 



The proper combination of emulsion sensitivities and filters permits 

 a detailed study of the structure of many astronomical objects such 

 as the extragalactic stellar system shown in plate 5. Only a portion 

 of the whole system is shown in order to concentrate attention on the 

 nebulous objects indicated by arrows. The first picture, taken on a 

 103a-D plate through a GG-11 filter, isolates the light emitted by 

 doubly ionized oxygen, which occurs in the so-called nebulium lines 



