138 AisiasruAL report Smithsonian institution,, 1942 



cleus, which is some 30,000 light-years from the smi in an accurately 

 measurable direction. Less certainly the view discloses that the Milky 

 Way system is a spiral, perhaps more open in structure than the An- 

 dromeda galaxy; it is rotating at high speed, but, even so, 2 million 

 centuries or possibly more will be required for the sun and its neigh- 

 bors to complete one circuit, to click oflF one cosmic year. Uncertainty 

 remains as to over-all dimensions of the discoidal galaxy and of its 

 stellar haze, and this uncertainty arises in part from the existence of 

 light-absorbing, mostly nonluminous, interstellar material and from 

 its irregular distribution and its dissimilar effectiveness on light at 

 various wave lengths. 



If, for a more distant view of this part of the universe we go off into 

 space several million light-years in a special direction, the Andromeda 

 nebula and our galactic system would look like a pair of galaxies, sepa- 

 rated by only a few diameters. And in the same field, apparently also 

 a part of our local group of galaxies, would be the great spiral. Mes- 

 sier 33. A closer inspection from this distant point, and a careful 

 measurement of distances, would show several fainter galaxies associ- 

 ated with these three large systems. Two of them would be the faint 

 companions of the Andromeda nebula — Messier 32 and NGC 205 ; two 

 of them would be our own satellite-companions, the Large and Small 

 Clouds of Magellan. And there would be at least four other dwarf 

 galaxies, two of them irregular in form, and two or more spheroidal. 



The existence of this local cloud of galaxies, in which our system 

 appears to be the big dominating member, seems to be now beyond 

 question, but the census of its membership is not complete. All the 

 known members are within a sphere of a million light-years diameter. 

 Those unknown, or of uncertain membership, include systems wholly 

 or partly concealed by the clouds of absorption near the Milky Way 

 plane. The rating of the great globular clusters is also not yet clear. 

 A hundred globular clusters surround our galaxy, apparently subordi- 

 nate members of the system, but the larger ones, like Omega Centauri 

 and 47 Tucanae, should perhaps be ranked with the dwarf galaxies. 

 In total luminosity and in mass they are comparable to NGC 205, the 

 faint spheroidal galaxy in the Andromeda group. Our hypothetical 

 Andromedan observer would probably record at least these two giant 

 clusters as dwarf galaxies, if our own procedure with regard to classi- 

 fying NGC 205 were followed. 



Groups like the local "supergalaxy" occur elsewhere in metagalactic 

 space. A dozen rich clusters are known, some of them with hundreds 

 of members, and a score or two of small groups, similar to our own, 

 are already on record. One such is a group of objects in Fornax, in 

 which the brightest are spheroidal ; in our group the brightest galaxies 

 are spiral or irregular in form. 



