304 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



away ; that is to say they are at such distances that the light from them 

 takes between 10 and 1,000 years to reach us. These distanc&s are 

 of course unimaginably vast by ordinary human standards, since light 

 takes a mere 8 minutes to traverse the distance of 93 million miles 

 between ourselves and the Sun. But they are quite small distances in 

 comparison with the size of the Galaxy — which is the name that we 

 give to the complete system of stars, or "island universe," that is de- 

 fined by the Milky Way and to which we ourselves and all the visible 

 stars belong. The Milky Way itself sweeps out a narrow belt in the 

 sky, which is simply the projection of a large thin disk containing the 

 Sun, many of the intrinsically bright stars (especially those that have 

 a high surface temperature) , and clouds of luminescent gas and dark 

 obscuring dust. 



A typical portion of the Milky Way, as seen through a telescope, 

 is shown in plate 1, and you can see how the stars are mixed together 

 with shining gas clouds and black clouds of dust that are generally 

 referred to briefly as interstellar matter. 



Recent work has shown that the Milky Way system is a typical 

 spiral galaxy, somewhat similar to the great spiral nebula in Ursa 

 Major that is shown in plate 2. This is a galaxy rather like our own 

 viewed from an oblique angle so that we can see something of its 

 structure. The brighest stars are concentrated in spiral arms, to- 

 gether with lanes of obscuring dust which appear as dark streaks 

 over the central blob. The position of the Sun in our own Galaxy 

 corresponds to a point in the outermost spiral arm but one, at a dis- 

 tance of about 30,000 light-years from the center, and the stars visible 

 to the naked eye are confined within a sphere whose diameter is 

 somewhat less than the thickness of one spiral arm. 



Plate 3 shows another similar galaxy seen edge-on, which is known 

 as the "Sombrero Hat" nebula. The spiral arms, defined by the hor- 

 izontal streak of dark matter, are seen to be concentrated in a flat disk, 

 which corresponds to what we see projected on the sky in our own 

 Galaxy as the Milky Way. The particular interest of this picture 

 is that it shows very clearly that we have at least two distinct popu- 

 lations of stars which interpenetrate : on one hand the disk and spiral 

 arm population, concentrated toward a central plane which turns out 

 to be exactly perpendicular to the axis of rotation of the whole system ; 

 and on the other hand a spherical or "halo" population which is dens- 

 est around the center of the galaxy but shows very little flattening 

 toward the plane. The disk and halo populations were christened 

 by the late Walter Baade as Population I and Population II respec- 

 tively; and the two populations differ from each other both in the 

 physical characteristics of their component stars and also in the lands 

 of orbits in which the stars revolve round the center of the Galaxy, 

 somewhat in the same way as the planets revolve round the Sun in the 



