312 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



a million times brighter and then gradually fades away over a period 

 of a few months; this is observed to happen quite frequently in 

 external galaxies, although the last outburst seen in our Galaxy was 

 in 1604. When the star explodes as a supernova, it forms a vast 

 expanding cloud of gas like tlie famous Crab nebula shown in plate 

 6. This nebula is the relic of a supernova outburst in the constella- 

 tion of Taurus witnessed by Chinese and Japanese astronomers in 

 A.D. 1054 and it has been steadily expanding since then. It is 

 supposed that such a cloud will eventually diffuse into the interstellar 

 medium together with the heavy elements that were manufactured in 

 the star just before the outburst, and in this way more and more 

 heavy elements become mixed with the original interstellar hydrogen 

 clouds as time goes on; these clouds in turn will eventually con- 

 dense into a new generation of stars, which will therefore contain 

 a larger admixture of metals than previous generations. In other 

 words, young stars will have a bigger proportion of heavy elements 

 than old stars ; and this prediction is partially borne out by observa- 

 tions of the spectra of stars in globular clusters. 



SPECTROSCOPIC OBSERVATIONS OF NEARBY STARS 



These general ideas have brought us up to a point where I should 

 like to say something about the very nearby stars, those within a 

 mere 60 light-years or so from the Sun, which have recently been 

 studied from various points of view at the Royal Greenwich Observa- 

 tory. These stars are near enough for us to be able to distinguish 

 between dwarfs and giants by the trigonometric surveying method of 

 finding their distances, and we can also see dwarfs that are compara- 

 tively cool and faint because they are not too far away. We can 

 also determine at what speed and in what direction they are traveling 

 through space, and thus judge whether they are going round the cen- 

 ter of the Galaxy in circular orbits, like the stars of Population I, or in 

 elliptical orbits like the older stars of Population II. On the other 

 hand, these stars do not belong to clusters, and so they form a confus- 

 ing jumble of different ages and chemical compositions, and the pur- 

 pose of this work is to sort out exactly what kind of star we are dealing 

 with in each case : that is, to determine its physical and chemical prop- 

 erties and its genetic relationship, if any, with the stars in the galactic 

 and globular clusters. 



Figure 3 shows the color-luminosity diagram for these nearby stars 

 determined by Olin J. Eggen. We see that most of the stars are 

 rather cool and faint as compared with the stars in young clusters, 

 but there is a group of rather faintish giants (known as 

 "subgiants") which must have been around for long enough to 

 have evolved away from the main sequence. In addition to stars like 

 the Sun, on the normal main sequence, a number of stars are below, 



