GALAXIES — SHAPLEY 141 



S Doradus are clustered around it, but unfortunately all of them 

 are difficult to study because, notwithstandino- their great intrinsic 

 luminosities, the intervening distance of 75,000 light-years dims the 

 light so that even large telescopes can with difficulty make detailed 

 analyses. None of the stars in the solar neighborhood is one-tenth 

 as bright as S Doradus. 



3. Ordinary novae. — Several times a year, if we pay close attention, 

 we find new stars in our own galactic system, especially in the direc- 

 tion toward the galactic center in Sagittarius. But daylight, clouds, 

 moonlight, and astronomical inactivity contribute to our failure to 

 record more than a few percent of those that we know, from sampling, 

 must be occurring. These new stars, or ordinary novae, behave much 

 like the supernovae described above. But the nova phenomenon is 

 much less violent, and a star is not sacrificed by each outburst. It is 

 likely that the ordinary nova represents the explosive instability of 

 the outer surfaces of a star. There is some indication that before 

 explosion the ordinary novae are slightly subnormal stars of ordinary 

 type. Perhaps this subnormality is at the bottom of the trigger action 

 that sets them off. 



But whatever the cause of the novae, it must be recorded as an 

 interesting phenomenon, and one that probably is important in the 

 general history of stars. In our own galaxy, and in the neighboring 

 Andromeda galaxy, these novafe appear so frequently that when one 

 thinks back over the past billion years that the earth's crust has 

 existed, one concludes that very many stars have blown up — a large 

 proportion of them. Frequently we have remarked that novation is 

 so common and time has been so long that every star might have 

 blown up once, or will explode during the next billion years — in other 

 words, that evolution, or development, by way of the nova outburst 

 is a major and not a negligible phenomenon in this world. 



Evidence is accruing, however, that novation is a recurrent phenom- 

 enon. Four or five stars have been novae more than once, and two of 

 them three times since 1860. If this be the true situation — that is, 

 only stars of peculiar character become novae — we may remain even 

 more at ease with respect to the immediate future of our sun. Its 

 character is good and normal. We like to believe that the sun is 

 and always will be only slightly variable (in the sunspot period), and 

 will remain quite dependable, undisturbed by interstellar clouds, 

 unsusceptible to nova-inciting disturbance — ati least for the next 

 thousand years while the astronomers are finding out about the uni- 

 verse. A nova-like change in the sun would promptly wipe biology 

 off the earth, if it did not erase the planet altogether. 



At maximum the ordinary novae are supergiant stars, more than 

 10,000 times the luminosity of the sun, but scarcely 1 percent as radiant 

 as the average supernova. When novae appear in external galaxies 



