THE GROWTH OF PHYSICAL IDEAS 117 



With the application of the spectroscope to the study of 

 the stars, the science of astrophysics was born. The chemical 

 composition of the atmospheres of the stars could be analyzed, 

 and in the case of the sun the most detailed investigations 

 were possible since the solar spectrum can be examined with 

 a dispersion and on a scale possible for no other source of 

 energy. With the increasing power of the great reflecting 

 telescopes and of the spectroscopes attached to them, it was 

 possible to learn much more about the structure of the 

 stellar universe. 



Scattered through the sky and appearing on photographic 

 plates among the stars are patches of radiant material to 

 which has been given the name nebulae. The spectroscope 

 shows that some of these nebulae are glowing masses of gas 

 because their spectra are quite different from those of the 

 stars. They show the bright emission lines corresponding to 

 those emitted by a gas through which electricity is passing 

 in a vacuum tube. But by far the greater number of the 

 nebulae have spectra that correspond to what might be 

 termed an average stellar spectrum, especially those nebulae 

 that have a definite shape, often a spiral. In the greatest of 

 all these nebulae, that in the constellation Andromeda, the 

 100-inch telescope at Mount Wilson has shown the existence 

 of stars. It was possible from the nature of the stars observed 

 to calculate the distance of the Andromeda nebula, and it 

 proved to be nearly a million light-years away. 



Man has traveled far from Tycho Brahe's picture of the 

 universe. First the earth lost its place at the center of the 

 solar system. Then it was realized that the sun was but one 

 star in the Milky Way, although, indeed, for a time it had 

 been believed that the sun and, therefore, the earth were near 

 the middle of the Milky Way. Now, with the Andromeda 

 nebula before our eyes, it is clear that the whole Milky Way 

 system is a great spiral nebula and that it is not alone in the 

 universe. There are other spiral nebulae composed of multi- 

 tudes of stars like those of the Milky Way. The Andromeda 

 nebula itself may be as great in its extension as our galaxy. 



