ANALYSIS OF STARLIGHT — PAGEL 307 



finally, at very high temperatures, to the ultraviolet. With a good 

 telescope, or even in good weather with the naked eye, the differences 

 in color between different stars are quite obvious when we look at 

 them. Cool stars like Betelgeuse, at the top left corner of the con- 

 stellation of Orion, are red, while hot stars like Rigel at the opposite 

 corner of Orion are bluish white. To study these differences in color 

 in a quantitative way, astronomers measure the brightnesses of stars 

 with photomultiplier cells attached to the end of the telescope which 

 are covered by optical filters of differently colored glass. Most com- 

 monly two such glass filters are used which let through mainly yellow 

 light and mainly blue light respectively, and in this way one obtains 

 a so-called color index which gives some indication of the distri- 

 bution of light in the continuous spectrum and hence of the surface 

 temperature. 



A second method of estimating surface temperatures is provided 

 by the spectroscope, which reveals the characteristic dark absorption 

 lines of the different substances present in gaseous form in the visible 

 surface layers of the stars. Most stellar spectra can be classified into 

 a continuous series of "spectral types" ranging from spectra with dark 

 lines of helium and hydrogen at one end of the scale to spectra with 

 lines of metals and molecules like CH, CN", and titanium oxide at the 

 other end. The different spectral classes do not, in general, represent 

 different chemical compositions, although we saw earlier that such 

 differences in composition do indeed exist; but the main cause of the 

 different kinds of spectra that we see is the effect of thermal excitation 

 of the atoms, molecules, and electrons, which prevents the molecules 

 and metallic atoms from showing up in the spectrum unless the 

 temperatures are comparatively low. At higher temperatures the 

 molecules dissociate into their constituent atoms and the metallic 

 atoms lose their electrons to become ionized, while hydrogen and 

 helium atoms become excited to the rather high energy levels in which 

 they need to be in order to absorb light in the visible part of the spec- 

 trum. The various spectral classes are arranged in order of diminish- 

 ing surface temperature from about 30,000° at one end of the scale 

 to about 3,000° at the other end, and for certain historical reasons 

 the different classes have come to be designated by a series of letters 

 of the alphabet : OBAFGIvMUNS ; these letters seem to be arranged 

 in a completely arbitrary order, but m fact they can easily be re- 

 membered as the initial letters of the sentence "Oh, be a fine girl ; kiss 

 me right now, sweetie." 



Plate 5 shows some typical spectra of the various different classes, 

 with the hot, bluish wliite 0-type stars at the top and cool, red M-type 

 stars at the bottom. The gradations are perfectly continuous, and 

 each class (corresponding to one of the letters) is subdivided further 

 into decimal fractions so that A5, for example, is halfway between 



