308 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 63 



AO and FO. We see that the hottest stars, at the top of the diagram, 

 have lines of ionized helium, He*, and of ordinary helium; and when 

 we go from class O to the slightly cooler class B, the ionized helium 

 fades away but hydrogen becomes stronger. In class A, hydrogen 

 reaches it maximum strengih and completely dominates the spectrum, 

 but this is not because hydrogen is any more abundant in these stars 

 than in the others, since in fact a detailed analysis shows that the 

 stars illustrated here have approximately the same chemical composi- 

 tion, with hydrogen much the most abmidant element. The reason 

 for the progression is simply that hydrogen is mostly ionized to 

 H* in the O and B stars, and since H"^ is merely a proton with no 

 electrons, it does not produce any line spectriun. When the tem- 

 perature is reduced to about 10,000°, corresponding to type AO, the 

 hydrogen is largely in its ordinaiy neutral state, but the temperature is 

 still high enough to keei:* an appreciable proportion of the atoms ex- 

 cited to their second quantiun level with a stored energy of about 10 

 electron volts for each excited atom; they have to be in this excited 

 level to produce the lines of the Balmer series in the visible spectrum, 

 whereas when they are in the ground state, with no stored energy, they 

 only absorb in the far ultraviolet Lyman series that can be detected 

 only from rockets and satellites above our atmosphere. "Wlien we 

 go further along the sequence to FO, corresponding to a surface 

 temperature of about 7,000°, there are correspondingly fewer hydrogen 

 atoms excited to an energy of 10 electron volts and the hydrogen 

 lines become weaker; but now two strong lines show up that arise 

 from ionized calcium in its lowest energy state — the so-called H and 

 K lines discovered by Fraunhofer in 1815. Wlien we come to type 

 GO, which represents stars that are just about as hot as the Sun, 

 the lines of ionized calcium are the strongest ones in the whole 

 spectrum and the hydrogen lines are still strong enough to be the 

 next runners up. But now we see a number of additional lines 

 arising from metals in the neutral state; that is, the atoms are being 

 bombarded less energetically by photons and electrons, and therefore 

 many of them are able to preserve their structure intact and show their 

 characteristic absorption spectrum, exactly as they do in the ordinary 

 electric arc. Most of the lines are due to iron, which is one of the most 

 abundant metals : strong lines of iron can be seen around 4400 A and 

 around 4050 A, and there is also a strong line of neutral calcium near 

 4200 A. At 4300 A, there is the first indication of an absorption 

 band caused by a molecule; this is the simplest hydrocarbon, CH, 

 and as we go on to still cooler stars of type K, with surface tem- 

 peratures of about 4,000°, we see the molecvilar bands and the lines 

 of iron and calcium growing steadily stronger, since the molecules 

 become more abundant and the atoms settle down more and more 



