The Birmingham age-herald., 1919-04-06

are grouped into families and genera on the earth, but we can also measure with high accuracy just how fast they are approaching toward or re'ceding rom us Incidentally, it may be pointed out that we definitely prove in this way that the earth is rotating and that it is nov ing around the sun; it is not until our own velocties arising from these two motions are allowed for that we obtain the true velocity of any star observed.
 This is entirely done by studying the light which we receive from any heavenly body When so much of the light from a bright star IS can be gathered in by a large lens, or by a convex mirror, is brought to a focus, it is allowed to pass through a mar row slit and then through a prism The effect of passing the light through the prism is to spread 1t out into the we'll known color strip or Spectrum, varying in color from vio- let at one end to blue at the other The little color strip formed by light from the sun, our nearest star, Is familiar to everyone. It is not, how ever, so generally known that if light from whatever source is ad- mitted to the prism only in a mar row, flat beam (which is secured by it's passing through the narrow slit, then the color strip will usually be crossed by an immense number of lines. which may be either bright or dark, and whose relative positions tell us exactly what substance is emitting the light of which examina- ton is being made
 Thus, among a multitude of differ ent lines which appear when the light of the sun is examined in this way there are more than 4000 Hines which can only be produced by the vibrat- ing molecules of iron, We are quite certain therefore that the s hn is part ly composed of iron, though the heas of our sun is so very great that the metal is there only in the form of a gas And similarly, when these lines appear in the color strips of the star Capella Cat l., Figure D. or from the star at II Gas they do), we are quite certain that on both of these stars there is iron also.
 In this way we can analyze the stars and nebulas and tell exactly of whaat substance they are com posed. But further than is, if the star is moving away from us, all of the lines will be shifted bodily toward the red end of the color strip; if the star is moving toward us they will all be shifted toward the violet. And by measuring the amount of this shift it can be told just how man miles second the distance is changing.
 Thns, both with Capella and with the star at H. the lines are all found to be displaced toward tho red end of the spectrum, but the fo Fmer shift is somewhat greater than the latter. It can. in fact, be shown that Capella is drawing away from us will tn a speed of 15.2 miles a second, while the star at H is increasing its distahce at the rate of 19:S-miles a second Arcturus and Polaris. on the other hand. are both approaching us, the former with a speed of 4.6 miles and the latter with a speed of 12.O miles al second.
 Sometimes, when the lines of the color strip are examined in this way it is found that the star alternately and reg ularly approaches and draws away from us. The only possible explanation of this is that the bright star is actually pass ing around an orbit, either abo ut a near by dark star or about bright one which is so near to it that no telescope is powerful enough to show the two stars separately. Capella itself is this kind of double star and so also is the sec- ond brightest star of the Chsrioteer at N. Capella revolves about it's unseen and possibly dark companion in period of 104 days, while in the smaller system a revolution is completed in only two days.
 As we can measure the varying veloci- tiel in these systems in miles, per sec ond, we can find the exact sizes of the orbits in miles and also the total masses of the systems. Thus the diameter of the orbit in which the star at l. moves is about 100000 miles, while the com bined mass of the two stars which make up this system is more than five times the mass of our sun. The small star, Mu, near the center of the constellation Scorpio, moves with speed of no less than 300 miles second in an orbit about eco coo miles across, the mass of the system being no less than IS times that