367. _Motion of Stars along the Line of Sight._--A motion of a star in the direction of the line of sight would produce no displacement of the star that could be detected with the telescope; but it would cause a change in the brightness of the star, which would become gradually fainter if moving from us, and brighter if approaching us. Motion along the line of sight has, however, been detected by the use of the tele-spectroscope (152), owing to the fact that it causes a displacement of the spectral lines. As has already been explained (169), a displacement of a spectral line towards the red end of the spectrum indicates a motion away from us, and a displacement towards the violet end, a motion towards us. * * * * * By means of these displacements of the spectral lines, Huggins has detected motion in the case of a large number of stars, and calculated its rate:-- STARS RECEDING FROM US. Sirius 20 miles per second. Betelgeuse 22 miles per second. Rigel 15 miles per second. Castor 25 miles per second. Regulus 15 miles per second. STARS APPROACHING US. Arcturus 55 miles per second. Vega 50 miles per second. Deneb 39 miles per second. Pollux 49 miles per second. Alpha Ursæ Majoris 46 miles per second. These results are confirmed by the fact that the amount of motion indicated is about what we should expect the stars to have, from their observed proper motions, combined with their probable distances. Again: the stars in the neighborhood of Hercules are mostly found to be approaching the earth, and those which lie in the opposite direction to be receding from it; which is exactly the effect which would result from the sun's motion through space. The five stars in the Dipper, which have a common proper motion, are also found to have a common motion in the line of sight. But the displacement of the spectral lines is so slight, and its measurement so difficult, that the velocities in the above table are to be accepted as only an approximation to the true values. Chemical and Physical Constitution of the Stars. 368. _The Constitution of the Stars Similar to that of the Sun._--The stellar spectra bear a general resemblance to that of the sun, with characteristic differences. These spectra all show Fraunhofer's lines, which indicate that their luminous surfaces are surrounded by atmospheres containing absorbent vapors, as in the case of the sun. The positions of these lines indicate that the stellar atmospheres contain elements which are also found in the sun's, and on the earth. [Illustration: Fig. 428.]