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.