One of the most remarkable astronomical discoveries in recent years was made by Kapteyn, who thereby as well as by other achievements has gained perhaps the highest rank among astronomers of today. He has shown that the stars rushing forth in the neighbourhood of the Sun belong to two great groups, one coming from the constellation Orion, and the other at nearly a right angle (100°) from the constellation Scorpio. In the former, we find nearly all the helium stars hitherto studied. We have previously seen that these stars stand almost still with reference to the Galaxy while the irregular nebulæ possess no motion at all relative to the same reference point--and the Galaxy is the natural datum-line for all astronomical measurements--so that the motion of the first-mentioned star-group toward the Sun is principally due to the Sun’s own motion. This group, according to Kapteyn, obeys the law of relative star velocities even better than the combined world of all stars; thus with reference to the Sun, the helium stars are the slowest, the yellow stars the fastest, while the hydrogen stars occupy a middle position, all a self-evident consequence of their own velocity with reference to the Galaxy which increases from helium stars to yellow stars.
Kapteyn has shown another regularity in this group which is easily explained. We have previously mentioned that the yellow stars are most, the helium stars least removed from their place of birth in the Milky Way. The result is that the yellow stars appear (on the average) to come from a point farther from the Galaxy than the apparent origin of the hydrogen stars and more remote yet than that of the helium stars. On account of the relatively high velocity of the yellow stars in different directions, their stream appears to be more divergent than the stream of hydrogen stars, and helium stars move in almost parallel paths (nearly directly opposed to the Sun’s true motion with reference to the Galaxy).
Similar regularities have been found by Kapteyn in the second star-drift which would lead us to think, as indeed Kapteyn assumes, that these stars also developed from an original nebulous mist, which arrived in our neighbourhood from the unknown distant, but is now used up in the formation of corresponding stars. Here again the yellow stars should have departed farther from their matrix than the white hydrogen stars. Helium stars are very rare in this drift, so that no reliable statistics have as yet been made for them.