Septentriones. Moreover, as the stars belonging to this particular type are certainly in many cases, and probably in all, very large orbs (referring here to real magnitude, not to apparent brilliancy), the inference seemed fairly deducible that the drifting five stars are not nearer than Alpha, and therefore (since we have seen that it is unlikely that all the Septentriones lie at nearly the same distance) the inference would be that the drifting stars lie much farther away than the rest.
It remained, however, that the crucial test of motion-measurement should be applied.
In the middle of May last I received a letter from Dr. Huggins announcing that the five are all receding from the earth. In all, the hydrogen line called F is "strong and broad." In the spectrum of Alpha the line F is "not very strong" (so faint, indeed, Dr. Huggins afterward informed me, that he preferred to determine the star's motion by one of the lines due to magnesium in the star's atmosphere). He found that Alpha is approaching. As to Eta, Dr. Huggins remarked that the line at F is "not so strong or so broad" as in the spectrum of "the five." He was uncertain as to the direction of motion, and mentioned that "the star was to be observed again." He subsequently found that this star is receding. But, whereas all the five are receding at the enormous rate of thirty miles per second, Eta's recession was so much smaller that, as we have seen, Dr. Huggins was unable to satisfy himself at a single observation that the star was receding at alL
It will be seen that my anticipations were more than fulfilled. The community of recessional motion was accompanied by evidence which might very well have been wanting—viz., by the discovery that neither Eta nor Alpha shared in the motion. Moreover, the physical association between the five stars was yet further evidenced by the close resemblance found to exist between the spectra of the five stars. Dr. Huggins remarked in his letter: "My expectation had nothing to do with the above results. At the moment, I thought Alpha was included in the group, and was therefore a little disappointed when I found Beta going the opposite way."
We have at length, then, evidence, which admits of no question—so obviously conclusive is it—to show not only that star-drift is a reality, but that subordinate systems exist within the sidereal system. We moreover recognize an unquestionable instance of a characteristic peculiarity of structure in a certain part of the heavens. For, though star-drift exists elsewhere, yet every instance of star-drift is quite distinct in character—the drift in Cancer unlike that in Ursa, and both these drifts unlike the drifts in Taurus, and equally unlike the drift in Aries or Leo. Much more, indeed, is contained in the fact now placed
- Sirius demonstrably gives out much more light than our sun, and according to the best determinations of his distance he must (if his surface is of equal intrinsic lustre) be from 2,000 to 8,000 times larger than the sun. Vega, Altair, and Rigel, are also certainly larger and may be very much larger than our sun.