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And since this is also visible in certain permanent, though peculiar objects notably in P Cygni, Beta Lyræ, and Eta Carinæ the acting cause must also evidently be permanent and inherent. The "new star of the new century" was a visual discovery. Dr. Anderson duplicated, with added éclat, his performance of nine years back.
A photograph of a Cygni, taken November, 26, 1886, shows that the H line is double, its two components having a difference in wave length of about one ten-millionth of a millimeter. A photograph of o Ceti shows that the lines G and h are bright, as are also four of the ultra-violet lines characteristic of spectra of the first type.
The great merit of having rendered photography available for the sounding of the celestial depths belongs to Professor Pritchard. The subject of his initial experiment was 61 Cygni.
Two of the stars, Betelgeux and Alpha Cygni, gave no certain sign of any perspective shifting; of the rest, Procyon, with a parallax of 0·334", proved the nearest to our system. At the mean distance concluded for these ten brilliant stars, the sun would show as of only fifth magnitude; hence it claims a very subordinate rank among the suns of space.
These successes led to renewed zeal, and now the distances of many stars are known more or less accurately. Several of the brightest stars, which might be expected to be the nearest, have not shown a parallax amounting to a twentieth of a second of arc. Among these are Canopus, alpha Orionis, alpha Cygni, beta Centauri, and gamma Cassiopeia. Oudemans has published a list of parallaxes observed.
Proper Motion. In 1718 Halley detected the proper motions of Arcturus and Sirius. In 1738 J. Cassinis showed that the former had moved five minutes of arc since Tycho Brahe fixed its position. In 1792 Piazzi noted the motion of 61 Cygni as given above.
It is a double which our three-inch will readily divide, the magnitudes being both six, distance 21", p. 122°. The distance of 61 Cygni, according to Hall's parallax of 0.27", is about 70,000,000,000,000 miles.
Further, the actual rate of proper motions, so far as regards that part of them which is projected upon the sphere, can be ascertained for stars at known distance. The annual journey, for instance, of 61 Cygni across the line of sight amounts to 1,000, and that of Alpha Centauri to 446 millions of miles.
The most inexperienced observer admires its beauty, and after an hour spent with doubtful results in trying to interest a tyro in double stars it is always with a sense of assured success that one turns the telescope to beta Cygni.
But Bessel's observations, between 1837 and 1840, of 61 Cygni, a star with the large proper motion of over 5", established its annual parallax to be 0".3483; and this was confirmed by Peters, who found the value 0".349. Later determinations for alpha2 Centauri, by Gill, make its parallax 0".75 This is the nearest known fixed star; and its light takes 4 1/3 years to reach us.
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