749
ASTRONOMY Parallaxes of Stars—continued. Star. jS Comee a Virginis /3 Centauri a Bootis a Centauri LI. 27298 a Scorpii 7j Hereulis 7T Herculis 5 Herculis v Draconis 0. A. 17415 70 OpMuchi 5 Urs. min. a Lyrse 0. A. 18609 31 Aquilae <r Draconis a Aquilse a Cygni 61 Cygni a Cephei e Indi a Gruis . a Piscis Austr Lac. 9352 Br. 3077 85 Pegasi
R. A. h. m. 13 7 13 20 13 57 14 11 14 33 14 52 16 23 16 39 17 11 17 11 17 30 17 37 18 0 18 5 18 33 18 42 19 20 19 33 19 46 20 38 21 2 21 16 21 56 22 2 22 52 22 59 23 8 23 57
Position for 1900. Decl. + 28-4 -10-6 + 59-9 + 19-7 -60-4 + 54-1 -26-2 + 39-1 + 36-9 + 24-9 + 55-3 + 68-4 + 2-5 + 86 6 + 387 + 59-5 + 11-7 + 69-5 + 8-6 + 44-9 + 38-2 + 62-1 -57-2 - 47‘4 -30-1 -36-4 + 56-6 + 26-6
•11 •00 g •03 g •03 •75 g •08 •02 g •40 •11 •05 •32 •22 •19 •03 : •11 •35 : •06 •26 : •23 •00 •39 •06: •27 g •02 g •13 g •28 g •15 •05
A desideratum of astronomy is a general survey of the heavens with a view of determining all the stars which have an appreciable parallax. Such a survey is Parallactic now ma(je possible by photography. To carry survey. p -g necessary to take two or three plates of each region of the heavens at those opposite seasons when the stars in the region have the maximum displacement by parallax in opposite directions. A comparison of the plates then shows whether any such displacement can be detected. The only attempt in this direction which has thus far been carried out was made by Professor Conner at Helsingfors, with the co-operation of Kapteyn, the photographic telescope being that used for the international chart of the heavens. Each plate taken for parallax was exposed three times, at intervals of six months. At the first exposure three images were made on the plate in slightly different positions. At the second, six months later, six images were made in the same way. At the third, a year after the first, three more were made. Thus, twelve impressions of each star were taken, forming together a rectangle of three stars on one side and four on the other. On such a plate the detection of a parallactic displacement is very easy. Unfortunately, only a region little more than 2° square, extending from 20 h. 1 m. to 20 h. 10 m. in right ascension, and from 35° to about 37° in declination, has yet been examined. The plates were carefully measured by Kapteyn. In three or four cases there is some presumption of a parallax of 0T", but in no case is it so strongly marked as to be beyond doubt. „ The probable error of a result from each plate is 0‘034 • but it is quite in accord with the theory of probabilities that errors of three or four times this amount should occur in one or more of the results. The extremely limited number of stars having a certainly measurable parallax renders it desirable to find other methods of estimating their distances. It was formerly supposed that the actual magnitude of the stars did not differ greatly from each other, and, if that were the case, an approximate estimate of the distance could be obtained from the apparent brightness. But it is now known that this is so far from the case that brightness alone scarcely affords any clue to the distance of a star. All that we can say with certainty
is that, in the general average, the fainter stars are more distant than the brighter ones in a ratio which admits of approximate determination, a statement which tells us nothing about the actual distance of any single star. More information is afforded by the proper motions of which we shall next speak. The work of cataloguing the stars and determining their exact positions, which is now being pursued on so large a scale, naturally involves the determinations of their proper motions. Every star may Motions be considered as having a proper motion of its own; but the number for which the proper motion has been determined is comparatively small, because, apart from the results of spectroscopic measurements of motion in the line of sight, which are necessarily extremely limited, the motion can be detected only by a comparison of observations taken at long intervals of time. The longer the interval the slower the motion that may be detected. We have now an interval of more than a century in the case of the stars observed by Bradley at Greenwich, about the year 1750-63, and one of the greater part of a century in the case of several other catalogues. From Auwers’s discussion of the Bradley stars, 3240 in number, it appears that a little more than one-half of these objects have a proper motion of 5" per century or more. A little more than one-half of Bradley’s stars are easily visible to the naked eye, and the greater part of the remaining half are between the 6th and 7th magnitude. Consequently, the conclusions derived from this work apply only to the brighter stars. Speaking in a general way, we may say that about one-half of these stars are known to have a measurable proper motion. In the case of the fainter stars the proportion would undoubtedly be much smaller than this, but it cannot be exactly stated at present. A few stars have a proper motion so abnormally rapid that it could be detected within a single year. Of these the most rapid known up to a recent time was that of Groombridge 1830, pointed out by Argelander. In 1897 Gill and Innes, at the Cape of Good Hope, found a star of yet greater proper motion. This case was carefully investigated by Kapteyn, who, by a comparison of the Cape photographs and Cordoba observations, found the motion to be the greatest yet known. The following is a list of the stars now known to have a proper motion, exceeding 3" per year :— Name. C. Z. Vh., 243 Gr. 1830 . Lac. 9352 Cor. 32416 61 Cygni . LI. 21185 e Indi LI. 21258 o2 Eridani /u, Cassiop. O. A. 15318 O. A. 15320 a Centauri Lac. 8760 e Erid ani. O. Arg. 11677
R. A. 1900.
Dec. 1900.
Prop. Motion.
Mag.
h. 5 11 22 0 21 10 21 11 4 1 15 15 14 21 3 11
-45-0 + 38-4 -36-4 -37-8 + 38-3 + 36-6 -57-2 + 44-0 - 7'8 + 54-4 -16 0 -15-9 -60-4 -39-2 -43-4 + 66-4
8-70 7-04 6-94 6-07 5-20 476 4-61 4-41 4-05 3-73 3-68 3-68 3-60 3 53 3-12 3-02
8'5 678'5 5-7 7'3 5-2 84599-1 1-0 7-3 4-4 9-0
m. 8 47 59 0 2 58 56 0
33 11 16 15
This table presents two notable features. One is the minuteness of many of the stars. That the star having the most rapid motion yet known should be below the 8th magnitude is illustrative of the great range of absolute brightness among the stars. The other feature is that two fourths of the circle of right ascension, namely, those from 5 h. to 11 h. and from 15 h. to 21 h., scarcely
