Lectures on Stellar Statistics | Page 2

braces to represent the italics.]
This notation has been introduced by PICKERING for variable stars and is used by him everywhere in the Annals of the Harvard Observatory, but it is also well suited to all stars. This notation gives, simultaneously, the characteristic numero of the stars. It is true that two or more stars may in this manner obtain the same characteristic numero. They are, however, easily distinguishable from each other through other attributes.
The galactic coordinates l and b are referred to the Milky Way (the Galaxy) as plane of reference. The pole of the Milky Way has according to HOUZEAU and GOULD the position ([alpha][delta]) = (124527). From the distribution of the stars of the spectral type B I have in L. M. II, 14[2] found a somewhat different position. But having ascertained later that the real position of the galactic plane requires a greater number of stars for an accurate determination of its value, I have preferred to employ the position used by PICKERING in the Harvard catalogues, namely ([alpha][delta]) = (124028), or
[alpha] = 12h 40m = 190°, [delta] = +28°,
which position is now exclusively used in the stellar statistical investigations at the Observatory of Lund and is also used in these lectures.
The galactic longitude (l) is reckoned from the ascending node of the Milky Way on the equator, which is situated in the constellation Aquila. The galactic latitude (b) gives the angular distance of the star from the Galaxy. On plate I, at the end of these lectures, will be found a fairly detailed diagram from which the conversion of [alpha] and [delta] of a star into l and b may be easily performed. All stars having an apparent magnitude brighter than 4m are directly drawn.
Instead of giving the galactic longitude and latitude of a star we may content ourselves with giving the galactic square in which the star is situated. For this purpose we assume the sky to be divided into 48 squares, all having the same surface. Two of these squares lie at the northern pole of the Galaxy and are designated GA1 and GA2. Twelve lie north of the galactic plane, between 0° and 30° galactic latitude, and are designated GC1, GC2, ..., GC12. The corresponding squares south of the galactic equator (the plane of the Galaxy) are called GD1, GD2, ..., GD12. The two polar squares at the south pole are called GF1 and GF2. Finally we have 10 B-squares, between the A- and C-squares and 10 corresponding E-squares in the southern hemisphere.
The distribution of the squares in the heavens is here graphically represented in the projection of FLAMSTEED, which has the advantage of giving areas proportional to the corresponding spherical areas, an arrangement necessary, or at least highly desirable, for all stellar statistical researches. It has also the advantage of affording a continuous representation of the whole sky.
The correspondence between squares and stellar constellations is seen from plate II. Arranging the constellations according to their galactic longitude we find north of the galactic equator (in the C-squares) the constellations:--
Hercules, Cygnus, Cepheus, Cassiop?a, Auriga, Gemini, Canis Minor, Pyxis, Vela, Centaurus, Scorpius, Ophiuchus,
and south of this equator (in the D-squares):--
Aquila, Cygnus, Lacerta, Andromeda, Perseus, Orion, Canis Major, Puppis, Carina, Circinus, Corona australis, Sagittarius,
mentioning only one constellation for each square.
At the north galactic pole (in the two A-squares) we have:--
Canes Venatici and Coma Berenices,
and at the south galactic pole (in the two F-squares):--
Cetus and Sculptor.
3. Changes in the position of a star. From the positions of a star on two or more occasions we obtain its apparent motion, also called the proper motion of the star. We may distinguish between a secular part of this motion and a periodical part. In both cases the motion may be either a reflex of the motion of the observer, and is then called parallactic motion, or it may be caused by a real motion of the star. From the parallactic motion of the star it is possible to deduce its distance from the sun, or its parallax. The periodic parallactic proper motion is caused by the motion of the earth around the sun, and gives the annual parallax ([pi]). In order to obtain available annual parallaxes of a star it is usually necessary for the star to be nearer to us than 5 siriometers, corresponding to a parallax greater than 0".04. More seldom we may in this manner obtain trustworthy values for a distance amounting to 10 siriometers ([pi] = 0".02), or even still greater values. For such large distances the secular parallax, which is caused by the progressive motion of the sun in space, may give better results, especially if the mean distance of a group of stars is simultaneously determined. Such a value of the secular parallax is also called, by KAPTEYN, the systematic parallax of