Lectures on Stellar Statistics

by C. V. L. Charlier

If δ is negative, for instance δ = -35°.84, we write

(αδ) = (174435),

so that the last two figures are in 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 (αδ) = (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 (αδ) = (124028), or

α = 12^h 40^m = 190°, δ = +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 α and δ of a star into l and b may be easily performed. All stars having an apparent magnitude brighter than 4^m 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.