Part 3 (1/2)

III. _The intensity of the light_ of the stars is expressed in magnitudes. We may distinguish between the _apparent_ magnitude (_m_) and the _absolute_ magnitude (_M_), the latter being equal to the value of the apparent magnitude supposing the star to be situated at a distance of one siriometer.

The apparent magnitude may be either the _photographic_ magnitude (_m'_), obtained from a photographic plate, or the _visual_ magnitude (_m_) obtained with the eye.

The difference between these magnitudes is called the _colour-index_ (_c_ = _m'_-_m_).

IV. _The characteristics of the stellar radiation_ are the mean wave-length (?_0) and the dispersion (s) in the wave-length. _The mean wave-length_ may be either directly determined (perhaps as _effective_ wave-length) or found from the spectral type (spectral index) or from the colour-index.

There are in all eight attributes of the stars which may be found from the observations:--the spherical position of the star (_l_, _b_), its distance (_r_), proper motion (_u_0_ and _v_0_), radial velocity (_W_), apparent magnitude (_m_ or _m'_), absolute magnitude (_M_), spectral type (_Sp_) or spectral index (_s_), and colour-index (_c_). Of these the colour-index, the spectral type, the absolute magnitude and also (to a certain degree) the radial velocity may be considered as independent of the place of the observer and may therefore be considered not as only apparent but also as _absolute_ attributes of the stars.

Between three of these attributes (_m_, _M_ and _r_) a mathematical relation exists so that one of them is known as soon as the other two have been found from observations.

FOOTNOTES:

[Footnote 1: Meddelanden frn Lunds Observatorium, No. 41.]

[Footnote 2: Meddelanden frn Lunds Astronomiska Observatorium, Serie II, No. 14.]

[Footnote 3: Annals of the Harvard Observatory, vol. 50.]

[Footnote 4: In order to deduce from _M_ the apparent magnitude at a distance corresponding to a parallax of 1? we may subtract 3m.48. To obtain the magnitude corresponding to a parallax of 0?.1 we may add 1.57. The latter distance is chosen by some writers on stellar statistics.]

[Footnote 5: The best colour-scale of the latter sort seems to be that of OSTHOFF.]

[Footnote 6: Compare H. A. 50 and H. A. 56 and the remarks in L. M. II, 19.]

CHAPTER II.

SOURCES OF OUR PRESENT KNOWLEDGE OF THE STARS.

18. In this chapter I shall give a short account of the publications in which the most complete information on the attributes of the stars may be obtained, with short notices of the contents and genesis of these publications. It is, however, not my intention to give a history of these researches. We shall consider more particularly the questions relating to the position of the stars, their motion, magnitude, and spectra.

19. _Place of the stars._ _Durchmusterungs._ The most complete data on the position of the stars are obtained from the star catalogues known as ”Durchmusterungs”. There are two such catalogues, which together cover the whole sky, one--visual--performed in Bonn and called the _Bonner Durchmusterung_ (B. D.), the other--photographic--performed in Cape _The Cape Photographic Durchmusterung_ (C. P. D.). As the first of these catalogues has long been--and is to some extent even now--our princ.i.p.al source for the study of the sky and is moreover the first enterprise of this kind, I shall give a somewhat detailed account of its origin and contents, as related by ARGELANDER in the introduction to the B. D.

B. D. was planned and performed by the Swedish-Finn ARGELANDER (born in Memel 1799). A scholar of BESSEL he was first called as director in bo, then in Halsingfors, and from there went in 1836 to Bonn, where in the years 1852 to 1856 he performed this great _Durchmusterung_. As instrument he used a FRAUENHOFER comet-seeker with an aperture of 76 mm, a focal length of 650 mm, and 10 times magnifying power. The field of sight had an extension of 6.

In the focus of the objective was a semicircular piece of thin gla.s.s, with the edge (= the diameter of the semicircle) parallel to the circle of declination. This edge was sharply ground, so that it formed a narrow dark line perceptible at star illumination. Perpendicular to this diameter (the ”hour-line”) were 10 lines, at each side of a middle line, drawn at a distance of 7'. These lines were drawn with black oil colour on the gla.s.s.

The observations are performed by the observer A and his a.s.sistant B. A is in a dark room, lies on a chair having the eye at the ocular and can easily look over 2 in declination. The a.s.sistant sits in the room below, separated by a board floor, at the _Thiede_ clock.

From the beginning of the observations the declination circle is fixed at a certain declination (whole degrees). All stars pa.s.sing the field at a distance smaller than one degree from the middle line are observed.

Hence the name ”Durchmusterung”. When a star pa.s.ses the ”hour line” the magnitude is called out by A, and noted by B together with the time of the clock. Simultaneously the declination is noted by A in the darkness.

On some occasions 30 stars may be observed in a minute.

The first observation was made on Febr. 25, 1852, the last on March 27, 1859. In all there were 625 observation nights with 1841 ”zones”. The total number of stars was 324198.

The catalogue was published by ARGELANDER in three parts in the years 1859, 1861 and 1862[7] and embraces all stars between the pole and 2 south of the equator brighter than 9m.5, according to the scale of ARGELANDER (his aim was to register all stars up to the 9th magnitude).

To this scale we return later. The catalogue is arranged in accordance with the declination-degrees, and for each degree according to the right ascension. Quotations from B. D. have the form B. D. 23.174, which signifies: Zone +23, star No. 174.