CHAPTER II.. Sources of Our Present Knowledge of the Stars.

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 principal 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 Abo, then in Haelsingfors, 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 deg..

In the focus of the objective was a semicircular piece of thin glass, 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 glass.

The observations are performed by the observer A and his assistant B. A is in a dark room, lies on a chair having the eye at the ocular and can easily look over 2 deg. in declination. The assistant 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 passing the field at a distance smaller than one degree from the middle line are observed. Hence the name "Durchmusterung". When a star passes 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 and embraces all stars between the pole and 2 deg. 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 deg..174, which signifies: Zone +23 deg., star No. 174.

ARGELANDER's work was continued for stars between [delta] = -2 deg. and [delta] = -23 deg. by SCHOeNFELD, according to much the same plan, but with a larger instrument (aperture 159 mm, focal length 1930 mm, magnifying power 26 times). The observations were made in the years 1876 to 1881 and include 133659 stars.

The positions in B. D. are given in tenths of a second in right ascension and in tenths of a minute in declination.

20. The Cape Photographic Durchmusterung (C. P. D.). This embraces the whole southern sky from -18 deg. to the south pole. Planned by GILL, the photographs were taken at the Cape Observatory with a DALLMEYER lens with 15 cm. aperture and a focal-length of 135 cm. Plates of 30 x 30 cm. give the coordinates for a surface of 5 x 5 square degrees. The photographs were taken in the years 1885 to 1890. The measurements of the plates were made by KAPTEYN in Groningen with a "parallactic" measuring-apparatus specially constructed for this purpose, which permits of the direct obtaining of the right ascension and the declination of the stars. The measurements were made in the years 1886 to 1898. The catalogue was published in three parts in the years 1896 to 1900.

The positions have the same accuracy as in B. D. The whole number of stars is 454875. KAPTEYN considers the catalogue complete to at least the magnitude 9m.2.

In the two great catalogues B. D. and C. P. D. we have all stars registered down to the magnitude 9.0 (visually) and a good way below this limit. Probably as far as to 10m.

A third great Durchmusterung has for some time been in preparation at Cordoba in Argentina. It continues the southern zones of SCHOeNFELD and is for the present completed up to 62 deg. southern declination.

All these Durchmusterungs are ultimately based on star catalogues of smaller extent and of great precision. Of these catalogues we shall not here speak (Compare, however, Sec.23).

A great "Durchmusterung", that will include all stars to the 11th magnitude, has for the last thirty years been in progress at different observatories proposed by the congress in Paris, 1888. The observations proceed very irregularly, and there is little prospect of getting the work finished in an appreciable time.

21. Star charts. For the present we possess two great photographic star charts, embracing the whole heaven:--The Harvard Map (H. M.) and the FRANKLIN-ADAMS Charts (F. A. C.).

The Harvard Map, of which a copy (or more correctly two copies) on glass has kindly been placed at the disposal of the Lund Observatory by Mr. PICKERING, embraces all stars down to the 11th magnitude. It consists of 55 plates, each embracing more than 900 square degrees of the sky. The photographs were taken with a small lens of only 2.5 cms. aperture and about 32.5 cms. focal-length. The time of exposure was one hour. These plates have been counted at the Lund Observatory by Hans HENIE. We return later to these counts.

The FRANKLIN-ADAMS Charts were made by an amateur astronomer FRANKLIN-ADAMS, partly at his own observatory (Mervel Hill) in England, partly in Cape and Johannesburg, Transvaal, in the years 1905-1912. The photographs were taken with a Taylor lens with 25 cm. aperture and a focal-length of 114 cm., which gives rather good images on a field of 15 x 15 square degrees.

The whole sky is here reproduced on, in all, 206 plates. Each plate was exposed for 2 hours and 20 minutes and gives images of the stars down to the 17th magnitude. The original plates are now at the observatory in Greenwich. Some copies on paper have been made, of which the Lund Observatory possesses one. It shows stars down to the 14th-15th magnitudes and gives a splendid survey of the whole sky more complete, indeed, than can be obtained, even for the north sky, by direct observation of the heavens with any telescope at present accessible in Sweden.

The F. A. C. have been counted by the astronomers of the Lund Observatory, so that thus a complete count of the number of stars for the whole heaven down to the 14th magnitude has been obtained. We shall later have an opportunity of discussing the results of these counts.

A great star map is planned in connection with the Paris catalogue mentioned in the preceding paragraph. This Carte du Ciel (C. d. C.) is still unfinished, but there seems to be a possibility that we shall one day see this work carried to completion. It will embrace stars down to the 14th magnitude and thus does not reach so far as the F. A. C., but on the other hand is carried out on a considerably greater scale and gives better images than F. A. C. and will therefore be of a great value in the future, especially for the study of the proper motions of the stars.

22. Distance of the stars. As the determination, from the annual parallax, of the distances of the stars is very precarious if the distance exceeds 5 sir. ( = 0".04), it is only natural that the catalogues of star-distances should be but few in number. The most complete catalogues are those of BIGOURDAN in the Bulletin astronomique XXVI (1909), of KAPTEYN and WEERSMA in the publications of Groningen Nr. 24 (1910), embracing 365 stars, and of WALKEY in the "Journal of the British Astronomical Association XXVII" (1917), embracing 625 stars. Through the spectroscopic method of ADAMS it will be possible to enlarge this number considerably, so that the distance of all stars, for which the spectrum is well known, may be determined with fair accuracy. ADAMS has up to now published 1646 parallax stars.

23. Proper motions. An excellent catalogue of the proper motions of the stars is LEWIS BOSS's "Preliminary General Catalogue of 6188 stars" (1910) (B. P. C.). It contains the proper motions of all stars down to the sixth magnitude (with few exceptions) and moreover some fainter stars. The catalogue is considered by the editor only as a preliminary to a greater catalogue, which is to embrace some 25000 stars and is now nearly completed.

24. Visual magnitudes. The Harvard observatory has, under the direction of PICKERING, made its principal aim to study the magnitudes of the stars, and the history of this observatory is at the same time the history of the treatment of this problem. PICKERING, in the genuine American manner, is not satisfied with the three thirds of the sky visible from the Harvard observatory, but has also founded a daughter observatory in South America, at Arequipa in Peru. It is therefore possible for him to publish catalogues embracing the whole heaven from pole to pole. The last complete catalogue (1908) of the magnitudes of the stars is found in the "Annals of the Harvard Observatory T. 50" (H. 50). It contains 9110 stars and can be considered as complete to the magnitude 6m.5. To this catalogue are generally referred the magnitudes which have been adopted at the Observatory of Lund, and which are treated in these lectures.

A very important, and in one respect even still more comprehensive, catalogue of visual magnitudes is the "Potsdam General Catalogue" (P. G. C.) by MUeLLER and KEMPF, which was published simultaneously with H. 50. It contains the magnitude of 14199 stars and embraces all stars on the northern hemisphere brighter than 7m.5 (according to B. D.). We have already seen that the zero-point of H. 50 and P. G. C. is somewhat different and that the magnitudes in P. G. C. must be increased by -0m.16 if they are to be reduced to the Harvard scale. The difference between the two catalogues however is due to some extent to the colour of the stars, as has been shown by Messrs. MUeLLER and KEMPF.

25. Photographic magnitudes. Our knowledge of this subject is still rather incomplete. The most comprehensive catalogue is the "Actinometrie" by SCHWARZSCHILD (1912), containing the photographic magnitudes of all stars in B. D. down to the magnitude 7m.5 between the equator and a declination of +20 deg.. In all, 3522 stars. The photographic magnitudes are however not reduced for the zero-point (compare Sec.6).

These is also a photometric photographic catalogue of the stars nearest to the pole in PARKHURST's "Yerkes actinometry" (1912), which contains all stars in B. D. brighter than 7m.5 between the pole and 73 deg. northern declination. The total number of stars is 672.

During the last few years the astronomers of Harvard and Mount Wilson have produced a collection of "standard photographic magnitudes" for faint stars. These stars, which are called the polar sequence, all lie in the immediate neighbourhood of the pole. The list is extended down to the 20th magnitude. Moreover similar standard photographic magnitudes are given in H. A. 71, 85 and 101.

A discussion of the colour-index (i.e., the difference between the photographic and the visual magnitudes) will be found in L. M. II, 19. When the visual magnitude and the type of spectrum are known, the photographic magnitude may be obtained, with a generally sufficient accuracy, by adding the colour-index according to the table 1 in Sec.15 above.

26. Stellar spectra. Here too we find the Harvard Observatory to be the leading one. The same volume of the Annals of the Harvard Observatory (H. 50) that contains the most complete catalogue of visual magnitudes, also gives the spectral types for all the stars there included, i.e., for all stars to 6m.5. Miss CANNON, at the Harvard Observatory, deserves the principal credit for this great work. Not content with this result she is now publishing a still greater work embracing more than 200000 stars. The first four volumes of this work are now published and contain the first twelve hours of right ascension, so that half the work is now printed.

27. Radial velocity. In this matter, again, we find America to be the leading nation, though, this time, it is not the Harvard or the Mount Wilson but the Lick Observatory to which we have to give the honour. The eminent director of this observatory, W. W. CAMPBELL, has in a high degree developed the accuracy in the determination of radial velocities and has moreover carried out such determinations in a large scale. The "Bulletin" No. 229 (1913) of the Lick Observatory contains the radial velocity of 915 stars. At the observatory of Lund, where as far as possible card catalogues of the attributes of the stars are collected, GYLLENBERG has made a catalogue of this kind for the radial velocities. The total number of stars in this catalogue now amounts to 1640.

28. Finally I shall briefly mention some comprehensive works on more special questions regarding the stellar system.

On variable stars there is published every year by HARTWIG in the "Vierteljahrschrift der astronomischen Gesellschaft" a catalogue of all known variable stars with needful information about their minima &c. This is the completest and most reliable of such catalogues, and is always up to date. A complete historical catalogue of the variables is given in "Geschichte und Literatur des Lichtwechsels der bis Ende 1915 als sicher veraenderlich anerkannten Sterne nebst einem Katalog der Elemente ihres Lichtwechsels" von G. MUeLLER und E. HARTWIG. Leipzig 1918, 1920.

On nebulae we have the excellent catalogues of DREYER, the "New General Catalogue" (N. G. C.) of 1890 in the "Memoirs of the Astronomical Society" vol. 49, the "Index catalogues" (I. C.) in the same memoirs, vols. 51 and 59 (1895 and 1908). These catalogues contain all together 13226 objects.

Regarding other special attributes I refer in the first place to the important Annals of the Harvard Observatory. Other references will be given in the following, as need arises.

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