Part 4 (1/2)

C.) by MuLLER 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. MuLLER 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. In all, 3522 stars. The photographic magnitudes are however not reduced for the zero-point (compare --6).

These is also a photometric photographic catalogue of the stars nearest to the pole in PARKHURST's ”Yerkes actinometry” (1912),[11] which contains all stars in B. D. brighter than 7m.5 between the pole and 73 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_,[12] 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 --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 princ.i.p.al credit for this great work. Not content with this result she is now publis.h.i.+ng 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.[13]

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.[14]

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 veranderlich anerkannten Sterne nebst einem Katalog der Elemente ihres Lichtwechsels” von G. MuLLER 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.

FOOTNOTES:

[Footnote 7: ”Bonner Sternverzeichnis” in den Astronomischen Beobachtungen auf der Sternwarte zu Bonn, Dritter bis Funfter Band. Bonn 1859-62.]

[Footnote 8: ”Bonner Durchmusterung”, Vierte Sektion. Achter Band der Astronomischen Beobachtungen zu Bonn, 1886.]

[Footnote 9: ”The Cape Photographic Durchmusterung” by DAVID GILL and J.

C. KAPTEYN, Annals of the Cape Observatory, vol. III-V (1896-1900).]

[Footnote 10: ”Cordoba Durchmusterung” by J. THOME. Results of the National Argentine Observatory, vol. 16, 17, 18, 21 (1894-1914).]

[Footnote 11: Aph. J., vol. 36.]

[Footnote 12: H. A., vol. 71.]

[Footnote 13: H. A., vol. 91, 92, 93, 94.]

[Footnote 14: A catalogue of radial velocities has this year been published by J. VOUTE, embracing 2071 stars. ”First catalogue of radial velocities”, by J. VOUTE. Weltevreden, 1920.]