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 Åbo,
then in Hälsingfors, 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 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° 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[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.
ARGELANDER's work was continued for stars between [delta] = -2° and
[delta] = -23° by SCHÖNFELD, 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.[8]
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_[9] (C. P. D.). This embraces
the whole southern sky from -18° 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 × 30 cm.
give the coordinates for a surface of 5 × 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.[10] It continues the southern zones of SCHÖNFELD
and is for the present completed up to 62° 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, §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
× 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. ([pi] = 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 MÜLLER 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. MÜLLER 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 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.[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 veränderlich anerkannten Sterne nebst einem Katalog der
Elemente ihres Lichtwechsels" von G. MÜLLER 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 Fünfter 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.]