Sitzungsberichte der Heidelberger Akademie der Wissenschaften Mathematisch-naturwissenschaftliche Klasse Die Jahrgänge bis1921 einschließlich erschienen im Verlag von Carl Winter, Universitäts fnt.chhandlung in Heidelberg, die Jahrgänge 1922-1933 im Verlag Walter de Gruyt.er & Co. in Berlin, die Jahrgänge 1934-1944 bei der Weiß'schen Universitätsbuchhandlung in Heidelberg. 1946, 1946 und 1947 Bind keine Sitzungsberichte erschienen. Jahrgang 1941. 1. Beiträge zur Petrographie des Odenwaldes. I. 0. H. ERDMANNSDÖRFFER. Schollen und Mischgesteine im Schriesheimer Granit. DM 1.-. 2. M. STECK. Unbekannte Briefe Frege's über die Grundlagen der Geometrie und Ant wortbrief Hilbert's an Frege. DM 1.--. 3. Studien im Gneisgebirge des Schwarzwaldes. XII. W. KLEBER. tlber das Amphi bolitvorkommen vom Bannstein bei Raslach im Kinzigtal. DM 1.60. 4. W. SoERGEL. Der Klimacharakter der als nordisch geltenden Säugetiere des Eis zeitalters. DM 1.40. Jahrgang 1942. 1. E. GoTSOHLIOH. Hygiene in der modernen Türkei. DM 0.60. 2. Studien im Gneisgebirge des Schwarzwaldes. XIII. 0. H. ERDMANNSDÖRFFER. tlber Granitstrukturen. DM 1.60. 3. J. D. AOHELIS. Die Überwindung der Alchemie in der paracelsischen Medizin. DM1.40. 4. A. BENNINGHOFF. Die biologische Feldtheorie. DM 1.-. Jahrgang 1943. I. A. BECKER. Zur Bewertung inkonstanter cr.-Strahlenquellen. DM 1.-. 2. W. BLASOHKE. Nicht-Euklidische Mechanik. DM 0.80. Jahrgang 1944. 1. C. 0Emt:E. tlber Altern und Tod. DM 1.-. 1945,1946 und 1947 sind keine Sitzungsberichte erschienen. Ab Jahrgang 1948 erscheinen die "Sitzungsberichte" im Springer-V erlag. Inhalt des Jahrgangs 1948: 1. P. CmusTJAN und R. BAAs. tlber ein Farbenphä.nomen. DM 1.50. 2. W. BLASOHKE. Zur Bewegungsgeometrie auf der Kugel. DM 1.-. 3. P. UHLENHUTH. Entwicklung und Ergebnisse der Chemotherapie. DM 2.-. 4. P. CmusTIAN. Die Willkürbewegung im Umgang mit beweglichen Mechanismen. DM1.50. 5. W. BoTHE. Der Streufehler bei der Ausmessung von Nebelkammerbahnen im Magnetfeld. DM 1.-. 6. W. TRoLL. Urbild und Ursache in der Biologie. DM 1.50. 7. H. WENDT. Die JANSEN-RAYLEIGHBche Näherung zur Berechnung von Unterschall strömungen. DM 2.40. 8. K. H. SCHUBERT. tlber die Entwicklung zulässiger Funktionen nach den Eigen funktionen bei definiten, seihstadjungierten Eigenwertaufgaben. DM 1.80. 9. W. ScHAAFF. Biegung mit Erhaltung konjugierter Systeme. DM 1.80. JO. A. SEYBOLD und H. MEHNER. tlber den Gehalt von Vitamin C in Pflanzen. DM 9.60. Sitzungsherich te der Heidelherger Akademie der Wissenschaften Mathematisch-naturwissenschaftliche Klasse Jahrgang 1960, 2. Abhandlung Problems of Astronomical Spectroscopy By P. Swings Universite de Liege, Institut d'Astrophysique, Cointe-Sclessin (Belgique) (Vorgelegt in der Sitzung vom 11. Juni 1960) 1960 Springer-Verlag Berlin Heidelberg GmbH Alle Rechte, insbesondere das der Ubersetzung in fremde Sprachen, vorbehalten Ohne ausdrückliche Genehmigung des Verlages ist es auch nieht ge stattet, diese Abhandlung oder Teile daraus auf photomechanischem Wege (Photokopie, Mikrokopie) zu vervielfältigen © by Springer-Verlag Berlin Heidelberg 1960 Ursprünglich erschienen bei Springer-Verlag OHG, Berlin • Göttingen • Heidelberg 1960 ISBN 978-3-662-23150-0 ISBN 978-3-662-25136-2 (eBook) DOI 10.1007/978-3-662-25136-2 Die Wiedergabe von Gebrauchsnamen, Hnndelsnamen, Warenbezeich nungen usw. in dieser Abhandlung berechtigt auch ohne besondere Kennzeichnung nicht zu der Annahme, daß solche Namen im Sinne der Warenzeichen-und Markenschutz-Gesetzgebung als frei zu betrachten wären und daher von jedermann benutzt werden dürften. Problems of Astronomical Spectroscopy By P. Swings The teamwork of BuNSEN, the chemist, and KIRCHHOFF, the physicist, one hundred years ago, represents an epoch in the development of astronomy. To be sure, FRAUNHOFER, in 1824, had already noticed that the spectrum of Sirins differed from that of the Sun, and thus established a basis for stellar spectroscopy. Y et it is the developmeht of the spectroscopic method of chemical analysis by KIRCHHOFFand BuNSEN which opened up the possibility of a determination of the chemical composition of the atmospheres of the sun and stars, as weil as of terrestrial substances. KIRCH HOFF's spectroscopic evidence for the presence of sodium in the sun represents a landmark in astronomy. Foraperiod of 80 or 90 years after KIRCHHOFF's and BUNSEN's spectacular work spectroscopy remained a glamorous part of physics and astrophysics, a status which it has partly lost in recent years in favor of nuclear science and other new developments. The spectroscopic discovery in Heidelberg, of cesium in 1860 and of rubidium in 1861, was followed by that of thallium by WILLIAM CROOKES in 1861, of indium by R. REICH and T. RICHTER in 1863, and of gallium, samarium and dysprosium by LECOCQ DE BOIS BAUDRAN a little later. The first new chemical element discovered in a celestial body is helium, which P. J. C. ]ANSSEN found by the observation of the D line at the solar eclipse of August 18, 1868. 3 The same year HUGGINS reported the presence in stellar spectra of lines of H, Na, Mg, Ca and Fe. An era of close and fruitful collaboration between laboratory and astronomical spectroscopy was beginning. All along, for a century, most new investigations in laboratory and theoretical spectroscopy have found applications in astronomy. But this has not been a one-way influence. Astro nomers have also led the spectroscopists to new concepts, new mechanisms or new compounds. It is true that the spectacular discovery of helium in the sun has not been followed by other 1* - 47 - 4 P. SWINGS: discoveries of unknown atoms in stars. But the assignment of the so-called nebulium lines to forbidden transitions opened important new possibilities to the spectroscopist, and this was followed a few years later by the interpretation of the coronium. The bands of the tricarbon radical C had been observed and studied in comets, 3 long before they were found and identified in the laboratory; the same is true for the SiC molecule. The observations on interstellar 2 lines led HERZBERG to the laboratory spectrum of CH+. Indeed the investigation of many atomic and molecular spectra was started at the urgent request of astronomical spectroscopists. It is not strange that astronomical spectra should have inspired important problems to the laboratory spectroscopists. Several million stellar spectra have been obtained, some two hundred thousand of them with slit and comparison spectrum. These concern stars in which conditions of temperature, gravity and even chemical composition vary. In addition other spectra concern inter stellar matter, nebulae, comets, planets, etc. The rangein physical conditions is extraordinarily wide. No wonder many spectroscopic problems arise which require the cooperation of the laboratory. For eighty or ninety years spectroscopy was the major source of information on atoms and molecules on the one hand, on celestial bodies on the other. In the last twenty years spectroscopy has lost much of its glamour among the young physicists, most of whom go now into other fields, such as nuclear physics. Similarly many young astrophysicists prefer new fields such as radio-astronomy, nuclear astrophysics, magnetohydrodynamical problems, etc. Yet there are a great number of unsolved problems in astronomical spectroscopy, and the help of the laboratory is required for many of them. It is most gratifying to note that a close cooperation con tinues to exist between many experimental and astronomical spectroscopists. Ten years ago at a meeting of the Joint Commission for Spectro scopy I gave a report on "Spectroscopic Problems of Astronomical Interest" in which I covered all types of spectra of celestial bodies and stressed some of the corresponding unsolved problems with emphasis on the identifications. Although considerable progress has been made during this decade quite a few of the most out standing puzzles which I mentioned in 1950 remain unsolved. Eloquent calls have been made recently, especially to the young scientists, in favor of astronomical spectroscopy because so much - 48 - Problems of Astronomical Spectroscopy 5 of the progress of astronomy depends on continued endeavors in spectroscopy. My present report to this conference isanother plea in the same direction. Let us start with a few remarks on the most spectacular recent spectroscopic progress, and especially on the prospects for progress in the fairly near future, i.e. the extension of astronomical observa tions into the far ultraviolet region from rockets and satellites. A symposium on this topic will take place in Liege next J uly; already now over a hundred astronomers have announced their active participation; this gives a clear indication of the interest shown by astronomers in this matter, which might have been con sidered a fairy tale only a few years ago, but which, owing to the accelerating pace of satellite development, appears now in the realm of possibilities within a few years. Indeed many of our col leagues who are devoting their efforts to rocket- or satellite astro nomy are convinced that the launehing of complex vehicles, suit able for astronomical observations from above the earth's atmo sphere is only a matter of a few years. Rockets have already given us extremely important information on the far ultraviolet spectrum of the sun. The NRL solar spectro grams taken from a height of about 19S Km show Fraunhofer lines longward of 17SO A, but below 1S SO no photospheric continuum is present, only emission lines. About one hundred emissions have been observed by the NRL group; among them fifty are clearly present between L"' (Ä 1216) and Ä SSO; they belong to the Lyman series (at least 8 members; but Lyman y is missing, which mean& that the atmosphere at 200 Km is still optically thick to Lyman y, the absorber being N CI, N II and III, 0 II-III-IV, Ne I and 2), VIII, Mg X, Si I and II, S I; the Lyman emission continuum is clearly present from Ä 910 to about Ä 820. Another group, at the University of Colorado, studied the spectrum in the region of shorter wavelength from heights ranging from 140 to 212 Km. Lines were measured down to 83.9 A, the resonance line of He II at Ä 304 being very intense. Actually the intensity of this He II line has been measured by another group (H. HINTEREGGER), as weil as its absorption in the atmosphere. Using a high resolution instrument (13th order of a grating of SO cm radius, 1200 lines per inch) the NRL group obtained the profile of Lyman cx. with a dis persion of 2.6 mm per A. Lyman cx. has a half-maximum width of the order of 1 A, with wings extending about one A on either side - 49 - 6 P. SWINGS: of the center. A broad central depression leaves two maxima separated by about 0.4 A. In the center of the broad weak reversal is a deep narrowcentral absorption core, with a width at half maximum of 0.03 or 0.04 A. This core is probably due to absorption by geocoronal hydrogen. If this hydrogen is located close to the earth the numbers of H-atoms per square centimeter column is about 2x1012. Other important results have been obtained on the X-ray emis sion of the sun, but much remains to be done before we have a clear picture of this X-ray spectrum. Rocket-astronomy will, of course, be continued and developed. Indeed Lyman oc-emission has already been observed during night launchings. We may, for example, hope that aurorae will be ob served from various heights and "from above". But the next step, satellite-astrophysics, will still be of much greater significance. We may try to foresee what could be expected from the far ultraviolet observation of celestial bodies, remembering however that the un expected results will probably be of greater importance and impact than the expected. Take, for example, the far ultraviolet spectrum of a comet. Assuming a fluorescence mechanism-which has been proved to be the only excitation process for the molecules of the usual spectral region-each strong solar emission line coinciding with an absorption line of a cometary molecule will give rise to a "resonance series". For example Lyman y will excite N -molecules 2 and give rise to a series of triplets. One may expect a major role to be played by the H -molecules. The Doppler shift of the solar 2 emission lines will be very effective: a slight change in radial velocity may suppress or create a resonance series. · Let us consider the stars in general. Are all stars built like the sun, with a corona at a million degrees, or are such non-thermal envelopes exceptional? What shall we actually see of the ultra violet region of stellar spectra? The interstellar matter will absorb wide regions. Assuming a hydrogen abundance of one atom per cm3 one finds that the optical thickness at A. 912 (threshold of the Lyman continuum) for 100 parsecs is about 2000! To reduce the optical thickness to 1, A. must be reduced to about 80 A. To this we still have to add the absorption in the continua of He and He+. At the limit A. S04 of the He I-continuum the absorption is about the sameasthat of H, but at shorter wavelengths it becomes more important, and it exceeds that of H by a factor of 20 at 10 A. We - 50 - Problems of Astronomical Spectroscopy 7 may thus expect that the stellar ultraviolet spectra will actually consist of two ranges: the optical from A. 2900 to A. 912, and the X-ray, below, say 30 A. The X-ray spectroscopy of starswill become of great importance. From measurements with the 21 cm-line of hydrogen we know that there are about 1020 atoms of H per cm2 column to the Crab nebula: we shall thus be able to detect the X-ray emission from the Crab nebula. Even for nearby stars the pro files of the stellar Lyman lines will be strongly perturbed by the inter stellar Lyman lines; there is very little likelihood that we shall ever observe the Lyman lines of gaseous nebulae or of bright line stars. But this unpleasant fogging effect of interstellar hydrogen and heliumwill be partially compensated by the important information which the interstellarabsorptionwill give us langward of the Lyman limit. Our present data on the physics of interstellar matter are still scanty because we can use the observations on only two inter stellar elemen ts : neutral Na and singly ionized Ca, and we know that interstellar Na and Ca are mainly in the form of Na+ and Ca++; moreover the corrections for ionization are very uncertain. We shall be able to observe many interstellar lines above A. 912: H, H2, CI to IV, NI to V, 0 I and IV, Mg I and II, Si I to IV, SI to IV, AI and II, Fe II and III. Many of these lines will be found with intensities as great as or greater than the D-doublet of Na. Even in the near ultraviolet, interesting information will be found in a comparison of the neighboring lines of Mg I 2852 and Mg II 2795 to 2805. Actually unexpected events may take place which may hinder the observation of the numerous lines indicated above: there may be absorption by molecules or by small interstellar grains, but this would again be of utmost interest! Our total ignorance of the abundance of H2-molecules is very serious. Quite a few astronomers believe now that H molecules are abundant in interstellar space. 2 Who knows? One may even find HD-lines, giving some clue on the abundance of deuterium. Comparison of the H I and H II regions will be most profitable. The possibility, sometimes raised, that there may be hot interstellar regions, say at temperatures of the order of 106 K, would be studied: one would find there absorp tions from highly ionized C, N and 0. I shall interrupt here these considerations on satellite-astro physics; such considerations if they involved the other spectral regions: infrared, radio, X- and y-ray, would indeed require much more time than is at my disposal. - 51 8 P. SWINGS: I want instead to speak now of present astronomical spectro scopy, and I shall even restriet myself to the photographic region, say from A. 3000 to A. 9000, with a possible short incursion into the near infrared. Hence I shall not speak on 21 cm-astronomy, a wide field in itself. Instrumental developments of tremendous importance have been made in the field of astronomical spectroscopy, and others may be expected soon. To start with the expected developments let me express the hope that we shall soon apply in astronomy, especially for the lang wavelengths, the recent Iabaratory work on SISAM and on the application of the Fabry-Perot interferometer. The most spectacular recent progress is the electron camera; this technique is opening extraordinary possibilities because its sensitivity and reso lution are so much greater than those of photographic emulsions. The spectral photoelectric scanning is also very important, and it is progressing satisfactorily. Scans with low resolutionarealready possible at many installations, down to the 1Oth magnitude, thus giving the absolute energy distributions in standard or variable stars. This technique fills a gap between the multi-color photo electric photometry and the difficult high resolution scan: the latter is only in a beginning stage. The progress in the receivers for the various spectral regions is eagerly awaited by stellar spectro scopists. From the considerations which will follow it will be abundantly clear that the present emphasis is on quantitative intensity deter minations of radiation. Hence the installation of the Rappel Laboratorium für Strahlungsmessung at the Heidelberg-Königstuhl Observatory was sincerely welcomed by all astrophysicsts, and its Director, Professor KIENLE, must be congratulated for his endeavors and his competence. Precise spectrophotometric data are, for example, needed for the determination of interstellar extinction, hence of absolute magnitudes. One cannot overemphasize the need of detailed accurate studies of the distribution of the magnetic field of the sun, including the active zones. The development by H. D. and H. W. BABCOCK of the solar magnetograph, based on the Zeeman effect, represents a great instrumental advance. Fine details in intensities higher than 0.3 gauss can be observed. The general field is a dipole whose mean intensity is 1 gauss. The emphasis is now on the study of the magnetic field of active regions. - 52