VOGEL, HERMANN CARL

(b. Leipzig, Germany, 3 April 1841; d. Potsdam, Germany, 13 August 1907), astrophysics

Vogel was the sixth child of Johann Carl Christoph Vogel, principal of a Leipzig Gymnasium, whose friends included Alexander von Humboldt, Robert Bunsen, and Carl Ritter. Vogel’s older brother Eduard, who later became an astronomer and African explorer, was a friend of Heinrich Louis d’Arrest, director of the Leipzig astronomical observatory. Through this friendship vogel came into contact with astronomy while still young.

After graduating from his father’s school, Vogel entered the Dresden Polytechnical School in 1860. Before he completed his training there, however, his parents died, leaving him with serious financial problems. He managed to support himself by doing odd jobs, supplemented by aid from his oldest brothers.

Vogel returned to Leipzig in 1863 and began to study natural science at the university. He immediately became second assistant at the university observatory, directed by Karl Bruhns. Vogel’s remarkable manual dexterity was very helpful in manipulating the instruments used to observe nebulae and star cluters. The Leipzig observatory was participating in the Astronomische Gesellschaft’s “zones project,” a great scanning operation of the northern skies, the goal of which was to ascertain the coordinates of all stars down to the ninth magnitude. At Bruhns’s suggestion Vogel agreed to make his nebulae observations in the zone+9°30' to 15°30', the area assigned to the Leipzig observatory. This work formed the basis of his inaugural dissertation, which contained a report of these observations and a detailed historical survey of the observation of nebulae (Jena, 1870).

While Vogel was a student at Leipzig , J.K.F. Zöllner obtained a professorship there (1866). Scarcely seven years older than Vogel, Zöllner exerted a lasting influence on his career, especially by insistent advocacy of astrophysics (the examination of celestial objects with the then new methods of photometry, spectroscopy, and photography). The period of Zöllner’s most important work in stellar photometry coincided with the years in which Vogel was working on his doctorate. At this time, for example, Zöllner proposed his ingenious design for a reversible spectroscope with which he sought to demonstrate the existence of Doppler shifts in stellar spectra.

At the recommendation of Zöllner and Bruhns, Vogel was named director of the observatory of F.G. von Bülow at Bothkamp, near Kiel, in 1870. Bülow, an ardent amateur astronomer, wished to finance the construction of an observatory suitable for serious scientific research, even though he himself seldom did such work. Thus Vogel had complete scientific freedom and, most important, he had sole discretion in determining the program of research and in procuring the necessary technical equipment to carry it out. The observatory had a considerable number of instruments, including a relatively large refracting telescope with an aperture of 293.5 mm. (11 1/2 inches) and fitted with an automatic guiding mechanism. Vogel soon acquired additional devices for it, including a spectroscope and a camera obscura, Among the other instruments at the observatory were a comet-seeker (aperture, 136 mm.; focal distance, 1670 mm.), a Fraunhofer refracting telescope (aperture, 75 mm., focal distance. 1 160 mm.), a Zöllner photometer, a ten-inch prismatic circle, two good pendulum clocks, and meteorological measuring devices. Vogel’s detailed description of the observatory’s equipment was published in Astronomische Nachrichten, 77 (1871), cols. 289–298.

Bülow’s generosity was of great significance for Vogel’s scientific development, and Vogel later stated that it was at Bothkamp that he really learned astrophysics. In particular, while there he worked intensively on the spectroscopic analysis of the stars. With the eleven-inch equatorial telescope he investigated the spectra of Mercury, Venus. Mars, Jupiter, and Uranus, as well as those of various nebulae, of Comet III 1871, of the northern lights, and of the sun. In addition, with a reversible spectroscope placed at his disposal by Zöllner, he tried to ascertain the rotation of the sun Following the attempts made by Huggins in England (1868), Vogel sought to determine spectroscopically the radial velocity of the fixed stars, a project on which he obviously was in close contact with Zöllner. The results, however, were uncertain; and Vogel, dissatisfied, temporarily abandoned this research. In the next two years he published the results he had achieved with his co-worker O. Lohse in Beobachtungen, angestellt auf der Sternwarte des Kammerhern v. Bülow zu Both kamp (1872-1875). Another work on the spectra of the planets won the prize of the Royal Danish Academy of Sciences and Letters at Copenhagen. Through these publications both the Bothkamp Observatory and its director became well-known in the scientific world.

In 1871, Wilhelm Foerster, clearly grasping the importance of astrophysical research, sent a memoir to the German crown prince and the minister of education that urged the construction of an astrophysical observatory at Potsdam, near Berlin. Vogel’s name naturally arose in the ensuing discussions, and in 1874 he was asked to become an observer at the future observatory. In his new post Vogel collaborated in planning the equipment for the new institution, a task for which he was well prepared by his experience at Bothkamp. In order to broaden his knowledge of recent developments in astronomy, he traveled to England, Scotland, and Ireland in 1875. On the trip he met and held scientific discussions with the leading astronomers of those countries, especially Huggins and Airy.

Even before the opening of the new observatory, Vogel pursued the astrophysical research he had begun at Bothkamp, although the time he could devote to it was limited by his responsibilities at Potsdam. It was hoped that Kirchhoff, whose research played a major role in the creation of astrophysics, would become director of the new observatory, but he refused, on the ground that the post would not allow him sufficient time for his theoretical studies because its incumbent would have sole responsibility for running the observatory. He agreed, however, to work at the observatory if he could be a codirector. His terms were accepted, and in July 1876 Kirchhoff, Wilhelm Foerster, and Arthur von Auwers (who served as business manager) were appointed codirectors of the observatory.

Vogel turned to research in spectrophotometry, a field that was to become of considerable importance. In 1876, through a study of Nova Cygni, Vogel obtained the first firm evidence of the changes that occur in a nova spectrum during the fading phase. He also began an extensive examination of the solar spectrum, intending to replace the solar spectrum tables of Kirchhoff (1861-1862) and Ångstrom (1868) with more precise ones. Vogel’s painstaking measurements, the reliability of which was further increased by the new absolute wavelength measurements of Gustav Müller and Kempf, constituted an outstanding achievement for the period with regard to both exactitude and abundance of lines. Unfortunately, they were soon superseded by Rowland’s tables, which were produced with diffraction gratings and therefore contained more precise measurements. In 1879, the year he completed the solar spectrum measurements, Vogel was named full professor; and on 15 March 1882 he was appointed director of the Potsdam Astrophysical Observatory, which had been officially put into service in 1879.

Vogel chose the spectroscopy of the fixed stars as his area of specialization. In response to Secchi’s proposal, he sought to classify the spectra of the fixed stars; believing that these spectra would reflect the stages of development through which the stars had passed. He decided to test his classification by a spectroscopic examination of the skies, which he executed during the following years. Vogel was disappointed by the extremely uniform distribution of the individual spectral classes of his rough schema–a distribution based on his own observations (of only 4,000 objects) — for he had hoped to obtain interesting results. Such results were later obtained through a study of the Harvard classes, which were established on the basis of a greater number of objects, some of which were less bright than those Vogel had examined. Dissatisfied with his findings, Vogel abandoned work in this area.

Meanwhile, Vogel returned to a problem that had intrigued him at Bothkamp: the determination of the radial components of stellar velocities from the Doppler shifts detectable in stellar spectra. His crucial fundamental idea was to employ photography, a technique that had recently been highly developed and that he had already used to record stellar spectra. In April 1887 Vogel attended the International Congress for Astrophotography in Paris, where, presumably, he became more convinced of the correctness of his ideas. Scarcely a year later he presented to the Royal Academy of Sciences at Berlin the first results of his research, in “Über die Bestimmung der Bewegung von Sternen in Visions radius durch spektrographische Beobachtung.” In it he showed that the spectrographic method yields more exact results than visual measurement of stellar spectra in the eyepiece. In a table derived from work done in collaboration with Julius Schemer, Vogel reported the Doppler shifts of the hydrogen γ lines in the spectra of Sirius (α Canis Majoris), Procyon (α Canis Minoris), Rigel (β Orionis), and Arcturus (α Bootis). Although this initial research showed traces of haste and was conducted with instruments that were not fully adequate to the task, the results evoked great interest; for this work ended the protracted controversy over the value of Doppler’s theory for the investigation of phenomena of motion in the universe and thereby gave astrophysics a new tool of immense value. Vogel worked with his collaborators to improve the observatory’s apparatus, and in 1892 he published “Untersuchung über die Eigenbewegung der Sterne im Visionsradius auf spektrographischem Wege” (Publikationen des Astrophysikalischen Observatorium zu Potsdam, 7, no. 25).

Vogel’s use of spectrography led to a sensational success: the discovery of the spectroscopic double stars. On the basis of periodic displacements in the spectral lines of Algol (β Persei) and Spica (α Virginis), Vogel proved that these objects are actually eclipsing binary stars, the components of which could not be detected as separate entities by means of optical devices. The establishment of periodic line displacements in the Algol spectrum and their well-defined relationship with the variation of exposure provided the first exact confirmation of the supposition that Algol is a component of a double star system. From the spectrographs of this system, Vogel and Schemer derived the orbital velocity of the brighter component. Employing data on the variation of exposure and several plausible hypotheses, they also determined the dimensions of the system, the diameter of both components, the total mass of the system, and the distances of the components from each other (1889). This was the first time that important new information was logically deduced by comparing measurements of Doppler shifts with data concerning the variation of exposure of a spectrographic double star system. Vogel’s introduction of these new techniques soon led to further discoveries, the first of which were Edward Pickering’s findings regarding Menkalinan (β Aurigae) and Mizar (ζ Ursae Majoris). The importance of such research is immediately evident from the relatively high number of spectrographic double stars accessible to the astronomer. (For example, probably half of the stars of spectrographic type B are of this kind.) In 1904 the spectrographic double stars played a decisive role in Hartmann’s discovery of the interstellar calcium absorption lines.

As director of the Potsdam Astrophysical Observatory, Vogel had an increasing number of organizational duties, which gradually obliged him to restrict his own scientific work. On the other hand, his role in the expansion of the observatory was of considerable importance for the progress of astrophysical research. His influence and tireless efforts ensured that the observatory equipment was adequate for its ambitious research programs — for instance, a very costly photographic double refractor that was put in service in 1899 and was used to determine the radial velocities of weaker stars. The lens, achromatized for photographic work, had an aperture of 800 mm. and was the largest photographic lens ever made. Shortly after this instrument went into service, Vogel became seriously ill and had to cease working for a while. Although he never completely recovered, he was able to do a further series of studies, including one on the operational possibilities of short-focal-length reflecting telescopes for research on nebulae.

Vogel’s scientific achievement brought him many international honors and assured him a place in the history of modern astronomy. The great scientific importance of the observatory he headed was due in large part to his willingness and ability to attract distinguished co-workers. Furthermore, to the extent that it was possible, given the research programs he had selected, Vogel usually assigned his co-workers to projects that corresponded to their scientific interests.

Vogel left a large sum of money to the Potsdam observatory, the interest on it to pay for study abroad and for the support of gifted children.

BIBLIOGRAPHY

I. Original Works. The writings that Vogel published while at Potsdam are listed in W. Hassenstein, “Das Astrophysikalische Observatorium Potsdam in den Jahren 1875-1939,” in Mitteilungen des Astrophysikalischen Observatoriums (Potsdam), 1 (1941). This article also contains a list of all the works by Vogel’s co-workers (pp. 13–55), both those that appeared elsewhere (pp. 13–53). Vogel’s earlier publications and his articles in Astronomische Nachrichten are listed in Generalregister der Bände 41 bis 80 der Astronomischen Nachrichten Nr. 961 bis 1920 (Kiel, 1938), col. 108; and in Generalregister der Bändle81 bis 120 der Astronomischen Nachrichten Nr. 1921 bis 2880 (Kiel, 1891), cols 121–122.

II. Secondary Literature. See W. Brunner, Pioniere der Weltallforschung (Stuttgart, n.d. [1954?]); A. F., “Hermann Carl Vogel,” in Monthly Notices of the Royal Astronomical Society, 68 , no.4 (Feb. 1908), 254–257; W. Foerster. Lebenserinnerungen und Lebenshoffnungen (Berlin, 1911), esp. 139–140; O. Lohse, “Hermann Carl Vogel (Todes-Anzeige),” in Astronomische Nachrichten, 175 (1907), cols. 373–378; and G. Müller, “Hermann Carl Vogel,” in Viertelijahrsschrift der Astronomischen Gesellschaft, 42 (1907), 323–339. See also D. B. Hermann, “Für Vorgeschichte der Astrophysikalischen Observatoriums Potsdam,” in Astronomische Nachrichten, 296 (1975), 245–259.

Dieter B. Herrmann