Hertzsprung, Ejnar

(b. Frederiksberg, Denmark, 8 October 1873; d. Roskilde, Denmark, 21 October 1967)

astronomy.

Hertzprung’s father, Severin Hertzsprung, had a graduate degree in astronomy from the University of Copenhagen but, for financial reasons, decided to accept a position in the Department of Finances of the Danish government and at a very early age became director of the state life insurance company. He instilled in his son his own interest in astronomy and mathematics; but because of his awareness of the lack of financial security, he did not encourage the boy to select these fields as a career. As a result Hertzsprung decided to study chemical engineering. His interest in chemistry resulted from his study of a small book on this subject by the Danish chemist Julius Thomsen. Hertzsprung graduated from the Polytechnical Institute in Copenhagen in 1898 and spent the next several years as a chemist in St. Petersburg. In 1901 he went to Leipzig to study photochemistry in Wilhelm Ostwald’s laboratory. He returned to Denmark the following year and began in earnest his study of astronomy.

During this period Hertzsprung began corresponding with the German astronomer Karl Schwarzschild, who invited Hertzsprung to visit him at Göttingen in 1909. Within a few months Hertzsprung was appointed associate professor at the university and during the same year, when Schwarzschild became the director of the astrophysical observatory at Potsdam, Hertzsprung joined him there as senior staff astronomer.

In 1919 Hertzsprung was appointed an associate director and associate professor of the observatory of the University of Leiden; he became its director in 1935. Upon retirement in 1944 he returned to Denmark, where he continued his research until 1966.

Hertzsprung received many honors for his outstanding contributions to astronomy. He was elected to eleven academies and societies in both Europe and the United States, and received honorary doctorates from Utrecht (1923), Copenhagen (1946), and Paris (1947). The Royal Astronomical Society awarded him its gold medal in 1929; in 1937 he received the Bruce Gold Medal of the Astronomical Society of the Pacific; and the city of Copenhagen honored him with its Ole Römer Medal in 1959.

Early in the twentieth century, when Hertzsprung entered the field of astronomy, study of the physical nature of stars was still in its infancy. Stellar astronomy during the nineteenth century had been directed mainly toward determining positions and motions of the stars. However, during the second half of the century pioneer work in spectroscopy had been initiated by Angelo Secchi and William Huggins, and the new technique of photography was introduced for making astronomical observations—primarily by Secchi, Warren de la Rue, and W. C. Bond. By 1875 Huggins had devised methods for photographing stellar spectra and had succeeded in determining the radial velocities of stars (that is, their motions in the line of sight) from shifts in the spectral lines.

From their visual observations of bright stars, Secchi and Huggins had already discovered that there were a few basically different types of stellar spectra and that they formed a series which distinctly connected one type to its neighbor. Secchi initially proposed four classes of spectra, and other classification schemes followed.

During the years 1890–1901 three catalogs of photographically determined stellar spectra were published by Harvard College Observatory. These form the basis for the original Henry Draper Catalog, in which Antonia C. Maury classified the brighter stars from the north pole to declination –30° and Annie Jump Cannon classified stars (mostly brighter than fifth magnitude) south of –30°. Two different systems of classification were adopted in the catalog. Miss Maury used the more detailed one—twenty-two main groups, each divided into seven different indexes with the use of the letters a, b, c, and four double letters to indicate detailed features in the spectra. Miss Cannon used a less detailed system still used today—with the exception that subdivisions and luminosity classes have since been added.

Hertzsprung said that it was his interest in the theory of blackbody radiation and its relation to the radiation of stars that initially stimulated his interest in astronomy. The problem of the radiation of a blackbody, one that absorbs all frequencies of light and, when heated, also radiates all frequencies, had first been posed by G. R. Kirchhoff and was finally solved by Max Planck in 1900 by means of his quantum theory.

Hertzsprung, with his background as a chemical engineer and a specialist in photochemistry, was without doubt better qualified to use photography in the solution of astronomical problems than most astronomers of that period. What knowledge he needed in the basic principles of observational astronomy he obtained working with H. E. Lau, a young astronomer. Between studying the contemporary astronomical literature and observing with the telescopes at the observatory of the University of Copenhagen and at the Urania Observatory in Frederiksberg, Hertzsprung kept himself fully occupied over the next several years.

During this early period of his astronomical career Hertzsprung published two now classic papers in Zeitschrift für wissenschaftliche Photographie, a journal devoted to photophysics and photochemistry. Both papers, published in 1905 and 1907, were entitled “Zur Strahlung der Sterne.” In examining the proper motions of stars with spectra classified by Miss Maury, he was able to show that the stars which she found to have exceptionally sharp and deep absorption lines (her index c) were more luminous than the rest. This discovery was the basis for measurement of luminosity by means of spectra—a method which, under the title of “spectroscopic parallaxes,” has become one of the most powerful means for determining stellar distances, galactic structure, and distances to other galactic systems.

These papers also contained Hertzsprung’s discovery of giant and dwarf stars. From his study of parallaxes, apparent magnitudes, proper motions, and colors, he determined that the stars could be divided into two series, one now known as the main sequence in the Hertzsprung-Russell diagram while the other constitutes the high-luminosity or giant stars. The diagram, developed from this discovery, is a plot of the intrinsic magnitude against temperature for a group of stars. It remains the cornerstone of all astronomical research related to the formation and evolution of stars. Hertzsprung’s original papers did not include an illustration of the diagram, possibly because he felt his study lacked sufficient data.

Hertzsprung actually constructed the first such diagram for the Pleiades star cluster in 1906, and he took it to Göttingen in 1909. The existence of such a diagram was not generally known until the American astronomer H. N. Russell presented it at a meeting of the Royal Astronomical Society in 1913 in an address on the subject of giant and dwarf stars, based on his own independent research—unaware of Hertzsprung’s earlier work.

In his 1907 paper Hertzsprung referred to the open star clusters as a method for deriving the relation between the radiation of a star and its color. Since the physical members of such a cluster would be of equal distance, or nearly so, their apparent magnitudes and colors should reveal this relation.

Before leaving Copenhagen, Hertzsprung photographed several clusters at the Urania Observatory, using coarse gratings in front of the objective of the telescope. By measuring the separation of the grating images from the central images, he obtained the effective wavelengths of the individual stars, which he used as an a index for their colors. This work was continued in Potsdam, and in 1911 he published color-magnitude diagrams of the Pleiades and the Hyades—the first diagrams of this type ever to be published.

During his stay at the Mount Wilson Observatory in 1912, Hertzsprung continued cluster work on NGC 1647 and the Pleiades, using coarse gratings in front of the sixty-inch reflector, the largest telescope in the world at that time. Work on the Pleiades alone involved measurements of nearly 10,000 effective wavelengths and was only the beginning of an extensive work that was to be carried out by Hertzsprung on this cluster. Over a period of twenty years he and his associates measured positions of stellar images on 161 photographs, taken at fifteen different observatories. These measurements were made to determine the relative proper motions of 2,920 stars in the region of the Pleiades and to establish membership in the cluster. The first-epoch plates had been taken with almost identical telescopes in the early period of the Carte du ciel program, which started in 1887. Because of the long interval between the first-and second-epoch plates, he could not only distinguish between members and nonmembers of the cluster but also was able to determine the upper limit for the internal motions and, in this way, to estimate that the total mass of the cluster did not exceed a few hundred solar masses.

Hertzsprung also found that the magnitudes and the colors of the member stars formed a narrow sequence, a result later to be corroborated by modern photoelectric observations. As early as 1929 he noted that the brighter Pleiades members were whiter than stars of the same brightness in the solar neighborhood, and that the Pleiades differed in stellar population from the Hyades and Praesepe clusters. These differences, first noted by Hertzsprung, are now interpreted to indicate that the Pleiades are younger than the other two clusters, as well as the stars in the solar neighborhood.

Another cluster which received Hertzsprung’s special attention was the Ursa Major cluster. In 1869 the English astronomer R. A. Proctor had discovered that the five bright stars in this constellation shared the same motion across the sky. That they actually shared the same motion in space was later confirmed by observation of their radial motion. In 1909 Hertzsprung noticed that two other stars, in widely separated regions of the sky, had motions directed toward the same convergent point of the sky as the five bright stars. This observation led him to make a systematic search for additional members. He succeeded in finding six among the bright stars, and two probable members. The most prominent new member was Sirius, the brightest star in the sky. This cluster of stars, sharing identical motions through space, surrounds the sun, without its being a member. The discovery by Hertzsprung resulted in a search by others for new members; to date resuled in a search by others of new members; to data results indicate that 135 stars are members of this remarkable cluster.

Hertzsprung’s effective use of objective gratings for high-precision photographic photometry is well demonstrated by his discovery of the variability of Polaris, which had been suspected by the Dutch astronomer Antonie Pannekoek in 1891. In order to demonstrate the reality of the variability, he took nearly 1,700 exposures on 400 plates during 50 nights. He succeeded in determining the amplitude of the light variation, which was only 0.171 magnitude, with an error of only 0.012 magnitude—an accuracy in stellar photometry unheard of in 1911.

One of the principal reasons why Hertzsprung was awarded the gold medal of the Royal Astronomical Society was his determination of the distance to the Small Magellanic Cloud in 1913. The method he introduced became the basis for all measurements of very large distances in our galactic system, as well as in the expanding universe of the galaxies. The distance determination was based on a very important discovery made by Henrietta S. Leavitt at the Harvard College Observatory the previous year. She had been studying the variable stars in the Small Magellanic Cloud and had found that a relation existed between the apparent magnitude and the period of light variation of the Cepheid variables, the light variation of which can be explained by a pulsation of the star as a whole. Hertzsprung realized that the stars in the cloud could be considered to be at the same distance and that, consequently, their period of variation could actually be related to their intrinsic brightness.

The next step was to select Cepheids close enough to our sun to evaluate their distances, from which their intrinsic brightnesses could be determined. Since no Cepheid was close enough to allow a direct determination of the distance, Hertzsprung used the bright Cepheids with known proper motions. From these he deduced the mean parallactic components of their motions, and thereby their distances and their intrinsic brightnesses. It was then a simple step to compute the intrinsic brightnesses (luminosities) of the Cepheids in the Small Magellanic Cloud from their periods. His value for the distance (10,000 parsecs) was larger than any distance determined in the universe at that time (1913) but about five times smaller than the presently accepted distance. There are a number of reasons for this discrepancy, the most important being the then unknown galactic absorption.

In the same paper Hertzsprung called attention to the asymmetric distribution of the bright Cepheids with respect to the sun, an asymmetry also shared by the very hot and bright stars of spectral class Oe5. He noticed that since the least concentration was in the best-observed part of the Milky Way, the distribution could not be attributed to observational selection. He found that the center of the distribution was in the direction which was much later discovered to be the direction toward the center of our galactic system.

During World War I, Hertzsprung began a program of photographic observations of double stars to which he later devoted much of his time. The ingenious photographic method that he developed either eliminated possible systematic errors or rendered them negligible, so that the results were ten times more accurate than the conventional visual observations with a micrometer. He later made observations of this kind in Johannesburg, South Africa, assisted by two of his former students; and in 1937, when he was at the Lick Observatory of the University of California, he used the large Lick refractor for such observations. After his retirement others took plates for him, but he continued to do the measuring even past his ninetieth birthday.

Hertzsprung’s other contributions to the field of double-star astronomy include his method of obtaining statistical distances (hypothetical parallaxes) for binaries of such slow orbital motion that the observed arcs are too short to permit the determination of their orbits. This method has developed into the so-called dynamical parallaxes, which has been of considerable significance in the statistical calibration of spectroscopic parallaxes.

In 1911 the English astronomer J.K.E. Halm had shown that there existed a statistical relation between the masses and the luminosities of spectroscopic binaries. Hertzsprung found the same relationship in 1915 for visual binaries and later provided the mathematical formulation in 1919—almost simultaneously with Arthur Eddington, who proved the relationship on the basis of theoretical investigations of the radiation equilibrium of the stars.

Hertzsprung returned to his early interest in the colors of the bright stars with a catalog (1922) of mean color equivalents of 734 stars brighter than the fifth magnitude and within 95° of the north celestial pole. In his reduction to a single scale of a range of color equivalents obtained by various methods, he solved the problem of finding the best linear relation between two quantities, both of which were affected by observational errors. His solution was later used by the Dutch cosmologist Willem de Sitter to discuss the velocity-distance relation for extragalactic nebulae.

In the same catalog Hertzsprung discussed the relation between color and luminosity of stars, using proper motions as distance indicators for lack of reliable parallaxes. The diagram illustrating this relationship showed, for the first time, the lack of bright stars of intermediate color, the famous “Hertzsprung gap” between the giants and main sequence stars.

Hertzsprung did not limit his study of variable stars to Polaris. His accurate light curves based upon extensive series of photographic observations of certain selected variables (S Sagittae, VV Orionis, and RR Lyrae) have only in recent years been surpassed in accuracy by photoelectric techniques.

Throughout the years 1924–1929 Hertzsprung concentrated on variable stars. In the first year and a half of this period he observed at the Union Observatory in Johannesburg with the Franklin-Adams telescope and took 1,792 plates, with a total exposure time of 638 hours. On the plates alone he made 36,000 estimates of brightness of variable stars and determined over a third of all the light curves of shortperiod variables published during that five-year period. He also visited Harvard for five months in 1926–1927 and made an additional 12,000 estimates of variables on the plate collection there.

Hertzsprung was deeply interested in the education of future astronomers. He said in the annual report of the Leiden observatory for 1933: “It is of importance that each student shall have the opportunity to get acquainted with as many different methods of observing as is possible with the means at his disposal, before choosing a particular branch of astronomy for his future specialty.” He always emphasized that it was important to plan and execute observational programs carefully, and that great care should be exercised in drawing conclusions from empirical data.

Hertzsprung’s guidance and inspiration and the example he set resulted in many of his students later occupying important positions in the astronomical world. He often said, “If one works hard, one always finds something and sometimes something important.” By following this principle, Hertzsprung made contributions to astronomy which place him among the great astronomers of all time.

BIBLIOGRAPHY

I. Origingal Works. Among many are “Zur Strahlung der Sterne,” in Zeitschrift für wissenschaftliche Photographie, 3 (1905), 429–422; and 5 (1907), 86–107; “On New Members of the System of the Stars β, γ, δ, ε, ζ Ursae Majoris,” in Astrophysical Journal, 30 (1909), 135–143; “Über die Vervendung photographischer effektiver Wellenlängen zur Bestimmung von Farbenäquivalenten,” in Publikationen des Astrophysikalischen Observatoriums zu Potsdam, 22 (1911), 1–40; “Nachweis der Veräderlidchkeit von α Ursae Minois,” in Astronomische Nachrichten189 (1911), 89–104; “Über Doppelstener mit eben merklicher Bahnbewegung,” ibid,. 190 (1912), 113–118; “Über ide räumliche Verteilung der Veräderlichen vom δ Cephei-Typus,” ibid., 196 (1914), 201–210; “Effective Wave-Lengths of 184 Stars in the Cluster N.G.C. 1647,” in Astrophysical Journal42 (1915), 92–110; “Bemerkungen zur Statistik der Sternparallaxen,” in Astronomische Nachrichten208 (1919), 89–96; “Photographische Messungen von Doppelsternen,” in Publikationen des Astrophysikalischen Observatoriums zu Potsdam,24 , pt. 2 (1920); “Mean Colour Equivalents and Hypothetical Angular Semi-Diameters of 734 Stars Brighter Than Fifth Magnitude and Within 95° of the North Pole,” In Annalen van de Sterrenwacht in Leiden, 14 pt, 1 (1922); “Effective Wavelengths of Stars in the Pleiades,” in Kongelige Danske Videnskabernes Selskabs Skrifters Sciences Section, 8th ser., 4 , no. 4 (1923); “On the Relation Between Mass and Absolute Brightness of Components of Double Stars,” in Bulletin of the Astronomical Institutes of the Netherlands, 2 (1923), 15–18; “The Pleiader,” in Monthly Notices of the Royal Astronomical Society,89 (1929), 660–678; and “Catalogue de 3259 ëtoiles dans les Pléiades in Annalen van de Sterrenwacht in Leiden19 , pt. 1 (1947).

II Ssecondary Lliterature. See A. O. Leuschner, “The Award of the Bruce Gold Medal to Professor Ejnar Hertzsprung,” in Publications of the Astronomical Society of the Pacific, 49 (1937), 65–81; and Rev. T. E. R. Phillips, “Address on the Award of the Gold Medal of the Royal Astronomical Society to E. Hertzsprung,” in Monthly Notices of the Royal Astronomical Society, 89 (1929), 404–417. Obituaries include Axel V. Nielsen, “Ejnar Hertzsprung—Measurer of stars,” in Sky and Telescope, 35 (January 1968), 4–6, K. Aa. Strand, “Ejnar Hertzsprung, 1873–1967,” in Publications of the Astronomical Society of the Pacific, 80 (1968), 51–56; and A. J. Wesselink, “Ejnar Hertzprung,” in Quarterly Journal of the Royal Astronomical Society, 9 (1968), 337–341.

K. Aa. Strand