Lindblad, Bertil

(b. Örebro, Sweden, 26 November 1895; d. Stockholm, Sweden, 25 June 1965)

astronomy.

Lindblad’s father. Lieutenant Colonel Birger Lindblad, was from Askersund. where four generations of his family had been merchants and active members of the magistrates court. Bertil Lindblad’s paternal grandmother, Jenny Hybhinette, belonged to a distinguished Walloon family that migrated to Sweden around 1630 to work in the iron industry. His mother Sara Gabriella Waldenström, belonged to an intellectual Swedish family whose members for several generations had distinguished themselves in theology and medicine.

Lindblad graduated in 1914 from the Karolinska Lärovcrket in Örebro. That same year he began studies in mathematics, physics, and astronomy al Uppsala University. Östen Bergstrand, director of the Uppsala observatory, soon realized that Lindblad was an unusually gifted student. Lindblad became interested in the study of the colors of stars, and he was encouraged in his early research not only by Bergstrand himself but also by Hugo von Zeipel, associate professor of astronomy.

In 1920 Lindblad defended his doctoral thesis, an important contribution to the theory of radiative transfer in the solar atmosphere with special application to the phenomenon of solar limb darkening. In the same paper he presented a first approach to the problem of determining absolute magnitudes of stars by combining two color indices from different parts of the spectrum. During a research period (1920–1922) at the Lick and Mount Wilson observatories, Lindblad was able to establish new, important spectroscopic criteria of stellar luminosity in addition to those which had been found somewhat earlier by Kohlschütter and Adams. He thus found that absorption bands of the cyanogen molecule CN are considerably stronger in the spectra of giants than in the spectra of dwarfs for stars with the same temperature as the sun or with a somewhat lower temperature. Compared with Kohlschütter’s and Adams’ criteria, the new criteria had the great advantage of being visible also in spectra for very low dispersion. Thus, faint stars could also be studied with this new spectroscopic method, and it soon became a most efficient tool in stellar research.

Upon his return to Uppsala in 1922 Lindblad continued his work with one of the observatory astrographs which had been especially equipped with an objective prism giving short dispersion spectra. He also included other principal spectrum lines and bands in his analysis and was able to use this astrograph for a first survey of faint stars in selected regions of the sky. In order to improve the calibration of his luminosity criteria he selected regions where the proper motions were known down to the limiting magnitude. In this way he improved considerably the basis of stellar statistics, and derived more accurately the density distribution of stars and the state of motion. This work gave a great stimulus to Swedish astronomy, and many young astronomers joined in the development of this field of research, which remains a major program at the observatories of Uppsala, Stockholm, and Lund.

Lindblad married Dagmar Bolin in 1924. The couple had four children.

In 1927 Lindblad was appointed director of the Sockholm observatory and was thus in charge of building the new observatory in Saltsjöbaden. The instruments for the new institution were selected to allow further applications of the spectroscopic criteria of stellar luminosity.

During his last years in Uppsala, Lindblad became increasingly interested in stellar dynamics and he introduced new concepts which could explain the asymmetric drift of high velocity stars. He advanced the fundamental idea that the galactic system is rotating around a distant center. In his detailed discussion he introduced a model of the galactic system consisting of a number of subsystems of different speeds of rotation and with different degrees of flatness and velocity dispersion.

Lindblad’s ideas were soon confirmed by J. H. Oort with his discovery of the differential rotation. Following that discovery, Lindblad became increasingly engaged in stellar dynamics and soon became a leading authority. He devoted much time to the difficult problem of explaining the spiral structure of rotating stellar systems assuming gravitational forces only. His first results indicated that spiral arms probably open up in their motion, but important numerical calculations made on his suggestion by his son Per Olof Lindblad (appointed director of the Stockholm observatory in 1967) have shown that trailing arms probably are the most frequent ones. This led Lindblad to a further development of his theory. He now introduced the concepts “dispersion orbits” and “density waves,” which were followed up with considerable success by C. C. Lin. Lindblad also found ways of explaining how spiral features may be preserved.

Despite his many official duties, Lindblad continued his scientific research; and even in his later years he found time to do observational work on eclipses and photometric studies of nebulae.

Lindblad was very active internationally; he was president of the International Astronomical Union from 1948 to 1952, when he became President of the International Council of Scientific Unions. A few months before his death, he was elected president of the council of the European Southern Observatory.

While director of the Stockholm observatory, Lindblad was also professor of astronomy at Stockholm University. He was elected to the Royal Swedish Academy of Sciences in 1928 and served as president in 1938 and 1960 and as vice-president for many years. He served as chairman of the Swedish Natural Science Research Council from 1951. Lindblad was honored with the Gold Medal of the Royal Astronomical Society in 1948, and he was a foreign member of academies in Denmark, Finland, Belgium, Portugal, Italy, France, Germany, Poland, and the United States. Al his death he was chairman of the Nobel Foundation.

BIBLIOGRAPHY

A comprehensive list of publications (about 100) has been given by J, H. Oort in his obituary notice printed in Quarterly Journal of the Royal Astronomical Society,7 (1966).

Yngve Öhman