The Royal Swedish Academy of Sciences has awarded Dr. Allan R. Sandage, The Observatories of the Carnegie Institution of Washington, Pasadena, USA the 1991 Crafoord Prize, SEK 1,5 million. The Prize Ceremony will take place on the Crafoord Day, 25 September 1991. Dr. Allan R. Sandage is awarded the prize for his very important contributions to the study of galaxies, their populations of stars, clusters and nebulae, their evolution, the velocity-distance relation (or Hubble relation), and its evolution with time.
Allan Sandage had the good fortune to start his research work together with the great observer Walter Baade and to know Edwin Hubble, “the father of observational cosmology”. Already early in his career Sandage could use the 5-meter telescope at Mount Palomar in California, which was then the largest telescope in the world. Since then Sandage has been at the forefront of observational cosmology with large optical telescopes for nearly 40 years.
The ages of stars and the Universe
One of his first achievements was a practical method for determining the age of stellar systems. He made careful observations of several globular clusters, which are systems of several hundred thousands of stars held together in a cluster by their mutual attraction. Studies of the spectra of the cluster stars revealed to him the under abundance of metals and other heavy elements in the globular cluster stars as compared to the abundances of the same elements in the Sun and stars in our neighborhood. Applying the new abundance values in the theories of stellar interior structure and stellar evolution of individual cluster stars with accurately measured brightness´s and colors he was able to derive the age of globular clusters stars. He found those stars to be much older than the Sun and he found the age to be at least 10 billion years. As a consequence, a lower limit to the age of the Universe must be 10 billion years.
The cosmic distance scale
Allan Sandage is known worldwide for his derivation of a distance scale in the Universe. He has built this scale step by step, staring with distance determinations to nearby galaxies. There he could use distance indicators calibrated in the Milky Way and the Magellanic Clouds. Of special importance are the Cepheid variables. The absolute brightness of these stars is related to the length of their period of variation. With the well known law that the observed brightness decreases with the square of the distance to the light source, the relation between absolute brightness and measured brightness of the Cepheids is used to derive the distance to a galaxy, where some Cepheids have been observed.
Once the distance to nearby galaxies have been accurately determined it is possible to calibrate the luminosities of new distance indicators found there, like super giant stars, supernovae and H II-regions (spheres of hot gas around hog giant stars) and to use them to take further steps out to somewhat more distant galaxies. Allan Sandage then discovered that the brightest elliptical galaxy in a rich cluster always has very nearly the same luminosity. Thus he could use the brightest elliptical galaxy in a rich cluster as a distance indicator for that cluster. With the abovementioned methods Sandage could measure very much larger distances in the Universe that had been possible to reach before.
The Hubble relation
Sandage also studied spectra of distant galaxies and compared the redshift of each spectrum with the distance to the same galaxy. The redshift is interpreted in terms of an expansion of the Universe. In this way Sandage could very substantially improve the determination of the Hubble parameter, which is the relation between the velocity of recession and the distance of a galaxy. The inverse value of the Hubble parameter is also a measure of the age of the Universe.
Allan Sandage took part in the first optical identifications of quasars and has later discovered a great class of radio-quiet QSOs. In fact, the whole idea of the quasi-stellar objects as the most distant objects ever observed rest on our interpretation of the extreme redshifts of their spectra in accordance with the Hubble relation. And the value of the Hubble parameter most often used is the one determined by Sandage.
Photographs of galaxies obtained with the large telescopes of California were published by Sandage in 1961 as “The Hubble Atlas of Galaxies”. These pictures as well as the classification of galaxies included in the Atlas have been very widely used by students and senior researchers, observers and theorists all over the world. The Atlas has been reprinted several times. More recently (1988) the very beautiful “Atlas of Galaxies Useful for Measuring the Cosmological Distance Scale” was published by Allan Sandage and John Bedke. Sandage has also contributed to two other books of great importance to many astronomers.
A sum of SEK 800 000 from the Anna-Greta and Holger Crafoord Fund are also awarded Swedish research within extragalactic astronomy.