Albrecht Otto Johannes Unsöld (20 April 1905 – 23 September 1995) was a German astrophysicist known for his contributions to spectroscopic analysis of stellar atmospheres.
Albrecht Unsöld was born in Bolheim (Württemberg), Germany. After school attendance in Heidenheim, Unsöld studied physics at the University of Tübingen and the University of Munich. At Munich, he studied under Arnold Sommerfeld, and was granted his doctorate in 1927. As a Fellow of the Rockefeller Foundation, he was an assistant in Potsdam and worked at the Mount Wilson Observatory in Pasadena, California. He then completed his Habilitation in Munich in 1929. In 1930, he was an assistant at the Institute of Theoretical Physics at the University of Hamburg. In September 1932, Unsöld became Ordinarius Professor and Director of the Institute for Theoretical Physics (and Observatory) at the University of Kiel – a position he held until his emeritus status was granted in 1973, after which he remained scientifically active for 15 years.
While a student at Munich, Unsöld was one of many of a long line of students who helped Sommerfeld explore and advance atomic theory. Unsold's theorem states that the square of the total electron wavefunction for a filled or half-filled sub-shell is spherically symmetric. Thus, like atoms containing a half-filled or filled s orbital (l = 0), atoms of the second period with 3 or 6 p (l = 1) electrons are spherically shaped. Likewise, so are atoms of the third period in which there are 5 or 10 d (l = 2) electrons. Hence, spherical atoms are those of the 1st, 2nd, 7th, 12th, 15th and 18th columns of the periodic table.
At Kiel, Unsöld made an intensive study of the effects of abundances, radiation damping, Doppler shifts, electric fields, and collisions on the formation and shape of spectral lines in stellar atmospheres. His analysis of the B0 star Tau Scorpii, obtained on his 1939 visit to Yerkes and McDonald Observatories, provided the first detailed analysis of a star other than the Sun, and he was able to determine the physics and composition of the star’s atmosphere.
From 1947 to 1948, Unsöld was President of the Astronomische Gesellschaft.
Unsöld edited the Zeitschrift für Astrophysik until it was merged with other European journals into Astronomy and Astrophysics. He was also the author of many books, and his book Physik der Sternatmosphären mit besonderer Berücksichtigung der Sonne was a bible for quantitative stellar spectroscopy and related fields, with special emphasis on the Sun.
Dr. Unsöld was named an honorary member of the Royal Astronomical Society of Canada on 1956-02-17.