Albrecht Otto Johannes Unsöld (20 April 1905 – 23 September 1995) was a Germanastrophysicist known for his contributions to spectroscopic analysis of stellar atmospheres.

Albrecht Unsöld was born inBolheim,Württemberg,Germany. After school attendance inHeidenheim, Unsöld studiedphysics at theUniversity of Tübingen and theUniversity of Munich. At Munich, he studied underArnold Sommerfeld, and was granted his doctorate in 1927.^[1] As a Fellow of the Rockefeller Foundation, he was an assistant inPotsdam and worked at theMount Wilson Observatory inPasadena,California. He then completed hisHabilitation in Munich in 1929.^[2] In 1930, he was an assistant at the Institute ofTheoretical Physics at theUniversity of Hamburg. In September 1932, Unsöld became Ordinarius Professor and Director of the Institute for Theoretical Physics (and Observatory) at theUniversity of Kiel – a position he held until his emeritus status was granted in 1973, after which he remained scientifically active for 15 years.^[3][4]

While a student at Munich, Unsöld was one of many of a long line of students^[5] who helped Sommerfeld explore and advanceatomic theory.^[6][7][8]Unsöld'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 starTau Scorpii, obtained on his 1939 visit toYerkes andMcDonald Observatories, provided the first detailed analysis of a star other than theSun, and he was able to determine the physics and composition of the star's atmosphere.^[9]

From 1947 to 1948, Unsöld was President of theAstronomische Gesellschaft.^[9]

Unsöld edited theZeitschrift für Astrophysik until it was merged with other European journals intoAstronomy and Astrophysics. He was also the author of many books, and his bookPhysik der Sternatmosphären mit besonderer Berücksichtigung der Sonne was a bible for quantitative stellar spectroscopy and related fields, with special emphasis on the Sun.^[9]

• 1956 –Bruce Medal
• 1957 –Gold Medal of the Royal Astronomical Society
• The asteroid2842 Unsöld is named in his honor.

• Albrecht UnsöldPhysik der Sternatmosphären mit besonderer Berücksichtigung der Sonne (Springer, 1955, 1968, and 1982)
• Albrecht UnsöldMax Planck (Hirt, 1958)
• Albrecht UnsöldÜber die mittleren Zustandsgrössen und Spektren der Sternatmosphären in Abhängigkeit von ihrem Wasserstoff- und Heliumgehalt (Vandenhoeck & Ruprecht, 1958)
• Albrecht UnsöldDer neue Kosmos (Springer, 1967 and 1981)
  • Albrecht UnsöldThe New Cosmos (Longmans, 1969)
• Albrecht UnsöldSterne und Menschen: Aufsätze u. Voträge (Springer, 1972)
• Albrecht UnsöldEvolution kosmischer, biologischer und geistiger Strukturen (Wissenschaftliche Verlagsgesellschaft, 1981 and 1983)
• Albrecht Unsöld und Bodo BaschekDer neue Kosmos. Einführung in die Astronomie und Astrophysik (Springer-Verlag GmbH, 1991, 2001, and 2004)
  • Albrecht Unsöld und Bodo BaschekThe New Cosmos (Heidelberg Science Library) (Springer-Verlag GmbH, 1991)
  • Albrecht Unsöld, Bodo Baschek, W.D. Brewer (Translator)The New Cosmos: An Introduction to Astronomy and Astrophysics (Springer, 5th Edition, 2005)ISBN 978-3-540-67877-9

• Albrecht UnsöldBeiträge zur Quantenmechanik des Atoms,Ann. d. Phys. (4)82 355-393 (1927). Received 22 December 1926, published in issue No. 3. of 8 February 1927. As cited in Mehra, Volume 5, Part 2, 2001, p. 959.

• Mehra, Jagdish, andHelmut Rechenberg The Historical Development of Quantum Theory. Volume 1 Part 1The Quantum Theory of Planck, Einstein, Bohr and Sommerfeld 1900 – 1925: Its Foundation and the Rise of Its Difficulties. (Springer, 2001)ISBN 0-387-95174-1
• Mehra, Jagdish, and Helmut RechenbergThe Historical Development of Quantum Theory. Volume 5 Erwin Schrödinger and the Rise of Wave Mechanics. Part 1 Schrödinger in Vienna and Zurich 1887-1925. (Springer, 2001)ISBN 0-387-95179-2
• Mehra, Jagdish, and Helmut RechenbergThe Historical Development of Quantum Theory. Volume 5 Erwin Schrödinger and the Rise of Wave Mechanics. Part 2 The Creation of Wave Mechanics: Early Response and Applications 1925 - 1926. (Springer, 2001)ISBN 0-387-95180-6

• 1905 births
• 1995 deaths
• German astrophysicists
• 20th-century German astronomers
• Recipients of the Gold Medal of the Royal Astronomical Society
• People from Heidenheim (district)
• Recipients of the Cothenius Medal
• University of Tübingen alumni

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