Steven A. Balbus (born 23 Nov 1953) is an American-born astrophysicist who is the Savilian Professor of Astronomy at Oxford University and a Professorial Fellow at New College, Oxford. In 2013, he shared the Shaw Prize for Astronomy with John F. Hawley.
Balbus was born in 1953 in Philadelphia, Pennsylvania. He attended the William Penn Charter School, and received S.B. degrees in mathematics and in physics from the Massachusetts Institute of Technology (MIT) in 1975. Balbus received a Ph.D. in theoretical astrophysics from the University of California at Berkeley in 1981. He then held postdoctoral appointments at MIT and Princeton University.
In 1985, Balbus joined the faculty of the University of Virginia. In 2004, he was appointed Professeur des Universités in the Physics Department of the École Normale Supérieure de Paris. He remained in Paris until 2012, when he moved to Oxford University as the Savilian Professor of Astronomy. At Oxford, he teaches astrophysical gas dynamics and supervises postdoctoral researchers and students. 
Balbus' research is in theoretical astrophysics. He has made discoveries related to gravitational instability in the interstellar medium and several contributions to the theory of thermal processes in magnetized dilute plasmas. He is best known for a 1991 paper, published with former colleague John F. Hawley, describing what is now known as magnetorotational instability (MRI). Most recently, Balbus has been working on a theory of the Sun's internal rotation.
Balbus was awarded a Chaire d'excellence in 2004 by the French Ministry of Higher Education. In 2013, he shared the Shaw Prize in Astronomy with Hawley for their work on the MRI. Considered one of the highest honors in astronomy, the prize included a US$1 million cash award. According to the Shaw selection committee the "discovery and elucidation of the magnetorotational instability (MRI)" solved the previously "elusive" problem of accretion, a widespread phenomenon in astrophysics and "provides what to this day remains the only viable mechanism for the outward transfer of angular momentum in accretion disks". 
Balbus is the recipient of a Wolfson Research Merit Award, and has held visiting faculty positions at Princeton University (Bohdan Paczynski Visitor and Spitzer Lecturer, 2011) and the University of California at Berkeley (Visiting Miller Professor, 2012).
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- Wolfson Award. "Royal Society announces new round of esteemed Wolfson Research Merit Awards". The Royal Society.