Professor of the Graduate School, Professor Emeritus; Astronomy
PhD in Astrophysics, University of Colorado, Boulder, 1979
BSc in Physics, Stanford University, 1973
- 605D Campbell Hall
stars; stellar activity; star formation; low mass objects
Major Themes in My Research Program
T Tauri Stars (very young stars)
We have been able to find and study stars in the process of formation. These are guides to the processes that formed our own Solar System. We use primarily high-resolution spectroscopy to diagnose the physical conditions in these systems. We are particularly known for our time-resolved studies of these stars; they actually change from night to night and this helps us understand what is going on in the near-stellar region.
Accretion Disks/ Star-Disk Interaction
Even before Hubble Space Telescope pictures of actual disks around newly forming stars, we had deduced their presence and developed means of understanding their properties. My particular interest is in the region where the stellar magnetic field interrupts the disk, and material takes its final plunge to the stellar surface.
Formation of red and brown dwarfs
Most stars are half the mass of our Sun or less. Below about 0.075 solar masses, the objects are considered "substellar" because they never acheive a stable luminosity (although they do experience some fusion). We have been studying the question of whether these objects form as stars do, or through another mechanism (as some have suggested).
Brown and Red Dwarfs (substellar/very low-mass stars)
Lithium Brown Dwarfs (discovery; lithium dating)
Until 1995, the existence of substellar objects was predicted but not confirmed. It was thought they might be a component of the "dark matter" (they aren't). I successfully applied the "lithium test" to discover the first young brown dwarf in 1995 (in the same year, an old brown dwarf too cool to be a star was also announced). In the process, I developed a technique to use lithium depletion to determine ages of young star clusters, and showed that previous estimates were too young. This method is now widely used. Hundreds of brown dwarfs are now known, and there are perhaps 1/10 as many of them as there are stars. We helped suggest and participated in the development of the "L" spectral type (the first new stellar classification since the early 20th century).
Brown and Red Dwarf properties
Brown dwarfs have only been known since 1995. Red dwarfs (stars less than half our Sun's mass) have been known for a long time, but they are very faint and did not receive the same attention as brighter stars. Using the 10-m telescope, I have applied high-resolution spectroscopy to these objects to diagnose their physical properties and move our level of understanding closer to that for the more massive stars.
Binary Brown and Red Dwarfs
Many stars come with companions. We have been studying the question of how this depends on stellar mass (low-mass stars have fewer companions) and what the binary systems look like (low-mass stars are in tighter systems). We discovered both the first brown dwarf spectroscopic (close) binary and the first brown dwarf wide (imaged) binary systems.
Stellar Magnetic Activity
Rotation and Activity
Our Sun exhibits magnetic activity in the form of sunspots, active regions, flares, and the hot corona. It turns out to be a relatively weak example of such phenomena - young stars are far more active (as it once was). The level of activity turns out to depend on the stellar rotation rate (fast rotation cranks up the magnetic dynamo), but magnetic activity acts to slow a star down. The study of this subject has long been my interest, and I have contributed to many of the basic conclusions. We are now extending these studies to red and brown dwarfs, where surprising new effects are showing up.
It is hard to measure magnetic fields on stars directly, so most of what we know comes from understanding how to use indirect proxies for the fields. I have done a lot of work in various aspects of how to accomplish this.
Measurement of Magnetic Fields
It is hard to measure magnetic fields on stars directly, but not impossible. I have been one of the leaders in this field, and have developed two new methods as well as improved upon existing ones. We recently have been able to extend direct measurements to very low-mass objects.
Searching for Earth-sized (Exo-)Planets
No extrasolar planets were known before 1995. Now well over 200 have been found, but none are small enough to be like the Earth and have a solid surface. Our best hope for determining the frequency of terrestrial planets is to perform a transit experiment in space. NASA's Kepler Mission is the first serious attempt to do this. I was a Co-Investigator on the mission (the Principal Investigator is Bill Borucki at NASA Ames). It launched in March 2009, and has found over 3500 planet candidates. My role in the mission was to understand the photometric variability caused by stellar activity, and to extract some of the treasure-trove of collateral stellar science.
Photometric Signatures of Activity
I have used the Kepler mission is to understand how stellar activity affects the brightness of stars. I have helped extract some of the stellar science inherent in the unprecedented new precision and time-coverage of stellar light-curves that the mission automatically provides. Among the topics I'm studying are the rotation and differential rotation of stars, starspot coverage and evolution, and using photometric "flicker" to measure stellar gravity.
GB was born May 3, 1951 in New York City to Saul and Phyllis Basri. He grew up in Fort Collins, Colorado, with younger brother David. Saul was a Physics professor at Colorado State Univ; Phyllis taught ballet and other dance. The family lived in Burma in 1957 and in Sri Lanka in 1965, both under Fullbright Lectureship appointments. GB attended Ft. Collins High School. He got a BSc in Physics from Stanford University in 1973, and a PhD in Astrophysics from the Univ. of Colorado, Boulder in 1979. His thesis was on stellar magnetic activity under the direction of Prof. Jeffrey Linsky, and he was an early user of the IUE satellite. An award of a Chancellor's Postdoctoral Fellowship then brought him to Berkeley, where he has been ever since. GB originally worked with Prof. Stu Bowyer on high energy observations of stellar activity, and Prof. Len Kuhi on newly forming stars. GB is married to Jessica Broitman, and has 1 son.
GB joined the faculty of the Berkeley Astronomy Dept. in 1982, received tenure in 1988, and became a full professor in 1994. His work in the 1980s concentrated on star formation and the study of T Tauri stars, as well as continuing studies of stellar magnetic activity. In the 1990s he continued work on these topics, as well as becoming an early pioneer and world expert in the study of brown dwarfs. He wrote an Annual Reviews of Astronomy and Astrophysics article on "Observations of Brown Dwarfs" in 2000, and delivered a plenary lecture to the American Astronomical Society entitled "Brown Dwarfs: Up Close and Physical" in 2004. His discovery work is described in the book 50 Years of Brown Dwarfs (Springer 2014; V. Joergens, Ed.). He was also lead author on an Annual Reviews of Earth and Planetary Science article entitled "What is a Planet?".
GB has written over 200 technical publications, including numerous review articles. There are more than 12000 citations to his works (h-index > 75). GB was awarded a Miller Research Professorship in 1997, and became a Sigma Xi Distinguished Lecturer in 2000. He has served on committees helping to award major NASA and NSF grants and projects, and awarding time on the (then world's largest) Keck telescopes. In Dec. 2001, NASA selected the Kepler mission to discover extrasolar terrestrial planets, and characterize inner extrasolar planetary systems. GB was a Co-investigator on the mission; his main task was to understand the "noise" that stellar variability introduces into the photometric detection of extrasolar planetary transits, and to help extract some of the stellar science that is also possible. The mission successfully showed that planets are very common around stars.
GB is involved in science education, and throughout his career has worked on encouraging the participation of minorities in science. His efforts in this, and on behalf of increasing diversity at the University, were recognized by the Chancellor's Award for Advancing Institutional Excellence in 2006. GB served as Acting Chair for the Astronomy Dept. in 2006-07. In 2007 he was selected by Chancellor Birgeneau after a national search as the founding Vice Chancellor for Equity and Inclusion at Berkeley. He stepped down in 2015, and was awarded the Berkeley Citation (the campus' highest honor). He became Professor of the Graduate School in 2016 (meaning he is an active Emeritus professor).