For my whole career I have worked on stellar magnetic activity and stellar atmospheres. Initially this work was on ultraviolet spectroscopy of stellar chromospheres, and computer modeling of semi-empirical model atmospheres and radiative transfer. It later expanded to include hot white dwarfs, rotation-activity relations and relations between diagnostics at different temperatures. My observational work has involved spacecraft including IUE, Hubble, and Kepler, and my ground-based work has been high resolution spectroscopy almost entirely at the Lick and Keck Observatories.
Later I began to work on direct measurement of stellar magnetic fields. I developed a new method based on equivalent widths and improved on a method based on high-resolution spectroscopy of line profiles with Marcy and students. This work was extended into the infrared by my students, especially for T Tauri stars. A while later Reiners and I developed another new method based on molecular spectra that could be used for low mass stars. This work showed that contrary to some previous theories, in fully convective stars the magnetic field still depends on rotation and remains strong.
I conducted an extensive program on stellar rotation over 3 decades, using Doppler broadening prior to 2010 then photometric periods from Kepler. After the discovery of brown dwarfs we proposed the new spectral class of L dwarfs, tested the hypotheses that their optical spectra were dominated by alkali resonance lines and dust, and established the first temperature scale for them. We discovered and studied the death of stellar chromospheres at the bottom of the main sequence, and the relation of that to rotational histories of very low mass stars. I collaborated on work regarding radio and optical emission from brown dwarfs and the auroral hypothesis.
Building on a long relation with the group at Ames, I became a Co-Investigator on NASA’s Kepler mission. This revolutionized the field of exoplanets by gathering ~200,000 precision stellar light curves and searching for planetary transits. It also greatly expanded the field of asteroseismology. Bill Borucki (PI) (for the mission) was awarded the Shaw Prize in Astronomy in 2015. My main role was to anticipate and understand the effects of stellar activity (particularly starspots) on light curves and to help extract the astrophysics that offers. I have continued to significantly advance that science, which is useful for the ongoing and upcoming photometric space missions.