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Astronomer Emeritus, Space Telescope Science Institute and Professor of Practice of Astronomy & Astrophysics, Penn State

Research Interests include:
V838 Monocerotis and the new classes of luminous red transients
Light echoes
Planetary nebulae and their central stars
Post-asymptotic-giant-branch (post-AGB) stars
Extragalactic distance scale
Extrasolar planets
Astrometry and trigonometric parallaxes
Novae and cataclysmic variable stars
White dwarfs
Close binary and variable stars; global monitoring
Globular clusters
Visual binary stars
Stellar chemical compositions
Peculiar red giants
Population II and III stars