A life of research
A Swiss physicist born in Varna, he joined the California Institute of Technology. He combined ambitious theoretical ideas with systematic surveys of the sky, particularly through work at Palomar Observatory.
With collaborators, he searched many galaxies for rare events. Building comparable sets of observations helped astronomers distinguish different kinds of stellar explosions and investigate their physical causes.
Understanding the contribution
In 1933 he found that galaxies in the Coma Cluster moved too quickly to be held together by the estimated luminous mass alone. He invoked additional dark matter to explain their collective behaviour.
Working with Walter Baade, he proposed in 1934 that supernovae were associated with the formation of neutron stars. This linked spectacularly bright explosions with tiny remnants containing matter at extraordinary density.