US astronomer who has worked particularly on the identification of galaxies.
He published Atlas of Peculiar Galaxies 1965.
He also carried out the first photometric work on the Magellanic Clouds - the nearest extragalactic system.
Arp was born in New York and educated at Harvard and the California Institute of Technology.
In 1953 he began research at the observatory on Mount Palomar, and after working at a number of other institutions returned there in 1969.
During the 1980s he put forward some unorthodox views on the red shifts of quasars.
In the ensuing controversy he left the Palomar Observatory and went to the Max Planck Institute for Astrophysics, near Munich, in Germany.
During his research on globular clusters, globular-cluster variable stars, novae, Cepheid variables, extragalactic nebulae, and so on, Arp has attempted to relate the listings of galaxies to radio sources; the optical identification of these sources can now be done fairly accurately.
He is working with other astronomers on the question whether the red shifts in the spectrum of quasars are due to the general expansion of the universe.