Halton Arp was an American astronomer known for his work in the field of galaxy morphology and the study of peculiar galaxies. He is best recognized for creating the Arp Catalog, a collection of 338 galaxies that exhibit unusual shapes and interactions. This catalog has been influential in understanding galaxy formation and evolution. Arp was born on March 21, 1927, and spent much of his career at the Mount Wilson Observatory in California. He often challenged conventional views about the relationship between galaxies and their redshifts, proposing alternative theories regarding the nature of dark matter and the expansion of the universe. Arp passed away on April 28, 2022.