Nemesis is a hypothetical small stellar companion of the Sun. Once every 30 million yr, it has been hypothesized, this dim or dark star would pass through the Oort Cloud, triggering comets that perhaps caused periodic mass extinctions on Earth. A leading champion of the idea is Richard A. Muller, a physicist at the University of California at Berkeley. Muller believes Nemesis is a red dwarf whose orbit ranges from 1 to 3 light-years away from the Sun. No convincing evidence for such an object has been found to date.