Nemesis is the name given to a hypothetical red dwarf star, magnitude between 7 and 12, orbiting the Sun at a distance of about 50,000 to 100,000 AU, somewhat beyond the Oort cloud.
Nemesis' existence was proposed by Richard A. Muller in 1984 (Nature, vol 308, pp 715-717, 1984) to explain an apparent 26-million year cycle in the occurrence of mass extinctions on Earth as noted by Raup and Sepkoski.
According to the theory, Nemesis periodically (roughly every 26 million years), passes through a denser region of the Oort cloud, disrupting the orbits of comets, and sending millions into the inner solar system and potential collision with the Earth. It was initially nicknamed "The Death Star" for this property, although this nickname has fallen out of usage due to its fictional use in the Star Wars universe.
No direct evidence for Nemesis has been found, however, and the existence of a periodicity in the Earth's series of mass extinctions is disputed.
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