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Nemesis, in Greek mythology, is divine retribution personified as a goddess. Such harsh divine justice is a major theme in the Hellenic world view, providing the unifying theme of the tragegies of Sophocles and many other mythological works. In some versions of the myth, Nemesis is the mother of Helen of Troy.

Nemesis is the name given to a theorized companion star orbiting the sun at a great distance beyond the Oort cloud, which periodically (on a scale of millions of years), passes through a denser region of the Oort cloud, disrupting the orbits of comets, sending them into collision wit the Earth and the other planets. Nemesis was proposed to explain the recurring mass extinctions on Earth, but this theory has been discredited. There is no credible evidence for the existence of Nemesis.

Nemesis was also a science fiction book written by Isaac Asimov, based on the hypothetical dwarf star companion of the Sun.