Nemesis is a theoretical dwarf star thought to be a companion to our sun. The theory was postulated to explain a perceived cycle of mass extinctions in Earth's history.
The Nemesis theory proposed that the mass extinctions were caused by comet impacts and that they occurred on a regular basis because there was some large mass object, dubbed the Nemesis Star, in a binary orbit with our Sun.
Dubbed "Nemesis" (after the Greek goddess of retribution), the companion star through its gravitational pull unleashes a furious storm of comets into the inner solar system that lasts anywhere from 100,000 years to two million years.