HENNING DALHOFF / SCIENCE PHOTO LIBRARY HENNING DALHOFF / SCIENCE PHOTO LIBRARY
Nemesis star. Computer artwork of the star Nemesis, a hypothetical companion star to the Sun (upper left). It is thought that Nemesis is either a red or brown dwarf that moves in an elliptical orbit at a distance from the sun that varies between 90,000 astronomical units and 20,000 astronomical units. This path would lead it through the Oort cloud, a cloud of comet nuclei. One theory is that this would send some comets on a path to Earth and so would explain the periodic mass extinctions of life, such as that which wiped out the dinosaurs. However Nemesis has not been found despite extensive searches.
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