Binary star systems are not unusual in our universe, and some scientists accept the possibility that our sun is a member of such as system — that our sun has an as—yet—unseen companion start. It is conjectured that this hypothetical star has an elliptical orbit that brings it close to the solar system approximately every twenty—six million years.

Astronomers believe that a cloud of virtually billions of comets circles the sun in an orbit far beyond that of Pluto's. It is speculated that when the approaching star gets close to the solar system enough gravitational pull is exerted on the comets to disrupt their orbits. This causes a few of them to careen toward the sun, creating the possibility of one or more of them impacting with the earth.

Mass extinctions of plant and animal life are believed to result from such collisions. Supporting this belief is the discovery by scientists that worldwide major biological extinctions do seem to occur approximately every twenty—six million years. There is growing evidence to indicate that dinosaur extinction may have resulted from the collision of the earth with a gigantic body from outer space.

Whether the extinction theory is true or not, it is small wonder that proponents of this hypothesis refer to this star as Nemesis!

From the book: 
Our Fascinating Earth