Zeta Cancri was a star system, or constellation, comprised of three stars in the Zeta Four Sector. When the Time Lords were monitoring that sector closely, they noted that it was beset by explosions — and that those explosions could make two of the three stars collide. If that happened, their computers predicted, Zeta Cancri would go nova. The simultaneous destruction of three stars in such close proximity would cause the destruction of the galaxy — and Gallifrey itself. Thus, they sent in K9 to help defuse the situation.
When he arrived in that solar system in K-Nel Mk 2, he discovered that the explosions were being caused by war. Whole planets were being destroyed intentionally, thus affecting the gravity in the solar system. Nevertheless, he found a way to disperse the gravity whirlpool that had been created, and thus prevented the stars of Zeta Cancri from colliding. (PROSE: K9 and the Zeta Rescue)