A planet orbiting the star Beta Niobe, whose people flee its coming nova by traveling into their own history.
Sarpeidon is the only planet of the star Beta Niobe. In All Our Yesterdays, the star is about to go nova, and the Enterprise arrives to find the planet empty except for the librarian Atoz. The population has used the atavachron to escape into the past.
Kirk is sent to an era of witch trials, while Spock and McCoy are sent to an ice age, where they meet the exiled Zarabeth and Spock reverts to the emotions of his ancestors. They return just before the star explodes.