The humanoid inhabitants of Sarpeidon, who escaped the imminent nova of their sun Beta Niobe by using a time portal called the atavachron to live out their lives in earlier periods of their history.
The people of Sarpeidon appear in the Original Series episode All Our Yesterdays. When the Enterprise arrives in 2269, hours before Beta Niobe explodes, the whole population has vanished. Only the librarian, Mr. Atoz, and his replicas remain, tending a library of discs that each show a period of the planet's past.
His machine, the atavachron, prepares a traveller's body for a chosen era and sends them through a doorway into it. Once prepared, a person cannot survive a return to the present.
Kirk stumbles unprepared into an age resembling seventeenth-century England and is accused of witchcraft. Spock and McCoy land in an ice age five thousand years back, where they meet Zarabeth, exiled there by the tyrant Zor Kahn. Spock begins to revert to the passions of his ancient ancestors before the three make it back and Atoz escapes to his own chosen time.