The sun of the planet Sarpeidon, which explodes as a nova in All Our Yesterdays and later serves as a gateway to a reversed universe in The Counter-Clock Incident.
Beta Niobe is the star orbited by Sarpeidon. In the Original Series episode All Our Yesterdays the Enterprise arrives a few hours before it is due to go nova, expecting to warn the inhabitants, and finds only the librarian Mr Atoz. Everyone else has fled into the planet's past through a time portal called the atavachron.
Kirk, Spock and McCoy are sent into the past by accident and barely return before the star explodes. The Enterprise leaves at warp as Sarpeidon is destroyed.
The star returns in the Animated Series episode The Counter-Clock Incident. There the nova, paired with a newborn star called Amphion in a universe where time runs backwards, forms a passage between the two universes through which the Enterprise is dragged.