The nebula in which Sarek's sabotaged ship drifts in 2256, forcing Burnham to reach her dying foster father through their shared katra before he can be found.
The Yridia Nebula is a region of space seen in the Discovery episode Lethe. Sarek is crossing it on his way to peace talks when his Vulcan pilot, a logic extremist, blows himself up, leaving the ambassador badly injured aboard a drifting, powerless ship.
Burnham feels his distress through the fragment of his katra she has carried since childhood. Lorca takes Discovery to the nebula against orders, but its interference hides the wreck from sensors, so Stamets builds a device that lets Burnham strengthen the link while she, Tilly and Tyler search by shuttle.
Inside Sarek's mind she keeps meeting the memory he is fixed on: the day the Vulcan Expeditionary Group forced him to choose between her and Spock. Once she understands it, she rouses him long enough to trigger his transponder, and the shuttle brings him back.