The Abode of Life (Star Trek #6)

ON THE PLANET MERCAN THERE ARE NO STARS, NO MOON, NO OUTER SPACE… The citizens of Mercan cannot conceive of worlds beyond their own. Their sun, Mercaniad, is prone to deadly, radioactive flare-ups, and the Mercans have organized their life around the need to survive The Ordeal — until a strange visitor appears from out of nowhere… The Enterprise, badly crippled and in desperate need of repairs, must seek help from a people who cannot believe in its existence. Mercaniad is about to blow, and James Kirk faces an impossible choice: to attack the sun itself and save his ship and crew — or let a people live in peace, in the only world they know…