Mystery of 'Hot, Young Stars' in Ancient Red Galaxies --Solved! New evidence from NASA's Wide-field Infrared Survey Explorer (WISE) and Galaxy Evolution Explorer (GALEX) missions provide support for the "inside-out" theory of galaxy evolution, which holds that star formation starts at the core of the galaxy and spreads outward. Like tree rings, inner and outer portions of a galaxy's disk are a historical record. Two NASA missions find evidence that star formation bursts started in galaxy centers and spread outward. Unexplained ultraviolet light might come from a late phase in the lives of older stars .