Cold star

A cold star was a celestial object that existed in a dream shared by Rory Williams, Amy Pond, and the Doctor, created by psychic pollen that had become lodged in the inner workings of the TARDIS.

Unlike a normal star, the cold star burned cold. Whereas other stars emitted warming radiation, the apparently endothermic cold star caused temperatures to plummet approaching the star itself.

The Doctor claimed he had never encountered a cold star prior to the psychic pollen episode, and he and Amy were skeptical such a thing could exist. After the resolution of the dream states, ending the apparent threat of the cold star, it remained unclear to the Doctor and his companions whether such an object could exist in reality, though the Doctor did say "Star burning cold? Do me a favour", implying that a cold star could not exist. (DW: Amy's Choice)