When its fuel runs out, an old star expands into a red giant and then collapses to become a white dwarf. Some dwarfs can go through a second period of gianthood because the collapse squeezes and heats leftover fuel, but astrophysicists expected the second red giant phase to last perhaps a few centuries. Now a white dwarf that reignited in 1996 has shown signs of heating up again, indicating that it has already passed through the cooler red giant stage. Radio telescope measurements of the star, known as Sakurai's object or V4334 Sgr, picked up a crackle characteristic of gases ionizing around it, which would require its temperature to have risen quite a bit since the late 1990s. The rapid evolution may be the result of the dwarf's innards mixing poorly, leading the star to burn only the fuel close to its surface, which would quickly run out, hypothesize University of Manchester researchers in the April 8 Science.
This article was originally published with the title "Twice Burned" in Scientific American 292, 6, 34 (June 2005)