White dwarf stars in globular cluster NGC 6397. (NASA, ESA, and H. Richer/University of British Columbia) SPACE ====================================================================== When stars like the Sun reach the end of their lives, the object that remains is a white dwarf. This is the star's shrunken, naked core, no longer capable of nuclear fusion. It shines, but only with residual heat, slowly cooling over billions of years until it's completely cold and dark. But not all white dwarfs cool the same way. Last year, astronomers found a certain type of massive white dwarf stars cool more slowly than others, as though they have...