Pictured here are the remains of a sun-like star in the constellation Gemini. As revealed by the Hubble Space Telescope, “the remnant star visible at the center of [planetary nebula] NGC 2371 is the super hot core of the former red giant now stripped of its outer layers.” Why do planetary nebulas glow? It all goes back to the extreme heat from the hot remnant star at the center of it all. But does that mean a nebula lasts forever? No, says NASA. Eventually the nebula will dissipate into space and the remains of this star will become a white dwarf. Hypothetically, a white dwarf may eventually cool to become a black dwarf, which means it is so dark it appears invisible in space.