... NASA Explanation: The two stars at the center of this nebula are very different. One is a white dwarf star with a mass similar to our Sun but with a radius as small as our Earth . ... The red giant star turns out to be a Mira variable , a pulsating star that is dumping mass into an accretion disk surrounding the white dwarf . Astronomers speculate that when enough mass falls onto the white dwarf, a thermonuclear explosion occurs creating another expanding hourglass-shaped nebula. ...