In the center of the cloud of gas, and completely dwarfed by the nebula, are the dying remnants of a red giant. During the final convulsive phase in the evolution of the stars, huge clouds of gas are ejected from the surface of the star before it emerges from its cocoon as a white dwarf . The intense ultraviolet radiation from the tiny remnant star then causes the surrounding gas to glow like a fluorescent sign.
by the standards of astronomical phenomena, planetary nebulas like NGC 3918 are very short-lived, with a lifespan of thousands of years.
Hubble's Wide Field and Planetary Camera 2.