Planetary nebulae (PNe) are shells of gas and dust that have been ejected from a star during the process of its evolution from a hydrogen-burning main sequence star into a red giant and eventually into a white dwarf. The lifetimes of PNe are relatively short, on the order of 20,000 years, so they are relatively rare, with about 1,500 known in the Galaxy. The nebulae are approximately a light year in size and are illuminated and ionized by the hot central star. The ejected material is an important source of mass return and enrichment of the interstellar medium.
There are many outstanding questions about the process of PNe formation. One puzzling aspect is how such a wide variety of shapes and nebula properties are form ed from material ejected from a spherical star. Very few PNe have the simplest structure of a round shell; most are bipolar or "butterfly"-shaped, or elliptical . Somehow the material from the star is transformed into complex shapes, with evidence of jets and clumps being ejected at various times during its formation. The complex structure of the Helix nebula is shown in the image to the right. The Helix is thought to have an overall bipolar shape, but appears ringlike because we are viewing it nearly pole-on.
Joseph Hora, Andrew Szentgyorgyi