A bright cloud of glowing gas and dust surrounding a highly evolved star. A planetary nebula forms when a red giant ejects its outer layers at speeds of about 10 km/s. The ejected gas is then ionized by ultraviolet light from the hot core of the star. As mass is lost this core is progressively exposed, and it ultimately turns into a white dwarf. Planetary nebulae are typically 0.5 l.y. across, and the amount of ejected material is 0.1 solar mass or more. Because the core is so hot the gas in the nebula is highly ionized. The planetary nebula exists for up to 100 000 years, during which time a sizable fraction of the star's mass is returned to interstellar space. Planetary nebulae were so named because they appeared to early observers to resemble a planetary disk. In fact the detailed shapes of planetary nebulae revealed by modern telescopes cover many different types, including ring-shaped (as in the Ring Nebula), dumbbell-shaped, or irregular. The various apparent shapes are now thought to be due to the angle at which we are viewing two lobes or cylinders of gas ejected in opposite directions from the central star in a bipolar outflow.
Subjects: Astronomy and Astrophysics.