Unformatted text preview: of doubly-ionized oxygen (that give them their green color), doubly-ionized neon, and singly-ionized helium. (The image of the Ring Nebula on the left is courtesy of Palomar Observatory .) AAO image Planetary nebula get their name because some looked like round, green planets in early telescopes. They are now known to be entirely different than the planets and are about one or more light years across (much larger than our solar system!). Originally, we thought planetary nebulae were simple expanding spherical shells that look like rings on the sky because when you look along the edge of the expanding spherical shell, you look through more material than when you look toward the center of the shell. The round soap bubbles you made as a child (or still do!) look like rings for the same reason. Indeed, many of the planetary nebulae first seen, like the Ring Nebula in Lyra and the Helix Nebula in Aquarius look like rings....
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- Fall '10
- Astronomy, outer layers, Ring Nebula