Pictured above is the planetary nebula NGC 5882, captured by the Hubble Space Telescope . Although planetary nebulae can appear similar to planets like Uranus and Neptune , they are actually gas clouds surrounding stars typically hundreds of light years away. Planetary nebula form when a typical star completes fusion in its core and ejects an outer envelope of gas - usually about 10 percent of the star's initial mass. This gas shell dims in about 50,000 years - short compared to the lifetimes of stars . Therefore, although only about 1000 planetary nebula are known in our Galaxy , it is thought that most stars go through this phase.
Green light is emitted when oxygen ions acquire electrons from the surrounding gas.
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