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.
NASA Web Site Statements, Warnings, and Disclaimers
NASA Official: Jay Norris. Specific rights apply.
A service of: LHEA at NASA / GSFC
& Michigan Tech. U.
Based on Astronomy Picture Of the Day