A planetary nebula forms when a star can no longer support itself by fusion reactions in its center. The gravity from the material in the outer part of the star takes its inevitable toll on the structure of the star, and forces the inner parts to condense and heat up. The high temperature central regions drive the outer half of the star away in a brisk stellar wind, lasting a few thousand years. When the process is complete, the remaining core remnant is uncovered and heats the now distant gases and causes them to glow.
Why "Planetary" Nebula?
Despite the name, these objects are totally unrelated to "planets". It is commonly thought that they may represent the final episode of the Sun's existence as a star. This concept has been questioned recently by Jacoby, Fullton, Morse, Kwitter and Henry ( 1997) and Bond (2001) - wherein evidence from globular cluster stars indicates that stars must be about 20% heavier than the Sun to form a PN. It is estimated that there are about 10,000 planetary nebulae in our galaxy, so they are a relatively common, although short-lived phase (about 25,000 years) of the stellar life cycle.
Here are several rather unusal examples of planetary nebulae. All were taken at Kitt Peak National Observatory (unless stated otherwise) by George Jacoby.
Why are the images in black and white?
Most of these pictures are shown in simple black and white because the cameras used on almost all telescopes only record a single color in a single exposure. Astronomers usually look at shades of grey rather than color images because the eye sees more detail in black and white. When images of these objects are taken in more than a single color, the pictures can be combined to form a color image (see Color Production info).
But even a color image created in this way is not what your eye would see when you look through a telescope. That's because these nebula emit light at very specific colors, mostly in blue-green and red and not much in-between. Your eye is tuned to seeing things like the Sun or light bulbs that emit light at all colors. For more information about the colors in nebulae, see this example of a PN in a globluar cluster.
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