There's nothing planetary about these nebulae. They were named by William Herschel in the late 18th century; he linked their romantic green hue to those of the methane-shrouded Neptune and Uranus. Nebulae consist of a gaseous shell of ionized elements edging away from a dying star at the rate of 10-30 kilometers per second. When giant stars (ones greater than a solar mass or two) begin to burn out, you can expect a supernova. But when medium and low mass stars (our dear one included) run low on helium, the star is no longer stable and the temperature of the core spikes so much that outer layers are forced away until the core is bared. This cooling core is now the central star of the glowing procession that is the nebulae, a parade that will persist until the star has nothing left--not even the UV radiation responsible for the nebulae's glow--and a white dwarf is born...And if all this star travel is giving you a cramp, the brewers at Oakham Ales have a lighter interpretation of the old W.D.
Planetary Nebulae: Our Galactic Pallbearers
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March 27, 2007 - 09:32am