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Unformatted text preview: because we have learned that planetary nebulae have extraordinarily bright oxygen emission lines. These oxygen emission lines have a wavelength of around 501 nanometers, so once we have ruled out any objects that do not share this specific characteristic, we will be a lot closer to our goal. We then have to factor in other specific properties of planetary nebulae that will bring us even further towards our goal. We have learned that planetary nebulae are typically elliptical in shape, while stars are typically circular. We also know that archetypal planetary nebulae are about one light year in diameter, and the density of the ionized gas in the nebulae is higher than that of the surrounding interstellar medium. Taking these attributes into account will help us establish a basic set of requirements for these nebulae and therefore will ultimately create a finalized list of the nebulae in the galaxy. Sources http://www.daviddarling.info/encyclopedia/P/planneb.html...
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