ocr: PLANETARY NEBULA Expanding, gaseous spheres moving away from a dying star, usually a red giant. The nebula has been ionized by the remnants of its central star, thus be- ing an emission nebula. The Ring Nebula in the constellation Lyra is very ypical planetary nebula. The ferm planetary nebula was coined by the German-English astronomer, William Herschel (1730 - 1822), as they reminded him fainfly of planets. He wrongly assumed that planetary nebulae develop into stars. More than 1,600 planetary nebulae have already been discovered in our Galaxy, but the total number is presumed to be twenty t ...
