M27: Not a Comet
While hunting for comets in the skies above 18th century France, astronomer Charles Messier diligently kept a list of the things he encountered that were definitely not comets. This is number 27 on his now famous not-a-comet list . In fact, 21st century astronomers would identify it as a planetary nebula , but it's not a planet either, even though it may appear round and planet-like in a small telescope. Messier 27 (M27) is an excellent example of a gaseous emission nebula created as a sun-like star runs out of nuclear fuel in its core. The nebula forms as the star's outer layers are expelled into space, with a visible glow generated by atoms excited by the dying star's intense but invisible ultraviolet light . Known by the popular name of the Dumbbell Nebula , the beautifully symmetric interstellar gas cloud is over 2.5 light-years across and about 1,200 light-years away in the constellation Vulpecula . This impressive color composite highlights details within the well-studied central region and fainter, seldom imaged features in the nebula's outer halo . It includes narrowband images recorded using filters sensitive to emission from oxygen atoms, shown in blue-green hues, and hydrogen atoms in red.