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Splashes of colour illustrate the clear difference between the two most populous type of bright nebulae in our Milky Way Galaxy. This image, taken by the wide-field OmegaCAM on the VLT Survey Telescope at the European Southern Observatory in Chile, is dominated by the wine-red hue of an emission nebula called IC 1284. An emission nebula quite literally emits light from the hydrogen gas that forms the vast majority of the composition of the nebula. Emission nebulae are star-forming regions, and when ultraviolet light from hot, young stars is absorbed by hydrogen, the hydrogen becomes ‘excited’ for a moment, and the hydrogen settles back down by emitting light at this characteristic red wavelength (known as hydrogen-alpha, a common wavelength of 659 nanometres used by professional and amateur astronomers alike). We can see some of these hot, young stars in the centre of IC 1284.
To the lower-right in the image, however, the hue turns decidedly blue with two additional nebulae, catalogued as NGC 6589 and 6590. These are reflection nebulae; they don’t emit the light themselves, but they reflect and scatter the light of nearby stars instead. Because blue light is scattered more easily than red, reflection nebulae appear blue. This principle of scattering is also why the sky appears blue during the day, because the atmosphere scatters more blue light than other colours.
Image: ESO/VPHAS+ Team.
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