The patches of colour illustrate the clear difference between the two most abundant types of bright nebulae in our Milky Way galaxy. This image, taken by the OmegaCAM wide-field camera on the European Southern Observatory's Very Large Telescope Survey in Chile, is dominated by the wine-red hue of an emission nebula called IC 1284. An emission nebula literally emits light from the hydrogen gas that makes up most of its composition. Emission nebulae are star-forming regions, and when ultraviolet light from hot, young stars is absorbed by hydrogen, it becomes “excited” for a moment and settles back down to emit light in 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.
However, at the bottom right of the image, the hue is made distinctly blue by two additional nebulae, cataloged as NGC 6589 and 6590. These are reflection nebulae; they do not emit light themselves, but instead reflect and scatter light from nearby stars. Because blue light is scattered more easily than red light, reflection nebulae appear blue. This scattering principle is also why the sky appears blue during the day, because the atmosphere scatters more blue light than other colors.
Image: ESO/VPHAS+ Team.
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