One of the fun things about covering so many different areas of science is that you often come across things you had no idea existed. This week's discovery goes by the name of "diffuse interstellar bands," a phenomenon first discovered back in the 1920s. Since then, dozens of these DIBs, which represent areas of the spectrum that are absorbed by the interstellar medium, have been discovered. Despite these discoveries—and this week's Nature describes a few more of the bands—we really don't know what's causing them.
Most stars emit radiation over a very broad spectrum, from the ultraviolet well down past the infrared. However, if there's some material between us and the star—say, for example, a cloud of hydrogen—then it can absorb some of that radiation. But it won't absorb it evenly. Instead, each element or compound absorbs a specific set of wavelengths that correspond to the energy gaps between its ground state electrons and their excited states. So a cloud of hydrogen that sits between us and a star will cause some very narrow gaps in the spectrum of the star when we observe it.
Diffuse interstellar gaps seem to operate on this principle, except they're a bit weirder. To start with, they're not the very narrow absorption peaks that typically result from a single element or a small mixture of them—hence the "diffuse" in their name. For another, there are dozens of them now cataloged, and none of them correspond to any known absorption pattern of an element or simple chemical. Finally, the strength of a single band will rarely correlate with the strength of any others, meaning that there must be a mixture of chemicals doing the absorbing, with the quantities of each varying depending on where we're actually doing the observing.
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Thank's!