 Here's an interesting story about unidentified infrared emission bands discovered in the early 1970s. Called UIR bands, they are found in a wide variety of celestial objects from planetary disks to starburst galaxies. And they have been observed as far back in time as when the universe was a mere 3.3 billion years old. This makes them very important for astronomers to understand. Here's a spectrum of the iris nebula taken by Spitzer showing the UIR bands. Just what could be causing these emissions was a mystery. Astronomer Chris Selgren, while working on her PhD at Caltech, made a discovery that turned out to be the key in unraveling the mystery. She found that sources of the unidentified infrared radiation also emitted continuously in the near infrared from dust grains heated to 1,000 degrees Kelvin. This temperature was too hot to be from a large number of dust grains, so she hypothesized that while most of the grains were far cooler than this, some would be heated for very short periods of time to 1,000 K by the absorption of a high-energy UV photon. These would then quickly cool to much lower temperatures, creating the unidentified near infrared emissions. The most likely source for the unidentified infrared radiation had to be a carbon-based dust grain, because they are the only material able to withstand the harsh environments where this radiation is observed. Selgren's discovery led the astrochemists at NASA and a competing group in France to investigate carbon-based soot particles in the laboratory. They found that the emission spectra from the soot produced a good match to the UIR spectrum. The molecules involved are called polycyclic aromatic hydrocarbon molecules, or PAHs or short. They're called aromatic, because they actually smell bad in the labs. When a high-energy ultraviolet photon hits the molecule, it is absorbed, creating a characteristic absorption line, heating the molecule to 1,000 K, and kicking it into its natural vibration modes. These vibrations accelerate electrons and produce infrared photons ejected in all directions. This drives the molecule's temperature back down to normal. Today, it is understood that this class of molecule is almost certainly responsible for the unidentified infrared emission bands. Research into this kind of radiation continues, with new images from web, adding significant additional data. In these three web mid-infrared images, the polycyclic aromatic hydrocarbons are found mostly within the main dust lanes, in the spiral arms. These images are providing insights into how PAH molecular dust depends on local interstellar medium conditions. This research is important because these particles account for up to 15% of the carbon in galaxies, and play a critical role in the formation of stars and planets. They are thought to be the starting material for the earliest life forms. In fact, they may have been vital in the formation of life here on Earth.