As seen on Astronomy Picture of the Day in this serene nightscape, the Milky Way’s graceful arc stretches over prominent peaks of Dolomites in the Italian Alps known as Tre Cime di Lavaredo. In this 180 degree wide-angle panorama the scene does look to the north and the sky is suffused with an eerie greenish light. Still, the subtle glowing bands are not aurorae, but airglow. Unlike aurorae powered by collisions with energetic charged particles and seen at high latitudes, airglow is due to chemiluminescence, the production of light in a chemical reaction, and found around the globe. The chemical energy is provided by the Sun’s extreme ultraviolet radiation. Like aurorae, the greenish hue of this airglow does originate at altitudes of 100 kilometers or so dominated by emission from excited oxygen atoms. More easily seen near the horizon, airglow keeps the night sky from ever being completely dark. Click on the second photo to see a day-time view of these peaks.



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