January 29, 2002
The oldest light in the universe still hums faintly through space, a relic of the beginning itself.
The Southern Sky in Warm Hydrogen
Birthday · January 29, 2002
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APOD published January 29, 2002. Full image credits are on the original APOD entry below. A publication-date match, not a photo taken on your date. Independent of NASA.
A robotic telescope with red sunglasses in Chile has been photographing the entire southern sky for years. The result, shown above, is the most complete sky map of the most common visible light emitted from the most abundant element in our Galaxy: hydrogen. A very specific red color emitted by warm ionized hydrogen was observed. Although spectacular emission nebulas glow brightly in this type of red light (H-alpha), a diffuse amount of warm hydrogen is spread throughout our Galaxy and its glow nicely indicates not only where darker hydrogen and other gasses may be located, but also the sometimes- complex history of interstellar gas. Gas tracking the plane of our Galaxy runs across the center, and huge gas clouds, some of which are the expanding shells of long dead stars, are also visible. The above map was derived from the Southern H-Alpha Sky Survey Atlas (SHASSA) and shows that our entire Galaxy is one big emission nebula, albeit at a quite faint level.