Astronomy Picture of the Day

Explanation: 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 colour 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 centre, 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.