Astronomy Picture of the Day

Explanation: This helmet-shaped cosmic cloud with wing-like appendages is popularly called Thor's Helmet. Heroically sized even for a Norse god, Thor's Helmet is about 30 light-years across. In fact, the helmet is actually more like an interstellar bubble, blown as a fast wind from the bright, massive star near the bubble's centre sweeps through a surrounding molecular cloud. Known as a Wolf-Rayet star, the central star is an extremely hot giant thought to be in a brief, pre-supernova stage of evolution. Catalogued as NGC 2359, the nebula is located about 15,000 light-years away in the constellation Canis Major. The sharp image, made using broadband and narrowband filters, captures striking details of the nebula's filamentary structures. It shows off a blue-green colour from strong emission due to oxygen atoms in the glowing gas.