When someone tells you «look, a flying donkey», you can answer: «you fool, it's a horse!».
The Horsehead Nebula is a dark nebula in the constellation of Orion, formed by a swirl of gas and dust channelled into the shape of a horse's head by a strong magnetic field. Dark nebulae block light: this one is visible because it stands out against the red emission of IC 434 — red because of hydrogen being ionized under the fierce light of Sigma Orionis, the third star to the left of Orion's Belt.
The nebulosity of the Belt is a tangle of gas and dust some 1,500 light years away, and one of the most active star-forming regions in our galaxy.
In the same field we find the Flame Nebula (NGC 2024), also emitting in ionized hydrogen, with a band of dark nebulae across its middle that gives it the shape of a flame; it shines under the ultraviolet radiation of Alnitak, the brightest star of the Belt. Then there are NGC 2023 and IC 435, two diffuse reflection nebulae — here the light comes from the central star — of fluorescent molecular hydrogen, emitting in the ultraviolet.
The gallery holds four versions of the same field: two colour framings, one starless — leaving only the gas — and one taken in the ionized hydrogen band alone (H-alpha), where the detail emerges despite the short exposure.
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