Orion's famous horse's head is a silhouette of dust, found on a photograph
Nobody first spotted the Horsehead Nebula through an eyepiece. In 1888 the Scottish astronomer Williamina Fleming noticed it on a photographic plate at Harvard College Observatory: a dark shape like a chess knight, outlined against a glowing red curtain of gas about 1,375 light-years away, just south of Orion's Belt.
The horse is not a glowing object at all. It is a cloud of dense dust thick enough to block the light of stars behind it, so it shows up as a hole cut into a brighter backdrop. That backdrop is IC 434, a sheet of hydrogen ionised by the nearby hot star Sigma Orionis, which gives colour photographs their deep red tone. The Horsehead juts from the edge of a larger dust cloud called Lynds 1630, part of the sprawling Orion molecular cloud complex, and the lower part of its neck throws a shadow to one side.
Soon after Fleming's find, E. E. Barnard described it in his catalogue of dark nebulae as a dark mass about four arcminutes wide on a nebulous strip south of Zeta Orionis, one of the Belt stars also known as Alnitak. It has been listed as Barnard 33 ever since.
Dark does not mean dead. Bright specks near the base of the Horsehead are young stars still forming, mostly of low mass. Magnetic fields steer gas escaping from the cloud into streams that show up as streaks against the background glow, and the density of stars differs noticeably on either side of the glowing hydrogen strip that marks the cloud's edge. Orion climbs high in winter evenings for northern observers and in summer evenings south of the equator.
The shape has even inspired a science fiction joke. In Isaac Asimov's 1951 novel The Stars, Like Dust, one character explains that it is named for its resemblance to a horse's head, while another, raised on a distant planet, insists it honours Horace Hedd, the first man to fly into it.
Source: Horsehead Nebula
