The Horsehead and Alnitak region
The Horsehead Nebula is one of the most famous nebulae on the sky. It is visible as the dark indentation to the red emission nebula seen above and to the right of center in the above photograph. The Horsehead is some 1,500 light-years distant, embedded in the vast Orion cloud complex. About five light-years "tall", the dark cloud is cataloged as Barnard 33 and is visible only because its obscuring dust is silhouetted against the glowing red emission nebula IC 434. Contrasting blue reflection nebula NGC 2023 is visible on the lower left. In this color image, both Horsehead and NGC 2023 seem to be caught in beams of light shining from above -- but the beams are actually just internal reflections from bright star Sigma Orionis, just off the upper edge of the view. The bright star on the left is located in the belt of the familiar constellation of Orion. The horse-head feature is dark because it is really an opaque dust cloud which lies in front of the bright red emission nebula. Like clouds in Earth's atmosphere, this cosmic cloud has assumed a recognizable shape by chance. After many thousands of years, the internal motions of the cloud will alter its appearance. The emission nebula's red color is caused by electrons recombining with protons to form hydrogen atoms. Also visible in the picture are blue reflection nebulae, which preferentially reflect the blue light from nearby stars. (Text adapted from APOD).
Apo TEC140 (140/f7.2) - FLI Proline 16803 - Ha (270m) L (200m) R (120m) G (120m) B (120m) - Warrumbungle Observatory, Coonabarabran, NSW, Australia