Astronomy Picture of the Day Index - Nebulae: Dark Nebulae

APOD: 2006 April 9 - Molecular Cloud Barnard 68
Explanation: Where did all the stars go? What used to be considered a hole in the sky is now known to astronomers as a dark molecular cloud. Here, a high concentration of dust and molecular gas absorb practically all the visible light emitted from background stars. The eerily dark surroundings help make the interiors of molecular clouds some of the coldest and most isolated places in the universe. One of the most notable of these dark absorption nebulae is a cloud toward the constellation Ophiuchus known as Barnard 68, pictured above. That no stars are visible in the centre indicates that Barnard 68 is relatively nearby, with measurements placing it about 500 light-years away and half a light-year across. It is not known exactly how molecular clouds like Barnard 68 form, but it is known that these clouds are themselves likely places for new stars to form. It is possible to look right through the cloud in infrared light.

APOD: 2005 May 21 - Snake in the Dark
Explanation: Dark nebulae snake across a gorgeous expanse of stars in this wide-field view toward the pronounceable constellation Ophiuchus and the centre of our Milky Way Galaxy. In fact, the central S-shape seen here is well known as the Snake Nebula. It is also listed as Barnard 72 (B72), one of 182 dark markings of the sky catalogued in the early 20th century by astronomer E. E. Barnard. Unlike bright emission nebulae and star clusters, Barnard's nebulae are interstellar dark clouds of obscuring gas and dust. Their shapes are visible in cosmic silhouette only because they lie in the foreground along the line of sight to rich star fields and glowing stellar nurseries near the plane of our Galaxy. Many of Barnard's dark nebulae are themselves likely sites of future star formation. Barnard 72 is a few light years across and about 650 light years away.

APOD: 2003 July 13 - The Horsehead Nebula
Explanation: One of the most identifiable nebulae in the sky, the Horsehead Nebula in Orion, is part of a large, dark, molecular cloud. Also known as Barnard 33, the unusual shape was first discovered on a photographic plate in the late 1800s. The red glow originates from hydrogen gas predominantly behind the nebula, ionized by the nearby bright star Sigma Orionis. The darkness of the Horsehead is caused mostly by thick dust, although the lower part of the Horsehead's neck casts a shadow to the left. Streams of gas leaving the nebula are funneled by a strong magnetic field. Bright spots in the Horsehead Nebula's base are young stars just in the process of forming. Light takes about 1500 years to reach us from the Horsehead Nebula. The above image was taken with the 0.9-metre telescope at Kitt Peak National Observatory.