Below, an image of the Horsehead Nebula, an 'elephant'-trunk structure in Orion. The nebula is a dark cloud of dust and gas silhoutted against hydrogen emission from gases in IC 434, which lies behind it. Many of the more prominent absorption nebulae were first cataloged in the 1890's by E. E. Barnard; because of this, the Horsehead is also known as Barnard 33. (Tracey and Russ Birch/Flynn Haase/AURA/NSF/NOAO)

Below: Since the Balmer-line (hydrogen) radiation of the emission nebula is in visible light the Horsehead nearly disappears in this infrared image. (A portion of an image by ESO/J. Emerson/VISTA/Cambridge Astronomical Survey Unit, posted as apod100112; posted with permission of ESO)

"Holes" in the sky: absorption nebulae in Ophiuchus. Barnard 72, the Snake Nebula, is near the center of this wide-field image. The small dark globule up to its right is Barnard 68, while the large diffuse absorption nebula near the bottom is Barnard 78. (Tom McQuillan/Adam Block/AURA/NSF/NOAO)

In visible light Barnard 68 completely obscures the stars behind it, and being relatively small (only 1/2 light year across) and close (about 500 light years away), there are no stars visible in front of it either. But as shown in the image below, at longer and longer infrared wavelengths, the nebula becomes more and more transparent, revealing a multitude of stars behind it. (ESO, SOFI)

     Although absorption nebulae are usually seen as dark obscuring clouds, space-based infrared observations can show them in a completely different way. This false-color composite of far infrared radiation in the southern Milky Way (near the Southern Cross) shows otherwise invisible radiation emitted by cold (approximately 10 Kelvin, or 440 degrees below zero Fahrenheit) and warmer clouds of gas and dust which are collapsing in complex stringlike networks to form new stars. (ESA, SPIRE & PACS Consortia, apod091016)