M42, the great Orion Nebula, is about 1,500 light years from Earth and about 30 light years in diameter - it is roughly spherical in shape, composed of gas and dust. The colors seen in long exposure photographic images comes from hydrogen, oxygen and nitrogen ionized by the intense energy emitted by the young, bright stars within the cloud. M42 and the other emission nebula in the universe are stellar nurseries, where gas and dust are condensing into new stars.

M42 was cataloged by French astronomer Charles Messier in 1769. It was the 42nd object cataloged by Messier, thus the designation "M42". Being the brightest emission nebula visible from earth it had been known to observers of the night sky for centuries before Messier cataloged it, but things like the size, distance and composition of M42 and the other deep space objects were unknown until the 20th century.

Visually, M42 can be observed with the naked eye in areas with dark skies - it is seen as a fuzzy, gray blob in the middle of the "sword" hanging below the prominent triple star "belt" in the constellation Orion. It can be seen with binoculars even in moderately light-polluted skies, and small amateur telescopes make it out easily from most earthly locations. But the human eye lacks sensitivity to low levels of color, so even with the largest telescopes under the darkest skies the nebula appears to be grey.

This image is a combination of 10 exposures lasting 3 minutes each, at ISO 800, taken with a Canon Digital Rebel XT on December 14, 2006. A broad-band "nebula filter" was used in the light path to minimize the effects of light pollution while maximizing transmission of the ionized hydrogen, oxygen and nitrogen wavelengths.