The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a diffuse nebula situated in the Milky Way, being south of Orion's Belt in the constellation of Orion. It is one of the brightest nebulae, and is visible to the naked eye in the night sky on Earth.

The Orion Nebula contains a very young open cluster, an association of about 2,800 stars within an ellipsoid with a diameter of 20 light years. This stellar nursery, as it is known, contains over 100 known kinds of organic and inorganic gases as well as dust; some of the latter is made up of large and complex organic molecules. In this region is also several notable stars including the white supergiant EZ Orionis.

Observers have long noted a distinctive greenish tint to the nebula, in addition to regions of red and of blue-violet. The red hue is an emmision from ionized hydrogen gases, while the blue-violet coloration is the reflected radiation from the massive O-class stars at the core of the nebula.

The green hue was a puzzle for astronomers in the early part of the 20th century because none of the known spectral lines at that time could explain it. There was some speculation that the lines were caused by a new element, and the name nebulium was coined for this mysterious material. With better understanding of atomic physics, however, it was later determined that the green spectrum was caused by a low-probability electron transition in doubly ionized oxygen, a so-called "forbidden transition". This radiation was all but impossible to reproduce in the laboratory at the time, because it depended on the quiescent and nearly collision-free environment found in the high vacuum of deep space (Source).