M42, the Great Orion Nebula

M 42, known as the Orion Nebula, is a diffuse nebula situated in the Milky Way in the constellation of Orion. It is one of the brightest nebulae with an apparent magnitude of 4.0 and is the closest region of massive star formation to Earth, approx. 1,350 light years away. M42 is estimated to be 24 light years across and has a mass around 2,000 times that if the Sun.

The nebula contains a very young open cluster, the Trapezium Cluster (named for the asterism of its primary four stars within a diameter of 1.5 light years - two of these can be resolved into their component binary systems on nights of good ‘seeing’. It is part of the larger Orion Nebula cluster, which contains around 2,800 stars within a region 20 light-years across.

The Orion Nebula harbours regions of ionized gas, neutral gas, dust clouds, reflection nebulae, and young stellar associations and star clusters and is part of the larger Orion Molecular Cloud Complex.

Nicolas-Claude Fabri de Peiresc is usually credited for the discovery of the Orion Nebula. The French astronomer observed it with a refractor telescope on November 24, 1610