Messier 42 — Emission Nebula in Orion

The Great Orion Nebula is one of the brightest diffuse nebulae in the sky and the closest major star-forming region to Earth, at roughly 1,344 light-years away. It spans about 24 light-years across and contains a young open cluster known as the Trapezium, whose hot O- and B-type stars ionize the surrounding hydrogen gas and make it glow. The nebula is part of the much larger Orion Molecular Cloud Complex, which also includes M43, the Horsehead Nebula region, and Barnard's Loop.

Visible to the naked eye as a fuzzy patch in Orion's Sword, even from moderately light-polluted skies. Binoculars reveal the bright core and sweeping wings of nebulosity. A small telescope at low power (around 40x) resolves the four stars of the Trapezium at the heart of the nebula. Use an OIII or UHC filter to dramatically enhance the extent of the nebulosity, especially the fainter outer regions. Averted vision helps reveal more structure. Best observed from October through March when Orion is high in the evening sky.

Known since antiquity as a hazy spot, its nebular nature was first noted by Nicolas-Claude Fabri de Peiresc in 1610 using an early telescope. Johann Baptist Cysat described it independently in 1611. Charles Messier cataloged it as M42 in 1769. It was one of the first objects photographed through a telescope by Henry Draper in 1880, and William Huggins confirmed its gaseous nature through spectroscopy in 1865.