The Orion Nebula (also known as Messier 42, M42, or NGC 1976)
is a diffuse nebula situated south[b] 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. M42 is
located at a distance of 1,344 ± 20 light years and is the
closest region of massive star formation to Earth. The M42
nebula is estimated to be 24 light years across. It has a
mass of about 2000 times the mass of the Sun. Older texts
frequently refer to the Orion Nebula as the Great Nebula
in Orion or the Great Orion Nebula.

The Orion Nebula is one of the most scrutinized and photographed
objects in the night sky, and is among the most intensely studied
celestial features. The nebula has revealed much about the process
of how stars and planetary systems are formed from collapsing clouds
of gas and dust. Astronomers have directly observed protoplanetary
disks, brown dwarfs, intense and turbulent motions of the gas, and
the photo-ionizing effects of massive nearby stars in the nebula.
There are also supersonic "bullets" of gas piercing the hydrogen
clouds of the Orion Nebula. Each bullet is ten times the diameter
of Pluto's orbit and tipped with iron atoms glowing bright blue.
They were probably formed one thousand years ago from an unknown
violent event.