M42 (NGC 1976): The Great Nebula in Orion

Constellation: Orion

Distance: 1350 light years

The Great Nebula in Orion is one of the most observed and photographed objects in the night sky. It is approximately 1350 light years away and about 24 light years across. It is a diffuse nebula, located in the middle of Orion’s “sword”. It is one of the brightest of all nebulae, visible to the naked eye under dark skies, and obvious through binoculars or telescopes of any size. Its bright core creates a challenge to photography not encountered in most other targets: exposures of varying lengths must be acquired to capture all of the detail in the various brightness ranges, and then blended together for the final product.

M42 is the prototypical stellar nursery, and is the closest region of massive star formation to Earth. Observations of the nebula have revealed approximately 700 stars in various stages of formation within the nebula. Stars form when clumps of hydrogen and other gases in an H II region contract under their own gravity. As the gas collapses, the central clump grows stronger and the gas heats to extreme temperatures by converting gravitational potential energy to thermal energy. If the temperature gets high enough, nuclear fusion will ignite and form a protostar. The protostar is 'born' when it begins to emit enough radiative energy to balance out its gravity and halt gravitational collapse. Recent observations with the Hubble Space Telescope have yielded the major discovery of protoplanetary disks within the Orion Nebula. HST has revealed more than 150 of these within the nebula, and they are considered to be systems in the earliest stages of solar system formation. The sheer numbers of them have been used as evidence that the formation of star systems is fairly common in our universe. Within about 100,000 years, most of the gas and dust in the nebula will be ejected. The remains will form a young open cluster, a cluster of bright, young stars surrounded by wispy filaments from the former cloud.

The youngest and brightest stars we now see in the Orion Nebula are thought to be less than 300,000 years old, and the brightest may be only 10,000 years in age. At its heart, the Orion Nebula contains a very young star cluster called the Trapezium (also known as Theta-1 Orionis), due to the shape outlined by its 4 main stars (see magnified view below). The Orion Nebula would be invisible if not for the four trapezium stars which together produce all of the ultraviolet radiation illuminating the nebula.

The Nebula is part of a much larger nebula that is known as the Orion Molecular Cloud Complex. The Orion Molecular Cloud Complex extends throughout the constellation of Orion and includes Barnard's Loop, the Horsehead Nebula, M43, M78, and the Flame Nebula. Just to the left of M42 in this photo is the reflection nebula NGC 1973/5/7 (The Running Man Nebula).

An added bonus for me is that the night that I acquired this photograph also happened to be the peak of the Geminid Meteor Shower, and a fairly clear and moonless night. So while all of the pieces of gear were doing their thing, I got to sit back, look up, and watch an awesome show.