Messier Index/M42

The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a diffuse nebula situated south of Orion's Belt. 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. Older texts frequently referred to the Orion Nebula as the Great Nebula in Orion or the Great Orion Nebula. Yet older, astrological texts refer to it as Ensis (Latin for "sword"), which was also the name given to the star Eta Orionis, which can be seen close to the nebula from Earth.

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 dense 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.

General information
The Nebula is in fact 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. Stars are forming throughout the Orion Nebula, and due to this heat-intensive process the region is particularly prominent in the infrared.

The nebula is visible with the naked eye even from areas affected by some light pollution. It is seen as the middle "star" in the sword of Orion, which are the three stars located south of Orion's Belt. The star appears fuzzy to sharp-eyed observers, and the nebulosity is obvious through binoculars or a small telescope.

The Orion Nebula contains a very young open cluster, known as the Trapezium due to the asterism of its primary four stars. Two of these can be resolved into their component binary systems on nights with good seeing, giving a total of six stars. The stars of the Trapezium, along with many other stars, are still in their early years. The Trapezium may be a component of the much-larger Orion Nebula Cluster, an association of about 2,000 stars within a diameter of 20 light years. Two million years ago this cluster may have been the home of the runaway stars AE Aurigae, 53 Arietis, and Mu Columbae, which are currently moving away from the nebula at velocities greater than 100 km/s.

Observers have long noted a distinctive greenish tint to the nebula, in addition to regions of red and areas of blue-violet. The red hue is well-understood to be caused by Hα recombination line radiation at a wavelength of 656.3 nm. 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 spectra 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 because it depended on the quiescent and nearly collision-free environment found in deep space.

History
The Maya of Central America had a folk tale which dealt with Orion's part of the sky, known as Xibalba. Their traditional hearths included in their middle a smudge of glowing fire that corresponded with the Orion nebula. This is clear pre-telescope evidence that the Maya detected a diffuse area of the sky contrary to the pin points of stars.

This nebula is currently visible to the unaided eye, yet oddly there is no mention of the nebulosity in the written astronomical records prior to the 17th century. In particular, neither Ptolemy in the Almagest nor Al Sufi in his Book of Fixed Stars noted this nebula, even though they both listed patches of nebulosity elsewhere in the night sky. Curiously this nebula was also not mentioned by Galileo, even though he made telescope observations of this part of the constellation in 1610 and 1617. This has led to some speculation that a flare-up of the illuminating stars may have increased the brightness of the nebula.

The Orion Nebula is generally credited as being first discovered in 1610 by Nicolas-Claude Fabri de Peiresc as noted in Peiresc's own records. Cysatus of Lucerne, a Jesuit astronomer, was the first to publish note of it (albeit somewhat ambiguously) in a book about a bright comet in 1618. It was independently discovered by several prominent astronomers in the following years, including Christiaan Huygens in 1656 (whose sketch was the first published in 1659). Charles Messier first noted the nebula on March 4, 1769, and he also noted three of the stars in Trapezium. (The first detection of these three stars is now credited to Galileo in 1617, but he did not notice the surrounding nebula&mdash;possibly due to the narrow field of vision of his early telescope.) Charles Messier published the first edition of his catalog of deep sky objects in 1774 (completed in 1771). As the Orion Nebula was the 42nd object in his list, it became identified as M42.

Spectroscopy done by William Huggins showed the gaseous nature of the nebula in 1865. Henry Draper took the first astrophoto of the Orion Nebula on September 30, 1880, which is credited with being the first instance of deep-sky astrophotography in history.

In 1902, Vogel and Eberhard discovered differing velocities within the nebula and by 1914 astronomers at Marseilles had used the interferometer to detect rotation and irregular motions. Campbell and Moore confirmed these results using the spectrograph, demonstrating turbulence within the nebula.

In 1931, Robert J. Trumpler noted that the fainter stars near the Trapezium formed a cluster, and he was the first to name them the Trapezium cluster. Based on their magnitudes and spectral types, he derived a distance estimate of 1,800 light years. This was three times further than the commonly accepted distance estimate of the period but was much closer to the modern value.

In 1993, the Hubble Space Telescope first observed the Orion Nebula. Since then, the nebula has been a frequent target for HST studies. The images have been used to build a detailed model of the nebula in three dimensions. Protoplanetary disks have been observed around most of the newly formed stars in the nebula, and the destructive effects of high levels of ultraviolet energy from the most massive stars have been studied.

In 2005, the Advanced Camera for Surveys instrument of the Hubble Space Telescope finished capturing the most detailed image of the nebula yet taken. The image was taken through 104 orbits of the telescope, capturing over 3,000 stars down to the 23rd magnitude, including infant brown dwarfs and possible brown dwarf binary stars. A year later, scientists working with the HST announced the first ever masses of a pair of eclipsing binary brown dwarfs, 2MASS J05352184–0546085. The pair are located in the Orion Nebula and have approximate masses of 0.054 M☉ and 0.034 M☉ respectively, with an orbital period of 9.8 days. Surprisingly, the more massive of the two also turned out to be the less luminous.

Structure
The entirety of the Orion Nebula extends across a 10° region of the sky, and includes neutral clouds of gas and dust, associations of stars, ionized volumes of gas and reflection nebulae.

The nebula forms a roughly spherical cloud that peaks in density near the core. The cloud has a temperature ranging up to 10,000 K, but this temperature falls dramatically near the edge of the nebula. Unlike the density distribution, the cloud displays a range of velocities and turbulence, particularly around the core region. Relative movements are up to 10 km/s (22,000 mi/h), with local variations of up to 50 km/s and possibly higher.

The current astronomical model for the nebula consists of an ionized region roughly centered on Theta1 Orionis C, the star responsible for most of the ultraviolet ionizing radiation. (It emits 3-4 times as much photoionizing light as the next brightest star, Theta2 Orionis A. ) This is surrounded by an irregular, concave bay of more neutral, high-density cloud, with clumps of neutral gas lying outside the bay area. This in turn lies on the perimeter of the Orion Molecular Cloud.

Observers have given names to various features in the Orion Nebula. The dark lane that extends from the north toward the bright region is called the "Fish's Mouth". The illuminated regions to both sides are called the "Wings". Other features include "The Sword", "The Thrust" and "The Sail".

Stellar formation
The Orion Nebula is an example of a stellar nursery where new stars are being born. Observations of the nebula have revealed approximately 700 stars in various stages of formation within the nebula.

Recent observations with the Hubble Space Telescope have yielded the major discovery of protoplanetary disks within the Orion Nebula, which have been dubbed proplyds. 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.

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.

Typically, a cloud of material remains a substantial distance from the star before the fusion reaction ignites. This remnant cloud is the protostar's protoplanetary disk, where planets may form. Recent infrared observations show that dust grains in these protoplanetary disks are growing, beginning on the path towards forming planetesimals.

Once the protostar enters into its main sequence phase, it is classified as a star. Even though most planetary disks can form planets, observations show that intense stellar radiation should have destroyed any proplyds that formed near the Trapezium group, if the group is as old as the low mass stars in the cluster. Since proplyds are found very close to the Trapezium group, it can be argued that those stars are much younger than the rest of the cluster members.

Stellar wind and effects
Once formed, the stars within the nebula emit a stream of charged particles known as a stellar wind. Massive stars and young stars have much stronger stellar winds than the Sun. The wind forms shock waves when it encounters the gas in the nebula, which then shapes the gas clouds. The shock waves from stellar wind also play a large part in stellar formation by compacting the gas clouds, creating density inhomogeneities that lead to gravitational collapse of the cloud.

There are three different kinds of shocks in the Orion Nebula. Many are featured in Herbig-Haro objects:


 * Bow shocks are stationary and are formed when two particle streams collide with each other. They are present near the hottest stars in the nebula where the stellar wind speed is estimated to be thousands of kilometers per second and in the outer parts of the nebula where the speeds are tens of kilometers per second. Bow shocks can also form at the front end of stellar jets when the jet hits interstellar particles.


 * Jet-driven shocks are formed from jets of material sprouting off newborn T Tauri stars. These narrow streams are traveling at hundreds of kilometers per second, and become shocks when they encounter relatively stationary gases.


 * Warped shocks appear bow-like to an observer. They are produced when a jet-driven shock encounters gas moving in a cross-current.

The dynamic gas motions in M42 are complex, but are trending out through the opening in the bay and toward the Earth. The large neutral area behind the ionized region is currently contracting under its own gravity.

Evolution
Interstellar clouds like the Orion Nebula are found throughout galaxies such as the Milky Way. They begin as gravitationally bound blobs of cold, neutral hydrogen, intermixed with traces of other elements. The cloud can contain hundreds of thousands of solar masses and extend for hundreds of light years. The tiny force of gravity that could compel the cloud to collapse is counter-balanced by the very faint pressure of the gas in the cloud.

Whether due to collisions with a spiral arm, or through the shock wave emitted from supernovae, the atoms are precipitated into heavier molecules and the result is a molecular cloud. This presages the formation of stars within the cloud, usually thought to be within a period of 10-30 million years, as regions pass the Jeans mass and the destabilized volumes collapse into disks. The disk concentrates at the core to form a star, which may be surrounded by a protoplanetary disk. This is the current stage of evolution of the nebula, with additional stars still forming from the collapsing molecular 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.

Some of these collapsing stars can be particularly massive, and can emit large quantities of ionizing ultraviolet radiation. An example of this is seen with the Trapezium cluster. Over time the ultraviolet light from the massive stars at the center of the nebula will push away the surrounding gas and dust in a process called photo evaporation. This process is responsible for creating the interior cavity of the nebula, allowing the stars at the core to be viewed from Earth. The largest of these stars have short life spans and will evolve to become supernovae.

Within about 100,000 years, most of the gas and dust 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 Pleiades is a famous example of such a cluster.