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Orion Nebula

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   CAPTION: Orion Nebula

       Diffuse nebula       Lists of nebulae

   The entire Orion Nebula in visible light
                Observation data
   ( Epoch J2000.0)
   Type                    Diffuse
   Right ascension         05^h 35^m 17.3^s
   Declination             -05° 23′ 28″
   Distance                1,500 ly (450 pc)
   Apparent magnitude (V)  +3.0
   Apparent dimensions (V) 65 × 60 arcmins
   Constellation           Orion
            Physical characteristics
   Radius                  15 ly
   Absolute magnitude (V)  —
   Notable features        Trapezium cluster
   Other designations      NGC 1976, M42,
                           LBN 974
                                         edit

   The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a
   diffuse nebula with a greenish hue and is situated below 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 about 1,500 light years
   away, and is the closest region of star formation to Earth. The M42
   nebula is estimated to be 30 light years across.

   The Orion Nebula is considered to be 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 discs, brown dwarfs, intense and turbulent motions of
   the gas, and the photo-ionizing effects of massive nearby stars in the
   nebula.

General information

   The Orion 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, and M78. M43 is also part of M42, as well
   as several nearby reflection nebulae noted in the New General
   Catalogue. 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 below Orion's Belt. The star
   appears fuzzy to sharp-eyed observers, and the nebulosity is obvious
   through a pair of 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 source of three
   runaway stars, AE Aurigae, 53 Arietis, and Mu Columbae, all of which
   are 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 H^α 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
   twentieth 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 "nebulum" 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 have a folk tale that deals with the Orion
   constellation's part of the sky. Their traditional hearths include in
   their middle a smudge of glowing fire that corresponds 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 seventeenth 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 Orion
   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. Johann Baptist Cysat, a Jesuit astronomer, was the first to
   publish note of it (albeit somewhat ambiguous) in a book on 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—possible 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
   level of ultraviolet energy from the most massive stars has 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 least luminous.

Structure

   Optical images reveal clouds of gas and dust in the Orion Nebula; an
   infrared image (right) reveals the new stars shining within. Credit: C.
   R. O'Dell-Vanderbilt University, NASA, and ESA.
   Enlarge
   Optical images reveal clouds of gas and dust in the Orion Nebula; an
   infrared image (right) reveals the new stars shining within. Credit: C.
   R. O'Dell-Vanderbilt University, NASA, and ESA.

   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 θ^1 C Orionis, the star responsible for most
   of the ultraviolet ionizing radiation. (It emits 3-4 times as much
   photoionizing light as the next brightest star, θ^2 A Orionis.) 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

   View of several proplyds within the Orion Nebula taken by the Hubble
   Space Telescope. Credit:NASA.
   Enlarge
   View of several proplyds within the Orion Nebula taken by the Hubble
   Space Telescope. Credit:NASA.

   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 solar 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.
   Herbig-Haro 47 seen with a bow shock and a series of jet-driven shocks.
   [1] Enlarge
   Herbig-Haro 47 seen with a bow shock and a series of jet-driven shocks.

   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 gasses.

     * 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

   Panoramic image of the center of the nebula, taken by the Hubble
   Telescope. This view is about 2.5 light years across. The Trapezium is
   at center left. Credit:NASA.
   Enlarge
   Panoramic image of the center of the nebula, taken by the Hubble
   Telescope. This view is about 2.5 light years across. The Trapezium is
   at centre left. Credit:NASA.

   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 centre 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.

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