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The entire Orion Nebula in visible light | |
| Observation data: J2000.0 epoch | |
| Right ascension | 05 32 49 |
| Declination | -05° 25′ |
| Distance | 1,500 ly ly |
| Apparent magnitude (V) | +4.0 |
| Apparent dimensions (V) | 85 × 60 arcmins |
| Constellation | Orion |
| Physical characteristics | |
| Radius | 15 lys ly |
| Absolute magnitude (V) | - |
| Notable features | Trapezium cluster |
| Designations | NGC 1976, M42 |
| See also: Lists of nebulae | |
The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a glowing emission nebula with a greenish hue and is situated below Orion's Belt. It is one of the brightest nebulae visible to the naked eye in the night sky. M42 is located at a distance of about 1,600 light years away, and is the closest region of stellar formation to Earth. The nebula is 25 light years across .
This 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 within the nebula, as well as brown dwarfs, intense and turbulent motions of the gas, and the photo-ionizing effects of massive nearby stars.
General information
M42 is part of a much larger nebula that extends throughout the constellation of Orion known as the Orion Molecular Cloud Complex, which 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 resoved into pairs on nights with good seeing, giving a total of six stars. It 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.
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 well-understood to be caused by H 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 that 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 Orion Nebula was discovered in 1610 by Nicolas-Claude Fabri de Peiresc, and it was independently discovered by several prominent astronomers in the following years, including Huygens in 1656. 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, thereafter it became identified as M-42.
Spectroscopy done by William Huggins showed the gaseous nature of the nebula in 1865. Henry Draper took the first astrophoto of the Orion Nebula in 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, and much closer to the modern value.
In 1993, the first observations of the Orion Nebula were performed by the Hubble Space Telescope. 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 over 104 orbits of the telescope, capturing over 3,000 stars down to 23rd magnitude, including infant brown dwarfs and possible brown dwarf binary stars.
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 optically-visible portion includes the bright M-42, which is one of the best-studied nebula in the sky. It is separated from the smaller M-43 by a dark lane.
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 falling dramatically near the edge. 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/sec. (22,000 miles/hr.), with local variations of up to 50 km/sec. and possibly higher.
The current astronomical model for the nebula consists of an ionized region roughly centered on θ 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 A Orionis, for example.) This is is surrounded by an irregular, concave bays of more neutral, high-density cloud, with clumps of neutral gas laying outside the bay area. This in turn lies on the perimeter of the Orion Molecular Cloud-1.
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 considered to be a primary 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 in 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, and the sheer numbers of them have been used as evidence that the formation of solar systems is fairly common in our universe.
Stars in the Orion Nebula form when clumps of hydrogen and other gases contract under their own gravity, experiments in space also show that particles can form together by sharing electrons, and forming bonds. Pressure in the clump heats to extreme temperatures, and if enough material is in the clump, nuclear ignitions may ignite and form a protostar. Once the protostar is born it creates enough energy to halt its own collapse.
As the protostar drifts away from its original birthplace, it carries a cloud of dust and other gasses with it. Inside the remnant cloud is the protostar's protoplanetary disk. Over millions of years excess material gets blown away by stellar wind from other stars. What is left of the protoplanetary disk forms objects such as planets. Recent infrared observations of the protoplanetary disks within the nebula have demonstrated just such an accretion of dust particles in these disks.
Once the protostar enters into its main sequence phase, it is classified as a star. Even though most planetary disk form planets, observations have shown that proplyds that form too near the Trapezium cluster end up being destroyed by the intense stellar radiation from the cluster, which means the stars lose the material to form planets.
Stellar Wind and Effects
Once formed, the stars within the nebula, such as those in Trapezium, emit large amounts of X-ray radiation, known as a stellar wind. Because of their massive sizes, this radiation is a million times more energetic than the solar wind from our own Sun. The wind forms shock waves when it encounters the gas in the nebula, which then shapes the gas clouds into various forms. The shock waves from stellar wind also play a large part in stellar formation by compacting the gas clouds.
There are three different kinds of shock waves in the Orion Nebula:
- Bow-shocks are stationary waves and are formed when two waves collide with each other. They are present near the hottest stars in the nebula where the wind speed is estimated to be thousands of kilometers per second and in the outer parts of the nebula where where the speeds are tens of kilometers per second.
- Jet-driven shocks are formed from jets of material sprouting off newborn stars. These narrow streams are travelling at hundreds of kilometers a second, and become shock waves 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
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 prestages 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, will 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 ultraviolet radiation that has the effect of ionizing the surrounding cloud. 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 what is responsible for creating the interior cavity of the nebula, allowing the stars at the core to be viewed from Earth. Within about 100,000 years, this process will disperse the remaining gas, leaving behind an open cluster. The largest of these stars have short life spans and will evolve to become supernovae.
Finally, once most of the gas and dust has been ejected, the remains will be a young open cluster that may resemble the Pleiades. That is, a cluster of bright, young stars surrounded by wispy filaments from the former cloud.
Notes and References
- Bowen, Ira S., 1927, "The Origin of the Nebulium Spectrum," Nature 120, 473
- Charles Messier, 1774, "Catalogue des Nébuleuses & des amas d'Étoiles, que l'on découvre parmi les Étoiles fixes sur l'horizon de Paris; observées à l'Observatoire de la Marine, avec differens instruments.", Mémoires de l'Académie Royale des Sciences, Paris.
- W.W. Campbell and J.H. Moore, 1917, "On the Radial Velocities of the Orion Nebula", Publications of the Astronomical Society of the Pacific, Vol. 29, No. 169.
- Trumpler, R. J., 1931, "The Distance of the Orion Nebula", Publications of the Astronomical Society of the Pacific, Vol. 43, No. 254.
- David F. Salisbury, 2001, "Latest investigations of Orion Nebula reduce odds of planet formation".
- M. Robberto, "An overview of the HST Treasury Program on the Orion Nebula", American Astronomical Society Meeting 207. Also see the NASA Press Release.
- B. Balick et al, 1974, "The structure of the Orion nebula", 1974, Astronomical Society of the Pacific, Vol. 86, Oct., p. 616.
- ibid, Balick, pg. 621.
- C. R. O'Dell, 2000, "Structure of the Orion Nebula", Publications of the Astronomical Society of the Pacific, 113:29-40.
- M.J. McCaughrean and C.R. O'dell, 1996, "Direct Imaging of Circumstellar Disks in the Orion Nebula", Astronomical Journal, v.111, p.1977.
- Marc Kassis et al, 2006, "Mid-Infrared Emission at Photodissociation Regions in the Orion Nebula", The Astrophysical Journal, 637:823-837. Also see the press release.
- Ker Than, 11 January 2006, "The Splendor of Orion: A Star Factory Unveiled", Space.com
- Mapping Orion's Winds, January 16, 2006, Vanderbilt News Service
- ibid, Balick, pp. 623 624.
- "Detail of the Orion Nebula", HST image and text.
- Press release, "Astronomers Spot The Great Orion Nebula's Successor]", Harvard-Smithsonian Center for Astrophysics, 2006.
See also
External links
- Messier 42, SEDS Messier pages
- January 2006 Hubble Space Telescope image of the Orion Nebula
- January 2006 Hubble Space Telescope image of the Trapezium cluster
- Orion Nebula M42, Hubble Images
- Remarkable new views captured of Orion Nebula, SpaceFlight Now, 2001.