|Observation data (J2000 epoch)|
|Right ascension||05h 35.4m|
|Distance||1.6 kly (490 pc)|
|Apparent magnitude (V)||4.0|
|Apparent dimensions (V)||47 (seconds of arc)|
|Estimated age||3 × 105 years|
The Trapezium or Orion Trapezium Cluster, also known by its Bayer designation of Theta1 Orionis, is a tight open cluster of stars in the heart of the Orion Nebula, in the constellation of Orion. It was discovered by Galileo Galilei. On February 4, 1617 he sketched three of the stars (A, C, D), but missed the surrounding nebulosity. The fourth component (B) was identified by several observers in 1673, and several more components were discovered later, for a total of eight by 1888. Subsequently several of the stars were determined to be binaries. Telescopes of amateur astronomers from about 5 inch aperture can resolve six stars under good seeing conditions.
The Trapezium is a relatively young cluster that has formed directly out of the parent nebula. The five brightest stars are on the order of 15-30 solar masses in size. They are within a diameter of 1.5 light-years of each other and are responsible for much of the illumination of the surrounding nebula. The Trapezium may be a sub-component of the larger Orion Nebula Cluster, a grouping of about 2,000 stars within a diameter of 20 light-years.
It is most readily identifiable by the asterism of four relatively bright stars. The four are often identified as A, B, C, and D in order of increasing right ascension. The brightest of the four stars is C, or Theta1 Orionis C, with an apparent magnitude of 5.13. Both A and B have been identified as eclipsing binaries.
Infrared images of the Trapezium are better able to penetrate the surrounding clouds of dust, and have located many more stellar components. About half the stars within the cluster have been found to contain evaporating circumstellar disks, a likely precursor to planetary formation. In addition, brown dwarfs and low-mass runaway stars have been identified.
The Chandra X-ray Observatory view of the Orion Nebula.
Possible black hole
A 2012 paper suggests an intermediate mass black hole with a mass >100 times larger than that of the Sun may be present within the Trapezium, something that could explain the large velocity dispersion of the stars of the cluster.
- Galileo Galilei: Siderius Nuncius, Venice, 1610. English Translation published at Bard College, Hudson NY" October 9, 2003 English Translation  Original Latin version 
- Tom Pope and Jim Mosher: Galilean telescope homepage" March 17, 2006 , "Some have expressed puzzlement that in his text Galileo does not mention the nebulosity (known in modern nomenclature as M42) enveloping these stars. ... Galileo believed, as he explains in Sidereus Nuncius, that what looks nebulous to the eye is resolved into stars by his telescope; what looks nebulous through his telescope could presumably also be resolved into stars by a still larger and more powerful telescope. Hence, a diffuse glow would be, more than anything, an indication of the limitations of his telescope and not particularly worthy of special note."
- Tom Pope and Jim Mosher: Page on Galileo's February 4, 1617 notebook drawing of the Trapezium region, May 2, 2006 "Perhaps significantly, Galileo makes no mention of having noticed the now well-known gas cloud, M42, surrounding the Trapezium stars."
- Lodriguss, Jerry. "The Trapezium, Theta Orionis". Retrieved 2009-06-02.
- Ladislav Subr; Pavel Kroupa; Holger Baumgardt. "Catch me if you can: is there a runaway-mass black hole in the Orion Nebula Cluster?". arXiv:1209.2114.
- Lada, E. A. et al. (1996). "Circumstellar Disks in the Trapezium Cluster". Bulletin of the American Astronomical Society 28: 1342. Bibcode:1996AAS...189.5301L.
- Poveda, Arcadio et al. (2005). "Low-Mass Runaway Stars from the Orion Trapezium Cluster". Astrophysical Journal 627 (1): L61–L64. arXiv:astro-ph/0506002. Bibcode:2005ApJ...627L..61P. doi:10.1086/432053.
- Chandra Observatory Uncovers Hot Stars In The Making, MIT Press Release, 2000.
- A detailed description of the Trapezium Cluster
- Astronomy Picture of the Day - In the Center of the Trapezium 2003 March 2
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