Trapezium (astronomy)

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This article is about the open cluster. For other uses, see Trapezium.

**Trapezium**
Observation data: J2000 epoch

Class:	?
Constellation:	Orion
Right ascension:	05^h 35.4^m
Declination:	−05° 27′
Distance:	1.6 kly (490 pc)
Apparent magnitude (V):	4.0
Apparent dimensions (V):	47 (seconds of arc)
Physical characteristics
Mass:	? (? M_☉)
Radius:	10 ly
V_HB:	?
Estimated age:	3 × 10⁵ years
Notable features:
Other designations:
See also: Open cluster, List of open clusters

The Trapezium, or Orion Trapezium Cluster is a tight open cluster of stars that lies within the heart of the Orion Nebula, in the constellation of Orion. It was discovered by Galileo Galilei in 1617, who sketched three of the stars (A, C, D) but somehow 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.

This is a relatively young cluster that has formed directly out of the nebula. The five brightest stars in this cluster have masses on the order of 15-30 Msun, lie within a diameter of 1.5 light-years 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, with masses on the order of 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 θ¹ C Orionis, with an apparent magnitude of 5.13. Both A and B have been identified as eclipsing binaries.

Trapezium in optical (left) and infrared light (right) from Hubble. NASA photo.

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.

[edit] References

E. A. Lada et al, "Circumstellar Disks in the Trapezium Cluster".
Arcadio Poveda et al, 2005, "Low-Mass Runaway Stars from the Orion Trapezium Cluster", Astrophysical Journal, 627.