The Trapezium, Theta 1 Orionis (θ1 Ori), is the most famous multiple star system in the night sky.

Located at the very heart of M42, the Orion Nebula, it is one of the youngest star clusters known. Its age is estimated to be less than a million years old and is located at a distance of 1,337 light-years. North is to the top in the above image.

The four brightest stars in the Trapezium (A, B, C and D) are easily visible in a four-inch telescope with decent optical quality under good seeing conditions. They range in brightness from magnitude 5 to magnitude 8. All are hot class O and B stars.

With more magnification and good seeing conditions, two fainter stars, E (mag 11.1) and F (mag 10.12) can also easily be seen in moderately sized amateur instruments. These stars are part of the Trapezium's multiple-star system. Because of their proximity to the much brighter stars in the Trapezium, their visibility in a telescope is dependent on good seeing, optical quality, and observer skill.

Three other faint stars, G, H and I are also part of the system, but are too faint to be seen except in very large amateur instruments. Other fainter stars are also involved in the system.

Many of these stars are themselves binary and multiple star systems.

θ Orionis - The Trapezium Multiple Star System Star Magnitude Notes A 6.72 - 7.65 Eclipsing Binary in 3-star system B 7.9 - 8.65 Eclipsing Binary in 5-star system C 5.13 Spectroscopic binary star D 6.71 Double star E 11.1 Spectroscopic binary star F 10.12 Binary star G 13.68 Single star H 15.8 Double star I 16.3 Single star?

Observational History

Galileo Galilei was apparently the first observer to draw the stars in the Trapezium. In his notebook dated February 4, 1617, he very accurately sketched stars A, C and D, although the drawing was not published in his lifetime. The Trapezium was also drawn as a triple star by Giovanni Batista Hodierna before the year 1654, and Christen Huygens described it in 1656.

Star D was independently discovered by both Abbe Jean Picard and Huygens in 1684.

Star E was discovered by William Struve in 1826 with a 9.5-inch refractor.

Star F was discovered by John Herschel in 1830 with a 12-inch refractor.

Star G was discovered by Alvan Clark in 1888 while testing the 36-inch refractor that he made for the Lick Observatory.

Star H was discovered by E.E. Barnard later in 1888, also with the 36-inch refractor at Lick. Barnard subsequently discovered that H was a double. It is separated by 1.6 arcseconds.

Stellar Systems in θ1 Orionis

Theta 1 Orionis, the Trapezium, is a complex star cluster called the Orion Nebula Cluster (ONC). It involves many multiple star systems. There are 1,000 young, hot stars involved in the cluster. They are crowded into a space 4 light-years in diameter, the same distance between the Sun and Proxima Centauri, our nearest neighbor star. Most of the stars in the cluster are hidden by dust in the nebula, but are visible at the infra-red wavelengths.

The main components of the Trapezium are:

• θ1 Ori A - a triple-star system. θ1 Ori A has a nominal magnitude of 6.73 V and a spectral classification of B0.5V. θ1 Ori A is also designated as HD 37020, V1016 Orionis, HR 1893, and Parenago 1865.

  A1, the primary, is a spectroscopic binary with a separation of 1 Astronomical Unit (AU), 93 million miles. At a distance of 1,350 light-years, the A1 spectroscopic binary would subtend an apparent angle of just 0.002416 arcseconds from Earth.

  θ1 Ori A is also an eclipsing binary star system. The spectroscopic binary θ1 Ori A1 has a companion star, A2 with a spectral class of AO, that blocks out some of the light from A1 every 65.43233 days. θ1 Ori A1 normally shines at magnitude 6.73, but fades to magnitude 7.65 during eclipse. The eclipse lasts 20 hours. Mid eclipse, when it is at minimum, lasts 2.5 hours. A2 orbits A1 at an apparent distance of 0.1881 arcseconds.

  
  
• θ1 Ori B - a 5-star system that is a mini-cluster. It has a nominal magnitude of 7.96 V and a spectral classification of B1V.

  θ1 Ori B is an Algol-type eclipsing spectroscopic binary variable star with a 6.47 day period. It has a magnitude range of 7.96 to 8.65 V for its brightness variability. θ1 Ori B is also designated as BM Orionis, HD 37021, HR 1894, and Parenago 1863.

  B1, the primary, is a spectroscopic binary with a separation of 0.13 AU. At a distance of 1,350 light-years, this would correspond to an apparent separation of just 0.00031408 arcseconds.

  B1 is also orbited by two other stars, B4, a single star, and another binary that orbits farther out, B2 and B3.

  B1 and its spectroscopic companion have a mass of 7 suns each for a total of 14 solar masses. B2 has a mass of 3 suns, B3 has a mass of 2.5 suns, and B4 has a mass of 0.2 suns.

  A paper by Larid M. Close, et al. in the Astrophysical Journal in 2013 titled Diffraction-limited Visible Light Images of Orion Trapezium Cluster With the Magellan Adaptive Secondary AO System (MagAO) presented data taken with the Adaptive Optics system of the 6.5 meter Magellan Telescope that resolved 4 components of θ1 Ori B with the following separations:

  • B1 - B2 separation = 0.9415 arcseconds
  • B2 - B3 separation = 0.1160 arcseconds
  • B1 - B4 separation = 0.6182 arcseconds
  
  
• θ1 Ori C - a triple star system. The primary has a magnitude of 5.13 V and a spectral classification of O7V. It is the brightest and most massive of the stars in the Trapezium with a mass of 38 solar masses. It is located at a distance of 1,350 light-years away from the Earth. θ1 Ori C is also designated as HD 37022, HR 1895, and Parenago 1891.

  C1, the primary, is a spectroscopic binary star with an angular separation of 0.0025 arcseconds.

  Another companion star, C2, with 9 solar masses, revolves around C1 with a period of 11.3 years, and is separated by 0.028 arcseconds at periastron. It has a spectral type of O9.5.

  θ1 Ori C is one of the most luminous stars known. It is more than 200,000 times more luminous than the Sun with a temperature of 39,000K. At the heart of the Trapezium Star Cluster, its hot ultraviolet radiation ionizes the Orion Nebula.

  
  
• θ1 Ori D - a double-star system. It has a nominal magnitude of 6.71 V and a spectral classification of B0.5V.

  Components D1 and D2 have a separation of 0.0184 arcseconds.

  θ1 Ori D is also designated as HD 37023, HR 1896, and Parenago 1889.

  
  

• θ1 Ori E - a spectroscopic binary. It has a nominal magnitude of 11.1 V and a spectral classification of G2IV. θ1 Ori E has a separation of 0.2 AU, that would correspond to an apparent separation of 0.00048 arcseconds. The two components have masses of 2.8 Suns each.

  θ1 Ori E is also designated as Parenago 1864, COUP 732 and WDS J05353-0523E.

  
  

• θ1 Ori F - a binary system. It has a nominal magnitude of 10.12 V and a spectral classification of B8.

  θ1 Ori F is also designated as Parenago 1892 and WDS J05353-0523F.

  
  

• θ1 Ori G - a single star. It has a nominal magnitude of 13.68 V and a spectral classification of K0.7. It is listed as a young stellar object associated with HH 726.

  θ1 Ori G is also designated as Parenago 1890, COUP 826 and MAX 116.

  
  

• θ1 Ori H - a close double star. It was discovered by Barnard, and is listed with a magnitude of 15.8 V.

  θ1 Ori H is also designated as Parenago 1866 and WDS J05353-0523H.

  
  

• θ1 Ori I - a single star with a magnitude of 16.3 V.

  
  

θ1 Orionis Catalogs: ORI 1d, ADS 4186, STF 748 Common Name: Trapezium Object Type: Multiple Star Magnitude: 5.1 to 16.3 Size: 20" Constellation: Orion Image Field Centered At: RA: 05h 35m 57s Dec: -05° 23' 08"