Theta2 Orionis

Visually, θ² Orionis consists of three stars in a line, each about an arc-minute from the next. The brightest of the three, θ² Orionis A, is itself a triple star system, while θ² Orionis B is a binary and θ² Orionis C is a triple, brining the total number of stars to eight.

In addition to the well-known three stars, the Washington Double Star Catalog confusingly lists a component D which is actually θ¹ Orionis C.^[18] There is one other star brighter than 10th magnitude in the region. V1073 Orionis is a B9.5 Orion variable that forms an equilateral triangle with θ² Ori B and C.

θ² Orionis C has a second entry in the Washington Double Star Catalog under the name S490. The companion is 10th magnitude and actually lies between θ² Ori B and V1073 Ori.^[18]

θ¹ Orionis, the well known Trapezium cluster, is only 2 arc minutes away from θ² Orionis A. Despite the names, θ² Orionis A is marginally brighter than the brightest star in the Trapezium. The Catalog of Components of Double and Multiple Stars includes the stars of θ¹ and θ² Ori within the same system of 13 components.

There are dozens of much fainter stars in the same field, many of them pre-main-sequence stars still forming from the Orion molecular cloud complex.

Theta² Orionis A, of apparent magnitude 5.02,^[2] is a triple star system. Its spectral lines were seen to change position periodically, indicating orbital motion. The first orbit was derived in 1924, indicating a period of 21 days and a rather eccentric orbit.^[19] This companion, named Theta² Orionis A₂, was later resolved interferometrically, yielding a secondary-to-primary flux ratio of 0.52±0.04 and a mass estimate of 10±2 solar masses, suggesting an early B spectrum.^[15] Aditionally, speckle interferometry has resolved a companion about 0.3" away, around 147 AU,^[20] named Theta² Orionis A₃.^[15] The presence of nearby companions helps to explain the unexpectedly high mass and visual luminosity for an O9.5 star at this distance, if it was assumed to be a single star. The three stars together have nearly the same mass as the O6 star θ¹ Orionis C and visually are even brighter.^[20]

Theta² Orionis A₁ is an O-type star of spectral type O9.5IVp,^[7] 29 times as massive as the Sun and 85,000 times as luminous, with a temperature of 35,000 K.^[13] The companion A₂ has an estimated mass of 12.6 times solar,^[14] while A₃ is estimated to be between three and seven times as massive as the Sun.^[13] The system shows unexplained rapidly variable X-ray emission. The X-rays are not thought to be caused by colliding winds or coronal emissions from the unseen companion,^[21] although bright x-ray flares have been observed in some young T Tauri stars.^[13]

Theta² Orionis B, of magnitude 6.38,^[4] is a binary star whose its components have been observed only once, using interferometry and at a projected separation of 40±1 au.^[15] The primary component, B₁, is an early B main sequence star nearly 30,000 K and over 10,000 times the luminosity of the Sun. The companion, B₂, has a mass of approximately 1.6±0.7 M_☉ and is thought to be an A or F-type star.^[15]

Theta² Orionis C, of apparent magnitude 8.18,^[6] is a triple star system consisting of an inner spectroscopic binary, with an orbital period of roughly 13 days and a semi-major axis of 0.165±0.007 au, and an outer companion resolved with interferometry at a projected separation of 15.7±0.2 au. The primary, Theta² Orionis C₁, has an estimated mass 4±1^[15] times that of the Sun and is 600 times as luminous, with a temperature of 13,800 K.^[16] The spectroscopic secondary, C₂, has no known properties. The tertiary, C₃, is an A-type star with 1.7 times the mass of the Sun.^[15]