Theta<sup>2</sup> Orionis (ø<sup>2</sup> Ori) is a multiple star system in the constellation Orion. It is a few arc minutes from its more famous neighbour the Trapezium Cluster, also known as ø<sup>1</sup> Orionis.
ø<sup>2</sup> Orionis consists of three stars in a line, each about an arc-minute from the next. The brightest of the three, ø<sup>2</sup> Orionis A, is itself a triple star system. The inner pair, A1 and A2, is a spectroscopic binary with the lines of the secondary difficult to observe because they are highly-broadened by its rapid rotation. The outer companion A3 is resolved by speckle interferometry at a separation of .
In addition to the well-known three stars, the Washington Double Star Catalog confusingly lists a component D which is actually ø<sup>1</sup> Orionis C. 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 ø<sup>2</sup> Ori B and C.
Bizarrely, ø<sup>2</sup> 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 ø<sup>2</sup> Ori B and V1073 Ori.
ø<sup>1</sup> Orionis, the well known Trapezium cluster, is only 2 arc minutes away from ø<sup>2</sup> Orionis A. Despite the names, ø<sup>2</sup> 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 ø<sup>1</sup> and ø<sup>2</sup> 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.
The primary component A appears as a 5th magnitude O class subgiant over 100,000 times as luminous as the Sun. The spectral type suggests it is evolving away from the main sequence, although it is thought to be less than 2 million years old.
The 6th magnitude component B is an early B main sequence star nearly and over 10,000 times the luminosity of the Sun.
Component C is another B class main sequence star, but cooler and less than a thousand times the luminosity of the Sun.
ø<sup>2</sup> Orionis A is itself 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.
Speckle interferometry has resolved a companion about 0.3" away, around 147 AU. High resolution spectroscopy shows that there is an even closer companion, only about 0.47 AU from the primary, for a total of three stars. Both companions are thought to be early A or late B with masses of . This helps to explain the high mass and visual luminosity for an O9.5 star at this distance. The three stars together have nearly the same mass as the O5.5 ø<sup>1</sup> Orionis C and visually are even brighter.
ø<sup>2</sup> Orionis A also 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, although bright x-ray flares have been observed in some young T Tauri stars.