Theta2 Orionis

Theta² Orionis (θ² 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 θ¹ Orionis.

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. 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 0.4″.

In addition to the well-known three stars, the Washington Double Star Catalog confusingly lists a component D which is actually θ¹ 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 θ² Ori B and C.

Bizarrely, 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.

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.

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 30,000 K and over 10,000 times the luminosity of the Sun.