Spot Mintaka at the top of Orion’s Belt

Mintaka (Delta (δ) Orionis) is easy to find. Shining at mag. +2.3, it marks the northwest end of Orion’s Belt, the star’s name deriving from the Arabic for ‘belt’. Sitting 17 arcminutes south of the celestial equator, Mintaka hides a great deal of complexity. Observations made by Johannes Franz Hartmann in 1904, using photographs taken at the Potsdam Observatory in Germany, showed Mintaka was a spectroscopic binary.

A spectroscopic binary shows periodic movement within its component star’s spectral lines. Hartmann noticed the Calcium-K line at 393.34 nanometres didn’t show the same periodicity as the Mintakan system and from this he deduced there must be a calcium cloud between us and the star. This was the first detection of what’s known as the interstellar medium.

Mintaka is a complex multiple star, comprising a mag. +6.8 star 52 arcseconds to the north of the primary and a far fainter 14th magnitude star in between. The mag. +2.3 primary is designated A, the 14th magnitude star is B and the mag. +6.8 star C.

Delta (δ) Orionis A is the spectroscopic binary, an O9.5 II giant (Aa1) and B1 V main sequence star (Aa2) in a 5.73- day orbit. A B0 IV subgiant (Ab) sits 0.26 arcseconds from the spectroscopic pair. The 14th magnitude companion (B) is a cool star, about 70 per cent the size of our Sun and around 40 per cent as luminous. The seventh magnitude companion (C) is another spectroscopic binary comprising an A-type primary and B-type companion in a 30-day orbit.