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Alnitak, Alnilam and Mintaka. These three prominent stars in the central regions of the constellation of Orion align to form the 'belt' of the Hunter.

Derivation

The identification of these three stars as a belt is a very old one. The Arabic names of two of the stars reflect this: Mintaka means 'the belt' and Alnitak means 'the girdle'. (The name of the third star, Alnilam, means 'the string of pearls'.) It has been suggested that the three Belt-stars influenced the placing of the Pyramids at Giza, and it is certainly true that there is a remarkable correspondence of position between the Pyramids and the stars.

Map of Orion\'s Belt

Stars

The three stars that make up the Belt are all immensely hot, massive and luminous bodies, of the brilliant blue O- and B-type spectral classifications. Both Alnilam and Alnitak are supergiants, and Mintaka, while not quite as massive as its companions, is still a true giant.

The linear alignment of the Belt is a line-of-sight effect that conceals somewhat the true relations of the stars making it up. In fact, the stars at either end of the Belt, Alnitak and Mintaka, are the closest together in space: Alnitak is a little over 800 light years from Earth, and Mintaka a hundred light years farther away. The central star, Alnilam, is much more distant than either of these, lying on the edge of the Orion Molecular Cloud more than 1,300 light years from the Solar System.

Diagram of Orion's Belt

Alnilam is the most massive and luminous of the three stars, so that despite its greater distance it still shines more brightly than its two companions. If the three stars were the same distance away, Alnilam would easily outshine the other Belt-stars. It is about 10,000 times more luminous than the Sun.

Nebulae

Like much of its parent constellation, Orion's Belt is a highly nebulous region of the sky. This is especially true in the direction of Alnitak, where numerous nebulous structures are found. The most famous of these is undoubtedly the dark Horsehead Nebula, a dark cloud blotting out the light from the red-pink streak of IC 434. Also occupying this region are the less well known Flaming Tree Nebula (NGC 2024) and the so-called Lump Star (NGC 2023).

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