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Sword of Orion

Map of the Sword of Orion

The common name for a group of bright objects forming a near-vertical line beneath Orion's Belt. From north to south, the most prominent objects in the Sword are the cluster NGC 1981, the star c Orionis, the famous Orion Nebula, and the bright star Hatysa.

None of the members of this chain of objects is particularly bright in itself, but their proximity to one another and the nebulosity across much of this region makes the Sword stand out clearly in the night sky. As a line descending from Orion's Belt, it is unsurprising that this asterism has been associated with a sword since antiquity.

The Stars of the Sword

The backbone of the Sword is formed by three stars: from north to south, these are c Orionis, Theta Orionis and Hatysa. Of these c Orionis, the faint northernmost star, is in the foreground of the structure as seen from Earth, at a distance of about 790 light years. The other components of the Sword lie relatively close together in space, occupying a region more than 1,300 light years from the Solar System.

At the southern end of the Sword is the brightest of its stars, Hatysa, sometimes known by the Arabic name Nair al Saif (which literally means 'bright one of the Sword'). This is an intensely luminous giant star of the comparatively rare O-type classification.

Between these two stars, in the central regions of the Sword's blade, is Theta Orionis, including the complex cluster known as the Trapezium. The stars of this group are the main source of illumination of the Orion Nebula, part of the complex of nebulae that make this part of southern Orion so distinctive.

Nebulae

Throughout much of southern Orion a vast dark cloud extends across the sky, the Orion Molecular Cloud. Parts of this cloud are illuminated by nearby stars or - as in the case of the Trapezium - stars that have actually formed within the nebulous gas and dust that fills this region. The largest of the nebulae of the Sword is the Orion Nebula, a vast cloud some twenty light years across, and covering an area of sky with about twice the diameter of the Moon as seen from Earth.

The Orion Nebula is not the only nebulous area in the Sword. Connected to it in a northerly direction is a smaller area of glowing dust, designated M43 (the Orion Nebula itself being M42). Further north still, filling the area behind the star c Orionis, is another nebulous region. This bright nebula is formally a complex of material catalogued as NGC 1973, 1975 and 1977, but the vaguely human-like figure formed by dark lanes of dust across its brighter background give it its common name: the Running Man Nebula.

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