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Orion


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Orion

(the Hunter; abbrev. Ori, gen. Orionis; area 594 sq. deg.)

An equatorial constellation which lies between Taurus and

Monoceros, and culminates at midnight in mid-December.

Its origin dates back to Sumerian times, when it was

identified with the hero Gilgamesh and his fight against

the Bull of Heaven (represented by Taurus), but today it is

associated with the son of Poseidon, in Greek mythology,

a mighty hunter who is accompanied in the sky by his two

dogs (the constellations Canis Major and Canis Minor) in

pursuit of a hare (the constellation Lepus). The brightest

stars of Orion were cataloged by Ptolemy (c. AD 100–175)

in the Almagest.

A conspicuous constellation and one of the few that

even remotely resembles the object after which it was

named, Orion is easily recognized by a quadrilateral of

first and second magnitude stars, α, γ , β and κ Orionis,

marking the hunter’s right and left shoulders, left foot

and right knee, respectively, within which lies a trio of

second-magnitude stars, δ, ε and ζ Orionis, in a straight

line, marking his belt, and below the belt another line of

fainter stars, 42, θ and ι Orionis, marking his sword.

The brightest stars in Orion are β Orionis (Rigel),

a triple system consisting of two bluish-white (B8 and

B5) components, magnitudes 0.2 and 6.8, separation 9.5,

the primary of which is the seventh brightest star in

the sky, and a third, unseen component which revolves

around the latter in a period of 9.86 days, α Orionis

(Betelgeuse), a red giant semiregular variable (range 0.0–

1.3, average period about 2335 days), which is usually

considered to be the tenth brightest star in the sky,

γ Orionis (Bellatrix), magnitude 1.6, ε Orionis (Alnilam),

magnitude 1.7, ζ Orionis (Alnitak), a close binary with

bluish-white (O9.5 and B0) components, magnitudes 1.8

and 4.0, separation 2.4, κ Orionis (Saiph), magnitude 2.1,

and δ Orionis (Mintaka), a multiple system consisting of

a bluish-white (O9.5) eclipsing binary (range 2.14–2.26,

period 5.73 days) with a third, very close companion,

magnitude 3.8, separation 0.27, and a fourth, bluish-white

(B2) component, magnitude 6.3, separation 53. There are

nine other stars brighter than fourth magnitude.

Another interesting multiple system is σ Orionis,

which consists of a very close binary with bluish-white

(O9.5 and B0.5) components, magnitudes 3.8 and 5.2,

separation 0.2, period about 170 years, and three other

bluish-white (A2, B2 and B2) components, magnitudes 8.8,

6.9 and 6.6, separations 11, 13 and 43 respectively.

Other interesting objects include the middle star in

the sword of Orion, θ Orionis, which is a wide double with

components θ1 and θ2 135 apart. Each component is itself

multiple, θ2 consisting of a bluish-white (O9.5) primary,

magnitude 5.0, and a fainter, magnitude 8.3, secondary,

separation 0.4, while θ1 has four main components:

A (also known as V1016 Orionis), a bluish-white (O7)

eclipsing binary (range 6.7–7.7, period 65.43 days), B (also

known asBMOrionis), another bluish-white (B0) eclipsing

binary (range 7.9–8.7, period 6.47 days), separation 8.8,

C, a bluish-white (O6) star, magnitude 5.1, separation

12.8, and D, another bluish-white (B0.5) star, magnitude

6.7, separation 21.5. The latter four stars, known as the

Trapezium, are the brightest members of an open cluster

at the heart of the Orion Nebula (M42, NGC 1976), one of

the brightest emission nebulae in the sky, just visible to the

unaided eye as a faint misty patch.

The Orion Nebula is an area of ionized hydrogen (HII)

associated with a region of mostly molecular hydrogen

(H2) that forms part of the Orion Molecular Clouds, which

extend across the constellation. These are believed to be

sites of star formation, and infrared observations have

detected the presence of several young stars and star

clusters, such as the Becklin–Neugebauer (BN) object and

the Kleinmann–Low (KL) cluster, embedded in the gas.

Another well-known nebula is the Horsehead Nebula

(Barnard 33), an protuberance of dark nebulosity

silhouetted against a swathe of emission nebulosity (IC

434) that extends south fromζ Orionis. There are no bright

galaxies in Orion.

The Orionid meteor shower appears to radiate from

the constellation.

See also: Becklin–Neugebauer object, Betelgeuse, Horsehead

Nebula, Orion Nebula, Orionids, Rigel.

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