One of the best known and most recognisable star-patterns in the sky, Orion represents an
heroic hunter of Greek myth. Lying on the edge of the Milky Way, this constellation is
rich in bright stars and nebulae.
In Greek myth, Orion was a giant who hunted the wild animals of the earth. He was the enemy of
Artemis the huntress, who according to some tales was responsible for his death. Other stories,
though, tell how he pursued the daughters of Pleione - the Pleiades -
and with them was turned into a constellation to chase them forever across the sky.
The origins of Orion as a celestial figure predate even the ancient Greeks. It is thought,
for example, that the Egyptians revered this constellation as
the heavenly embodiment of their god Osiris.
Orion contains some of the best known stars in the sky, with perhaps the
most famous being the variable red
giant Betelgeuse, which marks
Orion's left shoulder. At his right shoulder is another variable
star, this time blue in colour, known as
Bellatrix. At Orion's right foot is yet another famous
star, blue like
Bellatrix: the supergiant
A key feature of Orion's constellation is his
Belt of three bright stars
that form a nearly straight line across its central parts. These stars are
Alnitak, Alnilam and Mintaka,
three very distant but very luminous bodies. Beneath Alnilam (the central star of
the Belt) is the Sword of Orion -
a vertical line of fainter stars and nebulosity.
In addition to its better known stellar features, Orion also contains a bow-shaped
array of less prominent stars to the west of the
constellation's main body, which are said to represent the hunter's cape or shield.
To the north, where the Milky Way passes through the
constellation, a less clearly defined star-group
supposedly describe's Orion's club.
The Sun and Solar System are situated on the inner edge of a
small spiral arm of the Milky Way Galaxy known as the
Orion Arm. In the direction of the constellation of
Orion is the dense heart of this galactic arm, and so this part of the sky is filled with
nebulous regions. In particular, a vast dark cloud known as the Orion Molecular Cloud
fills a significant portion of the constellation. The Cloud, which lies about
1,300 light years away, is generally invisible from Earth.
Where conditions are right, though, areas of this matter are illuminated by nearby stars to
become visible as nebulae.
The most prominent of these is the diffuse nebula at the centre of the
Sword of Orion, sometimes simply called the
Great Nebula, but more commonly known as the
Orion Nebula. This is a region of star formation
just within the inner edge of the Cloud, where the newly formed stars that make up
the Trapezium formation light up the gas and dust that surrounds them.
Another very well known object in Orion is the so-called Horsehead Nebula. This is
a plume of interstellar matter that cuts out some of the light cast by the bright
nebula IC 434, just south of Alnitak. The fact that this plume has a shape irresistably reminiscent of a
horse's head makes it one of the most recognisable such phenomena in the sky.
The region around the Belt and
Sword of Orion, on the edge of the Molecular Cloud, is so active that it
exerts immense pressure on the surrounding interstellar material. The result is an apparent spherical 'wave' some three
hundred light years across. This phenomenon is visible as a faint and
broken ring known as Barnard's Loop, extending outwards as much as 14° from the central regions of Orion.