Orion

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.

Derivation

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.

Stars

Orion contains some of the best known stars in the sky, with perhaps the most famous being the variable red supergiant 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 Rigel.

A key feature of Orion's constellation is its 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. Southward of Alnilam (the central star of the Belt) is the Sword of Orion - a vertical north-south 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 or upraised sword.

Nebulae

The Solar System lies at the inner edge of the Orion Arm of our Galaxy. When we look at Orion, then, we are looking outward through the stars of our own galactic arm, towards the Galaxy's edge.

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.