The Orion Nebula

The constellation of Orion is one of the most familiar in the night sky, and it is unusual in being visible from both the northern and southern hemispheres, depending on the time of year. Its most notable feature is the chain of three stars of similar brightness in a short straight line (Orion’s belt), and two of the most familiar single stars in the sky, namely the red supergiant Betelgeuse at the top left-hand corner, and Rigel, a blue supergiant which is the sixth brightest star, at the bottom right.

However, probably the most intriguing feature of Orion is the nebula that is just visible with the naked eye (depending on how good your eyesight is), in the region below the belt that the ancients designated as Orion’s sword. The word “nebula” means mist or vapour, and that is how it appears, namely as an indistinct patch of fuzziness. With a reasonable telescope it is possible to see bright patches within the mist, and when seen through the orbiting Hubble telescope (see photo) it is truly spectacular, with huge regions of dust and gas illuminated by the stars within it. To make the images even more remarkable, the different light wavelengths are normally translated into “false colours” that give the nebula an extra level of beauty. 

As the Orion Nebula is the closest nebula to Earth, at around 1,300 light years away, it is not surprising that it has been known about for hundreds of years. There are four bright stars at the heart of the nebula that were first found to be surrounded by nebulosity by Nicholas Pieresc in 1611. Galileo had observed the stars but not the nebula in 1617, and Christian Huygens produced a sketch of the object in 1656. William Herschel, who later discovered Uranus, made detailed observations of the nebula in 1774, describing it as “an unformed fiery mist, the chaotic material of future suns”. It was recorded by Charles Messier in his 1774 catalogue of deep sky objects at number 42 on his list, and it has been known as M42 ever since (it is also listed in the “New General Catalogue” as NGC 1976).

Herschel’s remark, made more than 200 years ago, was particularly prescient, given that this is precisely what the Orion Nebula is, as are many other similar regions of space. It is a stellar nursery where new stars are being born, coalescing from vast quantities of dust and gas and eventually “igniting”. The material in question may well have come from the disintegration of a much older star (or stars) that had reached the end of their life cycle, so the nebula is a half-way point in the process of the death and re-birth of stars.

Seen in the visible light spectrum, which is all that the early astronomers had available to them, the most prominent feature of the Orion Nebula is the group of stars mentioned above as being at the heart of the nebula. Originally thought to be a single star, and named Theta Orionis, Galileo and Huygens both noted that it comprised three stars. When a fourth star was detected later in the 17^th century it was given the name The Trapezium, but later observations have increased the number still further, with some of the stars proving to be binaries (two stars in orbit around each other). This is an open cluster of young stars that have started to burn their way out of the dust and gas surrounding them, which is why they can be seen and why the surrounding nebula is illuminated.

However, when other means of observation became available to astronomers in the 20^th century, it became clear that there was much more to the Orion Nebula than the Trapezium. When viewed in the infrared, the nebula is seen to be much larger, in that there are features that are invisible in the “ordinary” spectrum. For example, in 1968 the “Becklin-Neugebauer object” was detected, this being a compact source of infrared emissions located behind the Orion Nebula as we see it. This appears to contain a massive, newly-formed star that is still so deeply embedded in the cloud of dust and gas from which it has formed that its visible light cannot escape. Instead, the light waves that are absorbed by the dust are re-emitted at infrared wavelengths. The object is so massive (about the size of the solar system) and active that it is the brightest object in the whole sky at those wavelengths.

As a stellar nursery, the Orion Nebula is extremely active, with as many as 700 stars at various stages of creation having been detected. The Hubble telescope has also discovered some 150 protoplanetary disks within the nebula that are believed to be an early stage of star formation, and “brown dwarfs” that are proto-stars that have not acquired enough mass to trigger nuclear fusion.

Many complex and violent processes have been detected within the nebula, which contains areas of ionized hydrogen (known as an H II region), molecular gas, and dust. As well as emissions in the infrared and visible wavelengths, there is ultraviolet radiation that is responsible for much of the glow of the nebula.

The Orion Nebula, itself about 24 light years across, is now regarded as being part of a general area of nebulosity that extends for hundreds of light years. This is known as the “Orion Molecular Cloud Complex” that includes much of the area around Orion’s sword and belt, notable features of which are the Horsehead Nebula, Barnard’s Loop and the Flame Nebula.

The Orion Nebula continues to fascinate astronomers and astrophysicists who, by studying it in detail, are learning a great deal about how stars are formed. We are fortunate to have such an active region of space so accessible to view by both earthbound and space-based instruments. Despite the long history of its study, the Orion Nebula doubtless has many more secrets yet to be revealed.

© John Welford