Mighty Orion worth a hunt
AS the evening twilight fades at this time of the year, the magnificent constellation of Orion, the Hunter, is seen towards the north.
Evenings in late summer offer a fine view of this famous star pattern, one of the few that really looks like the object or creature after which it was named.
As with many of the famous constellations, Orion appears upside down to us, because we are standing upside down compared with skywatchers in the northern hemisphere.
The most distinctive feature of Orion is his belt, formed by three bright stars in an almost straight line.
Above are two bright stars that mark his legs, and below are his shoulders, with one of them being marked by the reddish-orange star Betelgeuse.
It is just above Orion's belt, as seen from our part of the world, that we find one of the most well-known of all objects for binoculars and telescopes: the Orion Nebula, also known as M42. The Orion Nebula is easily located, as it coincides with the middle of what appear to be three stars pointing upwards from the belt.
These three form Orion's sword — which for us, of course, hangs upward.
Even with the unaided eye, the middle one of these “stars” appears rather fuzzy, as if it were out of focus. Aiming your binoculars at this will reveal it to be a little “cloud” of light set among the stars.
You will be looking at a huge cloud of material in space — largely ionised hydrogen, mixed in with smaller amounts of other elements.
This type of structure is sometimes called an “HII region”, because “HII” is the way scientists denote a hydrogen atom with its lone electron removed — that is, ionised. (Neutral hydrogen is called HI).
As well as being the most famous nebula in the sky, the Orion Nebula has been a major object for astronomical research.
It is the closest significant star-formation region to Earth, even though it is about 1350 light years away — meaning that the light of the nebula that we see today left on its journey to us in about the year 670.
It is a place where clouds of material are collapsing to form stars, so it could rightly be called a “stellar nursery”.
Beginning in the 1990s, the Hubble Space Telescope (HST) has been used to image stars in formation, called protostars, and even detect protoplanetary discs of material around them.
These are discs of material that will form the planetary systems of these new stars.
The Orion Nebula is a favourite target for astrophotographers. With the great advances in CCD-based astrophotography, many amateur astronomers have produced magnificent images of the nebula, as has the famous HST.
One of my favourite images of the Orion Nebula is that made from observations by the HST. It was put together by combining 520 separate images of the object, in various different wavelengths of light.
It reveals the nebula in stunning detail, with some of that detail familiar to people who gaze at the nebula through telescopes. A 20cm-diameter telescope easily shows several of the general features of the nebula that are captured in detail in the HST image, and even a small telescope reveals the tight little group of stars in the middle, known as the Trapezium.
The designation “M42” was given to the Orion Nebula by the 18th-century French astronomer Charles Messier, who observed the sky from Paris. (When we're travelling again and if you can go to Paris, you can visit the location where he did his work. It is the Hotel de Cluny, just off the Boulevard St Michael in the central part of the city.)
He was an avid comet hunter, watching for previously unknown ones as they moved against the backdrop of the
stars. Along the way, he made a list of objects such as nebulae, star clusters and galaxies, many of which looked like comets, and the Orion Nebula was the 42nd one in his list. Unfortunately for Messier, he is now known more for his list of these objects than for the comets he actually found!