Tuesday, 2 February 2016

M42, the mighty Orion Nebula

Orion's certainly the most prominent constellation of winter and it holds one of the greatest night sky treasures visible in the northern hemisphere: the famous Orion Nebula. It's easy to find, below the three stars in Orion's belt, at the middle of a string of small stars we call the "sword". If you look carefully, you can already see it with the naked eye under a not too much polluted sky. The total nebula extends over an area of 1°, or about two full moons across, but in reality it's just a part of a much larger nebula complex, including the also famous Horsehead Nebula, the Flame Nebula, M78 and gigantic Barnard's Loop which draws half a circle around the entire constellation. 

The Orion Nebula is a schoolbook example of a stellar nursery and observations have revealed up to 3.000 newborn stars at several stages of their formation within it. Stars are born when large clouds of gas and dust collapse under their own gravity and the heat generated by their compression ignites nuclear fusion. The best known baby stars within this nebula are without any doubt the so-called "trapezium", four bright stars which are so close to one another that in the large field of view of my binoculars they could hardly be separated. Some of the brightest stars we see in this nebula may only be 10.000 years old, which in stellar terms is extremely young. The Hubble space telescope also revealed so-called protoplanetary disks, which is a disk of hot gas and dust surrounding a newly formed star. Gravitational collapse within these disks results in the formation of planets. Given the sheer number of these disks that we've already discovered, scientists conclude that stellar and planetary formation is very common in our universe.

The nebula will have evaporated completely within 100.000 years due to stellar radiation and the star cluster which was formed within it will start its journey through space, eventually falling apart as well. The Pleiades of which I spoke in an earlier post are a perfect example of such a young cluster that's just emerged out of a similar nebula, the remains of which have already evaporated. Although the evolution of our universe appears extremely slow in comparison to the lifespan of a man or even the entire human civilisation, the total lifespan of such a nebula would be no longer than 30 million years, meaning that the Orion Nebula must've formed much later than the age of the dinosaurs and even later than the Eocene. It is believed that some stars which can now be found in nearby constellations such as Auriga, Aries and Columba in fact originated from this nebula and are still zooming away from it at a speed of 100 km/s. It's also odd that not a trace of this nebula can be found in any written account before the beginning of the 17th century. Ptolemy for example didn't mention it, although he did mention other (fainter) nebulous objects in the sky. Galilei noted the trapezium but didn't mention the nebula either. It is therefore believed that a current flare-up of young stars causes a temporary increase of the nebula's brightness.

The Orion Nebula is one of the few large nebulae with a high enough surface brightness to reveal colour to the human eye. With my binoculars I noticed a very faint blue-green hue, which is actually the first colour that our eyes can perceive in darkness and which is caused by doubly ionised oxygen in the nebula. But with my old 18" Dobsonian reflector I was also able to see a very faint reddish tint near the brightest edges.

Almost attached to the great Orion Nebula, you can see its smaller companion: M43, and a bit more towards the top the "Running Man Nebula". The dark dustlanes within it do show the silhouette of a running man in a serious telescope but obviously my binoculars are not powerful enough to show all of that detail. The faint glow around the bright star below the nebula, is NGC1980, another part of the Orion cloud complex.

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