The Coalsack is one thing, travelling throughout the Sagittarius spiral arm is another. You begin to see the ‘big picture’ and how all things in space are related to one another. It’s really hard not to give in and divert from one’s course just to see that seemingly unique jewel you have spotted on your Galaxy Map. I did it a couple of times, going here, going there. Most of the time, it left me baffled about the beauty of the ‘out here’. I remember some of my Astrophysics lessons back in the day realised that even that ‘unique jewel’ is in truth a puzzle piece of some greater thing.
Since I had decided to explore the so-called Carina Molecular Cloud (CMC) I knew already what the ‘greater thing’ would be: The CMC is one of the biggest structures in the known galaxy, spanning hundreds of Lightyears in diameter. It is four times larger than the Orion Complex. And it’s also one of the more dynamic regions in space where due to the various meeting forces the environment is subject to constant change. The complex is believed to have been formed by a huge gravitational density wave hitting an area of intensely tumultous gases and magnetic fields, forming an elongated cloud of active star formation and clustering.
The result: Although the Carina Nebula still being more than 2,000 light years out, you are already moving through its namesake molecular complex, the CMC. It’s full of open clusters and the younger and more massive OB structures like the Carina OB1 and OB2 associations. Additionally, there are quite a few silent witnesses of stellar death, like the occasional Neutron Star, White Dwarf or Black Hole. Speaking about Open Clusters, it is generally assumed that these all formed out of the CMC, although not simultaneously. Maybe their formation was triggered by a cascade of supernova blasts, who knows?
These clusters are generally some ten to fifty LY across and very densely packed with stars, either in the formof a clump or more of a string. Some of their stars were so short-lived however, that they already stopped their hydrogen fusion or even went supernova, explaining the occasional stellar corpse. The clusters’ cores mainly consist of the more younger and massive types of stars like O, B and also A types. They are still gravitationally bound to each other, and they can be easily spotted on the Galaxy Map by setting the star type filters accordingly.
Over time the cluster is subject to what astronomers call mass segregation: The more massive stars burn away the surrounding gas masses with their heavy radiation and the gravitational grip on the cluster lessens. The lower mass stars are then ejected into the stellar streams of the spiral arm, leaving only the massive stars behind. The most massive ones eventually go supernova, weakening overall gravity even further until the mid range mass stars are flung out as well. The cluster ceases to exist, all of its remaining stars being on their own now. It is thought that Sol also once was a member of a long gone cluster, although that is another story…
Understanding molecular clouds and the various entities and evolutionary stages they contain took some time. After all, for a long time, people couldn’t just fly there and have a look. They had to rely on their, alas often primitive, instruments. If the documentation of early 20th century astronomy is to be believed it took a while to realise that most prominent nebulae are in fact only the illuminated ‘hot spots’ within those bigger molecular clouds. The Orion Nebula is a perfect example for this, and so is the Eta Carina Nebula. In ancient pre-spaceflight times, however, most nebulae were seen as separate entities. Today we know better: The bigger picture is, simply put, that molecular clouds are so hard to detect, because they are very cold (just a few Kelvin) and hence emitt barely any visible light or radiation. Also, the interstellar material is spread very thin. But when a star goes nova or supernova the ejected stellar material blasts outwards at great speed and thus compresses the surrounding dark clouds, making them collapse on their own gravity. The resulting thick patches of matter can be observed as dark clouds and are the seeds of a new generation of stars. These can be either low to mid mass stars or rather massive stars if there is enough material around to nurture them. These stars won’t form solitary, they almost always form in groups: A cluster of stars is born.
Young and massive stars in turn emitt heavy energetic radiation, as mentioned above. This radiation ionizes the interstellar medium around. It agitates it and makes it ‘shine’ in different colours, depending on its chemical components. So for the non-poetic or non-aesthetic people, these beautiful nebulae are just the relatively small ionized patches (called H II regions) of a much larger molecular cloud, made detectable by some young stars’ heavy radiation, much like an oceanic carpet of iridescent algae glowing in the night. Ionized hydrogen is most prominent, shedding the characteristic red colour as can be seen in the region’s magnificient Eta Carina and Statue of Liberty Nebulae. Other elements shine in blue, green and yellow and it all adds up to the nebulae’s colourplay.