New study shows the science behind a famous astronomical image
Posted by Science Oxford on October 2, 2009 | comments
You must have seen this image before? It is the Eagle Nebula. The below article explains in detail what is exactly going on in there…
Visit this page »Massive stars are born in the densest parts of Giant Molecular Clouds, dense cold and dusty regions of the universe. They are very luminous, and in particular emit huge quantities of ultra-violet (UV) radiation. This ionises and heats the surrounding gas, generating a rapidly expanding bubble known as an HII region.
This process does not happen very evenly, leading to strange and sometimes beautiful structures forming on the walls of this bubble. The famous Eagle Nebula pillars (right) are examples of these structures, but there are many others, often called “Elephant Trunks” by astronomers because of their elongated appearance.
There are a number of possible formation mechanisms for these pillars. Instabilities in the bubble’s expansion could lead to corrugations forming in the expanding ionisation front, which could develop into elephant trunks. Observations show that molecular clouds are very clumpy though, so it is also possible that the densest clumps act as seeds for the formation of pillars. This is the idea we are modelling with our computer simulations.
2D Simulations
Real space is 3D, but often a lot can be learned from constructing simple 2D models of physical processes. As well as being simpler to visualise and analyse, they are also far less costly to simulate in terms of time and computing power. A desktop computer can run very detailed 2D models of star forming regions in a matter of hours.We first looked at the effect of ionising radiation from a single massive star on a single dense clump of gas placed in a lower density ambient gas. The idea is that if we put a number of dense clumps near each other, in a random distribution, some of them might move into the shadowed regions and organise themselves into something like a pillar.
Photo-ionised clumps are accelerated by the Rocket Effect, where hot gas “exhausts” away from illuminated surfaces in a photo-evaporation flow, and it gives an equal and opposite “kick” to the dense neutral gas (from Newton’s 3rd Law). If a clump is fully exposed to radiation, then it is pushed away from the star, but if it is partially shadowed, then the direction of the kick will be partly away from the star, but also away from the illuminated side and into the shadow. In this way it is possible that much more gas could get added to the shadowed region quite quickly, and a pillar could be built up much more quickly than with just a single dense clump. This is indeed what we see in our 2D models.
3D Simulations
2D simulations have a number of limitations, and bigger 3D simulations are needed to check the results. We have done a number of 3D simulations at low resolution (128 grid cells along each of the 3 directions), and we resimulated the most promising one at higher resolution (256^3 grid cells). This model has a background gas density of 100 atoms per c.c., and 30 dense clumps ranging from 3-10 times the mass of the sun. We placed a radiation source (star) off the grid about 6 light-years away and allowed the gas to evolve due to the UV radiation.This modelling only considers radiation and fluid dynamics. We plan to run similar models with various magnetic field configurations. We would also like to include the effects of gravity which would allow us to track the evolution of the elephant trunks much further, to the point where the densest parts may start to collapse under their own weight to form new stars. The chemistry and thermal physics in our models is relatively crude, and improving this is probably the next stage in our work.
What do you think?