Actually, the title of this blog post's a bit misleading because in reality the explosion caused the jellyfish and not the other way around, but I thought it sounded nicer this way. Now what on Earth is going on here? The nebulous filaments you see on this sketch are the remnants of a supernova explosion, quite similar to the ones that caused the Crab and the Veil nebulas. The Jellyfish Nebula (or IC443 if you like) decorates the winter constellation of Gemini and contrary to its two more famous counterparts it's a lot more difficult to observe. For starters, it lies much further away from us - some 5.000 lightyears compared to 1.470 for the Veil - and it's probably much older and therefore more dissolved into space. Given its large size and considerable distance we find that the Jellyfish extends 70 lightyears across, much more than the Veil's 50 lightyears. But this doesn't mean that the Jellyfish isn't an interesting object at all; much on the contrary I would say. Yes, it does take a reasonable amount of telescope and a dark sky to spot it, but as you can see on my sketch it does have a quality that is no second to many of the brighter nebulae in the sky. If you get the chance to observe it through a really big telescope under a really dark sky the delicate whisps and filaments reveal themselves just in the same way as they do in the Veil. All that you see is gas and other matter that's blown away by the incredible shockwave that the supernova explosion generated. Remember that a supernova releases more energy in a fraction of a second as our Sun produces in its entire life! The initial velocities of such a shockwave can be as high as 30.000km/s, or one tenth on the speed of light, and the resulting gas bubble's heated up to millions of °C! Of course, our Jellyfish has slowed down (30km/s) and also cooled down (10.000°C) considerably over time and probably within the next centuries its expansion speed may fall below the local speed of sound. Unfortunately there's still great uncertainty about the age of this nebula with estimates varying between 3.000 and 30.000 years. Recent observations with NASA's sophisticated X-ray telescope, however, seem to confirm the latter figure.
The Jellyfish is also an interesting object from another perspective. It lies within a dense molecular cloud complex and therefore scientists have studies this nebula to a great extent in order to see how the expanding blastwave interacts with the surrounding gas clouds. It's also been suggested that the star that caused the Jellyfish explosion had such a short life - perhaps as short as 30 million years - that it was still enveloped by its original birth nebula when the supernova went off. All that's left of it now is a tiny neutron star, merely 10km across but with a mass that's perhaps twice as much as our Sun's. Imagine how heavy it must be and how great the force must have been that was able to compress all of that matter into such a tiny ball! It's spinning rapidly whilst emitting radiation in a beam, much like a lighthouse on speed. Such pulsating neutron stars (or pulsars) can turn at a speed of several hundred rotations per second and they do this with such an accuracy that make them the most precise clocks in the universe.