What happens after a supernova occurs depends on a number of things, but hydrogen isn’t even close to the fastest thing that gets blown away from the dying star. You’re thinking along the right lines though, in that hydrogen is the lightest element, and therefore the easiest to accelerate of all of the periodic table, but when we’re dealing with supernovae, we’re not limited to just the pieces that are listed as a standalone element.
The former star is producing a huge amount of energy in a very short period of time by definition, as a supernova – with this kind of energy around, atoms are not in their neutral state as we might find them on earth. The vast majority of them will be ionized in some form, meaning that one or more of their electrons has absorbed so much energy from the surroundings that they have escaped their atom and are careening around as loose particles. Electrons with this sort of energy are moving much faster than the protons and neutrons they left behind, but even they aren’t the fastest things coming out of the supernova.
That award goes to the neutrino, which is a super tiny fundamental particle which very rarely interacts with matter through any other force than gravity. Because its mass is so small (for a while we thought they might be entirely massless, like the photon), they can travel at speeds that are very close to the speed of light. Neutrinos are produced in our sun, and are produced in much larger quantities in supernova. Because they’re traveling so fast and interacting so rarely, we can often detect the burst of neutrinos arriving at Earth before we can spot the rise in brightness from the explosion, as we did with Supernova 1987A, pictured above. The neutrinos can escape from the core of the star quickly without interacting with the rest of the star, but the burst of light that we can observe has to wait until the explosion, which begins at the very center of the star, reaches the star’s surface. (This is not a long delay; for Supernova 1987A it was only a few hours.)
Electrons are still zipping around pretty quickly though – they’re moving fast enough to create X-rays, which can then be observed with X-ray telescopes out in space. In order to do this, they have to be moving at relativistic speeds, where talking about their speeds in terms of the fraction of the speed of light is actually a useful metric.
In terms of where the matter goes, what we typically see as the supernova remnant in these images is the shock front of the ejected material hitting the gas and dust which surrounded that star. As the surrounding gas and dust will be a little differently arranged for each star, the shock front will have a slightly different shape for each supernova as well. And depending on the density of the material the shock front is running into, the expansion of the shock front will continue at different speeds. Inside that edge, there can be a reverse shock, where material has raced up behind the shock front, and bounced off of it, and is reflected back towards the inside, heating up the material on the inside of the supernova bubble to very high temperatures.
Once the material is expelled outwards into the shock front, it’s unlikely to fall back to the center; it’s too far away and moving too fast. It’s the material closer to the stellar remnant, which wasn’t initially collected into the remnant, but also didn’t get pushed very far away, still close to the center of the supernova bubble, which has the potential to fall back down over time.
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