It is a light echo effect. Expanding nebula that you see is a light front of the burst, reflected from stationary dust clouds surrounded the star. In my TODO list I type "light echo" task. I thought about new nebula lighting model, where it will be possible to light every sprite of nebula with time shifted light function, so it will be possible to make light echo effect.
Supernovae explosion is different task - it reqires animation of the nebula: sprites must move avay, twist or so. It is also possible, and in my TODO list.
Though wouldn't it be possible to just model how it looked before and after (for a supernova, maybe this), and then have a button or something to switch back and forth per object?
Anyways, glad to see it's on your to do list.
And i'm kinda scared of what v838 will look like once it's done, it already looks pretty... evil in the pictures alone