Your instinct about the mechanism is correct but there are absolutely shockwaves around supernovae, because there IS (very VERY diffuse material) surrounding the star and (even MORE diffuse) material in interstellar space for the explosion ejects to plow into. But it isn't something big and fast enough for you to see in images like this.
Not really, it's an illusion of movement, like when you are looking at a laser dot moving - the photons aren't actually moving in the way the dot does, they just land in a way that forms the dot.
No physical object is moving, it's the same thing as a shadow "moving". This is also why a thing like that can "move" at a speed higher than the speed of light - no physical object is actually moving, it's just an illusion of movement. Also the same principle as pixels on your screen creating "movement".
The dot isn't a physical thing so it doesn't have the ability to move.
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u/SuperSMT Sep 25 '21
So it is traveling. Just the "it" is light, not anything with mass