Question

[WyreWizard]

Yes, I have a little question about something seen in sci-fi movies.  When a massive body like a moon, planet or star explodes, why is the shock wave from that seen only as a ring radiating out from the point of origin?  I mean, space is three-dimensional.  If a shock wave were visible, it would appear as a sphere ballooning out from the point of origin.

I have seen these in Star Trek VI The Undiscovered Country and Star Trek Generations.

Why do movie writers make it this way?

[Zapranoth]

I've wondered that myself at times.

I can't really come up with anything even that teleologically works.

Maybe if you consider the observer of the explosion seeing the point source with one star nearby providing the incident light, you'd see refraction off of part of the sphere, and it'd look like a ring?   But it seems stylistic rather than probably realistic.

Someone who knows some physics might be able to give it a stab.

Googling for other similar discussions I found this:

"Not only is this depiction inaccurate, but I think it's retroactively inaccurate. As far as I recall, the original Death Star explosion was more of a fireworks style explosion -- you only got the annular ring explosion after the reedit. (I will now get corrected by legions of SW fans.)

Now in terms of a hollow, artificial structure such as the Death Star or the alien ship in Independence Day, I think the shape of the explosion would be dependent on both the force of the explosion and the shape/construction of the object. If the object is significantly more rigid in some dimension, you could get a shaped discharge. For example, if the DS had a ring of vents around its circumference, then you might get the annular shaped explosion. More likely you'd get something vaguely globular with protuberences.

Of course, the DS had some kind of artificial gravity, so that could also have affected the shape of the explosion in the microseconds before the gravity generator ceased to exist.

In the case of a planet, well, planets don't explode, so it's kind of a moot point. If you could somehow get the interior of a planet sufficiently molten/gaseous fast enough to cause it to explode, the explosion would probably be lost in comparison to the energy causing the explosion. Sort of like a marshmallow in a blow torch. But let's say that instead of pumping in energy, the Death Star did something more technologically clever such as transmuting half of the planet's core into antimatter. I imagine by focusing your beam properly, you could get any shaped explosion you want. Intuitively, I don't think that the rotational energy of the planet would make much difference in the shape of the original explosion -- we're talking a force that's not strong enough to reduce the weight of an object at the equator more than a hair less than at the pole vs forces that are sufficient to send that object hurtling into space. Insignificant."

[Hammock Rider]

My guess is that it looks cool.

[WyreWizard]

Well you are right that planets realistically don't explode, but some stars do.  Novas, Supernovas and Hypernovas all explode.

I have read a theoretical cause of shock waves from a massive stellar body exploding is the fabric of space readjusting itself to the loss of that body.  That body was holding some of the fabric of space around it inside of itself and space was partially collapsed around it.  But when the body explodes, the space is released and readjusts local space.  Thus, you have a shock wave which is the periphery of space readjustment.

TYVM for presenting your theories.

[tommex84]

Good question 

Here's my theory:
Shock waves are somewhat like waves in the water. They occur when you rapidly displace huge amounts of air or water if we're talking about water waves. Since the space is vacuum, there is no possible way to see shock waves. The only thing that you probably could see are particles of the exploded object, but they would probably be traveling near light speed or even faster if there was no gravity field nearby slowing them down.

Why the filmmakers are showing them that way? Probably because it's easier and looks cooler.