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kucharek
2002-Apr-03, 09:45 AM
I recently saw the updated version of the original "Star Wars" movie and recognized that they enhanced the explosions of Alderaan and the Death Star with this fancy shockwave I think I first saw in Star Trek VI when Chernobyl happened to the Klingons.
These explosions have not a simple, spherical shockfront, but a spherical explosion and a disc-shaped shockfront travelling out of the center of the explosion. Looks pretty nice.
Questions:
As many effects are today inspired by real stuff like spectacular Hubble photos, what started the use of these shockfronts for explosions in movies? Or are they pure fiction?
What kind of explosion must you have to get a shockwave like in the movies? A simple explosion surely has a simple spherical shockwave. Maybe the effect was inspired by old movies of nuclear test explosions in the atmosphere, as you get a ring-effect where the shockwave hits the ground.

Harald

Donnie B.
2002-Apr-03, 12:19 PM
Hmmm, I kinda like your "shock front intersecting the ground" theory.

Of course, it is possible to produce non-spherical shock fronts. All it takes is for some material to be present to resist the expanding wavefront in certain directions. The effect would be greater if the explosion were relatively small and the structure relatively strong. It would also tend to damp itself out and become more spherical at greater distances from the explosion (I think... or would this hold true in the vacuum of space?)

Someone has pointed out that the Death Star design includes an equatorial trench, which might account for the ring-shaped shock front -- especially if the trench were just the visible surface of some deeper structure.

Astrophotography has revealed a lot of ring-shaped objects, including one around the scene of the recent supernova in the Magellanic Cloud (1987a?). However, many of these objects are optical illusions - diffuse spheres seen edge-on (so we see only the thicker parts around the limb), or points of intersection between spherical shock fronts and other objects like gas clouds.

There are other kinds of non-symmetrical emissions, though, notably jets, which can be produced by intense magnetic fields or gravity wells.

SeanF
2002-Apr-03, 12:25 PM
On 2002-04-03 07:19, Donnie B. wrote:

Someone has pointed out that the Death Star design includes an equatorial trench, which might account for the ring-shaped shock front -- especially if the trench were just the visible surface of some deeper structure.



Good point . . . except I believe the shock wave ring from the Death Star was "polar" - it was at right angles to the equatorial trench rather than coinciding with it . . .

David Hall
2002-Apr-03, 12:29 PM
On 2002-04-03 07:25, SeanF wrote:
Good point . . . except I believe the shock wave ring from the Death Star was "polar" - it was at right angles to the equatorial trench rather than coinciding with it . . .


Right you were. It was off-center from the equator. I remember thinking how ridiculously odd it looked when I saw it first. I also thought it was overkill to have two explosions with the same effect in the same movie. I think it would have been cooler to leave the Alderaan ring out and just have an equatorial ring in the Death Star destruction.

Firefox
2002-Apr-03, 12:33 PM
I found it interesting how the ring expanded from each Death Star differently (polar for DS1, equatorial for DS2.) I don't know what the mechanics would be, though I do know the first one was ~100 miles across, while DS2 was in the neighborhood of 550 miles in diameter, though not yet completed. Again, I don't know if that has anything to do with the ring waves.

Perhaps the filmmakers were inspired by Hubble images of the Ring Nebula and similar planetary nebulae?

informant
2002-Apr-03, 12:56 PM
They did seem to take inspiration from some Hubble pictures of planetary nebulae, like Eta Carinae:
(Gave up on inserting picture.)
I remember thinking that when I watched the special edition. I though that was a bit lame.


<font size=-1>[ This Message was edited by: informant on 2002-04-03 12:18 ]</font>

GrapesOfWrath
2002-Apr-03, 12:59 PM
The BA covers this in his book. He spends a page and a half on it (pp. 254-255), in the chapter Bad Astronomy Goes Hollywood. From his Top Ten list of Bad Astronomy in movies and television: "9. ...Our Hero gets a lock on them and fires! A huge ball of expanding light erupts past us, accompanied by an even faster expanding ring of material as the Dreaded Enemy's engines explode."

He gives the credit to Star Trek VI also.

Silas
2002-Apr-03, 02:27 PM
In Trek VI, when the shock wave hits the Enterprise -- does anyone else see this as an ocean wave hitting a floating model?

I can't put my finger on it -- but there is just something about the motion, the up and down and rising and tilting -- that just says "ocean" to my visual cortex.

Silas

Firefox
2002-Apr-03, 02:40 PM
I did get that impression when she (it was Excelsior, btw) was hit by the wave. Another problem with it was that the ship was supposedly a good number of light years away. The way the writers went around this problem was to call it a "subspace shockwave." Guess that works for Trek, I suppose.

ToSeek
2002-Apr-03, 03:53 PM
On 2002-04-03 07:33, Firefox wrote:
I found it interesting how the ring expanded from each Death Star differently (polar for DS1, equatorial for DS2.) I don't know what the mechanics would be, though I do know the first one was ~100 miles across, while DS2 was in the neighborhood of 550 miles in diameter, though not yet completed. Again, I don't know if that has anything to do with the ring waves.


I would think it might be possible that one could get the illusion of a ring at the periphery just because there's more material in the way at that point. But that means the ring would appear perpendicular to one's position, and I don't think that's what they've been depicting.

Silas
2002-Apr-03, 04:32 PM
On 2002-04-03 09:40, Firefox wrote:
I did get that impression when she (it was Excelsior, btw) was hit by the wave.


Oops... Now I have to give back my Star Trek Merit Badge... /phpBB/images/smiles/icon_wink.gif

I just had another thought about "rings" and explosions; could they be inspired perhaps by the rings around the "stalk" in the mushroom clouds in some of the old H-Bomb tests? Or perhaps from the expanding circular shock wave that you can see in aerial footage of bombing runs from WWII or Vietnam? Or perhaps its borrowed from Japanese Animation, such as Akira?

(Alas, the new "explosion paradigm" is probably the effect of the weapon piercing the target at the same time of the explosion, as is seen in slow-motion shots of cruise missiles slamming into targets, and, of course, as was stunningly clear in the airline collisions at the WTC. I predict we'll see this used in action movies because, one, it's visually impressive, and, two, it has emotional resonance with the intended audiences of such movies.)

Still, think how far ahead we are of the old Hanna-Barbera "roiling cloud" explosion.

Ben Benoy
2002-Apr-04, 03:57 AM
Um... do I get to be the first to mention that the fx guys think it looks cool, so everything explodes with a ring? Yeah.

Ben Benoy

Also, I think in Trek VI, Sulu actually recommends that they surf the wave out. I may be imagining this, though.

Bob S.
2002-Apr-04, 10:24 PM
On 2002-04-03 22:57, Ben Benoy wrote:
http://antwrp.gsfc.nasa.gov/apod/ap960705.html with its triple rings (equatorial and 2 expanding rings moving along its axis);


Actually, the outer rings are probably expanding more quickly in diameter than they are up and down from the star. They are almost certainly density enhancements in a double-lobed expanding bubble of material blown out by the star before it blew up. See here for more (http://www.badastronomy.com/bitesize/sn87a_threering.html)

<font size=-1>[ This Message was edited by: The Bad Astronomer on 2002-04-04 17:45 ]</font>

The Bad Astronomer
2002-Apr-05, 12:34 AM
Oops! I think I hit the edit button again! Nuts. Sorry about that. I actually wrote the previous post about the rings around SN87A.

odysseus0101
2002-Apr-05, 01:49 AM
Also, I think in Trek VI, Sulu actually recommends that they surf the wave out. I may be imagining this, though.


If the shockwave really was a ring, then they could have just changed position "up or down" (yeah yeah, it's space and all, you know what I mean) to avoid the shockwave.

Simon
2002-Apr-05, 09:49 AM
I just had another thought about "rings" and explosions; could they be inspired perhaps by the rings around the "stalk" in the mushroom clouds in some of the old H-Bomb tests?

I think that's probably the most probable source. I remember my first thought when seeing an aerial photo of an H-bomb test was "Oh, so THAT'S where they got the idea!"

Darkwing
2002-Apr-05, 12:32 PM
In Star Trek: Generations, they had three really nice shockwaves--and all spherical: The Amargosa star exploding the Veridian star exploding, and the Enterprise's warp core breach...all were very cool in my opinion.

Firefox
2002-Apr-05, 12:53 PM
Yeah, I remember those shockwaves. They seemed almost too neat in my opinion, though I suppose the one from the Armagosa star hadn't expanded enough to let density enhancements to take over.

I could nitpick about their being able to see the star explode before they actually would have, but I'll abstain.

Darkwing
2002-Apr-05, 01:12 PM
Oh, sure, there are plenty of nitpicks you could make about the stellar explosions, but just terms of a cool spherical shockwave special effect, they were pretty neat.

Darkwing
2002-Apr-05, 01:49 PM
Oh, sure, there are plenty of nitpicks you could make about the stellar explosions, but just terms of a cool spherical shockwave special effect, they were pretty neat.

Wally
2002-Apr-05, 01:54 PM
On 2002-04-05 07:32, Darkwing wrote:
In Star Trek: Generations, they had three really nice shockwaves--and all spherical: The Amargosa star exploding the Veridian star exploding, and the Enterprise's warp core breach...all were very cool in my opinion.


Yeah, but can anyone explain just how that rocket used to blow up the star got there so quickly??? Seems it was just leaving the atmosphere, then whammo!! the Sun explodes.

Darkwing
2002-Apr-05, 02:27 PM
I know--the rocket got there pretty fast...but that was a "hollywood" compromise. Would you really would have wanted to just stare at the movie screen for a long time with nothing happening but the rocket travelling to the star, doing its thing, then waiting for the information of the explosion to get to the planet? For all the time required for all that to happen, most people would have long since left the theater.

All this said, I still think the shockwaves themselves looked cool, which was my original point.

kucharek
2002-Apr-08, 05:37 AM
And I never understood how turning a star into a black hole should change trajectories of bodies passing by. I think everyone is pretty happy about the fact that you always can consider the mass of a body concentrated in one point when you want to do orbit/trajectorie calculations.
Okay, ideally this applies only to a homogenous, spherical body, but this assumption is enough for most purposes.

Harald

Firefox
2002-Apr-08, 11:38 AM
I know--the rocket got there pretty fast...but that was a "hollywood" compromise. Would you really would have wanted to just stare at the movie screen for a long time with nothing happening but the rocket travelling to the star, doing its thing, then waiting for the information of the explosion to get to the planet?

I suppose they could have staged more fighting between Picard and Soran. /phpBB/images/smiles/icon_smile.gif

David Hall
2002-Apr-08, 12:05 PM
On 2002-04-05 08:54, Wally wrote:

Yeah, but can anyone explain just how that rocket used to blow up the star got there so quickly??? Seems it was just leaving the atmosphere, then whammo!! the Sun explodes.


Well, dunno for sure, but of course the missle could have had a warp engine built in. Would zap it there in no time. However, that still wouldn't explain the almost immediate darkening and destructive shockwave, which would also have to have gone faster than light to reach the planet.

David Hall
2002-Apr-08, 12:11 PM
On 2002-04-08 01:37, kucharek wrote:
And I never understood how turning a star into a black hole should change trajectories of bodies passing by.

I suppose in Trek at least, the missle that destroyed the star didn't just cause a supernova, but actually destroyed the mass of it, or converted the entirety of it into energy and radiated it away, which would have altered the gravitational effect. But once again, theory says that gravitational effects also travel at c, so this was inaccurate as well.

Hey, it was only a movie. /phpBB/images/smiles/icon_smile.gif

Darasen
2002-Apr-17, 02:26 PM
Easy answer: they look nifty and are not very hard to animate in a decent CG package.
http://www.the3dstudio.com/tutorials.asp?id=21

Darasen