On the left, you can see the destruction of Praxis, the moon of the Klingon homeworld. It blows up in the beginning of Star Trek VI: The Undiscovered Country. On the right, you can see the end Death Star from the 1997 re-release Star Wars: A New Hope. One astronomer, Phil Plait, argues that the rings appearing in both explosions show a poor understanding of physics, but the one appearing in Star Wars is somewhat more plausible.
In his 2002 book Bad Astronomy: Misconceptions And Misuses Revealed, From Astrology To The Moon Landing 'Hoax', Plait casts doubt on the verisimilitude of (Lucasfilm effects house) Industrial Light And Magic's seminal CGI effect depicting the explosion of the Klingon moon Praxis in Undiscovered Country. Since explosions in space tend toward spherical shape unless impeded, Plait concludes that the blast pattern resulting from the explosion of the Klingon mining operation has no credible reason to resolve into a ring form, even if everyone thinks the effect is far cooler here than in 1997's revised New Hope.
Conversely, the surface integrity of the Death Star hull is interrupted by a perfect ring in the form of the gargantuan maintenance trench which encircles it, meaning that at this point of interrupted stress, a growing explosion would find the least resistance. This makes the highly criticised 'ring effect' far more plausible in New Hope [V.2.0] than its predecessor in Star Trek VI.
Link via slashdot | Images: Paramount and Lucasfilm, respectively, modified by Shadowlocked