In Star Wars, the primary planet-destroying weapon of the Death Star. Despite the weapon's name, its fundamental nature is certainly not that of a laser or ordinary light. It more closely resembles the blasters, turbolasers, and "laser cannons" used throughout the Star Wars universe which are obviously not laser weapons either from their operation. The visible light emitted by the beam seems to be merely a by-product of the weapon's operation, whatever its underlying physics might be.

The destruction of an earth-sized planet such as Alderaan as displayed in the film requires tremendous energy. Assuming Alderaan is an approximately similar to the earth in its structure and composition, a rough approximate figure for the gravitational binding energy of Y = 2.4 x 1032 joules. This is the minimum energy needed to destroy a terrestrial planet, and given this much energy it would not explode in the catastrophic fashion we see, but would gradually break apart over the course of several hours. The Death Star's energy output must be far greater. The superlaser blast that destroyed Alderaan took about 0.21 seconds, so the power of the bolt should be of the order of 1032 watts.

It has been said that the Death Star would have had the capability to release a planet-destroying blast at most once every single day. So the main reactor that powers the superlaser must have a power output of roughly 2 x 1027 W. This is about five times the continuous power output of a star like the sun, and is far, far greater than any other fictional power source ever imagined in any other science fiction universe.

The actual explosion of Alderaan in the dramatic fashion seen in A New Hope allows us to set upper limits on the actual energy of the superlaser. A look at the explosion shows that the outermost parts of the blast are moving at a speed of about 1.7 x 107 m/s, about 6% of the speed of light. This places the energy of the beam at about 2.6 million times the lower limit threshold given above, or of the order of 3.4 x 1038 J. This is the equivalent of converting 3.8 x 1021 kg of mass into energy, roughly 10,000 times the energy that would be released if the entire mass of the station (assuming it were made completely of iron) were annihilated with an equal quantity of antimatter.

Source: Curtis Saxton's STAR WARS Technical Commentaries, from TheForce.Net:

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