Atari's only game released for the 2600 that featured a light gun!
From the manual:
Locate and Absorb Energy!
You're a scientist, and your invention, a pulsating, glowing orb called the Sentinel, absorbs and stores energy from a variety of sources. Now Earth is threatened by deadly aliens, and you must use your creation to save your planet.
You must defend the Sentinel against alien attacks as it glides over four enemy planets, absorbing all alien energy sources in its path.
The Sentinel absorbs the energy from every power pod or energy capsule you shoot with your light gun. When an energy source is hit, the Sentinel absorbs the target's energy.
Each of the four alien planets is more difficult to conquer than the last. You must find and destroy the alien lords who guard each planet's power station. Destroy the station to destroy the planet. Then move on to the next world.
A single alien entity controls the other alien lords. When you have destroyed the four worlds, the Sentinel will come face to face with this evil entity. Defeat the final evil to complete your quest!