1. A planetary nebula is a nebula formed when a star blows off its outer layers late in their lives. These gaseous emissions expand into space, forming a nebula which is generally the shape of a ring. Two centuries ago, German astronomer William Herschel called these nebulae planetary nebulae because they were round like the planets.
2. Direct Current is preferred over Alternating Current because AC would undo exactly as much electroplating as it accomplishes!
3. Simple harmonic motion is a type of periodic motion in which the restoring force acts directly proportional to the displacement and acts in the direction opposite to that of displacement. This is what happens in the motion of a simple pendulum.