What is the source of power for the Green Lantern?
Green Lantern
The character of Harold "Hal" Jordan was a second-generation test pilot, having followed in the footsteps of his father. He was given the power ring and battery (lantern) by a dying alien named Abin Sur, whose spaceship crashed on Earth. Abin Sur used his ring to seek out an individual who was "utterly honest and born without fear" to take his place as a member of the corps. At one point, when Hal Jordan was incapacitated, it was revealed that there were two individuals matching the specified criteria on Earth, the other being Guy Gardner, and the ring chose Jordan solely because of his proximity to Abin Sur. Gardner then became listed as Hal's "backup", even though he had a strong friendship with Barry Allen (The Flash). Gardner would fill in if Jordan was unavailable or otherwise incapacitated. Later, when Gardner was put into a coma, it turned out that by then there was a third human suitable for the task, John Stewart, who was designated as the Earth Sector's "backup" Lantern. Jordan, as Green Lantern, became a founding member of the Justice League of America and as of the mid-2000s is, along with John Stewart, one of the two active-duty Lanterns in Earth's sector of space.
List of Green Lanterns
When the Guardians of the Universe sent TO-T-U-K to a planet to find his replacement as the Green Lantern of Sector 1287, he couldn't find another sentient creature on the entire planet. When he observed the Collective trap and consume a bird-like creature, he learned that the Collective's mental aptitudes increased. TO-T-U-K realizing that he if allowed himself and the power battery to be consumed by the Collective then it would gain the ability to think and act. He also gave the Collective a power battery.