The Guardians of the Universe partitioned and designated 3600 Space Sectors for their Green Lantern Corps, with the goal of combatting evil throughout their reality. This was due to the wave of evil unleashed by one of their own, Krona, which spawned the Multiverse. Throughout countless centuries, generation after generation of these power-ring protectors would monitor their section of the universe, of which the first official Corps member from Earth-One was Hal Jordan. He was tasked with guarding sector 2814, and as Green Lantern inspired others such as Guy Gardner, John Stewart and Charlie Vicker to likewise assume this identity at various times.
In fact, Charlie was assigned sector 3319, although he eventually returned to his homeworld. The renegade Green Lantern known as Sinestro had been given stewartship over sector 1417. Tragically, the Green Lantern of sector 2813 Tomar-Re was unable to save a prominent world there, Krypton-One, from exploding due to internal pressures whose origin was a mystery for years. These are but a few examples, as several other sectors have been identified progressively over the years, made more widely known by their Green Lanterns interactions with Hal Jordan over time.
On a parallel dimensional plane, the Guardians released a portion of their energy into that other universe, spawning the Starheart. For a time it would be alone, as a sentient star. Eventually, it would find fellowship in similar cosmically imbued sentient beings within stellar bodies throughout their universe’s numerous galaxies.

In fact, both Krypton-One and Krypton-Two were designed by the Sun-Thrivers, gaseous beings whose existence had depended on their encasing themselves within an artificial sun. However, the sun needed a counter-balance to maintain its internal gravity, and so pulled together enough mass to form a large planet within that red sun's orbit. Given the nature of its creation, each Krypton was prone to be doomed eventually. With the aid of Kal-El, the Superman now of Earth-One, they built an orange sun to sustain them.
Kal-L, aka the Superman now of Earth-Two, likewise encountered twin Kryptonite powered green suns orbiting Colu-Two. Utlizing his universe's Sun-Thrivers technology to create them, the immortal villain Vandal Savage used these green suns to siphon off energy from Superman and Power Girl. When he failed and escaped capture with help from mystery beings (probably Savage’s allies the Sun-Thrivers) the twin suns remained.
Throughout the centuries since its creation in the 6th century, the Sun-Thriver within one of these two Kryptonite suns turned insane. Known as Aquarius, now a member with other sentient suns of the self-titled Council of Living Stars, it was stripped of power and banned for its insidious actions. After briefly regaining power through Starman's Cosmic Rod before being defeated by the Supermen and their allies in the Justice League and Justice Society, Aquarius was no more.
The remaining members of the Council of Living Stars, each inhabited by a Sun-Thriver and imbued with power like that of the Starheart, took a more active role in policing their reality than had their counterparts. They assumed the role of intergalactic protectors for sections of their universe... as had the Green Lantern Corps!