A YJA Moment - Kryptonians

Like many of the worlds in Known Space, they had been seeded by the Celestials, who returned after they evolved to sentience in order to grant them even more power, creating what would be known as Eternals. Due to unique evolutionary properties, Kryptonian Eternals drew sustenance and power from the Rao system's then-yellow sun.

Unlike other seed-worlds, Krypton was geometrically unstable and because the Kryptonian Eternals were better suited to survive such conditions and eventually outlived all of their unenhanced and meta-brethren.

Millennia before the other races developed interstellar travel, Kryptonians discovered the Negative Zone, which they called the Phantom Zone.  Besides using it as a prison, they constructed stardrives that all allowed them to explore their galaxy.  Among those that were colonized was Daxam and the Daxamites continue to be members of the galactic community.

Experiments on Krypton's sun, Rao caused it to change and become a Red Dwarf.  The cataclysmic effect on the homeworld required the full attention of the Kryptonians.  Red-spectrum radiation no longer empowered their cells and they soon found themselves powerless.  A deep winter fell on the planet, killing millions.

In response, the Scientist Guild leaders were disposed of and a new militant regime was installed by the Military Guild.  They recalled their forces back to Krypton to spearhead the construction of domed habitations for the surviving populace. Even though a form of democratic rule was established years later through the Lawmakers Guild, xenophobia had set into the public consciousness.  The people of Krypton would only look inward from that point onward.

The end came for Krypton at the hand of Galactus, who had been drawn to the unstable core of the planet.  Determined to make a stand, the forces of the Military Guild set its fleet on a suicide mission to stop the Devourer of Worlds.  It was not enough and the planet was consumed by Galactus' unquenchable hunger.

Only a few individuals survived the destruction of Krypton.  A few ships managed to escape the planet, saving those who would become Superman, Supergirl, General Zod, and the Sword of Rao, the High, and H'el are a few of those which would make their way to Earth.  In addition, their genetic legacy continues within the Daxamites, the Strontians, the Viltrumites, and the Royal House of Khera. 

Years prior, Brainiac had visited Krypton, shrunk the city of Kandor, and stored in it his spaceship's vaults.  After being defeated by the Justice League, Superman took the city and to the Fortress of Solitude and remains the only remaining society of pureblood Kryptonians.