wildcat

As usual, I'm wasting the day by drowning myself in Wikipedia and thinking about Superman too much.

Unbihexium, the placeholding name for the unknown element that shares the same atomic number - 126 - as green Kryptonite.

Information about Planet Krypton, the original source of Kryptonite. Specifically, there's a section on the survivors of Krypton:

"When the planet exploded, one entire city of Krypton survived the cataclysm. This city, named Argo City, drifted through space on an asteroid-sized fragment of Krypton, which had been transformed into kryptonite by the explosion. The super-advanced technology of its Kryptonian inhabitants gave the denizens of Argo City the ability to construct a life-sustaining dome that allowed them to survive for several years, in addition to building a lead shield that protected their city from the kryptonite radiation of their asteroid. However, the protective shield was destroyed in a meteor storm, exposing the inhabitants to the deadly radiation. One sole survivor of Argo City, Kara Zor-El, was sent to Earth by her scientist father to live with her cousin Kal-El, who had become known as Superman. Kara adjusted to her new life on Earth and became known as Supergirl."

This version of Supergirl ended up dying to save the universe in 1986, and since then there's been far too many off-shoot derivative versions and I can't even begin to explain what writers did to try and make them all fall in line with the established reality, without giving myself a headache. Best to focus on the story-by-story.