The Core is the scaled-down, raw, form of Trance Gemini. The Core is a miniature sun with numerous tentacles radiating from it.
Trance Gemini transformed into the Core because of the effort entailed in transporting the entire Andromeda Ascendant crew and ship into an alternate universe, which was too much for her to handle. Even though Trance was able to transport the crew through dimensions, she was only able to concentrate on where to transport them, not when. As a result, the crew arrived in the Seefra System at different times, some people like Seamus Harper, landing years before others, like Dylan Hunt. The amount of energy that Trance expended made her turn into the Core. The man who stumbled across Andromeda after it arrived in the Seefra System discovered the Core, wandering through the Andromeda's corridors, and kept her in a shielded chamber, which was usually hidden from sight. She was found by Dylan and released from her chamber, and at his coaxing turned back into Trance, who had amnesia.