When David Bowman went through the Star Gate after discovering Jupiter's monolith (Saturn's monolith in the book), he found himself in a white room in which he saw various life stages of himself. After that, he transformed into a fetus, and became a Star Child, an immortal being that resides within space.
The New Seed
The 2001: A Space Odyssey monthly comics by Marvel Comics featured human astronauts transformed into Star Children by the Monolith, though then they are called New Seeds.