In his book "Autobiography of a Yogi", Paramhansa Yogananda provides details about the astral planes learned from his guru. Yogananda claims that nearly all individuals enter the astral planes after death. There they work out the seeds of past karma through astral incarnations, or (if their karma requires) they return to earthly incarnations for further refinement. Once an individual has attained the meditative state of nirvikalpa samadhi in an earthy or astral incarnation, the soul may progress upward to the "illumined astral planet" of Hiranyaloka. After this transitional stage, the soul may then move upward to the more subtle causal spheres where many more incarnations allow them to further refine before final unification.