Source: Assault on the Crucible, pg(s). 59
A rheonnaghan appears as a pulsing geometric shape of pure energy. No two look identical; the meaning of the difference in their shapes is unknown. Many theories have been posited, from age to a representation of the rheonnaghan's home star. When asked about it, rheonnaghans fail to understand and become offended that their existence is just simple mathematics. A typical rheonnaghan is 20 feet across and has almost no weight.
Sometimes, a crystalline mass might precipitate from a star's plasma and coalesce around a portal that connects that star to the Positive Energy Plane. In a few cases, the energy from the Positive Energy Plane then infuses this matter with life and ejects it out of the star, giving birth to a rheonnaghan.
Rheonnaghans are unshakably fascinated with stars. Though not immune to stellar heat and radiation, they tend to hover near stars and study their phenomena. Many naturally develop solarian-like powers, and some conduct experiments into their own nature with the cocoons from which they were born.