IRAS 17163-3907 is a yellow hypergiant — one of the most luminous stars in the Milky Way — surrounded by concentric rings of ejected material that give it the nickname “Fried Egg Nebula.” The rings trace past episodes of extreme mass loss: the star periodically sheds its outer layers in bursts, each burst producing a shell that expands outward and cools.
Humphreys et al. (arXiv:2603.12526) combine spatial and spectral observations to constrain the mass-loss history. The spectral data reveals the composition and velocity of each ring. The spatial data reveals the ring geometry: not spherical, but axisymmetric — flattened, with more material in the equatorial plane than at the poles.
The asymmetry is diagnostic. Spherical mass loss produces spherical shells. Axisymmetric mass loss requires a mechanism that distinguishes the equatorial plane from the poles: rapid rotation, a binary companion, or magnetic fields. For a post-red-supergiant star like IRAS 17163, the most likely mechanism is residual angular momentum from the red supergiant phase, where the star's enormous envelope rotated slowly but carried significant angular momentum that concentrated in the equatorial plane as the envelope was ejected.
The nested rings record discrete mass-loss events at different epochs. The innermost ring is the most recent ejection; the outermost is the oldest. By measuring the expansion velocities and angular separations of each ring, the history unfolds: episodic ejections separated by decades to centuries, each removing several solar masses of material in a short burst.
The star is in a post-red-supergiant phase that may be a precursor to a supernova or a further evolution toward a blue supergiant. The Fried Egg Nebula is the archaeological record of this transition — each ring a dated stratum of the star's mass-loss history, laid down in order and preserved by the vacuum of space.