An S-brain, popularly known as known as a Godstar, is an archailect based upon a computational substrate comparable in mass and size to the most massive stars, at minimum 10 solar masses. The computronium basis usually corresponds to a colloidal structure of plasma, monopolium/magmatter, and/or diamondoid/feroid, but unlike a J-Brain, the majority of computronium is plasma-based. It is not know to what toposophic level an S-Brain can reach, but simple information theory would suggest S5+, possibly with spikes to S6 or beyond. S-Brains can exceed the computational resources of an N-Brain, but as superheavy stars in the 100+ solar mass level (such as the nearby Eta Carinae) do not last for long on the galactic timescale, the archai based upon them either willingly expire or presumably have some means to backup their consciousness.
Many S-Brains are surrounded by Matrioshka layers of computation (which may also be confusingly called M-Brains). Such systems are sometimes called SM-Brains to distinguish them from pure S-Brains (which look and mostly act like large bright stars), and to distinguish M-Brains from Matrioshka-Brains, though technically a Matrioshka-Brain can be deployed around any star.
A cluster of young stars converted to S-Brains is known as a Cluster Brain; famous examples include the Prediction Cluster and the Enigma Cluster.
Neuron Star - Text by Damien Broderick, in Anders Sandberg's Transhuman Terminology A neutron star used as a basis for a densely packed mind (similar to a jupiter brain or omegon) exploiting neutronium or quark matter for computation.