A Matrioshka brain is a hypothetical product of astroengineering, making use of one or more Dyson spheres and nanoscale information processing technology to produce a staggeringly powerful computer. Such a computer, it has been suggested might be capable of simulating alternate universes or as serving as a receptacle into which previously organic minds (such as human consciousness) could be uploaded, The term "Matrioshka brain" was coined by the nanotechnologist Robert Bradbury, who noted the analogy between nested Dyson spheres around a central star and a Russian Matrioshka doll.
In a Matrioshka brain, the Dyson spheres would be composed largely of material engineered at the atomic scale to function as a processing medium ("computronium".) The innermost sphere would absorb energy coming directly from the central star, use it to power its computational matrix and radiate the residue, which would be collected by the next sphere to repeat the process, and so on. The processing material of each sphere would be set up to function optimally at different temperatures, ranging from as much as several thousand degrees kelvin on the inner surface of the smallest sphere to little above the temperature of interstellar space at the outermost sphere.
A Matrioshka brain is an example of a class B stellar engine, making use of almost the entire energy output of its host star. It is similar in principle, but much larger in size and computational capacity, than a so-called Jupiter brain, which is "only" planetary in scale.