Free streaming

In astronomy, a free streaming particle is one that propagates through a medium without scattering.

Defining an exact surface for an object such as the Sun is made difficult by the diffuse nature of matter which constitutes the Sun at distances far from the stellar core.

An often used definition for the surface of a star is based on the path that photons take.

[1] The light which constitutes the cosmic microwave background comes from the surface of last scattering.

[2] Similarly, the surface of the cosmic neutrino background, if it could be observed, would mark when neutrinos decoupled and began to stream freely through the rest of the matter in the universe.

Diagram with blue circle representing the cosmic microwave background surface which the observer (at centre, at present time) observes at the time of last scattering . The yellow lines describe how photons were scattered before the epoch of recombination and were free-streaming after.