I was wondering: From the point of view of an observer who is in motion relative to the sun, the sun has a certain measurable amount of momentum. However, since the sun is constantly turning mass into energy via a nuclear reaction, yet its velocity remains more or less stable, that momentum should be very slowly decreasing. That would violate conservation of momentum. The only way I can see around this is if the energy being produced has momentum of its own. So can energy have momentum? Does anyone know if it's possible to measure the momentum of, say, a beam of light?