An abrupt boundary between regions of a fluid (gas or liquid) which are travelling at relative rates greater than the local speed of sound, such as that formed by the solar wind when it encounters a planet, comet, or other body. It is similar to the wave that forms at the bow of a ship. An aircraft travelling faster than sound generates a bow shock and a wake whose passage is heard as a sonic boom. Similar situations arise in space when flowing gas meets a fluid or a solid obstacle at supersonic speeds. A fast wind from a young star meeting dense condensations of gas forms bow shocks around them.