Bow-shock in Orionis - HST

Named for the crescent-shaped wave a ship makes as it moves through water, a "bow-shock" can form in space when two gas streams collide. In this case, the young star, LL Orionis, emits a vigorous wind, a stream of charged particles moving rapidly outward from the star. Our own Sun has a less energetic version of this wind that is responsible for auroral displays on the Earth. The material spewed from LL Ori collides with slow-moving gas evaporating away from the center of the Orion nebula, located to the lower right of the image. The surface where the two winds collide is seen as the crescent-shaped bow shock. Unlike a water wave from a ship, this interstellar bow shock is three-dimensional. The filamentary emission has a distinct boundary on the side facing away from LL Ori, but is diffuse on the side closest to the star, a trait common to many bow shocks.