The heliopause is the boundary of the heliosphere, where the pressure at the outer part of the heliosphere equals that of the surrounding interstellar medium (ISM). It is believed to lie between 100 and 150 astronomical units (AU) from the Sun in the direction in which the Sun is moving, but much further out downstream. Additionally, the heliopause distance probably fluctuates in response to changes in ISM and solar wind conditions. The thickness of the heliopause is unknown but could be tens of AU.
Upstream of the heliopause, in the ISM, theory suggests the existence of a bow shock where the incoming interstellar wind first reacts to its impending collision with the Sun's magnetosphere. Downstream from the Sun is thought to be a long, turbulent tail. The extent to which interstellar matter penetrates inside the heliopause is open to question but may be partially determined by the first interstellar probes.