Heliopause

The heliopause is the boundary where the Sun's influence ends and the interstellar medium begins. It marks the point where the solar wind—a stream of charged particles from the Sun—slows down and balances with the pressure of the surrounding galaxy. This boundary separates the solar system's protective bubble, called the heliosphere, from the rest of space. Beyond the heliopause lies interstellar space, filled with gas, dust, and cosmic rays from other stars. It’s a critical marker for understanding our Sun’s reach and how our solar system interacts with the galaxy.