The heliopause is a boundary in space where the outward pressure of our own sun's solar wind, made up of gases and particles "blown" outwards by the forces generated by the solar furnace, reaches an equilibrium with the inward pressure of interstellar particles pushing against the heliosphere. The heliosphere is the region around the Sun which is influenced by the Sun's magnetic field and in which the solar wind exerts a discernible outward pressure from the Sun. An analogy would be a balloon, with the Sun inside. The solar wind is the air inflating the balloon, the outside air is the pressure of the interstellar medium (ISM), and the balloon itself is the heliopause.
The existence of this boundary is accepted as fact by modern science. What is still undetermined is its exact position and how it affects interaction between the heliosphere and the interstellar medium, which is the interstellar space between solar systems. Much has been inferred by astronomers and physicists as to the exact position and profile of the heliosphere, and by extension, the heliopause, but exact data is thus far not available. It is hoped that as man made probes such as Voyager 1 and 2 and Pioneer 10 continue outward on their way from the solar system to interstellar space that more precise information will be available.
It is also generally accepted by science that the heliosphere is not a perfect sphere. This is because the Sun moves through interstellar space, and as with a flowing river, the "upstream" side of the heliosphere, in the direction of the Sun's travel, is compressed, while the "downstream" side is elongated. Astronomers estimate the heliopause to be located approximately 100 to 150 astronomical units (AU) from the sun in the "upstream" direction. An astronomical unit is the average distance from the Earth to the Sun, or about 93 million miles (149 million kilometers). The distance to the "downstream" boundary is unknown, but is thought to be somewhat further away, perhaps in the range of 170 AU.
As a boundary, the heliopause is not absolute. Particles from outside the heliosphere do penetrate inwards, and it is believed that some particles of solar wind pass beyond the boundary. Near the heliopause, on the "upstream" side, two areas of compression and turbulence are believed to exist, where magnetic fields form "bubbles". The solar wind, as it approaches the heliopause, reaches a region known as the termination shock, where it slows down significantly. Outside the heliopause, as the incoming flow of interstellar medium approaches the boundary, it also slows down considerably, a region known as the bow shock.