The heliosphere is an area surrounding the solar system extending well beyond the planet Pluto. It could be called the boundary of the solar system where the sun’s gravity has a little effect. This is the region where sun’s influence becomes negligible and interstellar space (space between stars) begins.
The heliosphere region is actually filled with particles given off by the sun and their magnetic field altogether called as solar winds. These solar winds spread out from the sun at around 400Km/s. The winds blow through the spaces between stars which are not ‘empty’ but contain traces of gas and dust. Blowing against this dust and gas, the solar winds creates a bubble like region called the heliosphere. This bubble is more like a cloud of fog.
Inside the Sun, millions of nuclear fusion reactions happen every now and then. As these reactions occur, the sun constantly sends out particles that fly into the space at very high speeds. Inside the heliosphere, as the solar winds travel from the sun, they hit the planets. Planets with their strong magnetic field divert the movement of the solar winds. The winds eventually slow down (due to the pressure from the interstellar medium) at a region of the heliosphere called heliosheath. The heliosheath has a few parts – the termination shock (the innermost part of the boundary), heliopause (the outermost boundary) and part in between them.
Astronomers have found that as the Solar System moves through the galaxy, the magnetic fields around the Sun are stretched. The final shape of heliosphere will look something like a teardrop with the round head in front and the tail in back. In some way, the heliosphere tends to act like a shield that protects the solar system from the dangerous cosmic rays which may come from any distant galaxies or exploding stars.