The interstellar space ( Latin inter stellas , 'between the stars') is the distant space within a galaxy . The astrospheres , the space around the stars , are interspersed in this area . Inside the astrospheres lies the denser interplanetary space filled by the stellar wind , outside the galaxies lies the thinner intergalactic space .
The boundary between interplanetary and interstellar space is defined by the astropause of the respective stars. In the case of the sun, this is the heliopause of the heliosphere , at which the solar wind is stopped by the interstellar medium . This limit area is approximately 120 astronomical units (AU) (18 billion kilometers) from the sun; that is four times the mean distance between the sun and Neptune .
Similarly, the boundary between interstellar and intergalactic space is defined by an outwardly driven galactic gas flow, which forms a boundary layer with the intergalactic matter in the outer area of the galaxies.
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