MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Illustration of the structure of the solar heliosphere. The heliosphere is a bubble-shaped region of outflowing gas surrounding the Sun, extending for at least 130 astronomical units (AU). Gases and particles flowing away from the Sun, the solar wind, expand outwards until they slow down to subsonic speeds at the termination shock, at 75 to 90 AU (teal). Beyond this, at 80 to 100 AU, is the heliopause (blue), where the Sun's wind is stopped by the interstellar medium (ISM), the gas between stars. Between the termination shock and the heliopause is the heliosheath. Where the heliosheath interfaces with the ISM, gases in that medium are brought to a stop at the bow shock (purple). The Voyager probes are believed to have approached and passed through several of these regions. Voyager 1, the most distant, passed through the heliopause and into interstellar space, at 121 AU, in 2012 (orange).
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