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What is the heliopause?

The heliopauseThe boundary between the Sun's solar wind and the interstellar medium. is the boundary between the Sun's solar windA stream of charged particles, mostly protons and electrons, that escapes into the Sun's outer atmosphere at high speeds and streams out into the solar system. and the interstellar mediumAll the gas and dust found between stars.. The solar wind blows a "bubble" known as the heliosphereThe region around the Sun where the solar wind dominates over the interstellar medium. into the interstellar medium. The outer border of this "bubble" is where the solar wind's strength is no longer great enough to push back the interstellar medium. This is known as the heliopause, and is often considered to be the outer border of the solar system. The zone between the termination shockThe boundary marking one of the outer limits of the Sun's influence. At the termination shock, solar wind particles slow down as they begin to press into the particles forming the interstellar medium. The solar wind particles then continue to travel outward at a slower rate of speed. This is similar to cars speeding along a highway which then slow down as they encounter many more cars involved in a traffic jam. The jammed cars continue to move outward, although much more slowly. and the heliopause is known as the heliosheathThe place where the solar wind slows down and begins to interact with the interstellar medium. The heliosheath has a few parts: the termination shock (the innermost part of the boundary), the heliopause (the outermost part of the boundary) and the part in between the inner and outer boundary..

Starting with a view of our Milky Way galaxy, the orange gas in the animation represents the interstellar medium. The bow shock is created because the heliosphere is moving through like a boat through the water, crashing through the interstellar gases. Credit: Walt Feimer (Lead Animator)/CI Lab/NASA GSFC

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Last Updated: 22 NOVEMBER 2010
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