Heliopause

Part of speech: noun

Pronunciation: /ˈhiː.li.ə.pɔːz/

Definitions

1. The boundary region where the solar wind from the Sun stops interacting with the interstellar medium defines a specific area
2. It marks the transition zone between the solar influence and the rest of the galaxy
3. This limit indicates where the Sun's particle flow ceases to effectively push against cosmic rays from other stars

Etymology: Heliopause is a modern scientific coinage, built from two parts: "helio-", from Greek "hēlios" meaning "sun", and "-pause", borrowed from the English word "pause" (a stop or interval), which here follows the pattern of "tropopause" and "magnetopause" in naming the boundary where one region ends and another begins. The term was formed on that pattern rather than handed down from classical antiquity, so no ancient writer ever used it. The "-pause" element carries the main idea. In the solar system, the solar wind, a stream of charged particles pouring out from the Sun, pushes outward until it meets the thin gas and dust filling the space between stars, the interstellar medium. Where the two push against each other and the wind stops flowing outward, astronomers draw the boundary, and the word names that zone. The word thus describes a place defined by where a flow comes to a halt, which is what "pause" suggests in everyday English.