![]() NASA's Hubble Space Telescope reveals the supercluster Westerlund 1, home of one of the largest known stars. The nebula of gas ejected from the star extends far beyond the orbit of Pluto, to 400 times the distance between the Earth and the sun. If UY Scuti replaced the sun in the center of the solar system, its photosphere would extend just beyond the orbit of Jupiter. ![]() ![]() "As far as an astrophysicist is concerned, this is the surface of the star, as this is the point at which photons can leave the star," Scudder wrote. The photosphere is where the star becomes transparent to light and the particles of light, or photons, can escape the star. ![]() Instead, astronomers rely on a star's photosphere to determine its size. "Most stars don't have a rigid surface where the gas ends and vacuum begins, which would have served as a harsh dividing line and easy marker of the end of the star." "The complication with stars is that they have diffuse edges," astronomer Jillian Scudder of the University of Sussex wrote for The Conversation. However, all stellar sizes are estimates. (Image credit: Philip Park (CC BY-SA 3.0)) The biggest known star is UY Scuti, about 1,700 times larger than the sun.
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