Stellar habitable zones. Diagram comparing the habitable zones (blue) around the Sun (left) and Barnard's Star (right). A star's habitable zone is a narrow region where liquid water could exist to support life. It depends on the temperature drop with distance from the star, but it also changes with planetary properties such as the atmospheric composition & pressure, and albedo (reflectance). For Earth, the Sun's habitable zone extends about 0.8-1.6 AU (astronomical unit, the mean Earth-Sun distance). Barnard's Star, a red dwarf star just six light years from the Sun, is cooler than the Sun. Its smaller habitable zone is closer to the star and estimated to range from 0.02-0.06 AU.
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