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After the hydrogen in a star's core has fused, stars with more than about 10 solar masses become red supergiants for the duration of their helium-fusing phase.23 These stars have very cool surface temperatures (3500–4500 K), and enormous radii. The five largest known red supergiants in the Galaxy are NML Cygni, VY Canis Majoris, VV Cephei A, V354 Cephei, and KW Sagittarii, which all have radii about 1500 times that of the Sun (about 7 astronomical units, or 7 times as far as the Earth is from the Sun). The radius of most red supergiants is between 200 and 800 times that of the Sun. They last 10 to 100 million years and are sometimes found in clusters. Luminosities can exceed 500,000 times that of the Sun. Several well-known red supergiants are: