On the Sun, sunspots are darker than the rest of the solar photosphere because they are a few thousand degrees cooler than their surroundings. But the Hubble Space Telescope has recently imaged the surface of the nearby red supergiant star Betelgeuse (above) in the constellation Orion, and found that its 'star spot' is actually hotter and brighter than the rest of the star's surface. There are some ideas why this might happen, but no one really understands how sunspots are formed even on the Sun. In the case of red giants or supergiants such as Betelgeuse, it is expected that because of their cooler temperatures, convection cells are enormous at the surface, so that there may be star spots that are a significant fraction of the full stellar surface. This might explain why Betelgeuse looks so unround.
Copyright 1997 Dr. Sten Odenwald
Return to Ask the Astronomer.