Larry Niven, "Bigger Than Worlds" (1974)

  ...assuming that the galaxy's most advanced civilizations are protoplasmic. But 
  beings whose chemistry is based on molten copper, say, would want a hotter 
  environment. They might have evolved faster, in temperatures where chemistry and 
  biochemistry would move far faster. There might be a lot more of them than of 
  us. And their red-hot Dyson spheres would look deceptively like red giant or 
  supergiant stars. One wonders.