Dense dead stars.
White dwarfs are stars that have consumed all the fuel normally used for fusion. Most are earth sized objects with at least half the mass of the Sun. About 94 percent of all stars become white dwarfs.
At one time scientists believed that as they cooled, white dwarfs would get redder and redder. However Brad M. S. Hansen of Princeton University and Saumon of Vanderbilt University independently predicted that white dwarfs should get bluer as they cool because of the hydrogen molecules that form in the stars atmosphere as the stars cool, would absorb infrared light from the surface and re-emit it as blue light.
Source:
http://www.sciforums.com/showthread.php?s=958769a6b30ca834c37bcda0d8644408&threadid=2758