Infrared spectrum of an extremely cool white-dwarf star

White dwarfs are the remnant cores of stars that initially had masses of le ss than 8 solar masses. They cool gradually over billions of years, and hav e been suggested(1,2) to make up much of the 'dark matter' in the halo of t he Milky way. But extremely cool white dwarfs have proved difficult to dete ct, owing to both their faintness and their anticipated similarity in colou r to other classes of dwarf stars. Recent improved models(3-5) indicate tha t white dwarfs are much more blue than previously supposed, suggesting that the earlier searches may have been looking for the wrong kinds of objects. Here we report an infrared spectrum of an extremely cool white dwarf that is consistent with the new models. We determine the star's temperature to b e 3,500 +/- 200 K, making it the coolest known white dwarf. The kinematics of this star indicate that it is in the halo of the Milky Way, and the dens ity of such objects implied by the serendipitous discovery of this star is consistent with white dwarfs dominating the dark matter in the halo.