  The brightness of type Ia supernovae, and their homogeneity as a class, makes
them powerful tools in cosmology, yet little is known about the progenitor
systems of these explosions. They are thought to arise when a white dwarf
accretes matter from a companion star, is compressed and undergoes a
thermonuclear explosion Unless the companion star is another white dwarf (in
which case it should be destroyed by the mass-transfer process itself), it
should survive and show distinguishing properties. Tycho's supernova is one of
the only two type Ia supernovae observed in our Galaxy, and so provides an
opportunity to address observationally the identification of the surviving
companion. Here we report a survey of the central region of its remnant, around
the position of the explosion, which excludes red giants as the mass donor of
the exploding white dwarf. We found a type G0--G2 star, similar to our Sun in
surface temperature and luminosity (but lower surface gravity), moving at more
than three times the mean velocity of the stars at that distance, which appears
to be the surviving companion of the supernova.
