An ordinary looking faint blue star (left) is actually a pair of white dwarfs that could someday light up the sky as a Type Ia supernova. Analysis of SDSS data shows that such white dwarf pairs merge often enough that they could explain the origins of these giant cosmic explosions.
We can't see the second white dwarf, but we know of its presence, and the future of both stars, from measuring multiple spectra of the star we can see. As the white dwarf orbits its unseen companion, it sometimes approaches us (blue in the drawing at the bottom) and sometimes receding from us (red).
As it moves, lines in its spectrum shift to shorter and longer wavelengths due to the Doppler Effect (blue and red jagged lines, respectively). The amount of shift tells astronomers the star's velocity, which in turn tells them how soon the pair of white dwarfs will go supernova.
Credit: Carles Badenes and the SDSS-III team