But by combining repeated observations from different phases of the satellite's operating life, Swedish astronomers made the succession of images shown above. Two stars shift their positions as they orbit around each other. The precision is astonishing. The stars are 34 light-years away in the Hydra constellation. They lie as close together as do the Sun and the planet Saturn.
Double stars are very useful in enabling astronomers to gauge the masses of stars.
In this case the mass of the upper star is 0.41 times the mass of the Sun, and that of its companion is 0.42. It was never easy to extract information from those small and distant lights in the sky. Now we're glad to show our colleagues some new tricks to make the Hipparcos results even more useful.