Visual Binary Stars
About half the stars in the sky have stellar companions, bound
together by gravity and in orbit around each other. Some of these can be seen
by the
eye or in a telescope, others are too close to be resolved.
You can see stars
together, but they must also share a common motion and be at the same distance away (but not too
far away).
If the orbital period is reasonable, orbital motion can be measured.
This allow us to determine its tilt.
