A BINARY SYSTEM is a pair of stars which are gravitationally bound to each other. Each star moves on an elliptical orbit about the binary system's center of mass.
The orbits of binary systems are much smaller than the distances between stars. Thus, the two stars in a binary system, as seen from Earth, will appear close together in the sky. HOWEVER, two stars which appear close together may be either
How to tell these two possibilities apart?
Watch the star for a long time, and look for orbital motion -- one star will trace out an elliptical path relative to the other.
Here's one visual binary:
Orbital period = 10 years
Maximum separation = 11 arcseconds = 30 AU
Companion was first seen in 1862. However, Sirius was known to be a binary in 1844, eighteen years earlier! A `wobble' was seen in the proper motion, which led the astronomer Bessel to deduce that Sirius A was being tugged on by an unseen companion.
When star A is moving toward us on its orbit, star B is moving away.
When star A is moving away, star B is moving toward us.
These orbital motions can be detected from the Doppler shift of the stars' absorption lines.
(Cautionary note: this method doesn't work if we're looking at the binary's orbit face-on. The Doppler shift only measures the radial velocity. The radial velocity is largest if we look at the orbit edge-on.)
(Actually, these classifications, which I've plucked from Burnham's Celestial Handbook, are quite uncertain). Capella A is cooler, but larger and more luminous, than Capella B.
Orbital period = 104 days (Shorter than Sirius)
From orbital period and orbital velocities, we deduce an orbital separation of 0.75 AU.
If we see the binary's orbit very nearly edge-on, the stars will sometimes cross in front of each other. When this happens, we say the nearer star eclipses the farther star.
There will be two eclipses per orbit; thus two dips in the intensity. The SMALLER dip occurs when the COOLER star is hidden; the LARGER dip occurs when the HOTTER star is hidden. The geometry of the eclipse, and the resulting light curve, is shown below:
(In the above illustration, the small blue star is hotter than the large yellow star.)
Orbital period = 69 hours (Shorter than Capella)
When the hotter star (Algol A) is hidden, the intensity drops greatly (by about 68%).
