Three methods capture binary stars

Binary stars orbit the same centre of gravity, but often only one star is visible. So, astronomers use several methods to identify the couples.

1 Astrometric binary stars:

Some binary star systems are spotted due to irregularities of the motions of the brightest star. One example is Sirius and Sirius B in the Canis Major constellation.

2 Eclipsing binary stars:

In other cases, the orbits of two stars block out each other's light. One example is Algol A in the Perseus constellation, whose brightness is clearly reduced by two thirds every third day.

3 Spectroscopic binary stars:

Two very close stars must often be spotted by means of a spectroscope, that analyses the light from the stars. Castor in the Gemini constellation was detected in this way.