This animation shows three different techniques that Gaia uses to detect binary stars.

Astrometry: binary stars are detected by a motion on the sky which is not uniform; this can be an elliptic motion or just a part of it for orbits with very long periods. The two sources can't be seen individually as they are too distant; either the two companions have a very different magnitude (an extreme example is a star and a planet) and only the motion of the bright one can be detected; or the sources have a similar magnitude and only the motion of the photocentre is seen. Astrometric binaries generally have long periods (months to years or decades) as the motion on the sky is too small to detect short-period binaries.

Photometry: eclipsing binaries are detected thanks to the periodic dimming of a star due to a (partial) eclipse by a companion. As the probability that the line of sight is precisely along the plane of the orbit is very small if the two sources are widely separated, the observed eclipsing binaries typically have small periods, in the order of hours to days.

Spectroscopy: these binaries have a radial velocity that varies periodically, depending on whether a star approaches or recedes from us. They are detected thanks to this variation. If the sources have a similar magnitude, the spectral lines of the two objects can be seen, though frequently only the lines from the brightest are seen. As the amplitude of the radial velocity variation increases when the period is shorter, short-period binaries are more frequent, typically from hours to months.

Thanks to these techniques, Gaia is able to detect thousands of binary stars. In the best cases, it is possible to estimate the mass of the companions and sometimes their individual magnitudes too. From this, astronomers may discover among normal stars some hidden treasures such as an exoplanet, or, on the heavy side, a white dwarf or compact companions such as neutron stars or even black holes.

Acknowledgements: ESA/Gaia/DPAC/CU4/NSS, G. Sadowski, A. Jorissen, C. Siopis

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