## Binary Star Astrometry

In the area of astronomy, the current research focus is on observational astrometry, especially short period, gravitationally bound pairs of binary stars. Binary astrometry is the repeated measurement, over time (often many years), of the position angles and separations of the two stellar components of a binary—the fainter secondary with respect to the brighter primary.

If a sufficient number of observations are plotted—a series of position angles and separations—an apparent orbit of the two stars as projected against the sky can be obtained. The apparent orbit, unless it is face on as seen from the Earth, will be tilted in space. However, knowing the dates of the observations allows this tilt to be removed.

We know the distance to any given binary, thanks to accurate parallaxes obtained by the European Space Agency’s Gaia astrometric space telescope. This allows us to scale up the apparent orbit to the real orbit. Kepler’s Third Law, which relates the orbital period (typically in years) to the actual physical separation between the two components and the sum of their masses, can be used to determine the summed masses of the two stars. Spectroscopy (radial velocity curves) or other means can then be used to parse the summed mass of the two stars into their individual masses.

Accurate knowledge of the masses of individual stars is key to understanding stellar evolution, as stellar mass is by far a star’s most sensitive life cycle parameter. The most accurate way to determine stellar masses is obtaining and analyzing binary orbits, which starts with locating short period binaries and precisely measuring their relative position angles and separations over a number of years to obtain an orbit.

Observations are being made of three classes of binaries:

** Known Binaries to add points to improve the quality of their orbits.

** Suspected Binaries to add enough points to determine whether or not they are actually gravitationally bound binaries.

** New-discovery Binaries which, if there is a hint of orbital curvature, will then be promoted to the “suspected binaries” category and hence candidates for further observation.