# Astrometric binary

An astrometric binary star is a binary star for which only one of the component stars can be visually observed. The visible star's position is carefully measured and detected to have a wobble, due to the gravitational influence from its counterpart. The position of the star is repeatedly measured relative to more distant stars, and then checked for periodic shifts in position. Typically this type of measurement can only be performed on nearby stars, such as those within 10 parsecs. Nearby stars often have a relatively high proper motion, so astrometric binaries will appear to follow a sinusoidal path across the sky.

If the companion is sufficiently massive to cause an observable shift in position of the star, then its presence can be deduced. From precise astrometric measurements of the movement of the visible star over a sufficiently long period of time, information about the mass of the companion and its orbital period can be determined.

Even though the companion is invisible, the characteristics of the system can be determined from the observations using Kepler's laws. The following equation gives the relation between the masses and the amount of motion observed:

[itex](m_1 + m_2) \cdot \left( \frac{m_2}{m_1 + m_2} \right) ^3 = \frac{a_1^3}{p^3 \cdot P^2}[itex]

where:

In 1844, Friedrich Bessel used observations with a meridian circle telescope to infer that Sirius has a hidden companion, due to the motions of the star. The mass of the hidden companion was determined using the formula above. After improved telescopes became available, Alvan G. Clark was able to directly observe the companion star 'Sirius B' in 1862. This companion was later determined to be a white dwarf by Kurt F. Bottlinger in 1923. Similar means were used to infer the presence of a companion orbiting Procyon.

This method of detecting binaries is also used to locate extrasolar planets orbiting a star. However the requirements to perform this measurement are very exacting, due to the great difference in the mass ratio, and the typically long period of the planet's orbit.

Detection of position shifts of a star is a very exacting science, and it is difficult to achieve the necessary precision. Various factors can introduce errors into the measurements that must be compensated by some means. These can include thermal effects on the telescope and equipment, flaws in the photographic plates, slight imperfections in the measuring devices, and human factors. Measurements of nearby star systems from the 1950s to the 1970s appeared to show that several nearby stars had companions. (An example of such is Barnard's star.) However these claims were later invalidated, and were demonstrated to be caused by measurement errors.

More recently orbital platforms, such as Hipparcos, were developed that provided exceptionally stable systems for performing very precise astrometric measurements. Space telescopes can avoid the blurring effect of the Earth's atmosphere, resulting in more precise resolution. In 2010 the ESA has proposed to launch the Gaia probe, the successor to the Hipparcos spacecraft. Gaia will be placed at the Earth L2 Lagrange point, where it will compile data on stars down to magnitude 20.

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