Editing Binary star (section)

====Astrometric binaries====
Astronomers have discovered some stars that seemingly orbit around an empty space. ''Astrometric binaries'' are relatively nearby stars which can be seen to wobble around a point in space, with no visible companion. The same mathematics used for ordinary binaries can be applied to infer the [[mass]] of the missing companion. The companion could be very dim, so that it is currently undetectable or masked by the glare of its primary, or it could be an object that emits little or no [[electromagnetic radiation]], for example a [[neutron star]].<ref>{{cite web |url=http://lantern.ncsa.uiuc.edu/~dbock/Vis/NeutronStar/Summary.html |title=Binary neutron star collision |first=D. |last=Bock |publisher=University of Illinois Urbana-Champaign |department=National Center for Supercomputing Applications |url-status=dead |archive-url=https://web.archive.org/web/20120426043619/http://lantern.ncsa.uiuc.edu/~dbock/Vis/NeutronStar/Summary.html |archive-date=2012-04-26 |df=dmy-all}}</ref>

The visible star's position is carefully measured and detected to vary, 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&nbsp;[[parsec]]s. Nearby stars often have a relatively high [[proper motion]], so astrometric binaries will appear to follow a ''wobbly'' 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 [[Astrometry|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.<ref>{{cite journal |first1=H. |last1=Asada |first2=T. |last2=Akasaka |first3=M. |last3=Kasai |title=Inversion formula for determining parameters of an astrometric binary |date=27 September 2004 |bibcode=2004PASJ...56L..35A |pages=L35–L38 |volume=56 |issue=6 |journal=Publ. Astron. Soc. Jpn. |arxiv=astro-ph/0409613 |doi=10.1093/pasj/56.6.L35|s2cid=15301393 }}</ref> Even though the companion is not visible, the characteristics of the system can be determined from the observations using [[Johannes Kepler|Kepler]]'s [[Kepler's laws of planetary motion|law]]s.<ref>{{cite web |url=http://csep10.phys.utk.edu/astr162/lect/binaries/astrometric.html |title=Astrometric Binaries |publisher=University of Tennessee}}</ref>

This method of detecting binaries is also [[Methods of detecting extrasolar planets#Astrometry|used to locate]] [[extrasolar planet]]s 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. Space telescopes can avoid the blurring effect of [[atmosphere of Earth|Earth's atmosphere]], resulting in more precise resolution.