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Stars which are sufficently close together to be affected by their mutual gravitational attraction are true double or multiple stars.
A pair (or group) of stars may appear to be a double (or multiple) system simply because they lie along the same line of sight from Earth (and therefore give the illusion of being physically close). If they are too far apart to be physically related then they are referred to as an optical double (or multiple).
Approximately one-fifth of the stars observed by Hipparcos belonged to systems with two or more stars. In addition to the five astrometric parameters which Hipparcos provided for each observed star (the two position components, the two proper motion components, and the parallax or distance), several extra parameters were produced for these double and multiple systems (e.g. orbital period, system geometry, etc.). The main Hipparcos Catalogue entry for each component provides a summary of the multiplicity data and points to the more extensive tabulation of the solutions in the Double and Multiple Systems Annex (Volume 10).
The Hipparcos Catalogue include charts - and the detailed properties - of more than 12000 such systems. Choose a system with the right level of complexity for your observational equipment, and see if you can resolve all of the components - or see some of the orbital systems change their configuration with time!
The following figures are extracts from Volume 10: Hipparcos Double and Multiple Systems Annex (Component solutions), which illustrate the rich variety of double system configurations measured by Hipparcos. Each figure is labelled with its CCDM identifier at the bottom left of the figure, followed by a `solution identifier' (in parentheses) for some specific systems. The scale of the chart is expressed at the bottom right of each chart, giving the length of each side in arcsec.
[The CCDM number is constructed from the right ascension and declination of the system (within the reference system J2000) and is used to classify the double and multiple systems within the Hipparcos Catalogue: thus each CCDM system may include two or more Hipparcos Catalogue (HIP) entries.]
The information given here for each system is a summary only. Hp refers to the Hipparcos magnitude. Clicking on the HIP number will return the Hipparcos and Tycho Catalogues entry for that number.
The following two systems form a sextuple (a six member system): In this first chart, four components are apparent. The brightest (A) has an Hipparcos magnitude of 4.9 mag. The other stars (in descending order of brightness) are B (Hp=6.3), C (Hp=7.2) and D (Hp=7.5).
These two stars are also part of the sextuple system. In brightness they cover the same range as the previous four stars (E is almost as bright as A at Hp=5 mag), while N is the faintest of the 6 (Hp=8.2 mag). They are separated by only 0.38 arcsec.