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Two stars orbiting around their center of mass are called a binary star. Binary stars are important in astrophysics for a number of reasons - not least since orbital studies allows the mass of the stars to be determined. Various sub-types of binary star exist such as optical binaries, spectroscopic binaries and eclipsing binaries.
Optical double stars are just “line of sight” arrangements of no particular astronomical significance.
Historically it was very difficult to distinguish between the wheat (binary stars) and the chaff (double stars). For this reason the Washington Double Star Catalog (WDS) is a mixture of three types of object:
Careful measurement over many years reveals that all stars are moving independently through space and this results in slow changes in their position relative to the earth. Proper Motion is a vector, that is it has both a magnitude and a direction. The magnitude has units of arc seconds per year and the direction is expressed in degrees with 0 degrees being north, 90 degrees being east and so on. Most catalogues present variable star information in the form of the magnitude of the motion in both right ascension and in declination since these are at right angles to each other.
For a pair to be considered a binary the two components would be expected to show very similar proper motion. Reliable and up-to-date results are not available for all stars but where such information is used it is clear that many systems previously reported as binaries are in fact "just" double stars. To complicate the matter still more: the fact that stars are moving in three dimensions, rather than in just two, has been ignored by most double star astronomers - both professional and amateur. The radial velocity just seems to get ignored - perhaps because it is not been measured for most stars?
So to summarise - the vast majority of pairs measured by amateurs using CCD's (and the distinction "by CCD's" is paramount) are of unproven status or are unquestionably line of sight groupings of little or no astronomical significance.
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As an example of the problem consider the data for GRV 48. In the WDS it is listed as having a separation of 24.7 arc seconds at 253 degrees. The two components are not particularly close together and the UCAC2 data does not suggest that the components are displaying common proper motion.
The image here is 5 x 5 arc minutes
So on what basis do the results that exist support the claim that this is a common proper motion pair?
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| Correcting WDS | Data Mining for doubles | Designed to fail? | Stellar Twins | Red Dwarf Binary Pairs |
In collaboration with the Remote Astronomical Society Observatory
Martin Nicholson - Daventry, United Kingdom.
This page was last updated on February 23rd 2008.