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Double stars imaged with the USNO's 12-inch refractor

The 12-inch refractor was originally tasked with measuring the astrometric properties of bright double stars.  A "moving wire" micrometer was installed to determine the separation of the two component stars (measured in seconds of arc), and the position angle of the fainter component with respect to the primary star, measured in degrees with 0° (celestial north) and increasing toward east.  These observations are now done with speckle interferometry employed on the 26-inch refractor.

We can still use the 12-inch for double star observations, though.  The visual appearance of many of these systems can be quite striking and are often observed when demonstrating the telescope's capabilities.  Here are some images of interesting double and multiple star systems made with the 12-inch telescope.

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Epsilon Lyrae is a very famous star system, known popularly as the "Double Double".  It consists of a wide pair of stars separated by 208 arc-seconds that is easily seen in binoculars.  Each of these components is itself a close double star.  Epsilon-2, at left, has a separation of 2.4 arc-seconds and an orbital period of around 724 years.  Epsilon-1 is slightly closer in separation at 2.3 arc-seconds and has a period of about 1800 years.  The entire system shares a common proper motion, so it is likely a true quadruple star system.  The two close pairs orbit each other with a period well in excess of 10,000 years.  The system is located about 160 light years from the solar system.

 

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Omicron-2 Eridani is a very interesting triple star system.  Located just over 16 light-years from the solar system, the bright component, A, is a "main sequence" star similar to bus slightly less luminous than the Sun.  In science fiction it is the parent star of the planet Vulcan, home of the "Star Trek" character Spock.  It is orbited by an unusual pair consisting of B, a "white dwarf", the super-dense remnant of the collapsed core of a dying star, and C, a "red dwarf', a sub-luminous star that barely has enough mass to sustain nuclear fusion.  Each of these stars are the most easily-visible stars of their class for owners of small amateur telescopes.  The separation of the A component from the B-C pair is 82 arc-seconds and the period is somewhere around 8,000 years.  Recent measurements of the B-C pair by USNO astronomers have revised the masses and orbital period of the close pair.  

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