Astronomy Web Site
EVAC Double Star Observation Program
Last Updated:   15-Dec-12
One of the EVAC (East Valley Astronomy Club) observation programs, is the "Double Star" program, which I completed on Dec 10, 2008. This project entails observing 100 sets of double stars, located in all of the 24 hour Right Ascension  values, and thus, requires considerable time commitment to complete. You can see my EVAC Double Star Observing Log Here, with links to the images. Click HERE for an interesting discussion of “What Magnification Is Good For Observing Double Stars”. But, what is a Double Star? The best definition I could find, comes from the web and Wikipedia, the free encyclopedia. Here then is their definition: From Wikipedia, the free ONLINE encyclopedia comes this Defination and History: Double star   In observational astronomy, a double star is a pair of stars which appear to lie close together in the sky. This can happen either because the pair forms a binary system of stars in mutual orbit, gravitationally bound to each other, or because it is an optical double, a chance alignment of two stars in the sky that lie at different distances. Since the beginning of the 1780s, both professional and amateur double star observers have telescopically measured the distances and angles between double stars to determine the relative motions of the pairs.[3] If the relative motion of a pair determines a curved arc of an orbit, or if the relative motion is small compared to the common proper motion of both stars, it may be concluded that the pair is in mutual orbit as a binary star. Otherwise, the pair is optical.[2] Multiple stars are also studied in this way, although the dynamics of multiple stellar systems are more complex than those of binary stars. History Mizar, in the Big Dipper (Ursa Major), was observed to be double by Giovanni Battista Riccioli in 1650(and probably earlier by Benedetto Castelli and Galileo). The bright southern star Acrux, in the Southern Cross, was discovered to be double by Father Fontenay in 1685. Since that time, the search has been carried out thoroughly and the entire sky has been examined for double stars down to a limiting apparent magnitude of about 9.0.At least 1 in 18 stars brighter than 9.0 magnitude in the northern half of the sky are known to be double stars visible with a 36-inch (910 mm) telescope. Observation Visual double stars can be defined as those double stars which are visible in an optical telescope. This is the majority of known double stars. If visual doubles show similar properties, such as similar proper motion through space, trigonometric parallaxes, or radial velocities, this is evidence that they are gravitationally attached and form a binary system; in this case, the visual double star is called a visual binary. Telescopic observation of a visual double star will yield the separation, or angular distance, between the two component stars in the sky and the position angle. The position angle specifies the direction in which the stars are separated and is defined as the bearing from the brighter component to the fainter, where north is 0°. These measurements are called measures. In the measures of a visual binary, the position angle will change progressively and the separation between the two stars will oscillate between maximum and minimum values. Plotting the measures in the plane will produce an ellipse. This is the apparent orbit, the projection of the orbit of the two stars onto the plane of the sky; the true orbit can be computed from it. Although it is expected that the majority of catalogued visual doubles are visual binaries, orbits have been computed for only a few thousand of the over 100,000 known visual double stars. Visual double stars can also be optical doubles, two stars that appear to be close together by chance. These pairs are actually separated by a great distance in space and are not gravitationally bound to each other, but happen to lie in the same line-of-sight when viewed from the Earth. Optical doubles may be distinguished from binary stars by observing their relative motion. If the motion is part of an orbit, or if the stars have similar radial velocities and the difference in their proper motions is small compared to their common proper motion, the pair is probably physical. When observed over a short period of time, the components of both optical doubles and long-period visual binaries will appear to be moving in straight lines; for this reason, it can be difficult to distinguish between these two possibilities. Return to Astronomy Home Return to EVAC Programs