Spectroscopic Binaries

A spectroscopic binary is a star whose nature is revealed through the periodic shifting of the lines in its spectrum resulting from the well-known Doppler effect. This causes the lines to be displaced toward the blue end of the spectrum as the primary star approaches us in its orbit, and then to the red end as it recedes. If both stars are of nearly the same luminosity, a double set of lines will appear, shifting back and forth in opposite directions as the two suns do their orbital dance around their common center of gravity.

The first spectroscopic binary to be discovered (in 1889) was the primary of the well-known visual triple star system Mizar at the "bend" of the Big Dipper's

Figure 2.7. Radial velocity plot of a typical doubleline spectroscopic binary -in this case, one having an orbital period of just four days. Plus values indicate motion in the line of sight away from us while minus ones reveal motion towards us, as each star revolves around the common center of gravity of the system.

Figure 2.7. Radial velocity plot of a typical doubleline spectroscopic binary -in this case, one having an orbital period of just four days. Plus values indicate motion in the line of sight away from us while minus ones reveal motion towards us, as each star revolves around the common center of gravity of the system.

handle. As previously mentioned, all three of its suns are spectroscopic binaries, making it an amazing sextuple system! Many of the brighter stars also have spec-troscopic companions, including a majority of the double and multiple stars listed in Chapter 7 and Appendix 3. Among them is Castor in Gemini, which is a visual triple and six-star system like Mizar.

Spectroscopic binaries cover a range of component separation from that of interferometric pairs all the way down to contact binaries (see below). Some of the wider, brighter ones bridge the gap between the two methods of detection. One of the brightest spectroscopic binaries in the sky is the radiant golden sun Capella (a Aurigae). It is composed of dual G-type suns in a 104-day orbit, discovered spec-troscopically over a century ago. The pair was later resolved using an interferometer on the 100-inch reflector at the Mount Wilson Observatory in California, and more recently directly imaged using the technique of speckle interferometry on a s number of smaller instruments. pe

The more massive, nearby spectroscopic binaries also reveal their presence y through perturbations in their proper motions and can be detected astrometrical-ly as well as spectroscopically. If the plane of their orbit is aligned nearly edge-on to our line of sight, the two stars will periodically eclipse each other and this can be detected photometrically (see below). Thus, the three basic techniques of observational astronomy - astrometry, spectroscopy and photometry - all come together in the study of these dynamic systems. Of the three, the author has conducted both astrometric and spectroscopic research on double stars. There's no question that the latter is the most fascinating of the two approaches to the observer. Knowing that contained in that beam of light falling on the spectroscope's slit, as seen through the telescope's guiding eyepiece, are two suns rapidly orbiting each other far out in the depths of space is quite wondrous! And the added bonus of knowing that the spectrograms taken that night will within a matter of weeks or months yield their orbital elements, spurs the observer on through the longest and coldest of nights at the telescope!

It should be mentioned before moving on that there is a relatively small class of spectroscopic doubles known as spectrum binaries (or sometimes as "symbiotic binaries"). In these objects, the presence of an unresolved pair is revealed by a unique spectrum consisting of the lines from two stars of different temperatures - as is often the case for spectroscopic binaries in general - but strangely showing no Doppler shifts. These are typically indicated in catalogues with a dual spectral type, such as A3+G7, as for normal spectroscopic pairs where the lines from both stars are seen.

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