Double stars are the rule, rather than the exception, in the solar neighbourhood and probably beyond. Current theories of star formation point to multiple stars or stars and planets as the preferential outcome of gravitating protostellar material. Stellar pairs can be detected at many wavelengths from X-rays, where modern satellites can resolve the two brightest components of Castor (separation 3.8''), to the radio where the precision of long baseline interferometry can also see the 4 milliarsecond (mas) "wobble" in the 2.87-day eclipsing system of Algol and can distinguish which of the two stars is emitting the radio waves. They come in a wide range of orbital sizes, periods and masses. From Groombridge 34, where the stars are separated by five times the distance of Pluto from the Sun and whose motion is barely perceptible, through the spectroscopic binaries with periods of weeks, down to exotic pairs like double white dwarf contact systems with periods of 5 minutes. From young O-star binaries like R136-38 in the Large Magellanic Cloud containing two extremely bright and hot stars, of 57 and 23 solar masses, down to the snappily named 2MASS J1426316+155701, a pair of brown dwarfs with masses only 0.074 and 0.066 times that of the Sun.
In this volume we are concentrating on only one aspect, the visual double stars, which we can define as those pairs which can be seen or imaged in a telescope of moderate aperture. The classic image of the doublestar observer as a professional scientist with a large refractor and a brass filar micrometer is no longer valid. Researchers cannot afford to spend a lifetime measuring a large number of pairs in order to get a few dozen orbits. The high precision astrometric satellites, ground-based interferometer arrays, and infrared speckle interferometry have all helped, respectively, to discover large numbers of new pairs, push direct detection into the spectroscopic regime with measurement of binaries with periods of a few days, and to probe the
near and mid-infrared where faint red and brown dwarf companions and, ultimately, planets appear. This has left a large number of wide, faint pairs which are under-observed.
There has been a common perception that doublestar observing is either not very interesting or does not afford any opportunities for useful work. The aim of this book is to dispel these views and indicate where observers might usefully direct their efforts. At the basic level, we give advice about how to observe them with binoculars and small telescopes. At a more serious level, chapters about micrometers, CCD cameras and other techniques have been included. For those who do not wish to spend several hundred pounds on a filar micrometer, the graticule eyepieces such as the Celestron Micro Guide available for catadioptric telescopes can be used effectively for relative position measurement of wider pairs, and for those who find observing too taxing, astrometry of faint pairs can be done by examination of some of the huge catalogues produced from the various Schmidt surveys.
Clearly, for the observer, the role of the telescope is very important. For casual viewing any optical aid can give reasonable views of wide and bright double stars. I spent several years accumulating visual estimates of colour, magnitudes, and relative positions of more than 1000 pairs using a 21-cm reflector using Webb's Celestial Objects for Common Telescopes (Volume II) and Norton's Star Atlas (15th edition, 1964). Even in Norton's many of the measures given were more than 30 years old and it was this that sparked an interest in obtaining a micrometer to bring them up-to-date.
On the whole, equatorially mounted telescopes are almost a necessity and although Dobsonian telescopes can give fine views of double stars, using them for measurement is not straightforward. Potential users should look at Chapter 22 where Michael Greaney shows how to calculate position angle in situations where the field rotates. Whilst the grating micrometer (described by Andreas Maurer in Chapter 14) is relatively insensitive to the lack of an RA drive the field rotation is an added problem.
Resolution is ultimately dependent on aperture and although many of the most interesting binaries are significantly closer than 1'' the aperture available to today's observers is no longer limited to the small sizes that were common about 30 or 40 years ago when the 12.5-inch reflector was the exception rather than the rule. These days no one is surprised to see amateur observers sporting 20, 30 or even 40-inch telescopes and for those who thought that refractors were the required telescope for double-star observing Christopher Taylor has other ideas.
In the last 10 years the CCD camera has become a dominant force in observational astronomy. As both a positional and photometric detector it has excellent applications in the observation of double stars and these will be discussed later by Doug West.
Filar micrometers are available commercially, costing about half the price of a CCD camera and usable up to the resolving limit of the telescope. The human eye is still the best all-round detector available for work on close pairs of images whether they are equally or unequally bright.
Those with the larger apertures, however, should consider the speckle interferometer as an alternative to the micrometer. With atmospheric effects becoming more significant with telescope size, the speckle camera can punch through the turbulence and produce diffraction-limit imaging. Nils Turner describes how this can be achieved at relatively low cost.
The availability of inexpensive and yet powerful personal computers has brought several other aspects of double-star astronomy within reach. The latest static version of the United States Naval Observatory (USNO) double star catalogue, WDS 2001.0, is now available on CD-ROM (the regularly updated WDS catalogue is available on-line only and incremental files can be downloaded to update the static version of the catalogue). It is no longer necessary to measure the bright pairs which appear in the popular observing guides. With the WDS the more neglected pairs can be selected for measurement and charting software makes finding even the most obscure pair much easier. The USNO have placed on their website several lists of neglected double stars which they would like observers either to confirm as double or to make new measures. Many of the catalogues available on the WDS CD-ROM can also be found on the CD-ROM available with this volume.
Orbital computation, once the province of specialists, can now be done by anyone but it is not to be taken lightly. Even if all the measures of a particular system can be rounded up it still requires an appreciation of the quality of the observations and the existence of systematic errors. How do you combine measures by Struve in 1828 with those by van
Biesbroeck in 1935 and speckle measures made in 1990? Perhaps most importantly is a new orbit necessary and is yours better than any others? Andreas Alzner has contributed two chapters on this important topic.
Finally, what about the double stars themselves? As we have seen, current research is pushing resolution to unprecedented limits but in the meantime who is paying any attention to the 90,000 plus pairs in the Washington Double Star (WDS) catalogue, the central repository for the subject? In particular, who is watching the southern binaries, many of which are being overlooked? I recently found four systems in the WDS catalogue which did not have orbits, one of which, 5 Velorum, is 2nd magnitude. Its 5th magnitude companion was not observed for 50 years and has recently passed through periastron. Thanks to Andreas Alzner, orbits for these pairs have now been computed, but confirming observations are also needed.
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