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Appendix: Spectroscopy of Variable Stars Appendix: Spectroscopy of Variable Stars As amateur astronomers gain ever-increasing access to professional tools, the science of spectroscopy of variable stars is now within reach of the experienced variable star observer. In this section we shall examine the basic tools used to perform spectroscopy and how to use the data collected in ways that augment our understanding of variable stars. Naturally, this section cannot cover every aspect of this vast subject, and we will concentrate just on the basics of this field so that the observer can come to grips with it. It will be noticed by experienced observers that variable stars often alter their spectral characteristics as they vary in light output. Cepheid variable stars can change from G types to F types during their periods of oscillation, and young variables can change from A to B types or vice versa. Spec­ troscopy enables observers to monitor these changes if their instrumentation is sensitive enough. However, this is not an easy field of study. It requires patience and dedication and access to resources that most amateurs do not possess. Nevertheless, it is an emerging field, and should the reader wish to get involved with this type of observation know that there are some excellent guides to variable star spectroscopy via the BAA and the AAVSO. Some of the workshops run by Robin Leadbeater of the BAA Variable Star section and others such as Christian Buil are a very good introduction to the field. © Springer Nature Switzerland AG 2018 M. Griffiths, Observer’s Guide to Variable Stars, The Patrick Moore 291 Practical Astronomy Series, https://doi.org/10.1007/978-3-030-00904-5 292 Appendix: Spectroscopy of Variable Stars Spectra, Spectroscopes and Image Acquisition What are spectra, and how are they observed? The spectra we see from stars is the result of the complete output in visible light of the star (in simple terms). This output produces a variety of colors, from blue at the short wavelength end of the spectrum to red at the longer wavelengths of the visible spectrum. To obtain the spectra of stars we need either a prism that will bend or refract the light into its composite colors or a diffraction grating, which is an instrument with hundreds of fine lines or grooves that split the light into its composite wavelengths. Most spectrographs are equipped with a diffraction grating. Some can be obtained cheaply, or commonly available filters can be obtained and then readjusted to perform the same function as a spectroscope. Alongside the usual equipment of CCD camera, telescopes, driven mounts and the software to control them all will be the consideration of what spectroscope to use. There are no spectroscopes in general use that will suffice for this exercise, so some observers use a Cokin filter (number 40), which has approximately 250 grooves per millimeter, that can act as a diffraction grating in front of your CCD camera. It is useful to remember that such equipment may not pick up the fainter stars, and so spectroscopy of brighter stars should be attempted first as part of the observers’ learning curve. Observers who perform spectroscopy have a variety of tools at their disposal. Many use nothing more than a relatively inexpensive spectrographic tool known as a Star Analyzer, which has a resolution of about 500 nm and can be fitted to the CCD in the same way as the Cokin filter above. However, most spectroscopes are expensive pieces of equipment, and some for laboratory spectroscopy can be modified for astronomical use. Such items as the Elliot Institute CCD Spec, the Shelyak LHIRES and the JTW L200 are quite pricey but have higher resolutions down to 5 nm. Some of these are shown in Fig. A.1. Depending on your preference, any spectroscope will smear the images from a light source, and the resolution of the image will depend on the FWHM of the images, the brightness of your subject variable star and the optical train of your telescope and camera. Observers in this field often add a small slit to the front of the grating or CCD so that the image is not overlaid with alternate stars, and the resolution is a little more constant per field. As a general rule of thumb, the higher the dispersion of the spectrum (the spread of light) the brighter the target variable star must be to get good information and data. Appendix: Spectroscopy of Variable Stars 293 Fig. A.1 LHIRES spectroscope. (Image from http://www.astronomy.com/observing/ product-reviews/2009/12/lhires-lite-spectroscope.) Software used by many observers include the ISIS image subtraction package for photometry and comparison with known spectra via the MILES software library, which can be accessed at http://www.iac.es/proyecto/ miles/pages/stellar-libraries/miles-library.php. There are some excellent guides to exploring and using this software available at http://www. astrosurf.com/buil/isis/guide_lhires/tuto1_en.htm. All files are in FITS format, so that immediate comparison can be made with your own spectrograph CCD files. ISIS can not only align the multiple CCD exposures you may have taken of your variable star but can also remove the sky background and reduce the data to a curve that enables the observer to see the most prevalent lines in the spectrum of the target star, as one can see in Fig. A.2 here. Using Wein’s law can then enable you to discover what type of star you are examining (although of course you will know this already!) and to confirm the spectral type for your own records. Although doing spectroscopy at this level seems like re-inventing the wheel, it is an important adjunct to amateur research, and we must remember that many variable stars are not followed by professional astronomers. Any information gleaned from such variable stars via spectroscopy can only add to the growing amount of knowledge that we have of the cosmos. Although the spectroscopy being done here is relatively low resolution, one never knows when such data may be useful in the future. Additionally, supernova research can gain a lot from the approach of amateurs using spectroscopy, 294 Appendix: Spectroscopy of Variable Stars Fig. A.2 ISIS spectrum. (Image from http://www.astrosurf.com/buil/isis/eshel/ reduction/echelle.htm.) and many of these observers are combing the skies looking for such events. In fact, it is mostly amateurs that spot supernovae, as they have the time, abilities and resources necessary to perform this function. Spectra of a supernova occurring is important in ascertaining the type of object it is (Type I or II); recognition of this will be a feather in the cap of any supernova patroller. Some basics in our approach to spectroscopy of variable stars follow the observational techniques provided earlier in the book. It is important in amateur spectroscopy to take the image when the object under study is at culmination, thus reducing air mass and ensuring maximum image quality. If using a CCD, make sure that the chip does not become saturated. As in all CCD imaging, flats and darks can be taken and subtracted. Binning is also useful to ensure that you have a reasonable pixel count. Additionally, one is still dealing with an image that requires as much input as possible in much the same way as photometry, so the FWHM of the source should be accounted for. Once you have the field of view of your subject star, your image may look a little like Fig. A.3, with stars smeared into rainbow-like trails. This is not a mistake. Hidden within the little rainbows (or if shooting in monochrome the extended trail) are all the bits of information you are looking for in the form of absorption bands characterizing the chemical makeup of the star. As this is low resolution, do not expect to get all the Appendix: Spectroscopy of Variable Stars 295 Fig. A.3 Spectra of field of view. (Image from https://www.rspec-astro.com.) absorption band that could be present, but bands such as H alpha, sodium and oxygen may well be present. Such details may also be spoiled a little by interstellar dust, making the image redder than it should appear. Remember, too, that hydrogen lines are visible only in the middle spectral ranges, so O- and B-type stars will not have these lines, but you may be able to pick up the lines of helium instead if you have that kind of sensitivity. Different types of variable stars also have special characteristics in their spectra, while dwarf nova and classical nova have very different traits that make them instantly recognizable if one knows something of the astrophysics of the subject. Variable star spectra should be apparent down to a visual magnitude of 13 or 14 if one is using a CCD and a telescope in the 20-cm range. This enables the observer to gain a large amount of data from several different types of variables, which will be good practice, but you may want to concentrate just on one type of variable star. If you have very good spectra reduced via ISIS or other software then you may wish to follow the next exercise in determining the distance to the source. Determining Red Shift or Blue Shift Considering that many variable stars show radial pulsations or have components that move toward or away from the observer, it may be possible with good resolution to make some calculations of the speed of movement of the stellar photosphere or orbiting object. It must be said however that in general, most spectrographs gained from amateur telescopes may not have 296 Appendix: Spectroscopy of Variable Stars sufficient resolution to do this exercise, as the changes in blue or red shift toward or away from an observer are going to be fractions of a nanometer at most.
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