Version 1.0 PAGE 5 The spectral type of a star allows the astronomer to know not only the temperature of the star, but also its luminosity (expressed often as the absolute magnitude of the star) and its color.  These properties, in turn, can help in determining the distance, mass, and many other physical quantities associated with the star, its surround- ing environment, and its past history.  Thus a knowledge of spectral classification is fundamental to understand- ing how we put together a description of the nature and evolution of the stars. Looked at on an even broader scale, the classification of stellar spectra is important, as is any classification system, because it enables us to reduce a large sample of diverse individuals to a manageable number of natural groups with similar characteristics.  Thus spectral classification is, in many ways, as fundamental to astronomy as is the Linnean system of classifying plants and animals by genus and species.  Since the group members presumably have similar physical characteristics, we can study them as groups, not isolated individuals.  By the same token, unusual individuals may readily be identified because of their obvious differences from the natural groups.  These peculiar objects then be subjected to intensive study in order to attempt to understand the reason for their unusual nature.  These exceptions to the rule often help us to understand broad features of the natural groups.  They may even provide evolutionary links between the groups. The appendices to this manual on pages 22, 23, 24 give the basic characteristics of the spectral types and luminosity classes in the MK system.  But the best way to learn about spectral classification is to do it, which is what this exercise is about. Introduction To The Exercise The computer program you will use consists of two parts.  The first is a spectrum display and classification tool.  This tool enables you to display a spectrum of a star and compare it with the spectra of standard stars of known spectral types. The tool makes it easy to measure the wavelengths and intensities of  spectral lines and provides a list of the wavelengths of known spectral lines to help you identify spectral features and to associate them with particular chemical elements. The second part of the computer program is a realistic simulation of an astronomical spectrometer attached to one of three research telescopes—one small, one medium-sized, and one large.  You will pick a telescope that is most appropri- ate to your needs.  A TV camera is attached to the telescope so you may see the star fields it is pointing to, and you can view the fields at high and low magnification.  You can steer the telescope so that light from a star will pass into the slit of the spectrometer and then turn on the spectrometer and begin to collect photons.  The spectrometer display will show the spectrum of the source as it builds up while you collect additional photons.  The spectrum is a record of the intensity of the light collected versus the wavelength.  When a sufficient number of photons are collected, you should be able to see the distinct spectral lines that will enable you to classify the spectrum. You can use the telescope to obtain spectra for a list of stars designated by your instructor.  You will then classify your spectra by comparing them with the spectra of standard stars stored in the computer, just as you did in the first part of the exercise. Figure 1 Digital Spectra of the Principal MK Types