The Nature of Spectral Classification [1]

In the “Atomic Spectra Lab” we learned to investigate spectra from gas discharge lamps. The spectra consisted of emission lines, light emitted by excited gas molecules.

In this lab we will investigate absorption spectra. Patterns of absorption lines were first observed in the spectrum of the sun by the German physicist Joseph von Fraunhofer early in the 1800’s, but it was not until late in the century that astronomers were able to routinely examine the spectra of stars in large numbers. Astronomers Angelo Secchi and E.C. Pickering were among the first to note that the stellar spectra could be divided into groups by the general appearance of their spectra. In the various classification schemes they proposed, stars were grouped together by the prominence of certain spectral lines.

In the course of the Harvard classification study, some of the old spectral types were consolidated together, and the types were rearranged to reflect a steady change in the strengths of representative spectral lines. The order of the spectral classes became O, B, A, F, G, K, and M, and though the letter designations have no meaning other than that imposed on them by history, the names have stuck to this day. Each spectral class is divided into tenths, so that a B0 star follows an O9, and an A0, a B9. In this scheme the sun is designated a type G2.

The spectral classification system used today is a refinement called the MK system, introduced in the 1940’s and 1950’s by W. W. Morgan and P.C. Keenan at Yerkes Observatory to take account of the fact that stars at the same temperature can have different sizes. A star a hundred times larger than the sun, for instance, but with the same surface temperature, will show subtle differences in its spectrum, and will have a much higher luminosity. The MK system adds a Roman numeral to the end of the spectral type to indicate the so-called luminosity class: a I indicates a supergiant, a III a giant star, and a V a main sequence star. Our sun, a typical main-sequence star, would be designated a G2V, for instance. In this exercise, we will be confining ourselves to the classification of main sequence stars, but the software allows you to examine spectra of varying luminosity class, too, if you are curious.

The spectral type of a star is so fundamental that an astronomer beginning the study of any star will first try to find out its spectral type. If it hasn’t already been cataloged (by the Harvard astronomers or the many who followed in their footsteps), or if there is some doubt about the listed classification, then the classification must be done by taking a spectrum of a star and comparing it with an Atlas of well-studied spectra of bright stars. Until recently, spectra were classified by taking photographs of the spectra of stars, but a modern spectrograph produces digital traces of intensity versus wavelength which are often more convenient to study. FIGURE 1 shows some sample digital spectra from the principal MK spectral types; the range of wavelength (the x axis) is 3900 Å to 4500 Å. The intensity (the y axis) of each spectrum is normalized, which means that it has been multiplied by a constant so that the spectrum fits into the picture, with a value of 1.0 for the maximum intensity, and 0 for no light at all.

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 surrounding environment, and its past history. Thus a knowledge of spectral classification is fundamental to understanding 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 Linnaean 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.

Figure 1: Digital Spectra of the Principal MK Types

[1] Material taken directly from The Classification of Stellar Spectra Student Manual Edited by Lucy Kulbago, John Carroll University, accessed on January 9, 2009 at http://www3.gettysburg.edu/~marschal/clea/CLEAhome.html.

To successfully complete this lab exercise, follow these steps:

• Read the introduction
• Complete the pre-lab activity and watch the linked video on stellar classification video at
• Download and read the lab answer sheet
• Print out the data for seven stars
• Go to the Sloan Digital Sky Survey at http://cas.sdss.org/dr7/en/proj/basic/spectraltypes/
• Perform all activities and answer all questions in the lab report
• Check the formatting of the report
• Submit the report in the assignment

Pre-Lab Activity

Before you start with classifying star spectra, go to the URL listed below, read the text and complete the practice exercise with the simulator at the bottom of the page..

http://astro.unl.edu/naap/hr/hr_background1.html

This is a self-contained exercise from the Sloan Digital sky survey. Be sure to read the material surrounding each part of the exericse. You will see how astronomers came up with the classification system we use today (and why the letters indicating star types are not in alphabetical order!

This lab illustrates how science (and even how things are sorted in science) can change with new information. Remember that the early group (mostly women) who sorted these stellar spectra learned to do it quickly and effectively and helped researchers understand key differences and their causes.

Begin here:

http://cas.sdss.org/dr7/en/proj/basic/spectraltypes/

https://openstax.org/books/astronomy/pages/1-introduction

.https://youtu.be/Y5VU3Mp6abI

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