7 Classification of Stellar Spectra 16f (1)

7 Classification of Stellar Spectra 16f (1)

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1 Astronomy 116 Lab 7 Classification of Stellar Spectra Revised 31 August 2015 Equipment PC computer with the CLEA program Spectral Classification (supplied) Objectives After doing this lab, you should be able to recognize the distinguishing characteristics of different spectral types of main-sequence stars, and assign spectral classifications with a precision of one tenth of a spectral type. You should also be able to understand the use of spectral classification in deriving the distances of stars, a process called spectroscopic parallax. Background: The History And Nature Of Spectral Classification When light passes through a prism, raindrops, ice crystals, or a grating, you often get a rainbow. This rainbow is a spectrum, where the different wavelengths of light get separated out. Close inspection of the sun’s spectrum reveals numerous dark lines, such that the spectrum looks somewhat like a colorful barcode. Patterns of these dark (absorption) lines were first observed in the spectrum of the sun early in the 1800s. Later in the century astronomers were able to routinely examine the spectra of stars in large numbers, and it was discovered that stellar spectra could be divided into groups by the general appearance of the spectra. In the various classification schemes they proposed, stars were grouped together by the prominence of certain spectral lines. In one scheme, for instance, stars with very strong hydrogen lines were called type I, stars with strong lines from metallic ions like iron and calcium were called type II, stars with wide bands of absorption that got darker toward the blue were called type III, and so on. Building upon this early work, astronomers at the Harvard Observatory refined the spectral types and renamed them with letters, A, B, C, etc. They also embarked on a massive project to classify spectra. This resulted in the Henry Draper Catalog , which was published between 1918 and 1924, and provided classifications of 225,300 stars. Even this study, however, represents only a tiny fraction of the stars in the sky. 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, etc.
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