Similar spectral lines showed up in laboratory experiments. They were later measured and cataloged by Josef von Fraunhofer, for whom they are still called "Fraunhofer lines." These dark lines stayed at exactly the same places in the colorful band from day to day and year to year. When seen this way, Wollaston noticed, the Sun's spectrum was marked by many narrow, black lines of various intensities. The slit provided a sharp, high-resolution view of the familiar rainbow spectrum, with no color overlapping another. Our tale began in 1802, when the English experimenter William Wollaston passed a beam of sunlight through a thin slit and then through a prism. Emission lines, by contrast, come from an energized, rarefied gas such as in a neon light or a glowing nebula. Whether in a star's atmosphere or in a laboratory, absorption lines are produced when a continuous rainbow of light from a hot, dense object (top left) passes through a cooler, more rarefied gas (top center). Important things to know about Spectral Types of Stars Dissecting Starlight Appended to the basic spectral type may be letters for chemical peculiarities, an extended atmosphere, unusual surface activity, fast rotation, or other special characteristics.Įvery starwatcher needs to have a feel for spectral types. The luminosity (when compared to the star's apparent brightness in our sky) also gives a good idea of the star's distance. ![]() The size and surface brightness in turn yield the star's luminosity (its total light output, or absolute magnitude) and often its evolutionary status (young, middle-aged, or nearing death). The atmospheric pressure depends on the star's surface gravity and therefore, roughly, on its size - telling whether it is a giant, dwarf, or something in between. The temperature sets the star's color and determines its surface brightness: how much light comes from each square meter of its surface. These reveal an abundance of information that paints the star's portrait and tells its life story. It is based on just two physical properties that imprint themselves on the spectrum of a star's light: the star's surface temperature and atmospheric pressure. The modern spectral classification system is so successful that it has hardly been changed since 1943. University of Michigan Department of Astronomy Click on this image to see some of these stellar spectra up close. ![]() ![]() Spectra contain the 'fingerprints' astronomers use to deduce myriad stellar properties. A thin prism was placed in front of the telescope to spread out starlight from the Hyades cluster into little rainbows, or spectra.
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