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12.12 Stellar Composition



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Representative stellar spectra from a range of spectral classes. Click here for original source URL.

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Cecilia Payne-Gaposchkin. Click here for original source URL.

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Solar spectrum showing the dark absorption lines. Click here for original source URL.

Astronomers derive what a star is made of using spectroscopy. There are two separate problems: detecting the presence of an element and measuring its amount. The presence of an element is detected by identifying at least one — or preferably several — of its absorption lines or emission lines in the spectrum of the star. Astronomers deduce the amount of the element from the appearance of the spectral lines. Generally, wider and darker (or deeper) absorption lines indicate that more atoms of the element are present. Likewise, wider and brighter emission lines indicate more atoms of a particular element. The relative proportions of all the different elements give the chemical composition of the star.The classification of enormous numbers of spectra at the Harvard College Observatory was the first step in discovering what elements stars are made of. Cecilia Payne-Gaposchkin was the first to demonstrate that nearby stars are composed primarily of hydrogen and helium, which makes them similar to the Sun in composition.

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Graphic representation of Wein's Law. Click here for original source URL.

 It is important to keep in mind, however, that the absence of spectral lines of a given element do not guarantee that the element is missing from the star. For instance, only the hottest stars, O and B stars, show spectral evidence for helium, because cooler stars photospheres are not hot enough to ionize helium. Physical conditions, like temperature, density, and pressure, need to be taken into account as well when using spectra to determine stellar composition. Similarly, the strength of a spectral feature is not a direct indication of the abudance of that element in the star. A sequence of spectral lines of the same element have relative strengths dictated by quantum physics, and the conversion of line strength to abundance requires a physical model for the star that incorporates temperature, density, and pressure. The width of the spectral feature is important too, because the spread in wavelength of a features maps to a velocity range for the gas — broader spectral features arise in gas of higher temperature and preserve.

Spectroscopy is extremely powerful. Identification of the spectral lines tells us what elements are present in the outermost layers of a star, because elements have patterns of lines as unique as fingerprints. Measurement of the relative line strengths of a particular element tells us about the temperature, density and pressure in a star's photosphere, or surface layer. This method supplements temperature information gained from other techniques, such as Wien's law applied to the peak of the radiation from the star.

Spectroscopy of stars tells us something very important about the universe we live in. Helium was a mysterious element when it was first discovered, because it is so rare on the Earth. Now we know that hydrogen and helium are the most abundant elements in the solar system. These two simple elements make up the bulk of the Sun and the bulk of the giant planets. It is the Earth that is unusual — Earth is a large rock that is rich in carbon and silicon and metals and is too low in mass to retain the light gases. When astronomers turned their attention on the stars, they did not know what to expect. Would there be strange elements or unusual states of matter at those vast distances? No. Most stars are put together just like the Sun. Hydrogen and helium are the most common elements everywhere in the universe that we have studied so far. This reassures us that the laws of physics and chemistry really are universal.

Stellar spectroscopy tells us about the surface layers of stars. To learn about interior composition we need to look elsewhere — the interstellar medium. Although we cannot see into the cores of stars, we can get an idea of their composition by looking at the remnants of stellar explosions. When stars age and die, they mix their gas by the churning motions of convection, then they blow off their outer layers, either slowly or in a violent supernova, and we are finally given a look at their interior composition. Hydrogen and helium are still the most abundant elements, but we also see heavier elements like carbon, nitrogen, oxygen, gold, silver, platinum, iron, and all the other elements on the periodic chart. This material was produced in stars, and will become the raw material that forms into new stars and planets