How to Measure the Composition of Stars
Amateur Astronomers Analyze Spectra With Affordable Spectroscopes
Oct 29, 2008
A spectroscope (also called a spectrograph or spectrometer) uses a prism or refraction grating to split white light into a continuous spectrum of wavelengths visualized as a rainbow of color. While all stars are roughly 95% hydrogen, slight differences in composition give astronomers information about the temperature, age, and history of individual stars thus shedding light on the evolutionary processes of the universe.
How to Measure the Composition of Stars- Types of Spectra
There are three types of spectra emitted by objects and detectable by spectroscopy. All give information about the composition and characteristics of a star. In the mid-1800s, Kirchoff described features of these distinct spectra:
- Continuous
- Emission
- Absorption
Continuous Stellar Spectra and the Planck Curve
Solid and liquid objects emit a continuous spectrum of light called a Planck curve. Two laws of radiation relate to the interpretation of a star’s Plank curve. The Stefan-Boltzmann law states that bodies at higher temperature emit more energy. Two stars with similar composition will emit different energies at the same wavelength with the hotter star having a stronger energy output.
Wien’s law dictates that the Planck curve of a higher temperature star demonstrates a shift towards shorter wavelengths (blue shift) compared to a cooler star of the same composition. This is visualized as a leftward shift of the spectrum on a graph called a line intensity profile.
Emission and Absorption Spectra
Hot gases emit a spectrum with strong, characteristic light bands at discrete wavelengths. This is termed an emission spectrum. By contrast, a light source passed through a cold gas emits a spectrum with dark bands at the discrete wavelengths characteristic of the gas. These dark bands represent peak wavelengths of light absorbed by the gas, thus these wavelengths are absent in an absorption spectrum.
An element's characteristic bands occur at the same location in absorption and emission spectra, but differ in appearance depending on whether the gas emits the light source or is illuminated by a more distant light source.
Measuring the Compostion of Stars- Spectral Classes
While the composition of all stars is 95% hydrogen, higher surface temperatures result in energetic atomic collisions resulting in the ionization of atoms through the loss of electrons. This results in characteristic absorption spectra evidencing lighter weight elements such as hydrogen and helium. The absorption spectra of cooler stars show banding evidencing the presence of heavier metals such as calcium and magnesium.
Astronomers classify normal stars by their peak absorption lines into seven spectral classes (O, B, A, F, G, K, M) with characteristic masses, temperatures, radii, and luminosities. Earth’s Sun is a relatively uninteresting type G star, exhibiting no extreme attributes.
Affordable Spectroscopes Open Spectroscopy to Amateur Astronomy
While the analytical science of spectroscopy long remained inaccessible to amateur astronomers, it is a critical tool for serious stargazing. Unlike other aspects of astronomy, spectroscopy is fairly tolerant of background light pollution and air turbulence, making it a desirable pursuit for the home astronomer.
Technological advances and market demand finally allow amateur astronomers to obtain and analyze stellar absorption spectra using telescopes, spectroscopy computer software, and spectroscopes costing from a few hundred to a few thousand dollars. High-end amateur spectroscopes even capture the rapid spectral shifts characteristic of binary systems; the technology does allow a broad array of investigation.
This article is generally informed by Keith Robinson's Spectroscopy: The Key to the Stars (Springer, 2007). The book is part of Patrick Moore's Practical Astronomy Series.
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