How to Measure the Composition of Stars
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Measuring the composition of stars with a homemade spectroscope
Purchase a compatible CCD camera and spectroscope system from a manufacturer such as SBIG Astronomical Instruments. Alternately, couple an existing camera can to a spectroscope. Construction of a homemade spectroscope could consist of a camera, a coupler, a five-filter holder with numerous transmission gratings, a 0.4x compressor lens to function as a culminating lens and a five-filter holder with multiple slits. Review the site under Additional Resources for details. - 2). Attach the connected spectrograph and camera to the telescope. At minimum, you will need a scope with an ability to track a celestial object as the Earth turns; a motorized right ascension drive is best. If you will be looking at deep space objects like nebulae or want to decrease exposure times when measuring the composition of stars, consider a telescope with a 10-inch or greater aperture. If you cannot afford this type of equipment, consider the technological limitations facing astronomers of the 1800s who developed spectroscopy.
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Absorption spectrum reveals stellar composition
Select appropriate slit width and grating based on the spectral target. SBIG's standard grating has 150 rulings per mm, yielding a single-exposure spectrum encompassing the banding range from hydrogen to calcium. - 4). Guide the telescope so that image of the star is visible through the spectrograph slit. Ensure both the slit and star are properly imaged on the CCD camera. A dual CCD self-guiding camera and spectroscope will track the star after it is properly fixed. Follow the manufacturer's recommendations for optimal imaging.
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Measuring composition of stars--hydrogen alpha peak
Interpret data from the spectral image using spectrometry software included with some commercial spectrographs or obtained separately. Each element on the periodic table exhibits a unique absorption spectrum consisting of bands (Fraunhaufer lines) at one or more wavelengths. The software helps determine the composition of stars by detecting absorption bands (or peaks on a line intensity profile) at specific wavelengths that characterize individual elements. - 6). Classify the star into a stellar class (O, B, A, F, G, K, M) based on measurements of the star's composition. For example, more massive O and B stars will exhibit spectral patterns for hydrogen and helium while less massive K and M stars will also include absorption peaks for metals such as calcium and helium. The sun belongs to stellar class G.
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