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How to Measure the Composition of Stars

Spectrometry (spectroscopy) is the essential tool used by astronomers since the 1800s to measure the composition, color and temperature of stars through analyses of emitted light spectra. Each chemical element reveals a distinct banding pattern in a star's absorption spectrum. When light from a star splits by prism or grating into a spectrum of wavelengths, the spectral pattern reflects the star's composition. While all stars are 95 percent hydrogen, variations in composition reveal age, luminosity and origin. Spectroscopy was out of reach for most amateur astronomers until the recent development of reasonably priced spectroscopes costing hundreds to thousands of dollars.

Things You'll Need

Telescope CCD camera Spectroscope (also called spectrograph or spectrometer) Gratings Slits Spectometry software

Instructions

- 1
  
  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.
- 3
  
  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.
- 5
  
  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.

Astronomy