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How does spectroscopy help astronomers learn about celestial objects?

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Astronomy Hubble Space Telescope, photographed by the space shuttle Discovery.

Spectroscopy is a fundamental tool in astronomy that allows scientists to analyze the physical properties of celestial objects by measuring the intensity of radiation as a function of wavelength or frequency.1

Analyzing Stellar Properties

By examining a star's spectrum, astronomers can determine its temperature, chemical composition, and intrinsic luminosity. Spectral lines are created by electron transitions within atoms or ions. Because these transitions are sensitive to environmental conditions, the resulting lines reveal:23
* Temperature: Variations in spectral features help categorize stars, as specific energy levels are only populated at certain temperatures.43
* Density and Gravity: The width of spectral lines—such as those of hydrogen—can indicate the density of a star's atmosphere, which is related to its surface gravity.4
* Magnetic Fields: High-resolution spectra can reveal the presence of strong magnetic fields.2

Motion and Cosmology

Spectroscopy also enables the measurement of relative motion through the Doppler effect. When a light source moves toward or away from Earth, its spectral lines shift in frequency. This technique was pivotal in discovering that the universe is expanding and helped identify the cosmic microwave background as a relic of the big bang.56