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How to Measure Velocities in the Heavens




Learn about Spectroscopy

When light from a glowing gas (for instance, the Sun) is sent through a slit, then through a prism that spreads it out into its spectrum, dark lines may be seen. In 1859 Gustav Kirchhoff discovered that the lines could be used to identify the chemical elements in the gas, as described here. The new spectroscopic techniques could reveal velocity as well as chemical composition. In 1842, even before Kirchhoff's work, the Austrian physicist Christian Doppler had argued that a spectrum would be shifted if the source of the light was moving. A more correct explanation was later furnished by the French physicist Hippolyte Fizeau, but the principle is named after Doppler — the Doppler velocity shift.

Not all scientists immediately accepted the prediction that the light from moving objects would show a Doppler shift. In 1868 the English amateur astronomer William Huggins found what appeared to be a slight shift for a hydrogen line in the spectrum of the bright star Sirius. By 1872, Huggins had more conclusive evidence of motions of Sirius and of several other stars.

Instrumental limitations prevented Huggins from extending his spectroscopic investigations to the faint spiral nebulae. Their velocities began to be measured only when astronomical entrepreneurship in America's Gilded Age (1880s and 1890s) led to the building of larger instruments, and the center of astronomical spectroscopy shifted to the United States.

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