Editing Observational astronomy (section)

=== Spectrograph ===
A vital instrument of observational astronomy is the [[spectroscope|spectrograph]]. The absorption of specific wavelengths of light by elements allows specific properties of distant bodies to be observed. This capability has resulted in the discovery of the element of [[helium]] in the Sun's [[emission spectrum]], and has allowed astronomers to determine a great deal of information concerning distant stars, galaxies, and other celestial bodies. [[Doppler shift]] (particularly "[[redshift]]") of spectra can also be used to determine the radial motion or distance with respect to the [[Earth]].

Early spectrographs employed banks of [[prism (optics)|prism]]s that split light into a broad spectrum. Later the [[diffraction grating|grating spectrograph]] was developed, which reduced the amount of light loss compared to prisms and provided higher spectral resolution. The spectrum can be photographed in a long exposure, allowing the spectrum of faint objects (such as distant galaxies) to be measured.

Stellar photometry came into use in 1861 as a means of measuring [[color index|stellar colors]]. This technique measured the magnitude of a star at specific frequency ranges, allowing a determination of the overall color, and therefore [[temperature]] of a star. By 1951 an internationally standardized system of UBV-[[Apparent magnitude|magnitudes]] (''U''ltraviolet-''B''lue-''V''isual) was adopted.