Spectrum of the 8th-magnitude Wolf-Rayet star WR 135 in Cygnus.
From its beginnings in the early 19th century, spectroscopy the analysis of starlight by wavelength has unlocked a treasure-trove of astronomical information. Most of what we know about a star's chemical composition, temperature, axial spin, magnetic field, and motion through space is deduced by studying its spectrum. Although light is greatly diluted when spread into a long spectrum, pioneers such as Angelo Secchi of the Vatican Observatory did remarkable work with visual spectroscopes. Furthermore, some bright stars present truly spectacular spectra to the eye. One example is the Wolf-Rayet star Gamma Velorum, about which the Scottish astronomer Ralph Copeland commented in 1883, "An intensely bright line in the blue, and the gorgeous group of three bright lines in the yellow and orange, render the spectrum...incomparably the most brilliant and striking in the whole heavens."
The author obtained this sampling of stellar spectra with one of his early spectrographs that used a 9-inch-diameter objective prism. Several prominent emission lines (and their wavelengths in angstroms) are marked in the spectrum of the star Beta Lyrae. The spectral classification of each star is given in parentheses. The prominent absorption line at 7584 angstroms in the near infrared is the A absorption due to oxygen in Earth's atmosphere. Because these spectra were formed by a prism, the dispersion varies according to wavelength with blue light at left spread out more than red light at right.
Even at that time William Huggins in England was using photography to record spectra of targets too faint to be studied visually. Many of spectroscopy's pioneers were amateur astronomers, but as the 19th century drew to a close, amateur interest was on the wane. By then spectroscopy had been adopted by professional observatories in North America and Europe, which had the large-aperture telescopes needed to exploit this powerful tool.
In 1999 we marked the 70th anniversary of Edwin Hubble's announcement that the spectra of distant galaxies exhibit a redshift, which we now interpret as due to the expansion of the universe. Even though he used the 100-inch telescope at Mount Wilson Observatory, Hubble required exposures up to 20 hours spread over several nights to record the spectra of some galaxies. Today the inefficient photographic plate has been superseded by electronic CCD detectors, and professional astronomers probe the depths of the universe with huge telescopes like the 10-meter Keck reflectors in Hawaii.