Telescopes and CCDs may gather and record starlight, but only a spectrograph can extract its secrets. A spectrograph separates white light into its component wavelengths— a spectrum—crossed by numerous dark and bright spectral lines.
It then records these lines as a kind of bar code of the object's physical properties. Spectroscopy is the basis of modern astrophysics— most of what we know about the chemical composition, temperature, and pressure in any astronomical object is encoded in its spectral lines.
One ofspectroscopy's most useful applications is in determining an object's motion toward or away from Earth. This was discovered in 1868 by William Huggins, who found that dark lines in the star Sirius were shifted toward the red end of the spectrum, implying a decrease in frequency. Based on work by Austrian physicist Christian Doppler, who theorized that a source exhibits a change in frequency if it is moving toward or away from the observer, Huggins determined that Sirius was moving away from the Sun at about 25 miles per second (40 km/s). Had the spectral lines been blueshifted, increasing in frequency, it would have indicated that Sirius was approaching Earth.
Edwin Hubble later applied this method to galaxies, demonstrating that most galaxies are redshifted and their recessional velocities increase with distance.
Fiber-optic technology is improving the efficiency of spectroscopy. Many optical fibers can be arranged so that each receives light simultaneously from a different star or galaxy. The opposite ends of the fibers can then be aligned so that they feed into the spectograph at once, allowing a hundred or more spectra to be gathered at the same time.