Thus, spectroscopic study of the stars has provided scientists with valuable theoretical knowledge, and is continuing to do so because the stars provide laboratories in which conditions unattainable on the earth are maintained, such as extremely high temperatures and extremely high and low pressures. Certain lines, for example, found in the spectra of nebulas were long thought to be due to an element, tentatively called nobelium, undiscovered on the earth. Scientists now know that these lines are produced by common elements under exceedingly high vacuum conditions. Late in 1969, for example, the Lunar and Planetary Laboratory at the University of Arizona announced that the spectral analysis of the rings surrounding the planet Saturn showed them to be largely formed of ammonia ice. Scientists also utilized spectroscopy to analyse the composition of the planet Jupiter and its atmosphere after fragments of Comet Shoemaker-Levy 9 crashed into the planet in July 1994. The collisions brought heated interior gases to the surface of the planet's atmosphere, where telescopes recorded them in detail.
A shift in the position of the spectrum lines occurs when the source of the radiation is moving toward or away from the observer. This shift in wavelength, known as the Doppler effect discussed above, provides a fairly accurate value for the relative speed of any source of radiation. In general, if all the lines in the spectrum of a star are shifted toward the red, that star is moving away from the earth, and the velocity of recession can be calculated from the amount of the shift. Conversely, if the star is moving toward the earth, the spectrum is shifted toward the violet. The Doppler shifts observed in the spectra of exterior galaxies indicate that the universe is expanding. The spectra of a few distant stars periodically split up; the doublets then combine into single lines again. This phenomenon is due to the presence of two stars called double stars, or spectroscopic binaries, that are revolving about each other so close together that a telescope cannot resolve them. When one of the stars is moving toward the earth and the other away, all the lines from one star are shifted toward the violet and all the lines of the other star are shifted toward the red. When both stars are moving transverse to the line of sight from the earth, the lines from the two stars coincide. All the molecules of a gas are in constant motion, so that at any instant some are moving toward a spectroscope and some away from it. The wavelengths of some of the photons are smaller, and those of others larger, than if all the atoms were at rest. Because of this variability of wavelength, each spectrum line is broadened slightly.