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Moreover they have significant absorption lines within the wavelength range discussed.
McCall's group uses a sophisticated laser-based technique called cavity ringdown spectroscopy to detect absorption lines.
Its formation depletes a stellar atmosphere of TiO and other oxides, wiping their absorption lines from the object's spectrum.
As a result, a typical quasar spectrum consists of a line of strong emission -- the Lyman-alpha radiation shifted to visible light -- followed by a series of weak absorption lines at shorter wavelengths.
Astronomers have long known that Achernar is unusual in that it is spinning very fast (judging from the broadening of the absorption lines in its spectrum).
The presence of many clouds at different distances creates a thicket of absorption lines, called the Lyman alpha forest, within the spectrum of light emitted by the quasar.
Przybylski's breakthrough came when he determined that the apparent lack of absorptions seen at low spectral resolution is actually due to the overlapping of myriad absorption lines of the heavy chemical elements known as rare earths.
Using Hubble's Goddard High-Resolution Spectrograph to precisely measure the ultraviolet absorption lines, Linsky and his co-workers calculated the deuterium-hydrogen ratio at about 15 parts per million, with an uncertainty of less than 10 percent.
For example, a graphic tries to explain redshift using a single spectrum with random absorption lines and a caption that includes, "Spectral lines formed by elements in the star shift to the red.
Together, the clouds create a thicket of absorption lines -- called the Lyman alpha forest -- within the spectra of quasars.
The tightly bound spectroscopic binaries have double sets of absorption lines, which allow both stars' orbital velocities to be measured.