Spectroscopy

One key development that brought astronomy into its modern age was the use of spectroscopy to study astronomical objects in the 19th century. Spectroscopy is carried out by splitting the light into separate colors, in the way a prism or raindrops can turn sunlight into a rainbow. If the light is split (dispersed) finely enough, it becomes possible to identify lines that are characteristic to specific atoms. This is how Helium was first discovered, by studying spectra of the Sun. The image above shows the spectra of stars of steadily decreasing temperature (historically named with letters, which is a relic of astronomy from before the meaning of the spectra were understood). The dark bands represent elements in the outer layers (photosphere) of the star, which absorb light that is traveling outward from the lower layers. The pattern of bands tells us which elements retain their electrons in the outer layer of the star, from which we can derive the temperature of the outer layers of the star.

Spectroscopy has become the single most important quantitative tool that an astronomer has. In the 19th century, it allowed astronomers to prove that the sun, rather than being a unique heavenly body, was simply one of many stars (a "G" star in the list above). This set astronomers on a path to determining how large our Galaxy, and eventually the universe, was. Spectroscopy eventually enabled astronomers to determine the composition of the sun and stars. This eventually led to theories of why stars shine (by fusing lighter elements into heavier ones, i.e., hydrogen into helium), where the elements come from (all those beside hydrogen, helium, and lithium are formed in stars), and the relative ages for stars (our star is at least the third or forth generation of stars to form in the Galaxy). It is used to study binary stars, and determine their masses, which is crucial to refining models for the internal structure of stars.

Spectroscopy is also applied to the study of distant Galaxies. It is used to study the motions of stars within Galaxies, in order to understand the properties of the dark matter that holds them together (see gravitational lensing below), and the black holes that lurk at the centers of galaxies.