Solar spectrum showing the dark absorption lines. Click here for original source URL.
A major goal of astronomy is to detect and understand nature's messages from space. Spreading out the wavelengths of the radiation from an astronomical object yields two types of information. The smooth thermal spectrum indicates the temperature of the atoms of the star or planet. The sharp spectral lines have patterns that indicate the chemical composition of the star or planet. These are powerful tools in understanding parts of the universe that we can never travel to and touch. Astronomy uses the techniques of remote sensing; with no physical samples to study we are limited to the information carried by electromagentic radiation. Spectroscopy gives that information enormous diagnostic power.
Electromagnetic transmittance, or opacity, of the Earth's atmosphere. Click here for original source URL.
All the radiation we can see is just a small slice of the much larger spectrum of electromagnetic radiation. However, not all of this radiation from space reaches us. Earth's atmosphere is opaque to most wavelengths of electromagnetic radiation. Only a thin band of optical and near infrared wavelengths and a wider band of radio wavelengths penetrate to the ground. For most of the history of astronomy, our knowledge of the universe has been confined to the messages coming through the narrow window of visible light. In the past forty years, this situation has been transformed by new technologies and the space programs of NASA and the European Space Agency. We are now free to explore the entire electromagnetic spectrum.