Tools of the Astronomer
Astronomers' data come in the form of electromagnetic radiation --light ranging from radio waves to high-energy gamma rays. We will stick to the description of light as waves and not use quanta, or photons here. Students should understand that the electromagnetic spectrum encompasses radiant energy of widely varying wavelengths and intensities. Different kinds of stars and other objects emit different amounts in different regions of the spectrum.
Researchers design telescopes and detectors to gather the dim light from distant objects, but each instrument can reveal only part of an object's full spectrum. The more efficient the telescope, the more objects they can see, and the further away, and more precise the measurements.
Much of astronomical research involves spectroscopy and spectrometry, using instruments to separate the wavelengths of light from an object and analyze their intensities. Here the spectrum of a source refers to the individual light output the object emits; what intensities at what wavelengths (or frequencies); which form a unique "fingerprint" for the object. The spectrum of a star is determined by its composition and its temperature. Astronomers can infer temperature and composition, given the spectrum. They can combine the spectrum with what is known about the physics of stars to determine other factors, such as the size and age of a star. The spectrum can also be a clue to the physical processes taking place in an object of unknown identity.