Astrophysics, the application of the theories and techniques of modern physics to astronomy. Astrophysics includes the study of the sun and other stars, planets, comets, nebulae, and the universe as a whole. It is a major branch of astronomy, covering all but a few astronomical fields of study.
One of the primary techniques used in astrophysics is spectroscopy. In spectroscopy, light is spread into a spectrum. The spectrum is then analyzed to determine various properties of the light source. For example, the spectrum of a star can be used to determine the star's chemical composition, temperature, surface conditions, and magnetic properties.
In addition to light, astrophysicists study other forms of radiation emitted by celestial objects. Observations of infrared radiation and radio waves can be made from the earth's surface, but other forms of radiation, such as ultraviolet radiation and X rays, are blocked by the earth's atmosphere and can only be studied with instruments that are carried high into the atmosphere or placed into earth orbit.
Astrophysics began in the latter part of the 19th century with studies of the spectra of the sun and other stars. With the development of modern atomic theory in the early part of the 20th century, great advances were made in understanding the nature of spectra. As various physical principles have been discovered or better understood, they have been used in astrophysics. For example, the discovery of nuclear reactions in which hydrogen atoms combine and form helium atoms, releasing energy, provided an explanation for the source of energy in stars; and the general theory of relativity has greatly aided astrophysicists in understanding astronomical phenomena involving objects of great mass and in describing the structure of the universe as a whole.