The study of the physical nature of the Universe and the objects in it, notably stars, galaxies, and the composition of the space between them. Astrophysics originated in the application of spectroscopy to the study of starlight in the nineteenth century. It complements the traditional branches of astronomy, astrometry and celestial mechanics, which are concerned with the positions and motions of objects.

Observational astrophysics interprets the electromagnetic and gravitational radiation emitted by celestial objects. Theoretical astrophysics attempts to explain the processes involved, which can lead to new understanding of the behaviour of matter under conditions not encountered on Earth. For example, nuclear physics had to develop before energy generation inside stars could be understood, and study of objects such as white dwarfs and neutron stars has helped confirm predictions about the behaviour of matter under extreme compression and intense gravitational fields.

Astrophysics can also probe the extremely tenuous gas between the stars, where complex molecules are formed and high-energy particles called cosmic rays move close to the speed of light. It also addresses ultimate questions about the origin of the Universe, the conditions shortly after its creation, and the origin of the chemical elements. Closer to home, astrophysics deals with matters such as the environment of the planets and the effects of the solar wind, which could explain short-term changes in our weather and long-term changes in climate. Many areas of physics are involved in such studies, including spectroscopy, plasma physics, atomic physics, and relativity.

The development of observatories in space since the 1960s has allowed astronomers to study the Universe at wavelengths that do not penetrate the Earth’s atmosphere, from radio waves to gamma rays, and to detect gravitational waves.