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Astrophysics began in the 1800s, when astronomers developed the spectroscope, which is used to determine the various properties of stars and planets. In spectroscopy, light is spread into a spectrum, and the resulting image can be used to determine a star's chemical composition, temperature, surface condition, and other properties. Astrophysicists knew that understanding the nature of stars would help them understand the larger question that all astrophysicists work toward answering: How did the universe begin?

A major advance in the field of astrophysics was the development of atomic theory. In 1803, a British chemist, John Dalton, proposed that each natural element consists of a particular kind of atom. In the early 1900s, scientists discovered that each atom has a nucleus, which contains protons, neutrons, and electrons that interact with each other. Today the atom is the basis of the study of physics. Physicists of all disciplines, from astrophysicists to nuclear physicists, use what we know about the atom and its parts to understand their respective fields.

In the case of astrophysicists, close examination of the parts of the atom will help to understand how matter and our universe formed. A widely held explanation today is the Big Bang theory, which hypothesizes that the universe was formed 15 to 20 billion years ago when a dense singular point of matter exploded and eventually formed stars and galaxies. Today most astrophysicists believe the universe is still expanding from that initial explosion.