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  The traditional distinction between astronomy (Greek, ‘naming the stars’) and astrophysics (Greek, ‘about the nature of stars’) was that astronomers observe celestial objects, while astrophysicists seek to explain them. This division has now been superceded as both astronomy and astrophysics broaden in scope. Specifically excluded from astrophysics are the study and measurement of the motions and positions of the stars and planets. Topics which lie mainly within the domain of astrophysics are general relativity, cosmology, the evolution and structure of stars, the composition of interstellar matter, and the study of certain very high energy particles produced by violent events.

General relativity is tested by astronomy and astrophysics. Studies of the motion of Mercury and the deflection of light by the gravitational field of the Sun are examples of astronomical verifications; an example of an astrophysical one is the slowing down of a rotating binary system. The two stars are orbiting each other, and the rate at which their orbital speed reduces is exactly that predicted by general relativity.

To investigate cosmology and the early universe, various techniques are employed. One of these involves careful measurement of what is known as the three degree background. This is the remnant of the Big Bang—all that remains of the violent event that created the universe. We see it as a uniform energy background. Recent observations of slight irregularities in this background have given us important clues to how the galaxies evolved.

By observing the frequencies of the light emitted by stars, we may discover which atoms they are composed of, as each atom emits a characteristic set of frequencies. Thus we may learn how stars create elements and evolve themselves.

Interstellar matter is a subject of great interest, because we do not see enough of it. The way that stars and galaxies move indicates that there is far more matter around than we can actually see. It is thought that so-called ‘dark matter’, matter which emits no radiation and which we therefore cannot see, may provide the missing mass, and this matter may exist in the voids between the stars.

Sources which emit high energy particles are supernova remnants, radio galaxies, pulsars and quasars. Most of these are objects from the early universe, whose light has only just reached us. By studying them we may learn about the evolution of the universe. JJ



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