Astrophysics is the study of how stars and planets work, and how we can learn about them. Astrophysicists use physics to explain what astronomers find and see.
Astrophysics is also the study of how the Universe started and how it is changing with time. This part of astrophysics is called cosmology. For a long time bodies in the sky seemed to be unchanging spheres moving in a circle. But on Earth growth and decay happened, and natural motion was in a straight line. Therefore, people thought the celestial region was made of a fundamentally different kind of matter from that found on Earth.
During the 16th and 17th century, natural philosophers such as Nicolaus Copernicus, Johannes Kepler, Galileo, Descartes, and Newton, began to maintain that the celestial and terrestrial regions were made of similar kinds of material and were subject to the same natural laws.

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