What is Astrophysics?
'''Astrophysics''' is the study of how [[star]]s, [[planet]]s, and other things in the Universe work, and how we can learn about them. Astrophysicists use [[physics]] and [[chemistry]] to explain what [[astronomer]]s find and see.<ref>Keeler, James E. 1897. The importance of astrophysical research and the relation of astrophysics to the other physical sciences. ''The Astrophysical Journal'', '''6''' (4): 271–288, Bibcode:1897ApJ.....6..271K, doi:10.1086/140401, PMID 17796068</ref> Astrophysics is also the study of how the [[Universe]] started and how it is changing with time. This part of astrophysics is called [[cosmology]]. == History == [[Image:NIEdot362.jpg|right|thumb|Early 20th-century comparison of elemental, solar, and stellar spectra]] 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 centuries, natural philosophers such as [[Nicolaus Copernicus]], [[Johannes Kepler]], [[Galileo]],<ref>{{Citation | last = Galilei | first = Galileo | author-link = Galileo Galilei | editor-last = Van Helden | editor-first = Albert | publication-date = 1989 | title = Sidereus Nuncius or The Sidereal Messenger | date = 15 April 1989 | publisher = University of Chicago Press | publication-place = Chicago | pages = 21, 47 | isbn = 0-226-27903-0 }}</ref> [[Descartes]],<ref>{{Cite web |author=Edward Slowik |title=Descartes' Physics |url=http://plato.stanford.edu/entries/descartes-physics/ |publisher=Stanford Encyclopedia of Philosophy |date=2013 |orig-year=2005 |accessdate=2015-07-18}}</ref> and [[Isaac Newton|Newton]],<ref>{{Citation | last = Westfall, Richard S. 1980 | title = Never at rest: a biography of Isaac Newton | date = 29 April 1983 | publisher = Cambridge University Press | publication-place = Cambridge | pages = 731–732 | isbn = 0-521-27435-