Editing Astrophysics (section)

==History==
Astronomy is an ancient science, long separated from the study of terrestrial physics. In the [[Aristotle|Aristotelian]] worldview, bodies in the sky appeared to be unchanging [[Celestial spheres|spheres]] whose only motion was uniform motion in a circle, while the earthly world was the realm which underwent [[On Generation and Corruption|growth and decay]] and in which natural motion was in a straight line and ended when the moving object reached its [[Telos|goal]].  Consequently, it was held that the celestial region was made of a fundamentally different kind of matter from that found in the terrestrial sphere; either [[Fire (classical element)|Fire]] as maintained by [[Plato]], or [[Aether (classical element)|Aether]] as maintained by [[Aristotle#Physics|Aristotle]].<ref>{{cite book | last = Lloyd | first = G. E. R. | author-link = G. E. R. Lloyd | title = Aristotle: The Growth and Structure of His Thought | publisher = Cambridge University Press | year = 1968 | location = Cambridge | pages = [https://archive.org/details/aristotlegrowths0000lloy/page/134 134–135] | url =https://archive.org/details/aristotlegrowths0000lloy| url-access = registration | isbn = 978-0-521-09456-6}}</ref><ref>{{cite book | last = Cornford | first = Francis MacDonald | author-link = F. M. Cornford | title = Plato's Cosmology: The ''Timaeus'' of Plato translated, with a running commentary | publisher = Bobbs Merrill Co | date = c. 1957 | orig-year = 1937 | location = Indianapolis | page = 118}}</ref>
During the 17th century, natural philosophers such as [[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 | publisher = University of Chicago Press | location = Chicago | pages = 21, 47 | isbn = 978-0-226-27903-9 | year = 1989 }}</ref> [[Descartes]],<ref>{{Cite encyclopedia |author=Edward Slowik |title=Descartes' Physics |url=http://plato.stanford.edu/entries/descartes-physics/ |encyclopedia=[[Stanford Encyclopedia of Philosophy]] |date=2013 |orig-year=2005 |access-date=2015-07-18}}</ref> and [[Isaac Newton|Newton]]<ref>{{Citation | last = Westfall | first = Richard S. | author-link = Richard S. Westfall | publication-date = 1980 | title = Never at Rest: A Biography of Isaac Newton | publisher = Cambridge University Press | location = Cambridge | pages = [https://archive.org/details/neveratrestbiogr00west/page/731 731–732] | isbn = 978-0-521-27435-7 | year = 1983 | url-access = registration | url = https://archive.org/details/neveratrestbiogr00west/page/731 }}</ref> began to maintain that the celestial and terrestrial regions were made of similar kinds of material and were subject to the same [[Physical law|natural laws]].<ref name = Burtt/>  Their challenge was that the tools had not yet been invented with which to prove these assertions.<ref>{{Cite journal |author=Ladislav Kvasz |title=Galileo, Descartes, and Newton – Founders of the Language of Physics |url=http://www.physics.sk/aps/pubs/2012/aps-12-06/aps-12-06.pdf |publisher=Institute of Philosophy, [[Academy of Sciences of the Czech Republic]] |journal=Acta Physica Slovaca |date=2013 |access-date=2015-07-18}}</ref>

For much of the nineteenth century, astronomical research was focused on the routine work of measuring the positions and computing the motions of astronomical objects.<ref>{{Citation | last = Case | first = Stephen | date = 2015 | title = 'Land-marks of the universe': John Herschel against the background of positional astronomy | journal = Annals of Science | volume = 72 | issue = 4 | pages = 417–434 | doi = 10.1080/00033790.2015.1034588 | pmid = 26221834 | quote = The great majority of astronomers working in the early nineteenth century were not interested in stars as physical objects. Far from being bodies with physical properties to be investigated, the stars were seen as markers measured in order to construct an accurate, detailed and precise background against which solar, lunar and planetary motions could be charted, primarily for terrestrial applications.|bibcode = 2015AnSci..72..417C | doi-access =  | s2cid = 205397708 }}</ref><ref>{{Citation | last = Donnelly | first = Kevin | date = September 2014 | title = On the boredom of science: positional astronomy in the nineteenth century | journal = The British Journal for the History of Science | volume = 47 | issue = 3 | pages = 479–503 | doi = 10.1017/S0007087413000915 | s2cid = 146382057 | url = https://zenodo.org/record/999531 }}</ref>  A new astronomy, soon to be called astrophysics, began to emerge when [[William Hyde Wollaston]] and [[Joseph von Fraunhofer]] independently discovered that, when decomposing the light from the Sun, a multitude of [[Fraunhofer lines|dark lines]] (regions where there was less or no light) were observed in the [[Visible spectrum|spectrum]].<ref>{{cite book | last=Hearnshaw|first=J.B. | title=The analysis of starlight | date=1986 | publisher=Cambridge University Press | location=Cambridge | isbn=978-0-521-39916-6 | pages=23–29}}</ref> By 1860 the physicist, [[Gustav Kirchhoff]], and the chemist, [[Robert Bunsen]], had demonstrated that the [[Absorption spectroscopy#Absorption spectrum|dark lines]] in the solar spectrum corresponded to [[Emission spectrum|bright lines]] in the spectra of known gases, specific lines corresponding to unique [[chemical element]]s.<ref>{{citation | last = Kirchhoff | first = Gustav | author-link = Gustav Kirchhoff |title=Ueber die Fraunhofer'schen Linien | journal=Annalen der Physik | volume=185 |issue=1 | pages=148–150 | date=1860 | bibcode = 1860AnP...185..148K | doi=10.1002/andp.18601850115| url = https://zenodo.org/record/1423666 }}</ref> Kirchhoff deduced that the dark lines in the solar spectrum are caused by [[absorption (optics)|absorption]] by [[chemical elements]] in the Solar atmosphere.<ref>{{citation | last = Kirchhoff | first = Gustav | author-link = Gustav Kirchhoff | title=Ueber das Verhältniss zwischen dem Emissionsvermögen und dem Absorptionsvermögen der Körper für Wärme und Licht  |journal=Annalen der Physik  |volume=185 | issue=2 | pages=275–301 | date=1860 | bibcode = 1860AnP...185..275K | doi=10.1002/andp.18601850205| url=https://zenodo.org/record/1423668 | doi-access=free }}</ref> In this way it was proved that the chemical elements found in the Sun and stars were also found on Earth.<!--belief was same ratio-->

Among those who extended the study of solar and stellar spectra was [[Norman Lockyer]], who in 1868 detected radiant, as well as dark lines in solar spectra. Working with chemist [[Edward Frankland]] to investigate the spectra of elements at various temperatures and pressures, he could not associate a yellow line in the solar spectrum with any known elements.  He thus claimed the line represented a new element, which was called [[helium]], after the Greek [[Helios]], the Sun personified.<ref>{{citation | last = Cortie | first = A. L. | title = Sir Norman Lockyer, 1836 – 1920 | journal = The Astrophysical Journal | year = 1921 | volume = 53 | pages = 233–248 | bibcode = 1921ApJ....53..233C | doi=10.1086/142602}}</ref><ref>{{Citation | last = Jensen | first = William B.|author1-link=William B. Jensen | title = Why Helium Ends in "-ium" | journal = Journal of Chemical Education | volume = 81 | issue = 7 | pages = 944–945 | year = 2004 | url = http://www.che.uc.edu/jensen/W.%20B.%20Jensen/Reprints/115.%20Helium.pdf | doi=10.1021/ed081p944 | bibcode = 2004JChEd..81..944J }}</ref>

In 1885, [[Edward C. Pickering]] undertook an ambitious program of stellar spectral classification at [[Harvard College Observatory]], in which a team of [[Harvard Computers|woman computers]], notably [[Williamina Fleming]], [[Antonia Maury]], and [[Annie Jump Cannon]], classified the spectra recorded on photographic plates. By 1890, a catalog of over 10,000 stars had been prepared that grouped them into thirteen spectral types. Following Pickering's vision, by 1924 Cannon expanded the [[Henry Draper Catalogue|catalog]] to nine volumes and over a quarter of a million stars, developing the [[Stellar classification#Harvard spectral classification|Harvard Classification Scheme]] which was accepted for worldwide use in 1922.<ref>{{Citation|last1=Hetherington |first1=Norriss S. |last2=McCray |first2=W. Patrick |author2-link=W. Patrick McCray |editor-last=Weart |editor-first=Spencer R. |editor-link=Spencer R. Weart |title=Spectroscopy and the Birth of Astrophysics |publisher=American Institute of Physics, Center for the History of Physics |url=https://www.aip.org/history/cosmology/tools/tools-spectroscopy.htm |access-date=July 19, 2015 |url-status=dead |archive-url=https://web.archive.org/web/20150907133751/https://www.aip.org/history/cosmology/tools/tools-spectroscopy.htm |archive-date=September 7, 2015 }}</ref>

In 1895, [[George Ellery Hale]] and [[James E. Keeler]], along with a group of ten associate editors from Europe and the United States,<ref name = Hale1895>{{Citation | last = Hale | first = George Ellery | title = The Astrophysical Journal | author-link = George Ellery Hale | journal = The Astrophysical Journal | volume = 1 | issue = 1 | pages = 80–84 | bibcode = 1895ApJ.....1...80H | doi = 10.1086/140011| year = 1895 }}</ref>  established [[The Astrophysical Journal|''The Astrophysical Journal: An International Review of Spectroscopy and Astronomical Physics'']].<ref>{{List journal | journal = The Astrophysical Journal | volume = 1 | issue = 1 }}</ref>  It was intended that the journal would fill the gap between journals in astronomy and physics, providing a venue for publication of articles on astronomical applications of the spectroscope; on laboratory research closely allied to astronomical physics, including wavelength determinations of metallic and gaseous spectra and experiments on radiation and absorption; on theories of the Sun, Moon, planets, comets, meteors, and nebulae; and on instrumentation for telescopes and laboratories.<ref name = Hale1895/>

Around 1920, following the discovery of the [[Hertzsprung–Russell diagram]] still used as the basis for classifying stars and their evolution, [[Arthur Eddington]] anticipated the discovery and mechanism of [[nuclear fusion]] processes in [[star]]s, in his paper ''The Internal Constitution of the Stars''.<ref name=eddington>{{ Citation | last = Eddington | first = A. S. | author-link = Arthur Eddington | date = October 1920 | title = The Internal Constitution of the Stars | journal = The Scientific Monthly | volume = 11 | issue = 4 | pages = 297–303 | doi = 10.1126/science.52.1341.233 | jstor = 6491 | pmid = 17747682 | bibcode = 1920Sci....52..233E | url = https://zenodo.org/record/1429642 }}</ref><ref name=eddington2>{{cite journal | bibcode = 1916MNRAS..77...16E | title = On the radiative equilibrium of the stars | journal=Monthly Notices of the Royal Astronomical Society | volume=77 | pages=16–35 | last1=Eddington|first1=A. S. | author-link = Arthur Eddington | year=1916 | doi=10.1093/mnras/77.1.16| doi-access=free }}</ref> At that time, the source of stellar energy was a complete mystery; Eddington correctly speculated that the source was [[nuclear fusion|fusion]] of hydrogen into helium, liberating enormous energy according to Einstein's equation ''E = mc<sup>2</sup>''. This was a particularly remarkable development since at that time fusion and thermonuclear energy, and even that stars are largely composed of [[hydrogen]] (see [[metallicity]]), had not yet been discovered.<ref>{{Cite book|title=Fusion|last1=McCracken|editor-last2=Stott|editor-first2=Peter |editor-last=McCracken|editor-first=Garry |last2=Stott|first2=Peter |first1=Garry|url=http://www.sciencedirect.com/science/article/pii/B9780123846563000027 |language=en|doi=10.1016/b978-0-12-384656-3.00002-7 |pages=13|isbn=978-0-12-384656-3|location=Boston|year=2013|publisher=Academic Press|quote=Eddington had realized that there would be a mass loss if four hydrogen atoms combined to form a single helium atom. Einstein's equivalence of mass and energy led directly to the suggestion that this could be the long-sought process that produces the energy in the stars! It was an inspired guess, all the more remarkable because the structure of the nucleus and the mechanisms of these reactions were not fully understood.|edition=Second}}</ref>

In 1925 Cecilia Helena Payne (later [[Cecilia Payne-Gaposchkin]]) wrote an influential doctoral dissertation at [[Radcliffe College]], in which she applied [[Saha ionization equation|Saha's ionization theory]] to stellar atmospheres to relate the spectral classes to the temperature of stars.<ref>{{citation  |last=Payne |first=C. H. |year=1925 |title=Stellar Atmospheres; A Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars  |type=PhD Thesis  |publisher=[[Radcliffe College]]  | location = Cambridge, Massachusetts | publication-date = 1925  |bibcode=1925PhDT.........6P }}</ref>  Most significantly, she discovered that hydrogen and helium were the principal components of stars, not the composition of Earth. Despite Eddington's suggestion, discovery was so unexpected that her dissertation readers (including [[Henry Norris Russell|Russell]]) convinced her to modify the conclusion before publication. However, later research confirmed her discovery.<ref>{{citation | last = Haramundanis | first = Katherine | editor-last = Hockey | editor-first = Thomas | editor2-last = Trimble | editor2-first = Virginia | editor2-link = Virginia Louise Trimble | editor3-last = Williams | editor3-first = Thomas R. | title = Biographical Encyclopedia of Astronomers | chapter = Payne-Gaposchkin [Payne], Cecilia Helena | access-date = July 19, 2015 |year=2007 | chapter-url=https://books.google.com/books?id=t-BF1CHkc50C | pages=876–878 | publisher = Springer | location = New York | isbn = 978-0-387-30400-7 }}</ref><ref>{{cite web |author1=Steven Soter and Neil deGrasse Tyson |title=Cecilia Payne and the Composition of the Stars |url=https://www.amnh.org/learn-teach/curriculum-collections/cosmic-horizons-book/cecilia-payne-profile |publisher=[[American Museum of Natural History]] |date=2000}}</ref>

By the end of the 20th century, studies of astronomical spectra had expanded to cover wavelengths extending from radio waves through optical, x-ray, and gamma wavelengths.<ref>{{cite conference |mode=cs2 |last1=Biermann |first1=Peter L. |last2=Falcke |first2=Heino |author2-link=Heino Falcke |editor-last=Panvini |editor-first=Robert S. |editor2-last=Weiler |editor2-first=Thomas J. |date=1998 |book-title=Fundamental particles and interactions: Frontiers in contemporary physics an international lecture and workshop series. AIP Conference Proceedings |title=Frontiers of Astrophysics: Workshop Summary |volume=423 |publisher=American Institute of Physics |pages=236–248 |isbn=1-56396-725-1 |bibcode=1998AIPC..423..236B |doi=10.1063/1.55085|arxiv=astro-ph/9711066 }}</ref> In the 21st century, it further expanded to include observations based on [[gravitational waves]].