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{{Short description|Subfield of astronomy}}
{{for|the academic journal|Astrophysics (journal)}}
[[Image:NIEdot362.jpg|thumb|Early 1900s comparison of elemental, solar, and stellar spectra]]
{{TopicTOC-Physics}}
'''Astrophysics''' is a science that employs the methods and principles of [[physics]] and [[chemistry]] in the study of [[astronomical object]]s and phenomena.<ref>{{cite book |title=Astrophysics in a Nutshell |first=Dan |last=Maoz |year= 2016 |publisher=Princeton University Press |pages=272 |isbn=978-1400881178 |url=https://books.google.com/books?id=bmBeCwAAQBAJ&pg=PA1}}</ref><ref>{{cite web | title=astrophysics | publisher=Merriam-Webster, Incorporated | url=http://www.merriam-webster.com/dictionary/astrophysics | access-date=2011-05-22 | archive-url= https://web.archive.org/web/20110610085146/http://www.merriam-webster.com/dictionary/astrophysics| archive-date= 10 June 2011 | url-status= live}}</ref> As one of the founders of the discipline, [[James Edward Keeler|James Keeler]], said,
Among the subjects studied are the [[Sun]] ([[solar physics]]), other [[star]]s, [[galaxy|galaxies]], [[extrasolar planet]]s, the [[interstellar medium]] and the [[cosmic microwave background]].<ref name="nasa.gov">{{cite web|url=https://science.nasa.gov/astrophysics/focus-areas/|title=Focus Areas – NASA Science|work=nasa.gov|access-date=2017-07-12|archive-date=2017-05-16|archive-url=https://web.archive.org/web/20170516154030/https://science.nasa.gov/astrophysics/focus-areas|url-status=dead}}</ref><ref>{{cite encyclopedia|url=https://www.britannica.com/EBchecked/topic/40047/astronomy|title=astronomy|encyclopedia=Encyclopædia Britannica|date=29 May 2023 }}</ref> Emissions from these objects are examined across all parts of the [[electromagnetic spectrum]], and the properties examined include [[luminosity]], [[density]], [[temperature]], and [[chemistry|chemical]] composition. Because astrophysics is a very broad subject, ''astrophysicists'' apply concepts and methods from many disciplines of physics, including [[classical mechanics]], [[electromagnetism]], [[statistical mechanics]], [[thermodynamics]], [[quantum mechanics]], [[theory of relativity|relativity]], [[nuclear physics|nuclear]] and [[particle physics]], and [[atomic, molecular, and optical physics|atomic and molecular physics]]. In practice, modern astronomical research often involves a substantial amount of work in the realms of [[Theoretical physics|theoretical]] and observational physics. Some areas of study for astrophysicists include their attempts to determine the properties of [[dark matter]], [[dark energy]], [[black holes]], and other [[celestial bodies]]; and the [[Cosmogony|origin]] and [[ultimate fate of the universe]].<ref name="nasa.gov"/> Topics also studied by theoretical astrophysicists include [[Formation and evolution of the Solar System|Solar System formation and evolution]]; [[stellar dynamics]] and [[Stellar evolution|evolution]]; [[galaxy formation and evolution]]; [[magnetohydrodynamics]]; [[large-scale structure of the universe|large-scale structure]] of [[matter]] in the universe; origin of [[cosmic ray]]s; [[general relativity]], [[special relativity]], [[quantum cosmology|quantum]] and [[physical cosmology]] (the physical study of the largest-scale structures of the universe), including [[string theory|string]] cosmology and [[astroparticle physics]].
==History==
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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]] |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>
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[[Observational astronomy]] is a division of the astronomical science that is concerned with recording and interpreting data, in contrast with [[theoretical astrophysics]], which is mainly concerned with finding out the measurable implications of physical [[model (abstract)|models]]. It is the practice of observing [[celestial object]]s by using [[telescope]]s and other astronomical apparatus.
* [[Radio astronomy]] studies radiation with a [[wavelength]] greater than a few millimeters. Example areas of study are [[radio waves]], usually emitted by cold objects such as [[interstellar gas]] and dust clouds; the cosmic microwave background radiation which is the [[redshift]]ed light from the [[Big Bang]]; [[pulsar]]s, which were first detected at [[microwave]] frequencies. The study of these waves requires very large [[radio telescope]]s.
* [[Infrared astronomy]] studies radiation with a wavelength that is too long to be visible to the naked eye but is shorter than radio waves. Infrared observations are usually made with telescopes similar to the familiar [[optical]] telescopes. Objects colder than stars (such as planets) are normally studied at infrared frequencies.
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== Popularization ==
The roots of astrophysics can be found in the seventeenth century emergence of a unified physics, in which the same laws applied to the celestial and terrestrial realms.<ref name = Burtt>{{Citation | last = Burtt | first = Edwin Arthur | author-link = Edwin Arthur Burtt | publication-date = 2003 | orig-year = First published 1924 | title = The Metaphysical Foundations of Modern Science | edition = second revised | publisher = Dover Publications | location = Mineola, NY | pages = 30, 41, 241–2 | isbn = 978-0-486-42551-1 | url = https://books.google.com/books?id=G9WBMa1Rz_kC | year = 2003 }}</ref> There were scientists who were qualified in both physics and astronomy who laid the firm foundation for the current science of astrophysics. In modern times, students continue to be drawn to astrophysics due to its popularization by the [[Royal Astronomical Society]] and notable [[Science education|educators]] such as prominent professors [[Lawrence Krauss]], [[Subrahmanyan Chandrasekhar]], [[Stephen Hawking]], [[Hubert Reeves]], [[Carl Sagan]] and [[Patrick Moore]]. The efforts of the early, late, and present scientists continue to attract young people to study the history and science of astrophysics.<ref>{{Cite web |author=D. Mark Manley |title=Famous Astronomers and Astrophysicists |url=http://cnr2.kent.edu/~manley/astronomers.html |publisher=[[Kent State University]] |date=2012 |access-date=2015-07-17}}</ref><ref>{{Cite web |author=The science.ca team |title=Hubert Reeves – Astronomy, Astrophysics and Space Science |url=http://www.science.ca/scientists/scientistprofile.php?pID=213 |publisher=GCS Research Society |date=2015 |access-date=2015-07-17}}</ref><ref>{{Cite web |title=Neil deGrasse Tyson |url=http://www.haydenplanetarium.org/tyson/ |publisher=[[Hayden Planetarium]] |date=2015 |access-date=2015-07-17}}</ref>
The television sitcom show ''[[The Big Bang Theory]]'' popularized the field of astrophysics with the general public, and featured some well known scientists like [[Stephen Hawking]] and [[Neil deGrasse Tyson
==See also==
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* {{Annotated link|High-energy astronomy}}
* {{Annotated link|List of important publications in physics#Astrophysics|Important publications in astrophysics}}
* {{Annotated link|List of astronomers|fallback=(includes astrophysicists)}}
* {{Annotated link|Neutrino astronomy|fallback=(future prospects)}}
* {{Annotated link|Timeline of gravitational physics and relativity}}
* {{Annotated link|Timeline of knowledge about galaxies, clusters of galaxies, and large-scale structure}}
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