Editing Astronomy (section)

== Specific subfields ==

=== Astrophysics ===
{{main |Astrophysics}}
[[File:Observable Universe logarithmic illustration (circular layout english annotations).png|thumb|Astrophysics applies [[physics]] and [[chemistry]] to understand the measurements made by astronomy. Representation of the Observable Universe that includes images from [[Hubble Space Telescope|Hubble]] and other [[List of optical telescopes|telescopes]].]]

Astrophysics is the branch of astronomy that employs the principles of physics and [[chemistry]] "to ascertain the nature of the [[astronomical object]]s, rather than their positions or motions in space".<ref>{{Cite journal | last = Keeler | first = James E. | author-link = James E. Keeler | title = The Importance of Astrophysical Research and the Relation of Astrophysics to the Other Physical Sciences | journal = The Astrophysical Journal | volume = 6 | issue = 4 | pages = 271–88 | date = November 1897 | bibcode = 1897ApJ.....6..271K |doi = 10.1086/140401| pmid = 17796068 | quote =[Astrophysics] is closely allied on the one hand to astronomy, of which it may properly be classed as a branch, and on the other hand to chemistry and physics.... It seeks to ascertain the nature of the heavenly bodies, rather than their positions or motions in space—''what'' they are, rather than ''where'' they are.... That which is perhaps most characteristic of astrophysics is the special prominence which it gives to the study of radiation.| doi-access = free }}</ref><ref>{{cite web | title=astrophysics | publisher=Merriam-Webster, Incorporated | url=http://www.merriam-webster.com/dictionary/astrophysics | access-date=22 May 2011 | archive-url= https://web.archive.org/web/20110610085146/http://www.merriam-webster.com/dictionary/astrophysics| archive-date= 10 June 2011 | url-status= live}}</ref> Among the objects studied are the [[Sun]], 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=12 November 2018|archive-date=16 May 2017|archive-url=https://web.archive.org/web/20170516154030/https://science.nasa.gov/astrophysics/focus-areas|url-status=live}}</ref><ref>{{cite encyclopedia|url=https://www.britannica.com/EBchecked/topic/40047/astronomy|title=astronomy|encyclopedia=Encyclopædia Britannica|access-date=12 November 2018|archive-date=10 May 2015|archive-url=https://web.archive.org/web/20150510024116/https://www.britannica.com/EBchecked/topic/40047/astronomy|url-status=live}}</ref> Their emissions 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'' typically apply many disciplines of physics, including [[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]].{{cn|date=March 2025}}

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]], and [[black holes]]; whether or not [[time travel]] is possible, [[wormhole]]s can form, or the [[multiverse]] exists; 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]] and [[physical cosmology]], including [[string theory|string]] cosmology and [[astroparticle physics]].{{cn|date=March 2025}}

=== Astrochemistry ===
{{main|Astrochemistry}}
Astrochemistry is the study of the abundance and reactions of [[molecule]]s in the [[Universe]], and their interaction with [[radiation]]. The discipline is an overlap of astronomy and [[chemistry]]. The word "astrochemistry" may be applied to both the [[Solar System]] and the [[interstellar medium]]. The study of the abundance of elements and [[isotope]] ratios in Solar System objects, such as [[meteorite]]s, is also called [[cosmochemistry]], while the study of interstellar atoms and molecules and their interaction with radiation is sometimes called molecular astrophysics. The formation, atomic and chemical composition, evolution and fate of [[molecular cloud|molecular gas clouds]] is of special interest, because it is from these clouds that solar systems form. Studies in this field contribute to the understanding of the [[formation of the Solar System]], Earth's origin and geology, [[abiogenesis]], and the origin of climate and oceans.<ref>{{Cite news|url=https://www.cfa.harvard.edu/research/amp-rg/astrochemistry|title=Astrochemistry|date=15 July 2013|newspaper=www.cfa.harvard.edu/|access-date=20 November 2016|archive-url=https://web.archive.org/web/20161120211934/https://www.cfa.harvard.edu/research/amp-rg/astrochemistry|archive-date=20 November 2016}}</ref>

=== Astrobiology ===
{{main|Astrobiology}}
Astrobiology is an interdisciplinary scientific field concerned with the [[abiogenesis|origins]], [[Protocell|early evolution]], distribution, and future of [[life]] in the [[universe]]. Astrobiology considers the question of whether [[extraterrestrial life]] exists, and how humans can detect it if it does.<ref name="about">{{cite web| url=http://astrobiology.nasa.gov/about-astrobiology/ |title=About Astrobiology |access-date=20 October 2008 |date=21 January 2008 |work=NASA Astrobiology Institute |publisher=NASA | archive-url= https://web.archive.org/web/20081011192341/http://astrobiology.nasa.gov/about-astrobiology/| archive-date= 11 October 2008}}</ref> The term exobiology is similar.<ref>[http://www.merriam-webster.com/dictionary/exobiology Merriam Webster Dictionary entry "Exobiology"] {{Webarchive|url=https://web.archive.org/web/20180904084642/https://www.merriam-webster.com/dictionary/exobiology |date=4 September 2018 }} (accessed 11 April 2013)</ref>

Astrobiology makes use of [[molecular biology]], [[biophysics]], [[biochemistry]], [[chemistry]], astronomy, [[physical cosmology]], [[exoplanetology]] and [[geology]] to investigate the possibility of life on other worlds and help recognize [[biosphere]]s that might be different from that on Earth.<ref>{{cite book |title=The life and death of planet Earth |last1=Ward |first1=P.D. |author2=Brownlee, D. |date=2004 |publisher=Owl Books |location=New York |isbn=978-0-8050-7512-0 }}</ref> [[Abiogenesis|The origin]] and early evolution of life is an inseparable part of the discipline of astrobiology.<ref>{{cite web |url=https://link.springer.com/journal/11084 |title=Origins of Life and Evolution of Biospheres |work=Journal: Origins of Life and Evolution of Biospheres |access-date=6 April 2015 |archive-date=8 February 2020 |archive-url=https://web.archive.org/web/20200208140912/https://link.springer.com/journal/11084 |url-status=live }}</ref> Astrobiology concerns itself with interpretation of existing [[Scientific method|scientific data]], and although speculation is entertained to give context, astrobiology concerns itself primarily with [[hypotheses]] that fit firmly into existing [[scientific theories]].{{cn|date=March 2025}}

This [[interdisciplinary]] field encompasses research on the origin of [[planetary system]]s, origins of [[List of interstellar and circumstellar molecules|organic compounds in space]], rock-water-carbon interactions, [[abiogenesis]] on Earth, [[planetary habitability]], research on [[biosignature]]s for life detection, and studies on the potential for [[extremophile|life to adapt to challenges]] on Earth and in [[outer space]].<ref name="Goals2016">{{cite news |url=http://astrobiology.com/2016/03/release-of-the-first-roadmap-for-european-astrobiology.html |title=Release of the First Roadmap for European Astrobiology |work=European Science Foundation |publisher=Astrobiology Web |date=29 March 2016 |access-date=2 April 2016 |archive-date=10 June 2020 |archive-url=https://web.archive.org/web/20200610010327/http://astrobiology.com/2016/03/release-of-the-first-roadmap-for-european-astrobiology.html |url-status=live }}</ref><ref name="NYT-20151218-jc">{{cite news |last=Corum |first=Jonathan |title=Mapping Saturn's Moons |url=https://www.nytimes.com/interactive/2015/12/18/science/space/nasa-cassini-maps-saturns-moons.html |date=18 December 2015 |work=[[The New York Times]] |access-date=18 December 2015 |archive-date=20 May 2020 |archive-url=https://web.archive.org/web/20200520124847/https://www.nytimes.com/interactive/2015/12/18/science/space/nasa-cassini-maps-saturns-moons.html |url-status=live }}</ref><ref>{{cite news | last = Cockell | first = Charles S. | title = How the search for aliens can help sustain life on Earth | date = 4 October 2012 | url = https://edition.cnn.com/2012/10/02/world/europe/astrobiology-aliens-environment-opinion/index.html?hpt=hp_c4 | work = CNN News | access-date = 8 October 2012 | archive-date = 10 September 2016 | archive-url = https://web.archive.org/web/20160910182606/http://edition.cnn.com/2012/10/02/world/europe/astrobiology-aliens-environment-opinion/index.html?hpt=hp_c4 | url-status = live }}</ref>

=== Physical cosmology ===
{{Nature timeline}}
{{Main|Physical cosmology}}

[[Cosmology]] (from the Greek {{lang|grc|κόσμος}} ({{transliteration|grc|kosmos}}) "world, universe" and {{lang|grc|λόγος}} ({{transliteration|grc|logos}}) "word, study" or literally "logic") could be considered the study of the Universe as a whole.{{cn|date=March 2025}}

[[File:Hubble Extreme Deep Field (full resolution).png|thumb|[[Hubble Extreme Deep Field]]]]

Observations of the [[large-scale structure of the Universe]], a branch known as [[physical cosmology]], have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the [[Big Bang]], wherein our Universe began at a single [[point in time]], and thereafter [[metric expansion of space|expanded]] over the course of 13.8 billion years<ref>{{cite web
|title = Cosmic Detectives
|url = http://www.esa.int/Our_Activities/Space_Science/Cosmic_detectives
|publisher = The European Space Agency (ESA)
|date = 2 April 2013
|access-date = 15 April 2013
|archive-date = 11 February 2019
|archive-url = https://web.archive.org/web/20190211204726/http://www.esa.int/Our_Activities/Space_Science/Cosmic_detectives
|url-status = live
}}</ref> to its present condition.<ref name=Dodelson2003/> The concept of the Big Bang can be traced back to the discovery of the [[Cosmic microwave background radiation|microwave background radiation]] in 1965.<ref name=Dodelson2003>{{cite book|last=Dodelson|first=Scott|title=Modern cosmology|date=2003|isbn=978-0-12-219141-1|publisher=[[Academic Press]]|pages=1–22}}</ref>

In the course of this expansion, the Universe underwent several evolutionary stages. In the very early moments, it is theorized that the Universe experienced a very rapid [[cosmic inflation]], which homogenized the starting conditions. Thereafter, [[Big Bang nucleosynthesis|nucleosynthesis]] produced the elemental abundance of the early Universe.<ref name=Dodelson2003/> (See also [[nucleocosmochronology]].){{cn|date=March 2025}}

When the first neutral [[atom]]s formed from a sea of primordial ions, space became transparent to radiation, releasing the energy viewed today as the microwave background radiation. The expanding Universe then underwent a Dark Age due to the lack of stellar energy sources.<ref name="cosmology 101">{{cite web|last = Hinshaw|first = Gary|date = 13 July 2006|url=http://map.gsfc.nasa.gov/m_uni.html|title = Cosmology 101: The Study of the Universe|publisher = NASA WMAP|access-date =10 August 2006| archive-url= https://web.archive.org/web/20060813053535/http://map.gsfc.nasa.gov/m_uni.html| archive-date= 13 August 2006 | url-status= live}}</ref>

A hierarchical structure of matter began to form from minute variations in the mass density of space. Matter accumulated in the densest regions, forming clouds of gas and the earliest stars, the [[Population III stars]]. These massive stars triggered the [[reionization]] process and are believed to have created many of the heavy elements in the early Universe, which, through nuclear decay, create lighter elements, allowing the cycle of nucleosynthesis to continue longer.<ref>Dodelson, 2003, pp. 216–61</ref>

Gravitational aggregations clustered into filaments, leaving voids in the gaps. Gradually, organizations of gas and dust merged to form the first primitive galaxies. Over time, these pulled in more matter, and were often organized into [[Galaxy groups and clusters|groups and clusters]] of galaxies, then into larger-scale superclusters.<ref>{{cite web|url=http://www.damtp.cam.ac.uk/user/gr/public/gal_lss.html|title = Galaxy Clusters and Large-Scale Structure|publisher = University of Cambridge|access-date =8 September 2006| archive-url= https://web.archive.org/web/20061010041120/http://www.damtp.cam.ac.uk/user/gr/public/gal_lss.html| archive-date= 10 October 2006 | url-status= live}}</ref>

Fundamental to the structure of the Universe is the existence of [[dark matter]] and [[dark energy]]. These are now thought to be its dominant components, forming 96% of the mass of the Universe. For this reason, much effort is expended in trying to understand the physics of these components.<ref>{{cite web|last = Preuss|first = Paul|url=http://www.lbl.gov/Science-Articles/Archive/dark-energy.html|title = Dark Energy Fills the Cosmos|publisher = U.S. Department of Energy, Berkeley Lab|access-date =8 September 2006| archive-url= https://web.archive.org/web/20060811215815/http://www.lbl.gov/Science-Articles/Archive/dark-energy.html| archive-date= 11 August 2006 | url-status= live}}</ref>

=== Extragalactic astronomy ===

[[File:grav.lens1.arp.750pix.jpg|thumb|This image shows several blue, loop-shaped objects that are multiple images of the same galaxy, duplicated by the [[gravitational lens]] effect of the cluster of yellow galaxies near the middle of the photograph. The lens is produced by the cluster's gravitational field that bends light to magnify and distort the image of a more distant object.]]
{{Main|Extragalactic astronomy}}
The study of objects outside our galaxy is a branch of astronomy concerned with the [[Galaxy formation and evolution|formation and evolution of galaxies]], their morphology (description) and [[Galaxy morphological classification|classification]], the observation of [[Active galaxy|active galaxies]], and at a larger scale, the [[Galaxy groups and clusters|groups and clusters of galaxies]]. Finally, the latter is important for the understanding of the [[large-scale structure of the cosmos]].<ref name=":0" />

Most [[galaxy|galaxies]] are organized into distinct shapes that allow for classification schemes. They are commonly divided into [[spiral galaxy|spiral]], [[elliptical galaxy|elliptical]] and [[irregular galaxy|Irregular]] galaxies.<ref>{{cite web|last = Keel|first = Bill|date = 1 August 2006|url=http://www.astr.ua.edu/keel/galaxies/classify.html|title = Galaxy Classification|publisher = University of Alabama|access-date =8 September 2006| archive-url= https://web.archive.org/web/20060901074027/http://www.astr.ua.edu/keel/galaxies/classify.html| archive-date= 1 September 2006 | url-status= live}}</ref>

As the name suggests, an elliptical galaxy has the cross-sectional shape of an [[ellipse]]. The stars move along [[randomness|random]] orbits with no preferred direction. These galaxies contain little or no interstellar dust, few star-forming regions, and older stars.<ref name=":0" />{{Rp|pages=877–878}} Elliptical galaxies may have been formed by other galaxies merging.<ref name=":0" />{{Rp|page=939}}

A spiral galaxy is organized into a flat, rotating disk, usually with a prominent bulge or bar at the center, and trailing bright arms that spiral outward. The arms are dusty regions of star formation within which massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the [[Milky Way]] and one of our nearest galaxy neighbors, the [[Andromeda Galaxy]], are spiral galaxies.<ref name=":0" />{{Rp|page=875}}

Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical.<ref name=":0" />{{Rp|page=879}} About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction.<ref>{{Cite web |date=2016-08-08 |title=A lopsided lynx |url=https://esahubble.org/images/potw1632a/ |access-date=2023-03-17 |website=esahubble.org |publisher=[[European Space Agency]] |language=en |archive-date=9 July 2021 |archive-url=https://web.archive.org/web/20210709183618/https://esahubble.org/images/potw1632a/ |url-status=live }}</ref>

An active galaxy is a formation that emits a significant amount of its energy from a source other than its stars, dust and gas. It is powered by a compact region at the core, thought to be a supermassive black hole that is emitting radiation from in-falling material.<ref name=":0" />{{Rp|page=907}} A [[radio galaxy]] is an active galaxy that is very luminous in the radio portion of the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies that emit shorter frequency, high-energy radiation include [[Seyfert galaxy|Seyfert galaxies]], [[quasar]]s, and [[blazar]]s. Quasars are believed to be the most consistently luminous objects in the known universe.<ref>{{cite web|url=http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html|title=Active Galaxies and Quasars|publisher=NASA|access-date=17 November 2016|archive-url=https://web.archive.org/web/20060831033713/http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html|archive-date=31 August 2006 }}</ref>

The [[large-scale structure of the cosmos]] is represented by groups and clusters of galaxies. This structure is organized into a hierarchy of groupings, with the largest being the [[supercluster]]s. The collective matter is formed into [[Galaxy filament|filaments]] and walls, leaving large [[Void (astronomy)|voids]] between.<ref name="evolving universe">{{cite book|author=[[Michael Zeilik]]|title=Astronomy: The Evolving Universe|edition=8th|publisher=Wiley|date=2002|isbn=978-0-521-80090-7}}</ref>

=== Galactic astronomy ===
[[File:Galactic longitude.JPG|thumb|A diagram of the Sun's location in the Milky Way, the angles represent longitudes in the [[galactic coordinate system]]]]

{{Main|Galactic astronomy}}
The [[Solar System]] orbits within the [[Milky Way]], a [[barred spiral galaxy]] that is a prominent member of the [[Local Group]] of galaxies. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is located within the dusty outer arms, there are large portions of the Milky Way that are obscured from view.<ref name=":0" />{{Rp|pages=837–842,944}}

In the center of the Milky Way is the core, a bar-shaped bulge with what is believed to be a [[supermassive black hole]] at its center. This is surrounded by four primary arms that spiral from the core. This is a region of active star formation that contains many younger, [[Stellar population|population I]] stars. The disk is surrounded by a [[Galactic spheroid|spheroid halo]] of older, [[population II]] stars, as well as relatively dense concentrations of stars known as [[globular cluster]]s.<ref>{{cite web|url=http://www.mpe.mpg.de/ir/GC/index.php|title=The Galactic Centre|last=Ott|first=Thomas|date=24 August 2006|publisher=Max-Planck-Institut für extraterrestrische Physik|access-date=17 November 2016|archive-url=https://web.archive.org/web/20060904140550/http://www.mpe.mpg.de/ir/GC/index.php|archive-date=4 September 2006 }}</ref>

Between the stars lies the [[interstellar medium]], a region of sparse matter. In the densest regions, [[molecular cloud]]s of [[Hydrogen|molecular hydrogen]] and other elements create star-forming regions. These begin as a compact [[pre-stellar core]] or [[dark nebula]]e, which concentrate and collapse (in volumes determined by the [[Jeans length]]) to form compact protostars.<ref name=Smith2004>{{cite book|first=Michael David|last=Smith|date=2004|pages=53–86|title=The Origin of Stars|chapter=Cloud formation, Evolution and Destruction|publisher=Imperial College Press|isbn=978-1-86094-501-4|chapter-url=https://books.google.com/books?id=UVgBoqZg8a4C|access-date=26 August 2020|archive-date=13 August 2021|archive-url=https://web.archive.org/web/20210813210429/https://books.google.com/books?id=UVgBoqZg8a4C|url-status=live}}</ref>

As the more massive stars appear, they transform the cloud into an [[H II region]] (ionized atomic hydrogen) of glowing gas and plasma. The [[Solar wind|stellar wind]] and supernova explosions from these stars eventually cause the cloud to disperse, often leaving behind one or more young [[open cluster]]s of stars. These clusters gradually disperse, and the stars join the population of the Milky Way.<ref>{{cite book|first=Michael David|last=Smith|date=2004|pages=185–99|title=The Origin of Stars|chapter=Massive stars|publisher=Imperial College Press|isbn=978-1-86094-501-4|chapter-url=https://books.google.com/books?id=UVgBoqZg8a4C|access-date=26 August 2020|archive-date=13 August 2021|archive-url=https://web.archive.org/web/20210813210429/https://books.google.com/books?id=UVgBoqZg8a4C|url-status=live}}</ref>

Kinematic studies of matter in the Milky Way and other galaxies have demonstrated that there is more mass than can be accounted for by visible matter. A [[dark matter halo]] appears to dominate the mass, although the nature of this dark matter remains undetermined.<ref>{{cite journal|author=Van den Bergh, Sidney|title=The Early History of Dark Matter|journal=Publications of the Astronomical Society of the Pacific|date=1999|volume=111|issue=760|pages=657–60|doi=10.1086/316369|arxiv = astro-ph/9904251 |bibcode = 1999PASP..111..657V |s2cid=5640064}}</ref>

=== Stellar astronomy ===
[[File:Ant Nebula.jpg|thumb|[[Mz 3]], often referred to as the Ant planetary nebula. Ejecting gas from the dying central star shows symmetrical patterns unlike the chaotic patterns of ordinary explosions.]]
{{Main|Star}}
{{see also|Solar astronomy}}
The study of stars and [[stellar evolution]] is fundamental to our understanding of the Universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior.<ref name=Amos7>Harpaz, 1994, pp. 7–18</ref> [[Star formation]] occurs in dense regions of dust and gas, known as [[Dark nebula|giant molecular clouds]]. When destabilized, cloud fragments can collapse under the influence of gravity, to form a [[protostar]]. A sufficiently dense, and hot, core region will trigger [[nuclear fusion]], thus creating a [[main-sequence star]].<ref name=Smith2004/>

Almost all elements heavier than [[hydrogen]] and [[helium]] were [[nucleosynthesis|created]] inside the cores of stars.<ref name=Amos7/>

The characteristics of the resulting star depend primarily upon its starting mass. The more massive the star, the greater its luminosity, and the more rapidly it fuses its hydrogen fuel into helium in its core. Over time, this hydrogen fuel is completely converted into helium, and the star begins to [[Stellar evolution|evolve]]. The fusion of helium requires a higher core temperature. A star with a high enough core temperature will push its outer layers outward while increasing its core density. The resulting [[red giant]] formed by the expanding outer layers enjoys a brief life span, before the helium fuel in the core is in turn consumed. Very massive stars can also undergo a series of evolutionary phases, as they fuse increasingly heavier elements.<ref name=Amos>Harpaz, 1994</ref>

The final fate of the star depends on its mass, with stars of mass greater than about eight times the Sun becoming core collapse [[supernova]]e;<ref>Harpaz, 1994, pp. 173–78</ref> while smaller stars blow off their outer layers and leave behind the inert core in the form of a [[white dwarf]]. The ejection of the outer layers forms a [[planetary nebula]].<ref>Harpaz, 1994, pp. 111–18</ref> The remnant of a supernova is a dense [[neutron star]], or, if the stellar mass was at least three times that of the Sun, a [[black hole]].<ref name="Cambridge atlas">{{cite book|editor= Audouze, Jean|editor2= Israel, Guy|title=The Cambridge Atlas of Astronomy|edition=3rd|publisher=Cambridge University Press|date=1994|isbn=978-0-521-43438-6}}</ref> Closely orbiting binary stars can follow more complex evolutionary paths, such as mass transfer onto a white dwarf companion that can potentially cause a supernova.<ref>Harpaz, 1994, pp. 189–210</ref> Planetary nebulae and supernovae distribute the "[[metallicity|metals]]" produced in the star by fusion to the interstellar medium; without them, all new stars (and their planetary systems) would be formed from hydrogen and helium alone.<ref>Harpaz, 1994, pp. 245–56</ref>

=== Solar astronomy ===
[[File:Uvsun trace big.jpg|thumb|An [[ultraviolet]] image of the Sun's active [[photosphere]] as viewed by the NASA's [[TRACE]] space telescope.]]
[[File:Observatórium Lomnický štít 1.jpg|thumb|Solar observatory [[Lomnický štít]] ([[Slovakia]]) built in 1962]]
{{See also|Solar telescope}}
At a distance of about eight light-minutes, the most frequently studied star is the [[Sun]], a typical main-sequence [[dwarf star]] of [[stellar class]] G2 V, and about 4.6 billion years (Gyr) old. The Sun is not considered a [[variable star]], but it does undergo periodic changes in activity known as the [[sunspot cycle]]. This is an 11-year oscillation in [[Wolf number|sunspot number]]. Sunspots are regions of lower-than-average temperatures that are associated with intense magnetic activity.<ref name="solar FAQ">{{cite web|url=http://www.talkorigins.org/faqs/faq-solar.html|title=The Solar FAQ|last=Johansson|first=Sverker|author-link=Sverker Johansson|date=27 July 2003|publisher=Talk.Origins Archive|access-date=11 August 2006|archive-url=https://web.archive.org/web/20060907235636/http://www.talkorigins.org/faqs/faq-solar.html|archive-date=7 September 2006 |url-status=live}}</ref>

The Sun has steadily increased in luminosity by 40% since it first became a main-sequence star. The Sun has also undergone periodic changes in luminosity that can have a significant impact on the Earth.<ref name="Environmental issues : essential primary sources.">{{cite web|url=http://catalog.loc.gov/cgi-bin/Pwebrecon.cgi?v3=1&DB=local&CMD=010a+2006000857&CNT=10+records+per+page|title=Environmental issues: essential primary sources|last1=Lerner|first1=K. Lee|first2=Brenda Wilmoth|date=2006|publisher=Thomson Gale|archive-url=https://archive.today/20120710152134/http://catalog.loc.gov/cgi-bin/Pwebrecon.cgi?v3=1&DB=local&CMD=010a+2006000857&CNT=10+records+per+page|archive-date=10 July 2012|last2=Lerner|access-date=17 November 2016}}</ref> The [[Maunder minimum]], for example, is believed to have caused the [[Little Ice Age]] phenomenon during the [[Middle Ages]].<ref name="future-sun">{{cite web|author=Pogge, Richard W. |date=1997 |url=http://www.astronomy.ohio-state.edu/~pogge/Lectures/vistas97.html |title=The Once & Future Sun |format=lecture notes |work=New Vistas in Astronomy |access-date=3 February 2010 |archive-url=https://web.archive.org/web/20050527094435/http://www-astronomy.mps.ohio-state.edu/Vistas/ |archive-date=27 May 2005 }}</ref>

At the center of the Sun is the core region, a volume of sufficient temperature and pressure for [[nuclear fusion]] to occur. Above the core is the [[radiation zone]], where the plasma conveys the energy flux by means of radiation. Above that is the [[convection zone]] where the gas material transports energy primarily through physical displacement of the gas known as convection. It is believed that the movement of mass within the convection zone creates the magnetic activity that generates sunspots.<ref name="solar FAQ" /> The visible outer surface of the Sun is called the [[photosphere]]. Above this layer is a thin region known as the [[chromosphere]]. This is surrounded by a transition region of rapidly increasing temperatures, and finally by the super-heated [[solar corona|corona]].<ref name=":0" />{{Rp|pages=498–502}}

A solar wind of plasma particles constantly streams outward from the Sun until, at the outermost limit of the Solar System, it reaches the [[heliopause (astronomy)|heliopause]]. As the solar wind passes the Earth, it interacts with the [[Earth's magnetic field]] ([[magnetosphere]]) and deflects the solar wind, but traps some creating the [[Van Allen radiation belt]]s that envelop the Earth. The [[aurora (astronomy)|aurora]] are created when solar wind particles are guided by the magnetic flux lines into the Earth's polar regions where the lines then descend into the [[Earth's atmosphere|atmosphere]].<ref>{{cite web|author = Stern, D.P.|author2 = Peredo, M.|date = 28 September 2004|url=http://www-istp.gsfc.nasa.gov/Education/Intro.html|title = The Exploration of the Earth's Magnetosphere|publisher = NASA|access-date =22 August 2006| archive-url= https://web.archive.org/web/20060824003619/http://www-istp.gsfc.nasa.gov/Education/Intro.html| archive-date= 24 August 2006 | url-status= live}}</ref>

=== Planetary science ===
[[File:dust.devil.mars.arp.750pix.jpg|thumb|The black spot at the top is a [[dust devil]] climbing a crater wall on [[Mars]]. This moving, swirling column of [[Atmosphere of Mars|Martian atmosphere]] (comparable to a terrestrial [[tornado]]) created the long, dark streak.]]
{{Main|Planetary science|Planetary geology}}
Planetary science is the study of the assemblage of [[planet]]s, [[natural satellite|moons]], [[dwarf planet]]s, [[comet]]s, [[asteroid]]s, and other bodies orbiting the Sun, as well as extrasolar planets. The [[Solar System]] has been relatively well-studied, initially through telescopes and then later by spacecraft. This has provided a good overall understanding of the formation and evolution of the Sun's planetary system, although many new discoveries are still being made.<ref name="geology">{{cite book|url=http://marswatch.tn.cornell.edu/rsm.html|title=Remote Sensing for the Earth Sciences: Manual of Remote Sensing|date=2004|publisher=John Wiley & Sons|edition=3rd|author=Bell III, J. F.|author2=Campbell, B.A.|author3=Robinson, M.S.|access-date=17 November 2016|archive-url=https://web.archive.org/web/20060811220029/http://marswatch.tn.cornell.edu/rsm.html|archive-date=11 August 2006 }}</ref>

The Solar System is divided into the [[inner Solar System]] (subdivided into the inner planets and the [[asteroid belt]]), the [[outer Solar System]] (subdivided into the outer planets and [[Centaurs (minor planets)|centaurs]]), comets, the trans-Neptunian region (subdivided into the [[Kuiper belt]], and the [[scattered disc]]) and the farthest regions (e.g., boundaries of the [[heliosphere]], and the [[Oort Cloud]], which may extend as far as a light-year). The inner [[terrestrial planet]]s consist of [[Mercury (planet)|Mercury]], [[Venus]], Earth, and [[Mars]]. The outer [[giant planet]]s are the [[gas giant]]s ([[Jupiter]] and [[Saturn]]) and the [[ice giant]]s ([[Uranus]] and [[Neptune]]).<ref name="planets">{{cite web|author = Grayzeck, E.|author2 = Williams, D.R.| date = 11 May 2006|url=http://nssdc.gsfc.nasa.gov/planetary/|title = Lunar and Planetary Science|publisher = NASA|access-date =21 August 2006| archive-url= https://web.archive.org/web/20060820173205/http://nssdc.gsfc.nasa.gov/planetary/| archive-date= 20 August 2006 | url-status= live}}</ref>

The planets were formed 4.6 billion years ago in the [[protoplanetary disk]] that surrounded the early Sun. Through a process that included gravitational attraction, collision, and accretion, the disk formed clumps of matter that, with time, became protoplanets. The [[radiation pressure]] of the [[solar wind]] then expelled most of the unaccreted matter, and only those planets with sufficient mass retained their gaseous atmosphere. The planets continued to sweep up, or eject, the remaining matter during a period of intense bombardment, evidenced by the many [[impact crater]]s on the Moon. During this period, some of the protoplanets may have collided and one such collision may have [[giant impact hypothesis|formed the Moon]].<ref name=Montmerle2006>{{cite journal|last=Montmerle|first=Thierry|author2=Augereau, Jean-Charles|author3= Chaussidon, Marc|title=Solar System Formation and Early Evolution: the First 100 Million Years|journal=Earth, Moon, and Planets|volume=98|issue=1–4|pages=39–95|date=2006|doi=10.1007/s11038-006-9087-5| bibcode=2006EM&P...98...39M|s2cid=120504344|display-authors=etal}}</ref>

Once a planet reaches sufficient mass, the materials of different densities segregate within, during [[planetary differentiation]]. This process can form a stony or metallic core, surrounded by a mantle and an outer crust. The core may include solid and liquid regions, and some planetary cores generate their own [[magnetic field]], which can protect their atmospheres from solar wind stripping.<ref>Montmerle, 2006, pp. 87–90</ref>

A planet or moon's interior heat is produced from the collisions that created the body, by the decay of radioactive materials (''e.g.'' [[uranium]], [[thorium]], and [[26Al|<sup>26</sup>Al]]), or [[tidal acceleration|tidal heating]] caused by interactions with other bodies. Some planets and moons accumulate enough heat to drive geologic processes such as [[volcanism]] and tectonics. Those that accumulate or retain an [[atmosphere]] can also undergo surface [[erosion]] from wind or water. Smaller bodies, without tidal heating, cool more quickly; and their geological activity ceases with the exception of impact cratering.<ref name="new solar system">{{cite book|editor=Beatty, J.K.|editor2=Petersen, C.C.|editor3=Chaikin, A.|title=The New Solar System|publisher=Cambridge press|url=https://books.google.com/books?id=iOezyHMVAMcC&pg=PA70|page=70edition = 4th|date=1999|isbn=978-0-521-64587-4|access-date=26 August 2020|archive-date=30 March 2015|archive-url=https://web.archive.org/web/20150330114739/http://books.google.com/books?id=iOezyHMVAMcC&pg=PA70|url-status=live}}</ref>