View source for Spiral galaxy

{{Short description|Class of galaxy that has spiral structures extending from their cores.}}
[[File:Messier 77 spiral galaxy by HST.jpg|thumb|An example of a spiral galaxy, the [[Astronomy:Messier 77|Messier 77]] (also known as NGC 1068)|upright=1.4]]

'''Spiral galaxies''' form a [[Astronomy:Galaxy morphological classification|class of galaxy]] originally described by [[Biography:Edwin Hubble|Edwin Hubble]] in his 1936 work ''The Realm of the Nebulae''<ref name="Hubble-1936">{{cite book|url=https://books.google.com/books?id=kgiXdDGLpFUC|title=The realm of the nebulae|last=Hubble|first=E.P.|publisher=Yale University Press|year=1936|isbn=9780300025002|series=Mrs. Hepsa Ely Silliman memorial lectures, 25|location=New Haven|oclc=611263346|author-link=Edwin Hubble}} [https://archive.org/details/TheRealmOfTheNebulae Alt URL](pp.&nbsp;124–151)</ref> and, as such, form part of the [[Hubble sequence]]. Most spiral galaxies consist of a flat, rotating disk containing [[Astronomy:Star|star]]s, [[Astronomy:Interstellar medium|gas and dust]], and a central concentration of stars known as the [[Astronomy:Bulge|bulge]]. These are often surrounded by a much fainter [[Astronomy:Galactic halo|halo]] of stars, many of which reside in [[Astronomy:Globular cluster|globular cluster]]s.

Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot [[Astronomy:OB star|OB star]]s that inhabit them.

Roughly two-thirds of all spirals are observed to have an additional component in the form of a bar-like structure,<ref name="mihalas1968">{{Cite book| author=D. Mihalas | date=1968 | title=Galactic Astronomy | publisher=W. H. Freeman | isbn=978-0-7167-0326-6}}</ref> extending from the central bulge, at the ends of which the spiral arms begin. The proportion of [[Astronomy:Barred spiral galaxy|barred spirals]] relative to [[Astronomy:Unbarred spiral galaxy|barless spirals]] has likely changed over the history of the [[Astronomy:Universe|universe]], with only about 10% containing bars about 8&nbsp;billion years ago, to roughly a quarter 2.5&nbsp;billion years ago, until present, where over two-thirds of the [[Astronomy:Galaxy|galaxies]] in the visible universe ([[Astronomy:Hubble volume|Hubble volume]]) have bars.<ref>{{cite web |url= https://www.sciencedaily.com/releases/2014/01/140116085103.htm |title= Hubble and Galaxy Zoo Find Bars and Baby Galaxies Don't Mix |date= 16 January 2014 |website= Science Daily }}</ref>

The [[Astronomy:Milky Way|Milky Way]] is a barred spiral, although the bar itself is difficult to observe from Earth's current position within the galactic disc.<ref>{{cite magazine|url=http://www.sciamdigital.com/index.cfm?fa=Products.ViewIssuePreview&ARTICLEID_CHAR=3BC08F0C-2B35-221B-67A9F2AE04AFC79A|url-status=dead|archive-url=https://web.archive.org/web/20130906193501/http://www.sciamdigital.com/index.cfm?fa=Products.ViewIssuePreview&ARTICLEID_CHAR=3BC08F0C-2B35-221B-67A9F2AE04AFC79A|title=Ripples in a Galactic Pond|magazine=Scientific American|date=October 2005|archive-date=6 September 2013}}</ref> The most convincing evidence for the stars forming a bar in the [[Galactic Center]] comes from several recent surveys, including the [[Astronomy:Spitzer Space Telescope|Spitzer Space Telescope]].<ref>{{cite journal|date=September 2005|title=First GLIMPSE Results on the Stellar Structure of the Galaxy|journal=The Astrophysical Journal Letters|volume=630|issue=2|pages=L149–L152|arxiv=astro-ph/0508325|bibcode=2005ApJ...630L.149B|doi=10.1086/491785|author1=R. A. Benjamin|author2=E. Churchwell|author3=B. L. Babler|author4=R. Indebetouw|author5=M. R. Meade|author6=B. A. Whitney|author7=C. Watson|author8=M. G. Wolfire|author9=M. J. Wolff|author10=R. Ignace|author11=T. M. Bania|author12=S. Bracker|author13=D. P. Clemens|author14=L. Chomiuk|author15=M. Cohen|author16=J. M. Dickey|author17=J. M. Jackson|author18=H. A. Kobulnicky|author19=E. P. Mercer|author20=J. S. Mathis|author21=S. R. Stolovy|author22=B. Uzpen|s2cid=14782284}}</ref>

Together with [[Astronomy:Irregular galaxy|irregular galaxies]], spiral galaxies make up approximately 60% of galaxies in today's universe.<ref>{{cite journal|date=February 1996|title=The APM Bright Galaxy Catalogue|journal=[[Organization:Monthly Notices of the Royal Astronomical Society|Monthly Notices of the Royal Astronomical Society]]|volume=278|issue=4|pages=1025–1048|arxiv=astro-ph/9603040|bibcode=1996MNRAS.278.1025L|doi=10.1093/mnras/278.4.1025|author=Loveday, J.}}</ref> They are mostly found in low-density regions and are rare in the centers of galaxy clusters.<ref>{{cite journal|date=March 1980|title=Galaxy morphology in rich clusters&nbsp;— Implications for the formation and evolution of galaxies|journal=The Astrophysical Journal|volume=236|pages=351–365|bibcode=1980ApJ...236..351D|doi=10.1086/157753|author=Dressler, A.|doi-access=free}}</ref>

==Structure==
[[File:HubbleTuningFork.jpg|thumb|Tuning-fork-style diagram of the [[Hubble sequence|Hubble sequence]]]]
Spiral galaxies may consist of several distinct components:
* A flat, rotating disc of [[Astronomy:Star|star]]s and [[Astronomy:Interstellar medium|interstellar matter]] of which spiral arms are prominent components
* A central stellar [[Astronomy:Bulge|bulge]] of mainly older stars, which resembles an [[Astronomy:Elliptical galaxy|elliptical galaxy]]
* A bar-shaped distribution of stars
* A near-spherical [[Astronomy:Galactic halo|halo]] of stars, including many in [[Astronomy:Globular cluster|globular cluster]]s
* A [[Astronomy:Supermassive black hole|supermassive black hole]] at the very center of the central bulge
* A near-spherical [[Astronomy:Dark matter|dark matter]] halo
The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy.

=== Spiral arms ===
[[File:UGC 12158.jpg|thumb|[[Astronomy:Barred spiral galaxy|Barred spiral galaxy]] UGC 12158]]

''Spiral arms'' are regions of [[Astronomy:Star|star]]s that extend from the center of barred and [[Astronomy:Unbarred spiral galaxy|unbarred]] spiral [[Astronomy:Galaxy|galaxies]]. These long, thin regions resemble a [[Spiral|spiral]] and thus give spiral galaxies their name. Naturally, different [[Astronomy:Galaxy morphological classification|classifications of spiral galaxies]] have distinct arm-structures. Sc and SBc galaxies, for instance, have very "loose" arms, whereas Sa and SBa galaxies have tightly wrapped arms (with reference to the Hubble sequence). Either way, spiral arms contain many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so bright.

=== Bulge ===
{{main article|Astronomy:Galactic bulge}}
A ''bulge'' is a large, tightly packed group of [[Astronomy:Star|star]]s. The term refers to the central group of stars found in most spiral galaxies, often defined as the excess of stellar light above the inward extrapolation of the outer (exponential) disk light.[[File:HAWK-I NGC 1300.jpg|thumb|[[Astronomy:NGC 1300|NGC 1300]] in infrared light]]

Using the Hubble classification, the bulge of Sa galaxies is usually composed of [[Astronomy:Population II star|Population II star]]s, which are old, red stars with low metal content. Further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller<ref>Alister W. Graham and C. Clare Worley (2008), [http://adsabs.harvard.edu/abs/2008MNRAS.388.1708G Inclination- and dust-corrected galaxy parameters: bulge-to-disc ratios and size-luminosity relations]</ref> and are composed of young, blue [[Astronomy:Population I star|Population I star]]s. Some bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity); others simply appear as higher density centers of disks, with properties similar to disk galaxies.

Many bulges are thought to host a [[Astronomy:Supermassive black hole|supermassive black hole]] at their centers. In our own galaxy, for instance, the object called [[Astronomy:Sagittarius A*|Sagittarius A*]] is a supermassive black hole. There are many lines of evidence for the existence of black holes in spiral galaxy centers, including the presence of [[Active galactic nucleus|active nuclei]] in some spiral galaxies, and dynamical measurements that find large compact central masses in galaxies such as [[Astronomy:Messier 106|Messier 106]].

=== Bar ===
[[File:Open Arms - potw2006a.jpg|thumb|Spiral galaxy NGC 2008]]
Bar-shaped elongations of stars are observed in roughly two-thirds of all spiral galaxies.<ref>de Vaucouleurs, G.; de Vaucouleurs, A.; Corwin, H. G., Jr.; Buta, R. J.; Paturel, G.; Fouqué, P. (2016), [http://adsabs.harvard.edu/abs/1991rc3..book.....D Third Reference Catalogue of Bright Galaxies]</ref><ref>B.D. Simmons et al. (2014), [http://adsabs.harvard.edu/abs/2014MNRAS.445.3466S Galaxy Zoo: CANDELS barred discs and bar fractions]</ref> Their presence may be either strong or weak. In edge-on spiral (and lenticular) galaxies, the presence of the bar can sometimes be discerned by the out-of-plane X-shaped or (peanut shell)-shaped structures<ref>Astronomy Now (8 May 2016), 
[https://astronomynow.com/2016/05/08/astronomers-detect-double-peanut-shell-galaxies/ Astronomers detect double ‘peanut shell’ galaxies]</ref><ref>Bogdan C. Ciambur and Alister W. Graham (2016), [http://adsabs.harvard.edu/abs/2016MNRAS.459.1276C Quantifying the (X/peanut)-shaped structure in edge-on disc galaxies: length, strength, and nested peanuts]</ref> which typically have a maximum visibility at half the length of the in-plane bar.

=== Spheroid{{anchor|Galactic spheroid}} ===
[[File:PHANGS image mosaic.jpg|thumb|upright=1.3|19 face-on spiral galaxies from the [[Astronomy:James Webb Space Telescope|James Webb Space Telescope]] in near- and mid-infrared light. Older stars appear blue here, and are clustered at the galaxies’ cores. Glowing dust, showing where it exists around and between stars – appearing in shades of red and orange. Stars that haven’t yet fully formed and are encased in gas and dust appear bright red.<ref>{{cite web |title=Webb reveals structure in 19 spiral galaxies |url=https://www.esa.int/Science_Exploration/Space_Science/Webb/Webb_reveals_structure_in_19_spiral_galaxies |website=www.esa.int |access-date=30 January 2024 |language=en}}</ref>]]

The bulk of the stars in a spiral galaxy are located either close to a single plane (the [[Astronomy:Galactic plane|galactic plane]]) in more or less conventional circular [[Physics:Orbit|orbit]]s around the center of the galaxy (the [[Galactic Center]]), or in a [[Spheroid|spheroid]]al galactic bulge around the galactic core.

However, some stars inhabit a ''spheroidal halo'' or ''galactic spheroid'', a type of [[Astronomy:Galactic halo|galactic halo]]. The orbital behaviour of these stars is disputed, but they may exhibit retrograde and/or highly inclined orbits, or not move in regular orbits at all. Halo stars may be acquired from small galaxies which fall into and [[Astronomy:Galaxy merger|merge]] with the spiral galaxy—for example, the [[Astronomy:Sagittarius Dwarf Spheroidal Galaxy|Sagittarius Dwarf Spheroidal Galaxy]] is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it.

Unlike the galactic disc, the halo seems to be free of dust, and in further contrast, stars in the galactic halo are of Population II, much older and with much lower [[Astronomy:Metallicity|metallicity]] than their Population I cousins in the galactic disc (but similar to those in the galactic bulge). The galactic halo also contains many [[Astronomy:Globular cluster|globular cluster]]s.

The motion of halo stars does bring them through the disc on occasion, and a number of small [[Astronomy:Red dwarf|red dwarf]]s close to the [[Astronomy:Sun|Sun]] are thought to belong to the galactic halo, for example [[Astronomy:Kapteyn's Star|Kapteyn's Star]] and [[Astronomy:Groombridge 1830|Groombridge 1830]]. Due to their irregular movement around the center of the galaxy, these stars often display unusually high [[Astronomy:Proper motion|proper motion]].

===Oldest spiral galaxies===
The oldest spiral galaxy on file is [[Astronomy:BX442|BX442]]. At eleven billion years old, it is more than two billion years older than any previous discovery. Researchers believe the galaxy's shape is caused by the gravitational influence of a companion [[Astronomy:Dwarf galaxy|dwarf galaxy]]. Computer models based on that assumption indicate that BX442's spiral structure will last about 100&nbsp;million years.<ref>Oldest spiral galaxy is a freak of cosmos http://www.zmescience.com/space/oldest-spiral-galaxy-31321/</ref><ref>{{cite web|last=Gonzalez|first=Robert T.|title=Hubble Has Spotted an Ancient Galaxy That Shouldn't Exist|url=http://io9.com/5927315/hubble-has-spotted-an-ancient-galaxy-that-shouldnt-exist|publisher=io9|access-date=10 September 2012|date=19 July 2012}}</ref>

[[Astronomy:A1689B11|A1689B11]] is an extremely old spiral galaxy located in the [[Astronomy:Abell 1689|Abell 1689]] galaxy cluster in the Virgo constellation.<ref>{{cite web|title=[BBC2005] Source 11 -- Galaxy|url=http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=%5BBBC2005%5D+Source+11&submit=SIMBAD+search|date=24 June 2018}}</ref> A1689B11 is 11 billion light years from the Earth, forming 2.6 billion years after the Big Bang.<ref>{{cite web|title=The most ancient spiral galaxy confirmed|url=https://phys.org/news/2017-11-ancient-spiral-galaxy.html|publisher=PhysOrg|date=3 November 2017}}</ref><ref name=Yuan>{{cite journal|arxiv=1710.11130|title=The most ancient spiral galaxy: a 2.6-Gyr-old disk with a tranquil velocity field|journal=The Astrophysical Journal|volume=850|pages=61|author1=Tiantian Yuan|author2=Johan Richard|author3=Anshu Gupta|author4=Christoph Federrath|author5=Soniya Sharma|author6=Brent A. Groves|author7=Lisa J. Kewley|author8=Renyue Cen|author9=Yuval Birnboim|author10=David B. Fisher|date=30 October 2017|doi=10.3847/1538-4357/aa951d|bibcode=2017ApJ...850...61Y|s2cid=119267114 |doi-access=free }}</ref>

[[Astronomy:BRI 1335-0417|BRI 1335-0417]] is the most distant known spiral galaxy, as of 2021. The galaxy has a [[Astronomy:Redshift|redshift]] of 4.4, meaning its light took 12.4 billion years to reach Earth.<ref>{{cite web|title=ALMA Spots Candidate for Most Distant Known Spiral Galaxy|publisher=www.sci-news.com|date=May 20, 2021|url=http://www.sci-news.com/astronomy/alma-most-distant-spiral-galaxy-09680.html|access-date=20 May 2021}}</ref>

===Related===
In June 2019, citizen scientists through [[Astronomy:Galaxy Zoo|Galaxy Zoo]] reported that the [[Hubble sequence|usual Hubble classification]], particularly concerning spiral galaxies, may not be supported, and may need updating.<ref name="EA-20190611">{{cite news |author=Royal Astronomical Society |title=Citizen scientists re-tune Hubble's galaxy classification |url=https://www.eurekalert.org/pub_releases/2019-06/ras-csr061119.php |date=11 June 2019 |work=EurekAlert! |access-date=11 June 2019 |author-link=Royal Astronomical Society }}</ref><ref name="MNRAS-20190430">{{cite journal |author=Masters, Karen L. |display-authors=et al. |title=Galaxy Zoo: unwinding the winding problem – observations of spiral bulge prominence and arm pitch angles suggest local spiral galaxies are winding |date=30 April 2019 |journal=[[Organization:Monthly Notices of the Royal Astronomical Society|Monthly Notices of the Royal Astronomical Society]] |volume=487 |issue=2 |pages=1808–1820 |doi=10.1093/mnras/stz1153 |bibcode=2019MNRAS.487.1808M |arxiv=1904.11436 }}</ref>

==Origin of the spiral structure==
[[File:NGC 6384 HST.jpg|thumb|left|Spiral galaxy [[Astronomy:NGC 6384|NGC 6384]] taken by Hubble Space Telescope]]
[[File:A spiral home to exploding stars.jpg|thumb|The spiral galaxy [[Astronomy:NGC 1084|NGC 1084]], home of five supernovae<ref>{{cite news|title=A spiral home to exploding stars|url=http://www.spacetelescope.org/images/potw1413a/|access-date=2 April 2014|newspaper=ESA / Hubble}}</ref>]]
The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925. He realized that the idea of stars arranged permanently in a spiral shape was untenable. Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy (via a standard solar system type of gravitational model), a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is called the ''winding problem''. Measurements in the late 1960s showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from [[Physics:Newtonian dynamics|Newtonian dynamics]] but still cannot explain the stability of the spiral structure.

Since the 1970s, there have been two leading hypotheses or models for the spiral structures of galaxies:
* star formation caused by [[Astronomy:Density wave theory|density waves]] in the galactic disk of the galaxy.
* the stochastic self-propagating star formation model ([[Astronomy:SSPSF model|SSPSF model]]) – star formation caused by shock waves in the [[Astronomy:Interstellar medium|interstellar medium]]. The shock waves are caused by the stellar winds and supernovae from recent previous star formation, leading to self-propagating and self-sustaining star formation. Spiral structure then arises from differential rotation of the galaxy's disk.

These different hypotheses are not mutually exclusive, as they may explain different types of spiral arms.

===Density wave model===
{{main|Astronomy:Density wave theory}}
[[File:Galaxy rotation wave.ogv|thumb|right|Animation of orbits as predicted by the density wave theory, which explains the existence of stable spiral arms. Stars move in and out of the spiral arms as they orbit the galaxy.]]
Bertil Lindblad proposed that the arms represent regions of enhanced density (density waves) that rotate more slowly than the galaxy's stars and gas. As gas enters a density wave, it gets squeezed and makes new stars, some of which are short-lived blue stars that light the arms.<ref name="Belkora2003">{{Cite book
|last1=Belkora |first1=L.
|date=2003
|title=Minding the Heavens: the Story of our Discovery of the Milky Way
|publisher=CRC Press
|page=355
|isbn=978-0-7503-0730-7
|url=https://books.google.com/books?id=cTdsuAEACAAJ}}</ref>

====Historical theory of Lin and Shu====
[[File:spiral galaxy arms diagram.svg|thumb|right|Exaggerated diagram illustrating Lin and Shu's explanation of spiral arms in terms of slightly elliptical orbits]]
The first acceptable theory for the spiral structure was devised by C. C. Lin and Frank Shu in 1964,<ref name="LinShu1964">{{cite journal|date=August 1964|title=On the spiral structure of disk galaxies|journal=The Astrophysical Journal|volume=140|pages=646–655|bibcode=1964ApJ...140..646L|doi=10.1086/147955|last1=Lin|first1=C. C.|author2=Shu, F. H.|doi-access=}}</ref> attempting to explain the large-scale structure of spirals in terms of a small-amplitude wave propagating with fixed angular velocity, that revolves around the galaxy at a speed different from that of the galaxy's gas and stars. They suggested that the spiral arms were manifestations of spiral density waves – they assumed that the stars travel in slightly elliptical orbits, and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic center. This is illustrated in the diagram to the right. It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits.<ref name="Henbest1994">{{citation|title=The Guide to the Galaxy|first=Nigel|last=Henbest|publisher={{wipe|Cambridge University Press}}|date=1994|isbn=9780521458825|url=https://books.google.com/books?id=-TI4AAAAIAAJ&pg=PA74|page=74|quote=Lin and Shu showed that this spiral pattern would persist more or less for ever, even though individual stars and gas clouds are always drifting into the arms and out again}}.</ref>

====Star formation caused by density waves====
The following hypotheses exist for star formation caused by density waves:
* As gas clouds move into the density wave, the local mass density increases. Since the criteria for cloud collapse (the [[Astronomy:Jeans instability|Jeans instability]]) depends on density, a higher density makes it more likely for clouds to collapse and form stars.
* As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.
* As clouds get swept up by the spiral arms, they collide with one another and drive shock waves through the gas, which in turn causes the gas to collapse and form stars.

===More young stars in spiral arms===
Spiral arms appear visually brighter because they contain both young stars and more massive and luminous stars than the rest of the galaxy. As massive stars evolve far more quickly,<ref name="Mainseqlife">{{cite web|title=Main Sequence Lifetime|url=http://astronomy.swin.edu.au/cosmos/M/Main+Sequence+Lifetime|work=Swinburne Astronomy Online|publisher=Swinburne University of Technology|access-date=8 June 2019}}</ref> their demise tends to leave a darker background of fainter stars immediately behind the density waves. This make the density waves much more prominent.<ref name="Belkora2003"/>

Spiral arms simply appear to pass through the older established stars as they travel in their galactic orbits, so they also do not necessarily follow the arms.<ref name="Belkora2003"/> As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the local higher density. Also the newly created stars do not remain forever fixed in the position within the spiral arms, where the average space velocity returns to normal after the stars depart on the other side of the arm.<ref name="Henbest1994"/>

===Gravitationally aligned orbits===
Charles Francis and Erik Anderson showed from observations of motions of over 20,000 local stars (within 300 parsecs) that stars do move along spiral arms, and described how mutual gravity between stars causes orbits to align on logarithmic spirals. When the theory is applied to gas, collisions between gas clouds generate the molecular clouds in which [[Astronomy:Protostar|new stars]] form, and evolution towards grand-design bisymmetric spirals is explained.<ref>{{cite journal|doi=10.1098/rspa.2009.0036|title=Galactic spiral structure|date=2009|last1=Francis|first1=C.|last2=Anderson|first2=E.|journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences|volume=465|issue=2111|pages=3425–3446|arxiv = 0901.3503 |bibcode = 2009RSPSA.465.3425F |s2cid=12461073}}</ref>

== Distribution of stars in spirals ==
[[File:Similar stellar distribution in spiral galaxies.jpeg|thumb|The similar distribution of stars in spirals]]
The stars in spirals are distributed in thin disks radial with intensity profiles such that<ref>F. Shirley Patterson (1940), [http://adsabs.harvard.edu/abs/1940BHarO.914....9P The Luminosity Gradient of Messier 33]</ref><ref>Gerard de Vaucouleurs (1957), [http://adsabs.harvard.edu/abs/1957AJ.....62...69D Studies of the Magellanic Clouds. III. Surface brightness, colors and integrated magnitudes of the Clouds.]</ref><ref name="fr-70">{{cite journal|author=Freeman, K. C.|title=On the Disks of Spiral and so Galaxies|journal=Astrophysical Journal|volume=160|page=811|date=1970|bibcode=1970ApJ...160..811F|doi=10.1086/150474|doi-access=free}}</ref>
:<math>
I(R) = I_0 e^{-R/h}
</math>
with <math>h</math> being the disk scale-length; <math>I_0</math> is the central value; it is useful to define: <math>R_{opt}= 3.2 h </math> as the size of the stellar disk, whose luminosity is

<math>
L_{tot} = 2\pi I_0 h^2
</math>.

The spiral galaxies light profiles, in terms of the coordinate <math>R/h</math>, do not depend on galaxy luminosity.

==Spiral nebula==
[[File:A Spiral Amongst Thousands (potm2301a).jpeg|thumb|right|Spiral galaxy LEDA 2046648, about one billion light-years away]]
<!---[[File:A loose spiral galaxy.jpg|thumb|Spiral galaxy ESO 499-G37, seen against a backdrop of distant galaxies, scattered with nearby stars.<ref>{{cite news|title=A loose spiral galaxy|url=http://www.spacetelescope.org/images/potw1247a/|access-date=19 November 2012|newspaper=ESA/Hubble Picture of the Week}}</ref> ]]-->

[[File:M51Sketch.jpg|thumb|left|Drawing of the Whirlpool Galaxy by Rosse in 1845]]

Before it was understood that spiral galaxies existed outside of our Milky Way galaxy, they were often referred to as ''spiral nebulae'', due to Lord Rosse, whose telescope Leviathan was the first to reveal the spiral structure of galaxies. In 1845 he discovered the spiral structure of M51, a galaxy nicknamed later as the "[[Astronomy:Whirlpool Galaxy|Whirlpool Galaxy]]", and his drawings of it closely resemble modern photographs.  In 1846 and in 1849 Lord Rosse identified similar pattern in [[Astronomy:Messier 99|Messier 99]] and [[Astronomy:Messier 33|Messier 33]] respectively.  In 1850 he made the first drawing of [[Astronomy:Andromeda Galaxy|Andromeda Galaxy]]'s spiral structure. In 1852 Stephen Alexander supposed that Milky Way is also a spiral nebula.<ref>Alexander, S. On the origin of the forms and the present condition of some of the clusters of stars, and several of the nebulae. Astronomical Journal, vol. 2, iss. 37, p. 97-103 (1852)</ref>

The question of whether such objects were separate galaxies independent of the Milky Way, or a type of [[Astronomy:Nebula|nebula]] existing within our own galaxy, was the subject of the [[Astronomy:Great Debate|Great Debate]] of 1920, between Heber Curtis of [[Organization:Lick Observatory|Lick Observatory]] and Harlow Shapley of Mount Wilson Observatory. Beginning in 1923, [[Biography:Edwin Hubble|Edwin Hubble]]<ref>{{cite web|url=http://www.nasa.gov/mission_pages/hubble/science/star-v1.html|title=NASA - Hubble Views the Star That Changed the Universe}}</ref><ref>{{cite journal|date=May 1926|title=A spiral nebula as a stellar system: Messier 33|journal=The Astrophysical Journal|volume=63|pages=236–274|bibcode=1926ApJ....63..236H|doi=10.1086/142976|author=Hubble, E. P.|author-link=Edwin Hubble}}</ref> observed Cepheid variables in several spiral nebulae, including the so-called [[Astronomy:Andromeda Galaxy|"Andromeda Nebula"]], proving that they are, in fact, entire galaxies outside our own. The term ''spiral nebula'' has since fallen out of use.

==Milky Way==

[[File:PIA19341-MilkyWayGalaxy-SpiralArmsData-WISE-20150603.jpg|thumb|Milky Way Galaxy's spiral arms and barred core – based on [[Astronomy:Wide-field Infrared Survey Explorer|WISE]] data]]

The [[Astronomy:Milky Way|Milky Way]] was once considered an ordinary spiral galaxy. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1960s.<ref>Gerard de Vaucouleurs (1964), [http://adsabs.harvard.edu/abs/1964IAUS...20..195D Interpretation of velocity distribution of the inner regions of the Galaxy]</ref><ref>{{cite journal|date=1996|title=On the spiral arm interpretation of COMPTEL <sup>26</sup>Al map features|journal=Space Science Reviews|volume=120|pages=315–316|bibcode=1996A&AS..120C.315C|author=Chen, W.|author2=Gehrels, N.|author3=Diehl, R.|author4=Hartmann, D.}}</ref> Their suspicions were confirmed by [[Astronomy:Spitzer Space Telescope|Spitzer Space Telescope]] observations in 2005,<ref name="fn3">{{cite magazine
 | first=Maggie
 | last=McKee
 | title=Bar at Milky Way's heart revealed
 | magazine=New Scientist
 | date=16 August 2005
 | url=https://www.newscientist.com/article/dn7854--bar-at-milky-ways-heart-revealed.html
 | access-date=17 June 2009}}</ref> which showed that the Milky Way's central bar is larger than what was previously suspected.

==Famous examples==
{{further|Astronomy:List of spiral galaxies}}
{{Colbegin|colwidth=30em}}
* {{annotated link|Astronomy:Andromeda Galaxy}}
* {{annotated link|Astronomy:Milky Way}}
* {{annotated link|Astronomy:Pinwheel Galaxy}}
* {{annotated link|Astronomy:Sunflower Galaxy}}
* {{annotated link|Astronomy:Triangulum Galaxy}}
* {{annotated link|Astronomy:Whirlpool Galaxy}}
* {{annotated link|Astronomy:Black Eye Galaxy}}
* {{annotated link|Astronomy:Malin 1}}
{{colend}}

==See also==
===Classification===
{{Colbegin|colwidth=30em}}
* {{annotated link|Astronomy:Disc galaxy}}
* {{annotated link|Astronomy:Dwarf elliptical galaxy}}
* {{annotated link|Astronomy:Dwarf spheroidal galaxy}}
* {{annotated link|Astronomy:Flocculent spiral galaxy}}
* {{annotated link|Astronomy:Galaxy color–magnitude diagram}}
* {{annotated link|Astronomy:Grand design spiral galaxy}}
* {{annotated link|Astronomy:Intermediate spiral galaxy}}
* {{annotated link|Astronomy:Lenticular galaxy}}
* {{annotated link|Astronomy:Ring galaxy}}
* {{annotated link|Astronomy:Starburst galaxy}}
* {{annotated link|Astronomy:Seyfert galaxy}}
{{colend}}

===Other===
{{Colbegin|colwidth=30em}}
* {{annotated link|Galactic coordinate system}}
* {{annotated link|Astronomy:Galactic corona}}
* {{annotated link|Astronomy:Galaxy formation and evolution}}
* {{annotated link|Astronomy:Galaxy rotation curve}}
* [[Astronomy:List of galaxies|List of galaxies]]
* [[Astronomy:List of nearest galaxies|List of nearest galaxies]]
* [[Astronomy:List of spiral galaxies|List of spiral galaxies]]
* {{annotated link|Astronomy:Stellar halo}}
* {{annotated link|Astronomy:Tully–Fisher relation}}
{{colend}}

==References==
{{reflist|35em}}

==External links==
* {{cite journal | author=Giudice, G.F. | author2=Mollerach, S. | author3=Roulet, E. | title=Can EROS/MACHO be detecting the galactic spheroid instead of the galactic halo? | journal=Physical Review D | date=1994 | volume=50 | issue=4 | pages=2406–2413 | arxiv=astro-ph/9312047 | doi=10.1103/PhysRevD.50.2406| pmid=10017873 |bibcode = 1994PhRvD..50.2406G | s2cid=14500715 }}
* {{cite news | last=Stephens | first=Tim | title=AEGIS survey reveals new principle governing galaxy formation and evolution | publisher=UC Santa Cruz | date=6 March 2007 | url=http://www.ucsc.edu/news_events/press_releases/text.asp?pid=1080 | access-date=24 May 2006 | url-status=dead | archive-url=https://web.archive.org/web/20070311210954/http://www.ucsc.edu/news_events/press_releases/text.asp?pid=1080 | archive-date=11 March 2007 | df=dmy-all }}
* [http://messier.seds.org/spir.html Spiral Galaxies @ SEDS Messier pages]
* [http://spiralzoom.com/Science/spiralgalaxies/SpiralGalaxies.html ''SpiralZoom.com''], an educational website about Spiral Galaxies and other spiral formations found in nature. For high school & general audience.
* [http://burro.cwru.edu/Academics/Astr222/Galaxies/Spiral/spiral.html Spiral Structure explained]
* [http://www.astro.wisc.edu/sirtf/ GLIMPSE: the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire]
* {{cite web|last=Merrifield|first=M. R.|title=Spiral Galaxies and Pattern Speed|url=http://www.sixtysymbols.com/videos/spiral_galaxies.htm|work=Sixty Symbols|publisher=[[Biography:Brady Haran|Brady Haran]] for the [[Organization:University of Nottingham|University of Nottingham]]}}

{{Galaxy}}
{{Milky Way}}

[[Category:Spiral galaxies| ]]
[[Category:Galaxy morphological types]]
[[Category:Spirals]]

{{Sourceattribution|Spiral galaxy}}