Grand design spiral galaxy
{{Short description|Spiral galaxy with prominent and well-defined spiral arms}}
Image:Grand Design Galaxy.png, a grand design spiral galaxy, with Integrated Flux Nebula.]]
A grand design spiral galaxy is a spiral galaxy with prominent and well-defined continuous spiral arms, as opposed to multi-arm, patchy and flocculent spirals which have subtler structural features. The spiral arms of a grand design galaxy extend clearly around the galaxy, covering a significant portion of the galaxy's circumference. Theses spiral arms host lot of star formation making them home to an abundance of bright, hot and short lived massive stars.{{Cite web |title=Science Data Portal |url=https://science.data.nasa.gov/science-discovery-engine/my-search-stories/grand-design-galaxy |access-date=2025-06-04 |website=science.data.nasa.gov}}
As of 2002, approximately 10 percent of all currently known spiral galaxies are classified as grand design type spirals,{{Citation|last = Mihos|first = Chris|title = Spiral Structure|date = 2002-01-11|url = http://burro.cwru.edu/Academics/Astr222/Galaxies/Spiral/spiral.html|access-date = 2007-05-30}} including M51 (Whirlpool Galaxy), M74 (Phantom Galaxy), M81 (Bode’s Galaxy), M83 (Southern pinwheel galaxy), M101 (Pinwheel Galaxy), NGC 6946 (Firworks Galaxy) and IC 342 (The Hidden Galaxy).
Origin of structure
Density wave theory is the preferred explanation for the well-defined structure of grand design spirals,{{Citation|last=Masters |first=Karen |title=What is the Origin of Spiral Structure in Galaxies |date=September 2002 |url=http://curious.astro.cornell.edu/question.php?number=199 |access-date=2007-05-30 |url-status=dead |archive-url=https://web.archive.org/web/20070609111626/http://curious.astro.cornell.edu/question.php?number=199 |archive-date=2007-06-09 }} first suggested by Chia-Chiao Lin and Frank Shu in 1964.{{cite journal | last1 = Lin | first1 = C. C. | last2 = Shu | first2 = Frank H. | title = On the Spiral Structure of Disk Galaxies. | journal = The Astrophysical Journal | date = August 1964 | volume = 140 | page = 646 | issn = 0004-637X | eissn = 1538-4357 | doi = 10.1086/147955 | pmid = | url = https://articles.adsabs.harvard.edu/pdf/1964ApJ...140..646L | doi-access = | bibcode = 1964ApJ...140..646L }} The term "grand design" was not used in this work, but appeared in the 1966 continuation paper;{{cite journal | last1 = Lin | first1 = C. C. | last2 = Shu | first2 = Frank H. | title = On the spiral structure of disk galaxies, II. Outline of a theory of density waves | journal = Proceedings of the National Academy of Sciences | date = February 1966 | volume = 55 | issue = 2 | pages = 229–234 | issn = 0027-8424 | eissn = 1091-6490 | doi = 10.1073/pnas.55.2.229 | pmid = 16591327 | pmc = 224127 | doi-access = free | bibcode = 1966PNAS...55..229L }} Lin{{cite book |author1=Basu |first=Baidyanath |url=https://books.google.com/books?id=WG-HkqCXhKgC |title=An introduction to astrophysics |author2=Chattopadhyay |first2=Tanuka |author3=Biswas |first3=Sudhindra Nath |date=1 January 2010 |publisher=PHI Learning Pvt. Limited |isbn=9788120340718 |page=454 |oclc=1038455319}} (along with Yuan and Shu{{cite book |author1=Buta |first=Ronald J. |url=https://books.google.com/books?id=g-P7dCbB5MEC&pg=PA28 |title=Atlas of Galaxies |author2=Corwin |first2=Harold G. |author3=Odewahn |first3=Stephen C. |date=8 March 2007 |publisher=Cambridge University Press |isbn=978-0-521-82048-6 |page=28 |oclc=1284385231}}) is usually credited with coining of the term.
According to the density wave theory, the spiral arms are created inside density waves that turn around the galaxy at different speeds from the stars in the galaxy's disk. Stars and gas are clumped in these dense regions due to gravitational attraction toward the dense material, though their location in the spiral arm may not be permanent. When they come close to the spiral arm, they are pulled toward the dense material by the force of gravity; and as they travel through the arm, they are slowed from exiting by the same gravitational pull. This causes the gas in particular to clump in the dense regions, which in turn causes gas clouds to collapse, resulting in star formation.