A Spitzer Space Telescope image of Messier 81, a grand design spiral.

A grand design spiral galaxy is a type of spiral galaxy with prominent and well-defined spiral arms, as opposed to multi-arm and flocculent spirals which have subtler structural features. The spiral arms of a grand design galaxy extend clearly around the galaxy through many radians and can be observed over a large fraction of the galaxy's radius. As of 2002, approximately 10 percent of all currently known spiral galaxies are classified as grand design type spirals,[1] including M51, M74, M81, M83, and M101.

Origin of structure

Density wave theory is the preferred explanation for the well-defined structure of grand design spirals,[2] first suggested by Chia-Chiao Lin and Frank Shu in 1964.[3] The term "grand design" was not used in this work, but appeared in the 1966 continuation paper;[4] Lin[5] (along with Yuan and Shu[6]) 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.

References

  1. ↑ Mihos, Chris (2002-01-11), Spiral Structure, retrieved 2007-05-30
  2. ↑ Masters, Karen (September 2002), What is the Origin of Spiral Structure in Galaxies, archived from the original on 2007-06-09, retrieved 2007-05-30
  3. ↑ Lin, C. C.; Shu, Frank H. (August 1964). "On the Spiral Structure of Disk Galaxies". The Astrophysical Journal. 140: 646. doi:10.1086/147955. eISSN 1538-4357. ISSN 0004-637X.
  4. ↑ Lin, C. C.; Shu, Frank H. (February 1966). "On the spiral structure of disk galaxies, II. Outline of a theory of density waves". Proceedings of the National Academy of Sciences. 55 (2): 229–234. doi:10.1073/pnas.55.2.229. eISSN 1091-6490. ISSN 0027-8424. PMC 224127. PMID 16591327.
  5. ↑ Baidyanath Basu; Tanuka Chattopadhyay; Sudhindra Nath Biswas (1 January 2010). An introduction to astrophysics. PHI Learning Pvt. Ltd. p. 454. ISBN 9788120340718. OCLC 1038455319.
  6. ↑ Ronald J. Buta; Harold G. Corwin; Stephen C. Odewahn (8 March 2007). Atlas of Galaxies. Cambridge University Press. p. 28. ISBN 978-0-521-82048-6. OCLC 1284385231.
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