Grand design spiral galaxy
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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. Approximately ten percent of spiral galaxies are classified as grand design type spirals[1], including M81 and M51, the Whirlpool Galaxy.
[edit] The origin of Grand Design structure
Density wave theory is the preferred explanation for the well-defined structure of grand design spirals[2]. According to this 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 are clumped in these dense regions due to gravitational attraction towards 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 towards the dense material by the force of gravity. However, as they travel through the arm, they are slowed from exiting by the same gravitational pull. This causes material to clump in the dense regions.
[edit] References
1. Mihos, Chris (2002-01-11), Spiral Structure, <http://burro.cwru.edu/Academics/Astr222/Galaxies/Spiral/spiral.html>. Retrieved on 30 May 2007
2. [|Masters, Karen] (2002-09), What is the Origin of Spiral Structure in Galaxies, <http://curious.astro.cornell.edu/question.php?number=199>. Retrieved on 30 May 2007
