Agrand design spiral galaxy is aspiral galaxy with prominent and well-defined continuousspiral arms, as opposed to multi-arm, patchy andflocculent spirals which have subtler structural features. The spiral arms of a grand design galaxy extend clearly around thegalaxy, covering a significant portion of the galaxy's circumference. These spiral arms host much star formation, making them home to an abundance of bright, hot, and short-lived massive stars.[1]
As of 2002, approximately 10 percent ofall currently known spiral galaxies are classified as grand design type spirals,[2] includingM51 (Whirlpool Galaxy),M74 (Phantom Galaxy),M81 (Bode’s Galaxy),M83 (Southern pinwheel galaxy),M101 (Pinwheel Galaxy),NGC 6946 (Fireworks Galaxy) andIC 342 (The Hidden Galaxy).
Density wave theory is the preferred explanation for the well-defined structure of grand design spirals,[3] first suggested byChia-Chiao Lin andFrank Shu in 1964.[4] The term "grand design" was not used in this work, but appeared in the 1966 continuation paper;[5] Lin[6] (along with Yuan and Shu[7]) 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 instar formation.
