A grand design spiral galaxy is a

spiral galaxy Spiral galaxies form a galaxy morphological classification, class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''

with prominent and well-defined continuous

spiral arms Spiral arms are a defining feature of spiral galaxies. They manifest as spiral-shaped regions of enhanced brightness within the galactic disc. Typically, spiral galaxies exhibit two or more spiral arms. The collective configuration of these 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 A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...

, 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. As of 2002, approximately 10 percent of all currently known spiral galaxies are classified as grand design type spirals, 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 Density wave theory or the Lin–Shu density wave theory is a theory proposed by C.C. Lin and Frank Shu in the mid-1960s to explain the spiral arm structure of spiral galaxies. The Lin–Shu theory introduces the idea of long-lived quasistati ...

is the preferred explanation for the well-defined structure of grand design spirals, first suggested by Chia-Chiao Lin and Frank Shu in 1964. The term "grand design" was not used in this work, but appeared in the 1966 continuation paper; Lin (along with Yuan and Shu) 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 Star formation is the process by which dense regions within molecular clouds in interstellar space—sometimes referred to as "stellar nurseries" or "star-forming regions"—Jeans instability, collapse and form stars. As a branch of astronomy, sta ...

References

{{Galaxy * Galaxy morphological types