Density wave theory

Density wave theory explains spiral arms as patterns of enhanced density that move through a galaxy’s disk, rather than as fixed groups of stars.

Spiral density wave: A propagating, organized increase in the density of stars and gas within a galactic disk. This is the pattern that the theory identifies with a galaxy’s spiral arms.
Differential rotation: Rotation in which the angular speed varies with distance from the rotation axis. Different orbital speeds explain why material arms would wind up without a wave pattern.
Milky Way spiral structure: The arrangement and geometry of spiral arms in the Milky Way’s disk. Its arm pattern and stellar motions are key tests of density-wave explanations.
Material arm theory: An explanation in which spiral arms are persistent arrangements of the same stars and gas moving with the disk. Differential rotation would wind material arms tightly, unlike a pattern that stars pass through.
Galactic rotation curve: A graph of orbital speed versus distance from the center of a galaxy. Differential rotation shears material around the disk, while waves can preserve spiral structure.
Epicyclic frequency: The frequency of small radial oscillations made by an object orbiting in an axisymmetric gravitational potential. It sets the orbital response that enters the theory’s resonance conditions.
Grand design spiral galaxy: A spiral galaxy with two prominent, coherent arms extending across much of its disk. Its orderly arms resemble the large-scale patterns the theory seeks to explain.
Transient spiral arms: Short-lived spiral patterns that grow and fade through local gravitational interactions in a galactic disk. They challenge the idea that spiral arms must be long-lived, global waves.
Corotation radius: The radius in a rotating galactic pattern where stars orbit at the pattern’s angular speed. Inside and outside this radius, stars move through the spiral pattern in opposite directions.
Jeans instability: The gravitational growth of density fluctuations when self-gravity overcomes pressure or velocity support. Self-gravity can reinforce the density perturbations that make up a spiral wave.

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