Spiral arm

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 is referred to as the spiral pattern or spiral structure of the galaxy.

The appearance of spiral sleeves is quite diverse. Grand design spiral galaxies exhibit a symmetrical and distinct pattern, comprising two spiral arms that extend throughout the galaxy. In contrast, the spiral structure of flocculent galaxies comprises numerous small fragments of arms that are not connected to each other. The appearance of spiral arms varies across the electromagnetic spectrum.

In addition to increased brightness, spiral arms are characterised by an increased concentration of interstellar gas and dust, bright stars and star clusters, active starburst, a bluer colour, and an enhanced magnetic field strength in galaxies. The contribution of spiral arms to the total galaxy luminosity can reach 40–50% for some galaxies. The characteristics of spiral arms are correlated with other properties of galaxies, for example, the twist angle of spiral arms is related to parameters such as the mass of the supermassive black hole at the centre and the contribution of the bulge to the total luminosity.

Two main theories have been proposed to explain the origin of spiral arms: the stochastic self-propagating star formation model and the density wave theory. These theories describe different variants of the spiral structure and do not exclude each other. In addition to these theories, there are other theories that can explain the appearance of spiral structure in some cases.

The spiral structure was first identified in 1850 by Lord Rosse in the galaxy M51. The nature of spiral structure in galaxies remained unresolved for a considerable period of time.