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Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae [1] and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars , gas and dust , and a central concentration of stars known as the bulge .
In spiral galaxies, the spiral arms do have the shape of approximate logarithmic spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms rotate around the center, but they do so with constant angular velocity .
When a galaxy forms, it has a disk shape and is called a spiral galaxy due to spiral-like "arm" structures located on the disk. There are different theories on how these disk-like distributions of stars develop from a cloud of matter: however, at present, none of them exactly predicts the results of observation.
A spiral galaxy consists of a flattened disk, with stars forming a (usually two-armed) spiral structure, and a central concentration of stars known as the bulge. Roughly half of all spirals are also observed to have a bar-like structure, with the bar extending from the central bulge, and the arms begin at the ends of the bar.
Galaxies have a variety of morphologies, with irregular, elliptical and disk-like shapes, depending on their formation and evolutionary histories, including interaction with other galaxies, which may lead to a merger. [4] Disc galaxies encompass lenticular and spiral galaxies with features, such as spiral arms and a distinct halo.
NGC 1300, viewed nearly face-on; Hubble Space Telescope image. A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. [1] Bars are found in about two thirds of all spiral galaxies in the local universe, [2] and generally affect both the motions of stars and interstellar gas within spiral galaxies and can affect spiral arms as well.
The Cartwheel Galaxy (also known as ESO 350-40 or PGC 2248) is a lenticular ring galaxy about 500 million light-years away in the constellation Sculptor. [1] It has a D 25 isophotal diameter of 44.23 kiloparsecs (144,300 light-years), and a mass of about 2.9–4.8 × 10 9 solar masses; its outer ring has a circular velocity of 217 km/s.
[13] [23] Spiral arms with a small pitch angle are called tightly wound, while those with a larger pitch angle are called open. [24] The shape of spiral arms is often described in a simplified manner as a logarithmic spiral. However, spiral arms may also be described as an Archimedean or hyperbolic spiral. In the case of the logarithmic spiral ...