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Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters. Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc.
An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. [1] Irregular galaxies do not fall into any of the regular classes of the Hubble sequence , and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure. [ 2 ]
The giant elliptical galaxy ESO 325-4. An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the three main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work The Realm of the Nebulae, [1] along with spiral and lenticular galaxies.
Galaxies are categorised according to their visual morphology as elliptical, [5] spiral, or irregular. [6] The Milky Way is an example of a spiral galaxy. It is estimated that there are between 200 billion [ 7 ] ( 2 × 10 11 ) to 2 trillion [ 8 ] galaxies in the observable universe .
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.
Grand design spiral galaxies exhibit a symmetrical and clear pattern comprising two spiral arms that extend throughout the galaxy. They account for 10% of the total number of spiral galaxies. In contrast, the spiral structure of flocculent galaxies consists of numerous small fragments of arms that are not connected to each other. Among spiral ...
Galaxies range in size from dwarfs with just a few billion (109) stars to giants with one hundred trillion (1014) stars, each orbiting its galaxy's center of mass. Galaxies are categorized according to their visual morphology as elliptical, spiral and irregular. Many galaxies are thought to have black holes at their active centers.
Rings. Galaxies are divided into those possessing ring-like structures (denoted ‘(r)’) and those without rings (denoted ‘(s)’). So-called ‘transition’ galaxies are given the symbol (rs). [16] Spiral arms. As in Hubble’s original scheme, spiral galaxies are assigned to a class based primarily on the tightness of their spiral arms.