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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.
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.
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.
A lenticular galaxy is an intermediate form that has properties of both elliptical and spiral galaxies. These are categorized as Hubble type S0, and they possess ill-defined spiral arms with an elliptical halo of stars [99] (barred lenticular galaxies receive Hubble classification SB0).
The evolutionary picture appears to be lent weight by the fact that the disks of spiral galaxies are observed to be home to many young stars and regions of active star formation, while elliptical galaxies are composed of predominantly old stellar populations. In fact, current evidence suggests the opposite: the early Universe appears to be ...
Many bulges have properties more similar to those of the central regions of spiral galaxies than elliptical galaxies. [6] [7] [8] They are often referred to as pseudobulges or disky-bulges. These bulges have stars that are not orbiting randomly, but rather orbit in an ordered fashion in the same plane as the stars in the outer disk.
Galaxies may contain hundreds of billions of stars and are categorized according to their visual morphology as elliptical, spiral, or irregular. Most of the galaxies in the observable universe are between 1,000 and 3,000 parsecs (3,300 and 9,800 ly) in diameter though some, including the Milky Way, are much larger. galaxy cluster
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.