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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.
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 ]
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.
Elliptical and lenticular galaxies are commonly referred to together as "early-type" galaxies, while spirals and irregular galaxies are referred to as "late types". This nomenclature is the source of the common, [ 18 ] but erroneous, belief that the Hubble sequence was intended to reflect a supposed evolutionary sequence, from elliptical ...
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 .
Supergiant elliptical galaxies are some of the largest galaxies known. The Condor Galaxy is a colossal spiral galaxy disturbed by the smaller IC 4970 . It is the largest known spiral galaxy with the isophotal diameter of over 717,000 light-years (220 kiloparsecs ).
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.
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.