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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.
Spiral galaxies consist of a flattened disk, with stars forming a (usually two-armed) spiral structure, and a central concentration of stars known as the bulge, which is similar in appearance to an elliptical galaxy. They are given the symbol "S".
Grand design spiral galaxy – Spiral galaxy with prominent and well-defined spiral arms; Intermediate spiral galaxy – Galaxy type intermediate between a spiral galaxy and barred spiral galaxy; Lenticular galaxy – Class of galaxy between an elliptical galaxy and a spiral galaxy; Ring galaxy – Galaxy with an annular appearance
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 ).
At the centre of the Hubble tuning fork, where the two spiral-galaxy branches and the elliptical branch join, lies an intermediate class of galaxies known as lenticulars and given the symbol S0. These galaxies consist of a bright central bulge, similar in appearance to an elliptical galaxy, surrounded by an extended, disk-like
Astronomers refer to the distinctive spiral-like bulge of galaxies such as ESO 498-G5 as disc-type bulges, or pseudobulges. 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.
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.
Although M87 is an elliptical galaxy and therefore lacks the dust lanes of a spiral galaxy, optical filaments have been observed in it, which arise from gas falling towards the core. Emission probably comes from shock-induced excitation as the falling gas streams encounter X-rays from the core region. [121]