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A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. [1] [2] The word is derived from the Greek galaxias (γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System.
As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (10 12) stars. [151] The Milky Way may contain ten billion white dwarfs, a billion neutron stars, and a hundred million stellar black holes. [f] [154] [155] Filling the space between the stars is a disk of gas and dust called the interstellar medium.
An example of a positively curved space would be the surface of a sphere such as the Earth. A triangle drawn from the equator to a pole will have at least two angles equal 90°, which makes the sum of the 3 angles greater than 180°. An example of a negatively curved surface would be the shape of a saddle or mountain pass. A triangle drawn on a ...
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 example of a spiral galaxy, Messier 77 (also known as NGC 1068) 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.
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.
The Spindle Galaxy (NGC 5866), a lenticular galaxy with a prominent dust lane in the constellation of Draco See also: Lenticular galaxy 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.
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.