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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.
NGC 4567 and NGC 4568 (nicknamed the Butterfly Galaxies [4] or Siamese Twins [NB 1] [5]) are a set of unbarred spiral galaxies about 60 million light-years away [1] in the constellation Virgo. They were both discovered by William Herschel in 1784. They are part of the Virgo Cluster of galaxies.
The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way. It was originally named the Andromeda Nebula and is cataloged as Messier 31 , M31 , and NGC 224 . Andromeda has a D 25 isophotal diameter of about 46.56 kiloparsecs (152,000 light-years ) [ 8 ] and is approximately 765 kpc (2.5 million light-years ...
An exact figure would depend on counting the number of very-low-mass stars, which are difficult to detect, especially at distances of more than 300 ly (90 pc) from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (10 12) stars. [151]
Below are lists of the largest stars currently known, ordered by radius and separated into categories by galaxy. The unit of measurement used is the radius of the Sun (approximately 695,700 km; 432,300 mi).
The Sigma-D relation (or Σ-D relation), used in elliptical galaxies, relates the angular diameter (D) of the galaxy to its velocity dispersion. It is important to describe exactly what D represents, in order to understand this method. It is, more precisely, the galaxy's angular diameter out to the surface brightness level of 20.75 B-mag arcsec ...
The second most massive galaxy cluster next to El Gordo is RCS2 J2327, a galaxy cluster with the mass of 2 quadrillion suns. Also has a systematic designation of ACT-CL J0102-4915. [6] [7] [8] Musket Ball Cluster: Named in comparison to the Bullet Cluster, as this one is older and slower galaxy cluster merger than the Bullet Cluster.
In some work regarding the distant past or future the galactic coordinate system is taken as rotating so that the x-axis always goes to the centre of the galaxy. [ 6 ] There are two major rectangular variations of galactic coordinates, commonly used for computing space velocities of galactic objects.