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The observable universe contains as many as an estimated 2 trillion galaxies [36] [37] [38] and, overall, as many as an estimated 10 24 stars [39] [40] – more stars (and, potentially, Earth-like planets) than all the grains of beach sand on planet Earth. [41] [42] [43] Other estimates are in the hundreds of billions rather than trillions.
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. Most galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3,000 to 300,000 light years) and
Size (left) and distance (right) of a few well-known galaxies put to scale. The following is a list of notable galaxies.. There are about 51 galaxies in the Local Group (see list of nearest galaxies for a complete list), on the order of 100,000 in the Local Supercluster, and an estimated 100 billion in all of the observable universe.
1912 — Vesto Slipher's spectrographic studies of spiral nebulae find high Doppler shifts indicating recessional velocity. 1917 — Heber Curtis finds novae in Andromeda Nebula M31 were ten magnitudes fainter than normal, giving a distance estimate of 150,000 parsecs supporting the "island universes" or independent galaxies hypothesis for spiral nebulae.
More recently, in November 2020, over 300 million habitable exoplanets are estimated to exist in the Milky Way Galaxy. [172] When compared to other more distant galaxies in the universe, the Milky Way galaxy has a below average amount of neutrino luminosity making our galaxy a "neutrino desert". [173]
The term "The Local Group" was introduced by Edwin Hubble in Chapter VI of his 1936 book The Realm of the Nebulae. [11] There, he described it as "a typical small group of nebulae which is isolated in the general field" and delineated, by decreasing luminosity, its members to be M31, Milky Way, M33, Large Magellanic Cloud, Small Magellanic Cloud, M32, NGC 205, NGC 6822, NGC 185, IC 1613 and ...
Many galaxies are thought to have black holes at their active centers. The Milky Way's central black hole, known as Sagittarius A* , has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, and observed as it existed ...
Groups are the most common structures of galaxies in the universe, comprising at least 50% of the galaxies in the local universe. Groups have a mass range between those of the very large elliptical galaxies and clusters of galaxies. [5] Our own galaxy, the Milky Way, is contained in the Local Group of more than 54 galaxies. [6]