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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.
This is a list of astronomical objects named after people.While topological features on Solar System bodies — such as craters, mountains, and valleys — are often named after famous or historical individuals, many stars and deep-sky objects are named after the individual(s) who discovered or otherwise studied it.
Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered. [107] Most of the information we have about dwarf galaxies come from observations of the local group, containing two spiral galaxies, the Milky Way and Andromeda, and many dwarf galaxies.
A comet is named after its first independent discoverers, up to a maximum of three names, separated by hyphens. [ 30 ] [ 31 ] The IAU prefers to credit at most two discoverers, and it credits more than three discoverers only when "in rare cases where named lost comets are identified with a rediscovery that has already received a new name."
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 ...
Observations indicate that there are SMBHs located near the center of most normal galaxies. [191] [192] The nature of the Milky Way's bar is actively debated, with estimates for its half-length and orientation spanning from 1 to 5 kpc (3,000–16,000 ly) and 10–50 degrees relative to the line of sight from Earth to the Galactic Center.
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.
This was followed by the first human spaceflight in 1961, when Yuri Gagarin was sent into orbit on Vostok 1. The first humans to escape low Earth orbit were Frank Borman, Jim Lovell and William Anders in 1968 on board the American Apollo 8, which achieved lunar orbit [177] and reached a maximum distance of 377,349 km (234,474 mi) from the Earth ...