Galaxy

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A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.wikipedia
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Milky Way

galaxyMilky Way Galaxyour galaxy
The word galaxy is derived from the Greek, literally "milky", a reference to the Milky Way. In the astronomical literature, the capitalized word "Galaxy" is often used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe.
The Milky Way is the galaxy that contains our Solar System.

Gravity

gravitationgravitationalgravitational force
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.
Gravity, or gravitation, is a natural phenomenon by which all things with mass or energy— including planets, stars, galaxies, and even light —are brought toward (or gravitate toward) one another.

Dwarf galaxy

dwarf galaxiesdwarfblue compact dwarf galaxy
Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass.
A dwarf galaxy is a small galaxy composed of about 100 million up to several billion stars, a small number compared to the Milky Way's 200–400 billion stars.

Interstellar medium

interstellar gasinterstellarinterstellar matter
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.
In astronomy, the interstellar medium (ISM) is the matter and radiation that exists in the space between the star systems in a galaxy.

Elliptical galaxy

elliptical galaxiesellipticalellipticals
Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular.
An elliptical galaxy is a type of galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless image.

Supermassive black hole

supermassive black holesblack holesuper massive black hole
Many galaxies are thought to have supermassive black holes at their centers.
Observational evidence indicates that all, or nearly all, massive galaxies contain a supermassive black hole, located at the galaxy's center.

Dark matter

dark matter detectiondark-mattermissing mass
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.
The primary evidence for dark matter is that calculations show that many galaxies would fly apart instead of rotating, or would not have formed or move as they do, if they did not contain a large amount of unseen matter.

Irregular galaxy

irregularirregular galaxiesdwarf irregular galaxy
Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular.
An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy.

Galaxy cluster

galaxy clusterscluster of galaxiescluster
The majority of galaxies are gravitationally organized into groups, clusters, and superclusters.
A galaxy cluster, or cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity with typical masses ranging from 10 14 –10 15 solar masses.

Galaxy group

group of galaxiesgroups of galaxiesgroup
The majority of galaxies are gravitationally organized into groups, clusters, and superclusters.
A galaxy group or group of galaxies (GrG ) is an aggregation of galaxies comprising about 50 or fewer gravitationally bound members, each at least as luminous as the Milky Way (about 10 10 times the luminosity of the Sun); collections of galaxies larger than groups that are first-order clustering are called galaxy clusters.

Universe

physical worldthe universeuniverses
In the astronomical literature, the capitalized word "Galaxy" is often used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe.
The Universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy.

Outer space

spaceinterstellar spaceintergalactic medium
The space between galaxies is filled with a tenuous gas (the intergalactic medium) having an average density of less than one atom per cubic meter.
The baseline temperature, as set by the background radiation from the Big Bang, is 2.7 K. The plasma between galaxies accounts for about half of the baryonic (ordinary) matter in the universe; it has a number density of less than one hydrogen atom per cubic metre and a temperature of millions of kelvins; local concentrations of this plasma have condensed into stars and galaxies.

Big Bang

Big Bang Theorybig-bangbirth
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 just 400 million years after the Big Bang.
Giant clouds of these primordial elements (mostly hydrogen, with some helium and lithium) later coalesced through gravity, eventually forming early stars and galaxies, the descendants of which are visible today.

GN-z11

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 just 400 million years after the Big Bang.
GN-z11 is a high-redshift galaxy found in the constellation Ursa Major.

Spiral galaxy

spiral galaxiesspiral armspiral
Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular.
Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence.

Light-year

light yearlight yearsly
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 just 400 million years after the Big Bang.
The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in nonspecialist and popular science publications.

Galaxy filament

filamentsfilamentgalactic filament
At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids.
Filaments consist of gravitationally bound galaxies.

Nebula

nebulaediffuse nebuladiffuse nebulae
Most 18th to 19th Century astronomers considered them as either unresolved star clusters or anagalactic nebulae, and were just thought as a part of the Milky Way, but their true composition and natures remained a mystery.
Originally, the term was used to describe any diffuse astronomical object, including galaxies beyond the Milky Way.

Void (astronomy)

voidvoidscosmic void
At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids.
Cosmic voids are vast spaces between filaments (the largest-scale structures in the universe), which contain very few or no galaxies.

Star cluster

star clustersclusterC
Most 18th to 19th Century astronomers considered them as either unresolved star clusters or anagalactic nebulae, and were just thought as a part of the Milky Way, but their true composition and natures remained a mystery.
Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space even though they are no longer gravitationally bound; they are then known as a stellar association, sometimes also referred to as a moving group.

Magellanic Clouds

Large and Small Magellanic CloudsClouds of MagellanLesser Magellanic Cloud
Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, and the Sombrero Galaxy.
The Magellanic Clouds (or Nubeculae Magellani ) are two irregular dwarf galaxies visible in the Southern Celestial Hemisphere; they are members of the Local Group and are orbiting the Milky Way galaxy.

Laniakea Supercluster

Laniakeaour own
The largest structure of galaxies yet recognised is a cluster of superclusters that has been named Laniakea, which contains the Virgo supercluster.
It was defined in September 2014, when a group of astronomers including R. Brent Tully of the University of Hawaii, Hélène Courtois of the University of Lyon, Yehuda Hoffman of the Hebrew University of Jerusalem, and Daniel Pomarède of CEA Université Paris-Saclay published a new way of defining superclusters according to the relative velocities of galaxies.

Supercluster

superclustersgalactic superclustergalaxy supercluster
The majority of galaxies are gravitationally organized into groups, clusters, and superclusters.
Galaxy

Globular cluster

globular clustersglobulartidal radius
A different method by Harlow Shapley based on the cataloguing of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the center.
Globular clusters are found in the halo of a galaxy and contain considerably more stars and are much older than the less dense open clusters, which are found in the disk of a galaxy.

Galactic plane

planeGalactic diskMilky Way
Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane, but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters, the present picture of our host galaxy, the Milky Way, emerged.
The galactic plane is the plane on which the majority of a disk-shaped galaxy's mass lies.