Disc galaxy

A disc galaxy is a galaxy characterized by a disc, a flattened circular volume of stars. These galaxies may or may not include a central non-disc-like region (a galactic bulge).

Junko Ueda observed that galaxy collisions result in disc galaxies, within 40 million light years from the Earth. While the galaxies are interacting, they change shape in cosmic time as they merge.[1]

Disc galaxy types include:

Galaxies that are not disc types include:

Ngc253 2mass barred spiral
The Sculptor Galaxy (NGC 253) is a disk galaxy


  1. ^ "Violent Origins of Disc Galaxies". Phys.
Andromeda Galaxy

The Andromeda Galaxy (), also known as Messier 31, M31, or NGC 224, is a spiral galaxy approximately 780 kiloparsecs (2.5 million light-years) from Earth, and the nearest major galaxy to the Milky Way. Its name stems from the area of the Earth's sky in which it appears, the constellation of Andromeda.

The 2006 observations by the Spitzer Space Telescope revealed that the Andromeda Galaxy contains approximately one trillion stars, more than twice the number of the Milky Way's estimated 200 to 400 billion stars. The Andromeda Galaxy's mass is estimated to be around 1.76 times that of the Milky Way Galaxy (~0.8-1.5×1012 solar masses vs the Milky Way's 8.5×1011 solar masses), though a 2018 study found that the Andromeda Galaxy's mass is roughly the same as the Milky Way's. The Andromeda Galaxy, spanning approximately 220,000 light-years, is the largest galaxy in the Local Group, which is also home to the Triangulum Galaxy and other minor galaxies.

The Milky Way and Andromeda galaxies are expected to collide in ~4.5 billion years, merging to form a giant elliptical galaxy or a large disc galaxy.

With an apparent magnitude of 3.4, the Andromeda Galaxy is among the brightest of the Messier objects making it visible to the naked eye from Earth on moonless nights, even when viewed from areas with moderate light pollution.

Bulge (astronomy)

In astronomy, a bulge is a tightly packed group of stars within a larger formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies (see galactic spheroid). Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy. It is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies.

Coma Filament

Coma Filament is a galaxy filament. The filament contains the Coma Supercluster of galaxies and forms a part of the CfA2 Great Wall.

Galactic corona

The terms galactic corona and gaseous corona have been used in the first decade of the 21st century to describe a hot, ionised, gaseous component in the Galactic halo of the Milky Way. A similar body of very hot and tenuous gas in the halo of any spiral galaxy may also be described by these terms.

This coronal gas may be sustained by the galactic fountain, in which superbubbles of ionised gas from supernova remnants expand vertically through galactic chimneys into the halo. As the gas cools, it is pulled back into the galactic disc of the galaxy by gravitational forces.

Galactic coronas have been and are currently being studied extensively, in the hope of gaining a further understanding of galaxy formation. Although, considering how galaxies differ in shaping and sizing, no particular theory has been able to adequately illustrate how the galaxies in the Universe originally formed.

Galactic disc

A galactic disc is a component of disc galaxies, such as spiral galaxies and lenticular galaxies. Galactic discs consist of a stellar component ( composed of most of the galaxy's stars) and a gaseous component (mostly composed of cool gas and dust). The stellar population of galactic discs tend to exhibit very little random motion with most of its stars undergoing nearly circular orbits about the galactic center. Discs can be fairly thin because the disc material's motion lies predominantly on the plane of the disc (very little vertical motion). The Milky Way's disc, for example is approximately 1 kpc thick but thickness can vary for discs in other galaxies.

Galactic halo

A galactic halo is an extended, roughly spherical component of a galaxy which extends beyond the main, visible component. Several distinct components of galaxies comprise the halo:

the stellar halo

the galactic corona (hot gas, i.e. a plasma)

the dark matter haloThe distinction between the halo and the main body of the galaxy is clearest in spiral galaxies, where the spherical shape of the halo contrasts with the flat disc. In an elliptical galaxy, there is no sharp transition between the other components of the galaxy and the halo.

Galaxy merger

Galaxy mergers can occur when two (or more) galaxies collide. They are the most violent type of galaxy interaction. The gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies involved. The exact effects of such mergers depend on a wide variety of parameters such as collision angles, speeds, and relative size/composition, and are currently an extremely active area of research. Galaxy mergers are important because the merger rate is fundamental measurement of galaxy evolution. The merger rate also provides astronomers with clues about how galaxies bulked up over time.

Galaxy rotation curve

The rotation curve of a disc galaxy (also called a velocity curve) is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre. It is typically rendered graphically as a plot, and the data observed from each side of a spiral galaxy are generally asymmetric, so that data from each side are averaged to create the curve. A significant discrepancy exists between the experimental curves observed, and a curve derived from theory. The theory of dark matter is currently postulated to account for the variance.

Lynx–Ursa Major Filament

Lynx–Ursa Major Filament (LUM Filament) is a galaxy filament.The filament is connected to and separate from the Lynx–Ursa Major Supercluster.

NGC 2985

NGC 2985 is a spiral galaxy located in the constellation Ursa Major. It is located at a distance of circa 70 million light years from Earth, which, given its apparent dimensions, means that NGC 2985 is about 95,000 light years across. It was discovered by William Herschel on April 3, 1785.The galaxy is seen with an inclination of 37 degrees. The galaxy has a bright nucleus from which emanate multiple tightly wound spiral fragments. Numerous blue knots are visible at the galactic disk. At the outer part of the galaxy lies a massive spiral arm that forms a pseudoring that encircles the galaxy. The inner part of the galaxy, where active star formation has been observed, has been found to be unstable, contrary to the outer stable one. It has been suggested that the presence of molecular clouds accounts for the instability of the region.The nucleus of NGC 2985 is active, and based on its spectrum has been categorised as a LINER. The most accepted theory for the activity source is the presence of an accretion disk around a supermassive black hole. The mass of the supermassive black hole at the centre of NGC 2985 is estimated to be 160 million (108.2) M☉, based on stellar velocity dispersion. The velocity dispersion is anisotropic, and changes with the azimuth. The rotational speed of the galaxy at its effective radius is 222.9 ± 31.2 km/s.NGC 2985 is the brightest member of a galaxy group known as the NGC 2985 group. Other members of the group include NGC 3027, 25 arcminutes away. Other nearby galaxies include NGC 3252, and NGC 3403.

NGC 3621

NGC 3621 is a field spiral galaxy about 22 Mly (6.7 Mpc) away in the equatorial constellation of Hydra. It is comparatively bright and can be well seen in moderate-sized telescopes. The galaxy is around 93,000 ly (29,000 pc) across and is inclined at an angle of 25° from being viewed edge on. It shines with a luminosity equal to 13 billion times that of the Sun. The morphological classification is SA(s)d, which indicates this is an ordinary spiral with loosely wound arms. There is no evidence for a bulge. Although it appears to be isolated, NGC 3621 belongs to the Leo spur.This galaxy has an active nucleus that matches a Seyfert 2 optical spectrum, suggesting that a low mass supermassive black hole is present at the core. Based upon the motion of stars in the nucleus, this object may have a mass of up to three million times the mass of the Sun.

NGC 4550

NGC 4550 is a barred lenticular galaxy located in the constellation of Virgo that can be seen with amateur telescopes. It lies at a distance of 50 million light-years (15.5 mega parsecs) from the Milky Way and is a member of the Virgo Cluster.

NGC 6670

NGC 6670 are a pair of interacting galaxies within the Draco constellation, which lie around 401 million light-years from Earth. Its shape resembles a Leaping Dolphin. NGC 6670 was discovered by Lewis A. Swift on July 31, 1886. NGC 6670 is a combination of two colliding disc galaxies which are known as NGC 6670E and NGC 6670W. The galaxy is 100 billion times brighter than our Sun. The galaxies have already collided once before and they are now moving towards each other again nearing a second collision. Its apparent magnitude is 14.3, its size is 1.0 arc minutes.

Perseus–Pegasus Filament

Perseus–Pegasus Filament is a galaxy filament containing the Perseus-Pisces Supercluster and stretching for roughly a billion light years (or over 300/h Mpc). Currently, it is considered to be one of the largest known structures in the universe. This filament is adjacent to the Pisces–Cetus Supercluster Complex.

Spiral galaxy

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. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.

Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them.

Roughly two-thirds of all spirals are observed to have an additional component in the form of a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. The proportion of barred spirals relative to their barless cousins has likely changed over the history of the Universe, with only about 10% containing bars about 8 billion years ago, to roughly a quarter 2.5 billion years ago, until present, where over two-thirds of the galaxies in the visible universe (Hubble volume) have bars.Our own Milky Way is a barred spiral, although the bar itself is difficult to observe from the Earth's current position within the galactic disc. The most convincing evidence for the stars forming a bar in the galactic center comes from several recent surveys, including the Spitzer Space Telescope.Together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in today's universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters.

Thin disk

The thin disk is a structural component of spiral and S0-type galaxies, composed of stars, gas and dust. The Milky Way's thin disk is thought to have a scale height of around 300–400 parsecs (980–1,300 ly) in the vertical axis perpendicular to the disk, and a scale length of around 2.5–4.5 kiloparsecs (8.2–14.7 kly) in the horizontal axis, in the direction of the radius. For comparison, the Sun is 8 kiloparsecs (26 kly) out from the center. The thin disk contributes about 85% of the stars in the Galactic plane and 95% of the total disk stars. It can be set apart from the thick disk of a galaxy since the latter is composed of older population stars created at an earlier stage of the galaxy formation and thus has less heavy elements. Stars in the thin disk, on the other hand, are created as a result of gas accretion at the later stages of a galaxy formation and are on average more metal-rich.The thin disk contains stars with a wide range of ages and may be divided into a series of sub-populations of increasing age. Notwithstanding, it is considered to be considerably younger than the thick disk.Based upon the emerging science of nucleocosmochronology, the Galactic thin disk of the Milky Way is estimated to have been formed 8.8 ± 1.7 billion years ago. It may have collided with a smaller satellite galaxy, causing the stars in the thin disk to be shaken up and creating the thick disk, while the gas would have settled into the galactic plane and reformed the thin disk.

Timeline of epochs in cosmology

The timeline of cosmological epochs outlines the formation and subsequent evolution of the Universe from the Big Bang (13.799 ± 0.021 billion years ago) to the present day. An epoch is a moment in time, from which nature or situations change to such a degree that, it marks the beginning of a new era or age.

Times on this list are measured from the moment of the Big Bang.

Ursa Major Filament

Ursa Major Filament is a galaxy filament. The filament is connected to the CfA Homunculus, a portion of the filament forms a portion of the "leg" of the Homunculus.

Active nuclei
Energetic galaxies
Low activity
See also

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