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Laniakea Supercluster
For other uses, see Laniakea. The Laniakea Supercluster (Laniakea; also called Local Supercluster or Local SCl) is the galaxy supercluster that is home to the Milky Way and 100,000 other nearby galaxies. It was defined in September 2014, when a group of astronomers including R. Brent Tully of the University of Hawaii and Hélène Courtois of the University of Lyon published a new way of defining superclusters according to the relative velocities of galaxies. The new definition of the local supercluster subsumes the prior defined local supercluster, the Virgo Supercluster, now an appendage.
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Galaxy rotation curve
Rotation curve of the typical spiral galaxy M 33 (yellow and blue points with errorbars) and the predicted one from distribution of the visible matter (white line). The discrepancy between the two curves is accounted for by adding a dark matter halo surrounding the galaxy. Play media Left: A simulated galaxy without dark matter. Right: Galaxy with a flat rotation curve that would be expected under the presence of dark matter. 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. The rotational/orbital speeds of galaxies/stars do not follow the rules found in other orbital systems such as stars/planets and planets/moons that have most of their mass at the centre. Stars revolve around their galaxy's centre at equal or increasing speed over a large range of distances. Instead, the orbital velocity of planets in solar systems and moons orbiting planets decline with distance. In the latter cases, this reflects the mass distributions within those systems. The mass estimations for galaxies based on the light they emit are far too low to explain the velocity observations. The rotation curves of spiral galaxies are asymmetric. The observational data from each side of a galaxy are generally averaged. Rotation curve asymmetry appears to be normal rather than exceptional. The galaxy rotation problem is the discrepancy between observed galaxy rotation curves and the theoretical prediction, assuming a centrally dominated mass associated with the observed luminous material. When mass profiles of galaxies are calculated from the distribution of stars in spirals and mass-to-light ratios in the stellar disks, they do not match with the masses derived from the observed rotation curves and the law of gravity. A solution to this conundrum is to hypothesize the existence of dark matter and to assume its distribution from the galaxy's center out to its halo. Though dark matter is by far the most accepted explanation of the rotation problem, other proposals have been offered with varying degrees of success. Of the possible alternatives, the most notable is Modified Newtonian Dynamics (MOND), which involves modifying the laws of gravity.
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Red supergiant
Hertzsprung–Russell diagram Spectral type Brown dwarfs White dwarfs Red dwarfs Subdwarfs Main sequence ("dwarfs") Subgiants Giants Bright giants Supergiants Hypergiants absolute magni- tude (MV) Red supergiants (RSGs) are supergiant stars (luminosity class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive. Betelgeuse and Antares are the brightest and best known red supergiants, indeed the only first magnitude red supergiants.
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Red dwarf
This article is about the type of star. For the television sitcom, see Red Dwarf. Proxima Centauri, the closest star to the Sun at 4.2 ly, is a red dwarf Hertzsprung–Russell diagram Spectral type Brown dwarfs White dwarfs Red dwarfs Subdwarfs Main sequence ("dwarfs") Subgiants Giants Bright giants Supergiants Hypergiants absolute magni- tude (MV) A red dwarf is a small and relatively cool star on the main sequence, of either K or M spectral type. Red dwarfs range in mass from a low of 0.075 solar masses (M☉) to about 0.50 M☉ and have a surface temperature of less than 4,000 K. Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun, but because of their low luminosity, individual red dwarfs cannot be easily observed. From Earth, not one is visible to the naked eye. Proxima Centauri, the nearest star to the Sun, is a red dwarf (Type M5, apparent magnitude 11.05), as are twenty of the next thirty nearest stars. According to some estimates, red dwarfs make up three-quarters of the stars in the Milky Way. Stellar models indicate that red dwarfs less than 0.35 M☉ are fully convective. Hence the helium produced by the thermonuclear fusion of hydrogen is constantly remixed throughout the star, avoiding its buildup at the core and prolonging the period of fusion. Red dwarfs therefore develop very slowly, maintaining a constant luminosity and spectral type for trillions of years, until their fuel is depleted. Because of the comparatively short age of the universe, no red dwarfs exist at advanced stages of evolution.
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Red giant
This article is about the type of star. For the racehorse, see Red Giant (horse). For the software company, see Red Giant Software. For the comic book publisher, see Red Giant Entertainment. Hertzsprung–Russell diagram Spectral type Brown dwarfs White dwarfs Red dwarfs Subdwarfs Main sequence ("dwarfs") Subgiants Giants Bright giants Supergiants Hypergiants absolute magni- tude (MV) A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses (M☉)) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature as low as 5,000 K and lower. The appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars. The most common red giants are stars on the red-giant branch (RGB) that are still fusing hydrogen into helium in a shell surrounding an inert helium core. Other red giants are the red-clump stars in the cool half of the horizontal branch, fusing helium into carbon in their cores via the triple-alpha process; and the asymptotic-giant-branch (AGB) stars with a helium burning shell outside a degenerate carbon–oxygen core, and a hydrogen burning shell just beyond that.
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Redshift
This article is about the astronomical phenomenon. For other uses, see Redshift (disambiguation). Lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to absorption lines in the optical spectrum of the Sun (left). Arrows indicate redshift. Wavelength increases up towards the red and beyond (frequency decreases). In physics, redshift happens when light or other electromagnetic radiation from an object is increased in wavelength, or shifted to the red end of the spectrum. In general, whether or not the radiation is within the visible spectrum, "redder" means an increase in wavelength – equivalent to a lower frequency and a lower photon energy, in accordance with, respectively, the wave and quantum theories of light. Some redshifts are an example of the Doppler effect, familiar in the change of apparent pitches of sirens and frequency of the sound waves emitted by speeding vehicles. A redshift occurs whenever a light source moves away from an observer. Another kind of redshift is cosmological redshift, which is due to the expansion of the universe, and sufficiently distant light sources (generally more than a few million light years away) show redshift corresponding to the rate of increase in their distance from Earth. Finally, gravitational redshift is a relativistic effect observed in electromagnetic radiation moving out of gravitational fields. Conversely, a decrease in wavelength is called blueshift and is generally seen when a light-emitting object moves toward an observer or when electromagnetic radiation moves into a gravitational field. However, redshift is a more common term and sometimes blueshift is referred to as negative redshift. Knowledge of redshifts and blueshifts has been applied to develop several terrestrial technologies such as Doppler radar and radar guns. Redshifts are also seen in the spectroscopic observations of astronomical objects. Its value is represented by the letter z. A special relativistic redshift formula (and its classical approximation) can be used to calculate the redshift of a nearby object when spacetime is flat. However, in many contexts, such as black holes and Big Bang cosmology, redshifts must be calculated using general relativity. Special relativistic, gravitational, and cosmological redshifts can be understood under the umbrella of frame transformation laws. There exist other physical processes that can lead to a shift in the frequency of electromagnetic radiation, including scattering and optical effects; however, the resulting changes are distinguishable from true redshift and are not generally referred to as such (see section on physical optics and radiative transfer). Redshift and blueshift
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Red clump
The red clump is the prominent group of red giant stars at about 5,000 K and 75 L☉. The red clump is a clustering of red giants in the Hertzsprung–Russell diagram at around 5,000 K and absolute magnitude (MV) +0.5, slightly hotter than most red-giant-branch stars of the same luminosity. It is visible as a more dense region of the red giant branch or a bulge towards hotter temperatures. It is most distinct in many, but not all, galactic open clusters, but it is also noticeable in many intermediate-age globular clusters and in general nearby field stars (e.g. the Hipparcos stars). The red clump giants are cool horizontal branch stars, stars originally similar to the sun which have undergone a helium flash and are now fusing helium in their cores.
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Cosmology
For other uses, see Cosmology (disambiguation). The Hubble eXtreme Deep Field (XDF) was completed in September 2012 and shows the farthest galaxies ever photographed. Except for the few stars in the foreground (which are bright and easily recognizable because only they have diffraction spikes), every speck of light in the photo is an individual galaxy, some of them as old as 13.2 billion years; the observable universe is estimated to contain more than 200 billion galaxies. Cosmology (from the Greek κόσμος, kosmos "world" and -λογία, -logia "study of"), is the study of the origin, evolution, and eventual fate of the universe. Physical cosmology is the scholarly and scientific study of the origin, evolution, large-scale structures and dynamics, and ultimate fate of the universe, as well as the scientific laws that govern these realities. The term cosmology was first used in English in 1656 in Thomas Blount's Glossographia, and in 1731 taken up in Latin by German philosopher Christian Wolff, in Cosmologia Generalis. Religious or mythological cosmology is a body of beliefs based on mythological, religious, and esoteric literature and traditions of creation and eschatology. Physical cosmology is studied by scientists, such as astronomers and physicists, as well as philosophers, such as metaphysicians, philosophers of physics, and philosophers of space and time. Because of this shared scope with philosophy, theories in physical cosmology may include both scientific and non-scientific propositions, and may depend upon assumptions that can not be tested. Cosmology differs from astronomy in that the former is concerned with the Universe as a whole while the latter deals with individual celestial objects. Modern physical cosmology is dominated by the Big Bang theory, which attempts to bring together observational astronomy and particle physics; more specifically, a standard parameterization of the Big Bang with dark matter and dark energy, known as the Lambda-CDM model. Theoretical astrophysicist David N. Spergel has described cosmology as a "historical science" because "when we look out in space, we look back in time" due to the finite nature of the speed of light.
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Astrophysical maser
An astrophysical maser is a naturally occurring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum. This emission may arise in molecular clouds, comets, planetary atmospheres, stellar atmospheres, or various other conditions in interstellar space.
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Coronal mass ejection
Play media This video shows the particle flow around Earth as solar ejecta associated with a coronal mass ejection strike. A coronal mass ejection (CME) is an unusually large release of plasma and magnetic field from the solar corona. They often follow solar flares and are normally present during a solar prominence eruption. The plasma is released into the solar wind, and can be observed in coronagraph imagery. Coronal mass ejections are often associated with other forms of solar activity, but a broadly accepted theoretical understanding of these relationships has not been established. CMEs most often originate from active regions on the Sun's surface, such as groupings of sunspots associated with frequent flares. Near solar maxima, the Sun produces about three CMEs every day, whereas near solar minima, there is about one CME every five days.
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Brown dwarf
Artist's concept of a T-type brown dwarf Comparison: most brown dwarfs are only slightly larger than Jupiter (10–15%) but up to 80 times heavier due to greater density. (Note: The Sun is not to scale and would be larger.) Hertzsprung–Russell diagram Spectral type Brown dwarfs White dwarfs Red dwarfs Subdwarfs Main sequence ("dwarfs") Subgiants Giants Bright giants Supergiants Hypergiants absolute magni- tude (MV) Brown dwarfs are substellar objects that occupy the mass range between the heaviest gas giants and the lightest stars, of approximately 13 to 75–80 Jupiter masses (MJ). Below this range are the sub-brown dwarfs, sometimes referred to as rogue planets, and above it are the lightest red dwarfs (M9 V). Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth. Unlike the stars in the main-sequence, brown dwarfs are not massive enough to sustain nuclear fusion of ordinary hydrogen (1H) to helium in their cores. They are, however, thought to fuse deuterium (2H) and to burn lithium (7Li) if their mass is above a debated threshold of 13 MJ and 65 MJ, respectively. It is also debated whether brown dwarfs would be better defined by their formation processes rather than by their supposed nuclear fusion reactions. Stars are categorized by spectral class, with brown dwarfs designated as types M, L, T, and Y. Despite their name, brown dwarfs are of different colors. Many brown dwarfs would likely appear magenta to the human eye, or possibly orange/red. Brown dwarfs are not very luminous at visible wavelengths. Planets are known to orbit some brown dwarfs: 2M1207b, MOA-2007-BLG-192Lb, and 2MASS J044144b At a distance of about 6.5 light years, the nearest known brown dwarf is Luhman 16, a binary system of brown dwarfs discovered in 2013. DENIS-P J082303.1-491201 b is listed as the most-massive known exoplanet (as of March 2014) in NASA's exoplanet archive, despite having a mass (7001285000000000000♠28.5±1.9 MJ) more than twice the 13-Jupiter-mass cutoff between planets and brown dwarfs.
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Pisces–Cetus Supercluster Complex
The Pisces–Cetus Supercluster Complex is a galaxy filament. It includes the Virgo Supercluster which in turn contains the Local Group, the galaxy cluster that includes the Milky Way. (However, a 2014 study indicates that the Virgo Supercluster is only a lobe of a greater supercluster, Laniakea.)
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Compact star
In astronomy, the term compact star (or compact object) is used to refer collectively to white dwarfs, neutron stars, and black holes. It would grow to include exotic stars if such hypothetical dense bodies are confirmed. Most compact stars are the endpoints of stellar evolution and are thus often referred to as stellar remnants, the form of the remnant depending primarily on the mass of the star when it formed. These objects are all small in volume for their mass, giving them a very high density. The term compact star is often used when the exact nature of the star is not known, but evidence suggests that it is very massive and has a small radius, thus implying one of the above-mentioned categories. A compact star that is not a black hole may be called a degenerate star.
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Cataclysmic variable star
Artist's conception of a cataclysmic variable system Cataclysmic variable stars (CV) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae, from the Latin 'new', since ones with an outburst brightness visible to the naked eye and an invisible quiescent brightness appeared as new stars in the sky. Cataclysmic variable stars are binary stars that consist of two components; a white dwarf primary, and a mass transferring secondary. The stars are so close to each other that the gravity of the white dwarf distorts the secondary, and the white dwarf accretes matter from the companion. Therefore, the secondary is often referred to as the donor star. The infalling matter, which is usually rich in hydrogen, forms in most cases an accretion disc around the white dwarf. Strong UV and X-ray emission is often seen from the accretion disc, powered by the loss of gravitational potential energy from the infalling material. Material at the inner edge of disc falls onto the surface of the white dwarf primary. A classical nova outburst occurs when the density and temperature at the bottom of the accumulated hydrogen layer rise high enough to ignite runaway hydrogen fusion reactions, which rapidly convert the hydrogen layer to helium. If the accretion process continues long enough to bring the white dwarf close to the Chandrasekhar limit, the increasing interior density may ignite runaway carbon fusion and trigger a Type Ia supernova explosion, which would completely destroy the white dwarf. The accretion disc may be prone to an instability leading to dwarf nova outbursts, when the outer portion of the disc changes from a cool, dull mode to a hotter, brighter mode for a time, before reverting to the cool mode. Dwarf novae can recur on a timescale of days to decades.
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List of black holes
This is a dynamic list and may never be able to satisfy particular standards for completeness. You can help by expanding it with reliably sourced entries. This is a list of black holes (and stars considered probable candidates) organized by size (including black holes of undetermined mass); some items in this list are galaxies or star clusters that are believed to be organized around a black hole. Messier and New General Catalogue designations are given where possible.
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Hawking radiation
Simulated view of a black hole (center) in front of the Large Magellanic Cloud. Note the gravitational lensing effect, which produces two enlarged but highly distorted views of the Cloud. Across the top, the Milky Way disk appears distorted into an arc. Hawking radiation is black-body radiation that is predicted to be released by black holes, due to quantum effects near the event horizon. It is named after the physicist Stephen Hawking, who provided a theoretical argument for its existence in 1974, and sometimes also after Jacob Bekenstein, who predicted that black holes should have a finite, non-zero temperature and entropy. Hawking's work followed his visit to Moscow in 1973 where the Soviet scientists Yakov Zeldovich and Alexei Starobinsky showed him that, according to the quantum mechanical uncertainty principle, rotating black holes should create and emit particles. Hawking radiation reduces the mass and energy of black holes and is therefore also known as black hole evaporation. Because of this, black holes that do not gain mass through other means are expected to shrink and ultimately vanish. Micro black holes are predicted to be larger net emitters of radiation than larger black holes and should shrink and dissipate faster. In June 2008, NASA launched the Fermi space telescope, which is searching for the terminal gamma-ray flashes expected from evaporating primordial black holes. In the event that speculative large extra dimension theories are correct, CERN's Large Hadron Collider may be able to create micro black holes and observe their evaporation. In September 2010, a signal that is closely related to black hole Hawking radiation (see analog gravity) was claimed to have been observed in a laboratory experiment involving optical light pulses. However, the results remain unverified and debatable. Other projects have been launched to look for this radiation within the framework of analog gravity.
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Herbig Ae/Be star
A Herbig Ae/Be star (HABe) is a pre-main-sequence star – a young (8 M☉) stars in pre-main-sequence stage are not observed, because they evolve very quickly: when they become visible (i.e. disperses surrounding circumstellar gas and dust cloud), the hydrogen in the center is already burning and they are main-sequence objects.
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