Active galactic nucleus

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Active_galactic_nucleus

An active galactic nucleus (AGN) is a compact region at the center of a galaxy that emits a significant amount of energy across the electromagnetic spectrum, with characteristics indicating that the luminosity is not produced by stars. Such excess, non-stellar emissions have been observed in the radio, microwave, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy. The non-stellar radiation from an AGN is theorized to result from the accretion of matter by a supermassive black hole at the center of its host galaxy.

Active galactic nuclei are the most luminous persistent sources of electromagnetic radiation in the universe and, as such, can be used as a means of discovering distant objects; their evolution as a function of cosmic time also puts constraints on models of the cosmos.

The observed characteristics of an AGN depend on several properties such as the mass of the central black hole, the rate of gas accretion onto the black hole, the orientation of the accretion disk, the degree of obscuration of the nucleus by dust, and presence or absence of jets.

Numerous subclasses of AGN have been defined on the basis of their observed characteristics; the most powerful AGN are classified as quasars. A blazar is an AGN with a jet pointed toward the Earth, in which radiation from the jet is enhanced by relativistic beaming.

History

Quasar 3C 273 observed by the Hubble Space Telescope. The relativistic jet of 3C 273 appears to the left of the bright quasar, and the four straight lines pointing outward from the central source are diffraction spikes caused by the telescope optics.

During the first half of the 20th century, photographic observations of nearby galaxies detected some characteristic signatures of AGN emission, although there was not yet a physical understanding of the nature of the AGN phenomenon. Some early observations included the first spectroscopic detection of emission lines from the nuclei of NGC 1068 and Messier 81 by Edward Fath (published in 1909), and the discovery of the jet in Messier 87 by Heber Curtis (published in 1918). Further spectroscopic studies by astronomers including Vesto Slipher, Milton Humason, and Nicholas Mayall noted the presence of unusual emission lines in some galaxy nuclei. In 1943, Carl Seyfert published a paper in which he described observations of nearby galaxies having bright nuclei that were sources of unusually broad emission lines. Galaxies observed as part of this study included NGC 1068, NGC 4151, NGC 3516, and NGC 7469. Active galaxies such as these are known as Seyfert galaxies in honor of Seyfert's pioneering work.

The development of radio astronomy was a major catalyst to understanding AGN. Some of the earliest detected radio sources are nearby active elliptical galaxies such as Messier 87 and Centaurus A. Another radio source, Cygnus A, was identified by Walter Baade and Rudolph Minkowski as a tidally distorted galaxy with an unusual emission-line spectrum, having a recessional velocity of 16,700 kilometers per second. The 3C radio survey led to further progress in discovery of new radio sources as well as identifying the visible-light sources associated with the radio emission. In photographic images, some of these objects were nearly point-like or quasi-stellar in appearance, and were classified as quasi-stellar radio sources (later abbreviated as "quasars").

Soviet Armenian astrophysicist Viktor Ambartsumian introduced Active Galactic Nuclei in the early 1950s.^[10] At the Solvay Conference on Physics in 1958, Ambartsumian presented a report arguing that "explosions in galactic nuclei cause large amounts of mass to be expelled. For these explosions to occur, galactic nuclei must contain bodies of huge mass and unknown nature. From this point forward Active Galactic Nuclei (AGN) became a key component in theories of galactic evolution." His idea was initially accepted skeptically.

A major breakthrough was the measurement of the redshift of the quasar 3C 273 by Maarten Schmidt, published in 1963. Schmidt noted that if this object was extragalactic (outside the Milky Way, at a cosmological distance) then its large redshift of 0.158 implied that it was the nuclear region of a galaxy about 100 times more powerful than other radio galaxies that had been identified. Shortly afterward, optical spectra were used to measure the redshifts of a growing number of quasars including 3C 48, even more distant at redshift 0.37.

The enormous luminosities of these quasars as well as their unusual spectral properties indicated that their power source could not be ordinary stars. Accretion of gas onto a supermassive black hole was suggested as the source of quasars' power in papers by Edwin Salpeter and Yakov Zeldovich in 1964. In 1969 Donald Lynden-Bell proposed that nearby galaxies contain supermassive black holes at their centers as relics of "dead" quasars, and that black hole accretion was the power source for the non-stellar emission in nearby Seyfert galaxies. In the 1960s and 1970s, early X-ray astronomy observations demonstrated that Seyfert galaxies and quasars are powerful sources of X-ray emission, which originates from the inner regions of black hole accretion disks.

Today, AGN are a major topic of astrophysical research, both observational and theoretical. AGN research encompasses observational surveys to find AGN over broad ranges of luminosity and redshift, examination of the cosmic evolution and growth of black holes, studies of the physics of black hole accretion and the emission of electromagnetic radiation from AGN, examination of the properties of jets and outflows of matter from AGN, and the impact of black hole accretion and quasar activity on galaxy evolution.

Models

UGC 6093 is classified as an active galaxy, which means that it hosts an active galactic nucleus.

For a long time it has been argued that an AGN must be powered by accretion of mass onto massive black holes (10⁶ to 10¹⁰ times the Solar mass). AGN are both compact and persistently extremely luminous. Accretion can potentially give very efficient conversion of potential and kinetic energy to radiation, and a massive black hole has a high Eddington luminosity, and as a result, it can provide the observed high persistent luminosity. Supermassive black holes are now believed to exist in the centres of most if not all massive galaxies since the mass of the black hole correlates well with the velocity dispersion of the galactic bulge (the M–sigma relation) or with bulge luminosity. Thus AGN-like characteristics are expected whenever a supply of material for accretion comes within the sphere of influence of the central black hole.

Accretion disc

Main article: Accretion disc

In the standard model of AGN, cold material close to a black hole forms an accretion disc. Dissipative processes in the accretion disc transport matter inwards and angular momentum outwards, while causing the accretion disc to heat up. The expected spectrum of an accretion disc peaks in the optical-ultraviolet waveband; in addition, a corona of hot material forms above the accretion disc and can inverse-Compton scatter photons up to X-ray energies. The radiation from the accretion disc excites cold atomic material close to the black hole and this in turn radiates at particular emission lines. A large fraction of the AGN's radiation may be obscured by interstellar gas and dust close to the accretion disc, but (in a steady-state situation) this will be re-radiated at some other waveband, most likely the infrared.

Relativistic jets

Main article: Relativistic jet

Image taken by the Hubble Space Telescope of a 5000-light-year-long jet ejected from the active galaxy M87. The blue synchrotron radiation contrasts with the yellow starlight from the host galaxy.

Some accretion discs produce jets of twin, highly collimated, and fast outflows that emerge in opposite directions from close to the disc. The direction of the jet ejection is determined either by the angular momentum axis of the accretion disc or the spin axis of the black hole. The jet production mechanism and indeed the jet composition on very small scales are not understood at present due to the resolution of astronomical instruments being too low. The jets have their most obvious observational effects in the radio waveband, where very-long-baseline interferometry can be used to study the synchrotron radiation they emit at resolutions of sub-parsec scales. However, they radiate in all wavebands from the radio through to the gamma-ray range via the synchrotron and the inverse-Compton scattering process, and so AGN jets are a second potential source of any observed continuum radiation.

Radiatively inefficient AGN

There exists a class of "radiatively inefficient" solutions to the equations that govern accretion. Several theories exist, but the most widely known of these is the Advection Dominated Accretion Flow (ADAF). In this type of accretion, which is important for accretion rates well below the Eddington limit, the accreting matter does not form a thin disc and consequently does not efficiently radiate away the energy that it acquired as it moved close to the black hole. Radiatively inefficient accretion has been used to explain the lack of strong AGN-type radiation from massive black holes at the centres of elliptical galaxies in clusters, where otherwise we might expect high accretion rates and correspondingly high luminosities. Radiatively inefficient AGN would be expected to lack many of the characteristic features of standard AGN with an accretion disc.

Particle acceleration

AGN are a candidate source of high and ultra-high energy cosmic rays (see also Centrifugal mechanism of acceleration).

Observational characteristics

There is no single observational signature of an AGN. The list below covers some of the features that have allowed systems to be identified as AGN.

• Nuclear optical continuum emission. This is visible whenever there is a direct view of the accretion disc. Jets can also contribute to this component of the AGN emission. The optical emission has a roughly power-law dependence on wavelength.
• Nuclear infrared emission. This is visible whenever the accretion disc and its environment are obscured by gas and dust close to the nucleus and then re-emitted ('reprocessing'). As it is thermal emission, it can be distinguished from any jet or disc-related emission.
• Broad optical emission lines. These come from cold material close to the central black hole. The lines are broad because the emitting material is revolving around the black hole with high speeds causing a range of Doppler shifts of the emitted photons.
• Narrow optical emission lines. These come from more distant cold material, and so are narrower than the broad lines.
• Radio continuum emission. This is always due to a jet. It shows a spectrum characteristic of synchrotron radiation.
• X-ray continuum emission. This can arise both from a jet and from the hot corona of the accretion disc via a scattering process: in both cases it shows a power-law spectrum. In some radio-quiet AGN there is an excess of soft X-ray emission in addition to the power-law component. The origin of the soft X-rays is not clear at present.
• X-ray line emission. This is a result of illumination of cold heavy elements by the X-ray continuum that causes fluorescence of X-ray emission lines, the best-known of which is the iron feature around 6.4 keV. This line may be narrow or broad: relativistically broadened iron lines can be used to study the dynamics of the accretion disc very close to the nucleus and therefore the nature of the central black hole.

Types of active galaxy

It is convenient to divide AGN into two classes, conventionally called radio-quiet and radio-loud. Radio-loud objects have emission contributions from both the jet(s) and the lobes that the jets inflate. These emission contributions dominate the luminosity of the AGN at radio wavelengths and possibly at some or all other wavelengths. Radio-quiet objects are simpler since jet and any jet-related emission can be neglected at all wavelengths.

AGN terminology is often confusing, since the distinctions between different types of AGN sometimes reflect historical differences in how the objects were discovered or initially classified, rather than real physical differences.

Radio-quiet AGN

• Low-ionization nuclear emission-line regions (LINERs). As the name suggests, these systems show only weak nuclear emission-line regions, and no other signatures of AGN emission. It is debatable whether all such systems are true AGN (powered by accretion on to a supermassive black hole). If they are, they constitute the lowest-luminosity class of radio-quiet AGN. Some may be radio-quiet analogues of the low-excitation radio galaxies (see below).
• Seyfert galaxies. Seyferts were the earliest distinct class of AGN to be identified. They show optical range nuclear continuum emission, narrow and occasionally broad emission lines, occasionally strong nuclear X-ray emission and sometimes a weak small-scale radio jet. Originally they were divided into two types known as Seyfert 1 and 2: Seyfert 1s show strong broad emission lines while Seyfert 2s do not, and Seyfert 1s are more likely to show strong low-energy X-ray emission. Various forms of elaboration on this scheme exist: for example, Seyfert 1s with relatively narrow broad lines are sometimes referred to as narrow-line Seyfert 1s. The host galaxies of Seyferts are usually spiral or irregular galaxies.
• Radio-quiet quasars/QSOs. These are essentially more luminous versions of Seyfert 1s: the distinction is arbitrary and is usually expressed in terms of a limiting optical magnitude. Quasars were originally 'quasi-stellar' in optical images as they had optical luminosities that were greater than that of their host galaxy. They always show strong optical continuum emission, X-ray continuum emission, and broad and narrow optical emission lines. Some astronomers use the term QSO (Quasi-Stellar Object) for this class of AGN, reserving 'quasar' for radio-loud objects, while others talk about radio-quiet and radio-loud quasars. The host galaxies of quasars can be spirals, irregulars or ellipticals. There is a correlation between the quasar's luminosity and the mass of its host galaxy, in that the most luminous quasars inhabit the most massive galaxies (ellipticals).
• 'Quasar 2s'. By analogy with Seyfert 2s, these are objects with quasar-like luminosities but without strong optical nuclear continuum emission or broad line emission. They are scarce in surveys, though a number of possible candidate quasar 2s have been identified.

Radio-loud AGN

See main article Radio galaxy for a discussion of the large-scale behaviour of the jets. Here, only the active nuclei are discussed.

• Radio-loud quasars behave exactly like radio-quiet quasars with the addition of emission from a jet. Thus they show strong optical continuum emission, broad and narrow emission lines, and strong X-ray emission, together with nuclear and often extended radio emission.
• "Blazars" (BL Lac objects and OVV quasars) classes are distinguished by rapidly variable, polarized optical, radio and X-ray emission. BL Lac objects show no optical emission lines, broad or narrow, so that their redshifts can only be determined from features in the spectra of their host galaxies. The emission-line features may be intrinsically absent or simply swamped by the additional variable component. In the latter case, emission lines may become visible when the variable component is at a low level. OVV quasars behave more like standard radio-loud quasars with the addition of a rapidly variable component. In both classes of source, the variable emission is believed to originate in a relativistic jet oriented close to the line of sight. Relativistic effects amplify both the luminosity of the jet and the amplitude of variability.
• Radio galaxies. These objects show nuclear and extended radio emission. Their other AGN properties are heterogeneous. They can broadly be divided into low-excitation and high-excitation classes. Low-excitation objects show no strong narrow or broad emission lines, and the emission lines they do have may be excited by a different mechanism. Their optical and X-ray nuclear emission is consistent with originating purely in a jet. They may be the best current candidates for AGN with radiatively inefficient accretion. By contrast, high-excitation objects (narrow-line radio galaxies) have emission-line spectra similar to those of Seyfert 2s. The small class of broad-line radio galaxies, which show relatively strong nuclear optical continuum emission probably includes some objects that are simply low-luminosity radio-loud quasars. The host galaxies of radio galaxies, whatever their emission-line type, are essentially always ellipticals.

Features of different types of galaxies

Galaxy type	Active nuclei	Emission lines		X-rays	Excess of		Strong radio	Jets	Variable	Radio loud
		Narrow	Broad		UV	Far-IR
Normal (non-AGN)	no	weak	no	weak	no	no	no	no	no	no
LINER	unknown	weak	weak	weak	no	no	no	no	no	no
Seyfert I	yes	yes	yes	some	some	yes	few	no	yes	no
Seyfert II	yes	yes	no	some	some	yes	few	no	yes	no
Quasar	yes	yes	yes	some	yes	yes	some	some	yes	some
Blazar	yes	no	some	yes	yes	no	yes	yes	yes	yes
BL Lac	yes	no	no/faint	yes	yes	no	yes	yes	yes	yes
OVV	yes	no	stronger than BL Lac	yes	yes	no	yes	yes	yes	yes
Radio galaxy	yes	some	some	some	some	yes	yes	yes	yes	yes

Unification of AGN species

Unified AGN models

Unified models propose that different observational classes of AGN are a single type of physical object observed under different conditions. The currently favoured unified models are 'orientation-based unified models' meaning that they propose that the apparent differences between different types of objects arise simply because of their different orientations to the observer. However, they are debated (see below).

Radio-quiet unification

At low luminosities, the objects to be unified are Seyfert galaxies. The unification models propose that in Seyfert 1s the observer has a direct view of the active nucleus. In Seyfert 2s the nucleus is observed through an obscuring structure which prevents a direct view of the optical continuum, broad-line region or (soft) X-ray emission. The key insight of orientation-dependent accretion models is that the two types of object can be the same if only certain angles to the line of sight are observed. The standard picture is of a torus of obscuring material surrounding the accretion disc. It must be large enough to obscure the broad-line region but not large enough to obscure the narrow-line region, which is seen in both classes of object. Seyfert 2s are seen through the torus. Outside the torus there is material that can scatter some of the nuclear emission into our line of sight, allowing us to see some optical and X-ray continuum and, in some cases, broad emission lines—which are strongly polarized, showing that they have been scattered and proving that some Seyfert 2s really do contain hidden Seyfert 1s. Infrared observations of the nuclei of Seyfert 2s also support this picture.

At higher luminosities, quasars take the place of Seyfert 1s, but, as already mentioned, the corresponding 'quasar 2s' are elusive at present. If they do not have the scattering component of Seyfert 2s they would be hard to detect except through their luminous narrow-line and hard X-ray emission.

Radio-loud unification

Historically, work on radio-loud unification has concentrated on high-luminosity radio-loud quasars. These can be unified with narrow-line radio galaxies in a manner directly analogous to the Seyfert 1/2 unification (but without the complication of much in the way of a reflection component: narrow-line radio galaxies show no nuclear optical continuum or reflected X-ray component, although they do occasionally show polarized broad-line emission). The large-scale radio structures of these objects provide compelling evidence that the orientation-based unified models really are true. X-ray evidence, where available, supports the unified picture: radio galaxies show evidence of obscuration from a torus, while quasars do not, although care must be taken since radio-loud objects also have a soft unabsorbed jet-related component, and high resolution is necessary to separate out thermal emission from the sources' large-scale hot-gas environment. At very small angles to the line of sight, relativistic beaming dominates, and we see a blazar of some variety.

However, the population of radio galaxies is completely dominated by low-luminosity, low-excitation objects. These do not show strong nuclear emission lines—broad or narrow—they have optical continua which appear to be entirely jet-related, and their X-ray emission is also consistent with coming purely from a jet, with no heavily absorbed nuclear component in general. These objects cannot be unified with quasars, even though they include some high-luminosity objects when looking at radio emission, since the torus can never hide the narrow-line region to the required extent, and since infrared studies show that they have no hidden nuclear component: in fact there is no evidence for a torus in these objects at all. Most likely, they form a separate class in which only jet-related emission is important. At small angles to the line of sight, they will appear as BL Lac objects.

Criticism of the radio-quiet unification

In the recent literature on AGN, being subject to an intense debate, an increasing set of observations appear to be in conflict with some of the key predictions of the Unified Model, e.g. that each Seyfert 2 has an obscured Seyfert 1 nucleus (a hidden broad-line region).

Therefore, one cannot know whether the gas in all Seyfert 2 galaxies is ionized due to photoionization from a single, non-stellar continuum source in the center or due to shock-ionization from e.g. intense, nuclear starbursts. Spectropolarimetric studies reveal that only 50% of Seyfert 2s show a hidden broad-line region and thus split Seyfert 2 galaxies into two populations. The two classes of populations appear to differ by their luminosity, where the Seyfert 2s without a hidden broad-line region are generally less luminous. This suggests absence of broad-line region is connected to low Eddington ratio, and not to obscuration.

The covering factor of the torus might play an important role. Some torus models predict how Seyfert 1s and Seyfert 2s can obtain different covering factors from a luminosity- and accretion rate- dependence of the torus covering factor, something supported by studies in the x-ray of AGN. The models also suggest an accretion-rate dependence of the broad-line region and provide a natural evolution from more active engines in Seyfert 1s to more "dead" Seyfert 2s and can explain the observed break-down of the unified model at low luminosities and the evolution of the broad-line region.

While studies of single AGN show important deviations from the expectations of the unified model, results from statistical tests have been contradictory. The most important short-coming of statistical tests by direct comparisons of statistical samples of Seyfert 1s and Seyfert 2s is the introduction of selection biases due to anisotropic selection criteria.

Studying neighbour galaxies rather than the AGN themselves first suggested the numbers of neighbours were larger for Seyfert 2s than for Seyfert 1s, in contradiction with the Unified Model. Today, having overcome the previous limitations of small sample sizes and anisotropic selection, studies of neighbours of hundreds to thousands of AGN have shown that the neighbours of Seyfert 2s are intrinsically dustier and more star-forming than Seyfert 1s and a connection between AGN type, host galaxy morphology and collision history. Moreover, angular clustering studies of the two AGN types confirm that they reside in different environments and show that they reside within dark matter halos of different masses. The AGN environment studies are in line with evolution-based unification models where Seyfert 2s transform into Seyfert 1s during merger, supporting earlier models of merger-driven activation of Seyfert 1 nuclei.

While controversy about the soundness of each individual study still prevails, they all agree on that the simplest viewing-angle based models of AGN Unification are incomplete. Seyfert-1 and Seyfert-2 seem to differ in star formation and AGN engine power.

While it still might be valid that an obscured Seyfert 1 can appear as a Seyfert 2, not all Seyfert 2s must host an obscured Seyfert 1. Understanding whether it is the same engine driving all Seyfert 2s, the connection to radio-loud AGN, the mechanisms of the variability of some AGN that vary between the two types at very short time scales, and the connection of the AGN type to small- and large-scale environment remain important issues to incorporate into any unified model of active galactic nuclei.

A study of Swift/BAT AGN published in July 2022 adds support to the "radiation-regulated unification model" outlined in 2017.  In this model, the relative accretion rate (termed the "Eddington ratio") of the black hole has a significant impact on the observed features of the AGN. Black Holes with higher Eddington ratios appear to be more likely to be unobscured, having cleared away locally obscuring material in a very short timescale.

Cosmological uses and evolution

For a long time, active galaxies held all the records for the highest-redshift objects known either in the optical or the radio spectrum, because of their high luminosity. They still have a role to play in studies of the early universe, but it is now recognised that an AGN gives a highly biased picture of the "typical" high-redshift galaxy.

Most luminous classes of AGN (radio-loud and radio-quiet) seem to have been much more numerous in the early universe. This suggests that massive black holes formed early on and that the conditions for the formation of luminous AGN were more common in the early universe, such as a much higher availability of cold gas near the centre of galaxies than at present. It also implies that many objects that were once luminous quasars are now much less luminous, or entirely quiescent. The evolution of the low-luminosity AGN population is much less well understood due to the difficulty of observing these objects at high redshifts.

Starburst galaxy

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Starburst_galaxy 

The Antennae Galaxies are an example of a starburst galaxy occurring from the collision of NGC 4038/NGC 4039. Credit: NASA/ESA.

A starburst galaxy is one undergoing an exceptionally high rate of star formation, as compared to the long-term average rate of star formation in the galaxy or the star formation rate observed in most other galaxies. For example, the star formation rate of the Milky Way galaxy is approximately 3 M_☉/yr, while starburst galaxies can experience star formation rates of 100 M_☉/yr or more. In a starburst galaxy, the rate of star formation is so large that the galaxy will consume all of its gas reservoir, from which the stars are forming, on a timescale much shorter than the age of the galaxy. As such, the starburst nature of a galaxy is a phase, and one that typically occupies a brief period of a galaxy's evolution. The majority of starburst galaxies are in the midst of a merger or close encounter with another galaxy. Starburst galaxies include M82, NGC 4038/NGC 4039 (the Antennae Galaxies), and IC 10.

Definition

Light and dust in a nearby starburst galaxy

Starburst galaxies are defined by these three interrelated factors:

1. The rate at which the galaxy is currently converting gas into stars (the star-formation rate, or SFR).
2. The available quantity of gas from which stars can be formed.
3. A comparison of the timescale on which star formation will consume the available gas with the age or rotation period of the galaxy.

Commonly used definitions include:

• Continued star-formation where the current SFR would exhaust the available gas reservoir in much less than the age of the Universe (the Hubble Time).
• Continued star-formation where the current SFR would exhaust the available gas reservoir in much less than the dynamical timescale of the galaxy (perhaps one rotation period in a disk type galaxy).
• The current SFR, normalized by the past-averaged SFR, is much greater than unity. This ratio is referred to as the "birthrate parameter".

Triggering mechanisms

Mergers and tidal interactions between gas-rich galaxies play a large role in driving starbursts. Galaxies in the midst of a starburst frequently show tidal tails, an indication of a close encounter with another galaxy, or are in the midst of a merger. Turbulence, along with variations of time and space, cause the dense gas within a galaxy to compress and rapidly increase star formation. The efficiency at which the galaxy forms also increases its SFR . These changes in the rate of star formation also led to variations with depletion time, and power a starburst with its own galactic mechanisms rather than merging with another galaxy. Interactions between galaxies that do not merge can trigger unstable rotation modes, such as the bar instability, which causes gas to be funneled towards the nucleus and ignites bursts of star formation near the galactic nucleus. It has been shown that there is a strong correlation between the lopsidedness of a galaxy and the youth of its stellar population, with more lopsided galaxies having younger central stellar populations. As lopsidedness can be caused by tidal interactions and mergers between galaxies, this result gives further evidence that mergers and tidal interactions can induce central star formation in a galaxy and drive a starburst.

Types

Artist's impression of a galaxy undergoing a starburst.

Classifying types of starburst galaxies is difficult since starburst galaxies do not represent a specific type in and of themselves. Starbursts can occur in disk galaxies, and irregular galaxies often exhibit knots of starburst spread throughout the irregular galaxy. Nevertheless, astronomers typically classify starburst galaxies based on their most distinct observational characteristics. Some of the categorizations include:

• Blue compact galaxies (BCGs). These galaxies are often low mass, low metallicity, dust-free objects. Because they are dust-free and contain a large number of hot, young stars, they are often blue in optical and ultraviolet colours. It was initially thought that BCGs were genuinely young galaxies in the process of forming their first generation of stars, thus explaining their low metal content. However, old stellar populations have been found in most BCGs, and it is thought that efficient mixing may explain the apparent lack of dust and metals. Most BCGs show signs of recent mergers and/or close interactions. Well-studied BCGs include IZw18 (the most metal poor galaxy known), ESO338-IG04 and Haro11.
  • Blue compact dwarf galaxies (BCD galaxies) are small compact galaxies.
  • Green Pea galaxies (GPs) are small compact galaxies resembling primordial starbursts. They were found by citizen scientists taking part in the Galaxy Zoo project.
• Luminous infrared galaxies (LIRGs).
  • Ultra-luminous Infrared Galaxies (ULIRGs). These galaxies are generally extremely dusty objects. The ultraviolet radiation produced by the obscured star-formation is absorbed by the dust and reradiated in the infrared spectrum at wavelengths of around 100 micrometres. This explains the extreme red colours associated with ULIRGs. It is not known for sure that the UV radiation is produced purely by star-formation, and some astronomers believe ULIRGs to be powered (at least in part) by active galactic nuclei (AGN). X-ray observations of many ULIRGs that penetrate the dust suggest that many starburst galaxies are double-cored systems, lending support to the hypothesis that ULIRGs are powered by star-formation triggered by major mergers. Well-studied ULIRGs include Arp 220.
  • Hyperluminous Infrared galaxies (HLIRGs), sometimes called submillimeter galaxies.

SBS 1415+437 is a WR galaxy located about 45 million light-years from Earth.

• Wolf–Rayet galaxies (WR galaxies), galaxies where a large portion of the bright stars are Wolf–Rayet stars. The Wolf–Rayet phase is a relatively short-lived phase in the life of massive stars, typically 10% of the total life-time of these stars and as such any galaxy is likely to contain few of these. However, because the stars are both very luminous and have very distinctive spectral features, it is possible to identify these stars in the spectra of entire galaxies and doing so allows good constraints to be placed on the properties of the starbursts in these galaxies.

Ingredients

Messier 82 is the prototype nearby starburst galaxy about 12 million light-years away in the constellation Ursa Major.

Firstly, a starburst galaxy must have a large supply of gas available to form stars. The burst itself may be triggered by a close encounter with another galaxy (such as M81/M82), a collision with another galaxy (such as the Antennae), or by another process which forces material into the centre of the galaxy (such as a stellar bar).

The inside of the starburst is quite an extreme environment. The large amounts of gas mean that very massive stars are formed. Young, hot stars ionize the gas (mainly hydrogen) around them, creating H II regions. Groups of very hot stars are known as OB associations. These stars burn very bright and very fast, and are quite likely to explode at the end of their lives as supernovae.

After the supernova explosion, the ejected material expands and becomes a supernova remnant. These remnants interact with the surrounding environment within the starburst (the interstellar medium) and can be the site of naturally occurring masers.

Studying nearby starburst galaxies can help us determine the history of galaxy formation and evolution. Large numbers of the very distant galaxies seen, for example, in the Hubble Deep Field are known to be starbursts, but they are too far away to be studied in any detail. Observing nearby examples and exploring their characteristics can give us an idea of what was happening in the early universe as the light we see from these distant galaxies left them when the universe was much younger (see redshift). However, starburst galaxies seem to be quite rare in our local universe, and are more common further away – indicating that there were more of them billions of years ago. All galaxies were closer together then, and therefore more likely to be influenced by each other's gravity. More frequent encounters produced more starbursts as galactic forms evolved with the expanding universe.

Examples

Artist's impression of gas fueling distant starburst galaxies.

M82 is the archetypal starburst galaxy. Its high level of star formation is due to a close encounter with the nearby spiral M81. Maps of the regions made with radio telescopes show large streams of neutral hydrogen connecting the two galaxies, also as a result of the encounter. Radio images of the central regions of M82 also show a large number of young supernova remnants, left behind when the more massive stars created in the starburst came to the end of their lives. The Antennae is another starburst system, detailed by a Hubble picture, released in 1997.

List of starburst galaxies

This is a dynamic list and may never be able to satisfy particular standards for completeness. You can help by adding missing items with reliable sources.

 

Galaxy	Type	Notes
M82	I0	Archetype starburst galaxy
Antennae Galaxies	SB(s)m pec / SA(s)m pec	Two colliding galaxies
IC 10	dIrr	Mild starburst galaxy
HXMM01		Extreme starburst merging galaxies
HFLS3		Unusually large intense starburst galaxy
NGC 1569	IBm	Dwarf galaxy undergoing a galaxy-wide starburst
NGC 2146	SB(s)ab pec
NGC 1705	SA0− pec
NGC 1614	SB(s)c pec	Merging with another galaxy
NGC 6946	SAB(rs)cd	Also known as fireworks galaxy for frequent supernovae
Baby Boom Galaxy		Brightest starburst galaxy in distant universe
Centaurus A	E(p)	Only known case of an "Elliptical Starburst" galaxy
Large Magellanic Cloud		Being disrupted by the Milky Way
Haro 11		Emits Lyman continuum photons
Sculptor Galaxy	SAB(s)c	Nearest starburst galaxy.
Kiso 5639		Also known as the 'Skyrocket Galaxy' due to its appearance

Magellanic Clouds

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Magellanic_Clouds 

The Large and Small Magellanic Clouds

ALMA antennae bathed in red light. In the background are the southern Milky Way on the left and the Magellanic Clouds at the top.

The Magellanic Clouds (Magellanic system or Nubeculae Magellani) are two irregular dwarf galaxies in the southern celestial hemisphere. Orbiting the Milky Way galaxy, these satellite galaxies are members of the Local Group. Because both show signs of a bar structure, they are often reclassified as Magellanic spiral galaxies.

The two galaxies are the following:

• Large Magellanic Cloud (LMC), about 163 kly (50 kpc) away
• Small Magellanic Cloud (SMC), about 206 kly (63 kpc) away

The Magellanic clouds are visible to the unaided eye from the Southern Hemisphere, but cannot be observed from the most northern latitudes.

History

The Magellanic Clouds have been known since ancient times to indigenous peoples across South America, Australia, and Africa, and from the first millennium in Western Asia. The first preserved mention of the Large Magellanic Cloud is believed to be in petroglyphs and rock drawings found in Chile. They may be the objects mentioned by the polymath Ibn Qutaybah (d. 889 CE), in his book on Al-Anwā̵’ (the stations of the Moon in pre-Islamic Arabian culture):

"وأسفل من سهيل قدما سهيل . وفى مجرى قدمى سهيل، من خلفهما كواكب زهر كبار، لا ترى بالعراق، يسميها أهل تهامة الأعبار

"And below Canopus, there are the feet of Canopus, and on their extension, behind them bright big stars, not seen in Iraq, the people of Tihama call them al-a‘bār."

Later Al Sufi, a professional astronomer, in 964 CE, in his Book of Fixed Stars, mentioned the same quote, but with a different spelling. Under Argo Navis, he quoted that "unnamed others have claimed that beneath Canopus there are two stars known as the 'feet of Canopus', and beneath those there are bright white stars that are unseen in Iraq nor Najd, and that the inhabitants of Tihama call them al-Baqar [cows], and Ptolemy did not mention any of this so we [Al-Sufi] do not know if this is true or false." Both Ibn Qutaybah and Al-Sufi were probably quoting from the former's contemporary (and compatriot) and famed scientist Abu Hanifa Dinawari's mostly lost work on Anwaa. Abu Hanifa was probably quoting earlier sources, which may be just travelers stories, and hence Al-Sufi's comments about their veracity.

In Europe, the Clouds were first reported by 16th century Italian authors Peter Martyr d'Anghiera and Andrea Corsali, both based on Portuguese voyages. Subsequently, they were reported by Antonio Pigafetta, who accompanied the expedition of Ferdinand Magellan on its circumnavigation of the world in 1519–1522. However, naming the clouds after Magellan did not become widespread until much later. In Bayer's Uranometria they are designated as nubecula major and nubecula minor. In the 1756 star map of the French astronomer Lacaille, they are designated as le Grand Nuage and le Petit Nuage ("the Large Cloud" and "the Small Cloud"). John Herschel studied the Magellanic Clouds from South Africa, writing an 1847 report detailing 919 objects in the Large Magellanic Cloud and 244 objects in the Small Magellanic Cloud. In 1867 Cleveland Abbe suggested that they were separate satellites of the Milky Way. Distances were first estimated by Ejnar Hertzsprung in 1913 using 1912 measurements of Cepheid variables in the SMC by Henrietta Leavitt. Recalibration of the Cepheid scales allowed Harlow Shapley to refine the measurement, and these were again revised in 1952 following further research. As of 2023, some astronomers believe the Magellanic Clouds should be renamed, alleging that Magellan was a murderer and neither an astronomer nor the discoverer of the dwarf galaxies.

Characteristics

The Large Magellanic Cloud (LMC).

Small Magellanic Cloud (SMC).

LMC and SMC rendered from Gaia EDR3 data with foreground stars removed

The Large Magellanic Cloud and its neighbour and relative, the Small Magellanic Cloud, are conspicuous objects in the southern hemisphere, looking like separated pieces of the Milky Way to the naked eye. Roughly 21° apart in the night sky, the true distance between them is roughly 75,000 light-years. Until the discovery of the Sagittarius Dwarf Elliptical Galaxy in 1994, they were the closest known galaxies to our own (since 2003, the Canis Major Dwarf Galaxy was discovered to be closer still, and is now considered the actual nearest neighbor). The LMC lies about 160,000 light years away, while the SMC is around 200,000. The LMC is about twice the diameter of the SMC (14,000 ly and 7,000 ly respectively). For comparison, the Milky Way is about 100,000 ly across.

The total mass of these two galaxies is uncertain. Only a fraction of their gas seems to have coalesced into stars and they probably both have large dark matter halos. One recent estimate of the total mass of the LMC is about 1/10 that of the Milky Way. That would make the LMC rather a large galaxy in the current observable universe. Since the sizes of relatively nearby galaxies are highly skewed, the average mass can be a misleading statistic. In terms of rank, the LMC appears to be the fourth most massive member of over 50 galaxies in the local group. Suggesting that the Magellanic cloud system is historically not a part of the Milky Way is evidence that the SMC has been in orbit about the LMC for a very long time. The Magellanic system seems most similar to the distinct NGC 3109 system, which is on the edge of the Local Group.

Astronomers have long assumed that the Magellanic Clouds have orbited the Milky Way at approximately their current distances, but evidence suggests that it is rare for them to come as close to the Milky Way as they are now. Observation and theoretical evidence suggest that the Magellanic Clouds have both been greatly distorted by tidal interaction with the Milky Way as they travel close to it. The LMC maintains a very clear spiral structure in radio-telescope images of neutral hydrogen. Streams of neutral hydrogen connect them to the Milky Way and to each other, and both resemble disrupted barred spiral galaxies. Their gravity has affected the Milky Way as well, distorting the outer parts of the galactic disk.

Aside from their different structure and lower mass, they differ from our galaxy in two major ways. They are gas-rich; a higher fraction of their mass is hydrogen and helium compared to the Milky Way. They are also more metal-poor than the Milky Way; the youngest stars in the LMC and SMC have a metallicity of 0.5 and 0.25 times solar, respectively. Both are noted for their nebulae and young stellar populations, but as in our own galaxy their stars range from the very young to the very old, indicating a long stellar formation history.

The Large Magellanic Cloud was the host galaxy to a supernova (SN 1987A), the brightest observed in over four centuries.

Measurements with the Hubble Space Telescope, announced in 2006, suggest the Magellanic Clouds may be moving too fast to be long term companions of the Milky Way. If they are in orbit, that orbit takes at least 4 billion years. They are possibly on a first approach and we are witnessing the start of a galactic merger that may overlap with the Milky Way's expected merger with the Andromeda Galaxy (and perhaps the Triangulum Galaxy) in the future.

In 2019, astronomers discovered the young star cluster Price-Whelan 1 using Gaia data. The star cluster has a low metallicity and belongs to the leading arm of the Magellanic Clouds. The existence of this star cluster suggests that the leading arm of the Magellanic Clouds is 90,000 light-years away from the Milky Way—closer than previously thought.

Mini Magellanic Cloud (MMC)

Astrophysicists D. S. Mathewson, V. L. Ford and N. Visvanathan proposed that the SMC may in fact be split in two, with a smaller section of this galaxy behind the main part of the SMC (as seen from Earth's perspective), and separated by about 30,000 light years. They suggest the reason for this is due to a past interaction with the LMC splitting the SMC, and that the two sections are still moving apart. They have dubbed this smaller remnant the Mini Magellanic Cloud.

Anapanasati

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Anapanasati 

Buddha statue doing anapanasati

Ānāpānasati (Pali; Sanskrit ānāpānasmṛti), meaning "mindfulness of breathing" ("sati" means mindfulness; "ānāpāna" refers to inhalation and exhalation), paying attention to the breath. It is the quintessential form of Buddhist meditation, attributed to Gautama Buddha, and described in several suttas, most notably the Ānāpānasati Sutta (MN 118).

Derivations of anāpānasati are common to Tibetan, Zen, Tiantai, and Theravada Buddhism as well as Western-based mindfulness programs.

Contemplation of bodily phenomena

The Ānāpānasati Sutta prescribes mindfulness of inhalation and exhalation as an element of mindfulness of the body, and recommends the practice of mindfulness of breathing as a means of cultivating the seven factors of awakening, which is an alternative formulation or description of the process of dhyana: sati (mindfulness), dhamma vicaya (analysis), viriya (persistence), pīti (rapture), passaddhi (serenity), samadhi (unification of mind), and upekkhā (equanimity). According to this and other sutras, the development of these factors leads to release (Pali: vimutti; Sanskrit mokṣa) from dukkha (suffering) and the attainment of nirvana.

Derivations of anāpānasati are a core meditation practice in Theravada, Tiantai, and Chan traditions of Buddhism as well as a part of many mindfulness programs. According to Anālayo, in both ancient and modern times anāpānasati by itself is likely the most widely used Buddhist method for contemplating bodily phenomena.

The practice

Ānāpānasati sutta

See also: Satipatthana Sutta

The mindfulness practice described in the Ānāpānasati Sutra is to go into the forest and sit beneath a tree and then to simply watch the breath:

Breathing in long, he discerns, 'I am breathing in long'; or breathing out long, he discerns, 'I am breathing out long.' Or breathing in short, he discerns, 'I am breathing in short'; or breathing out short, he discerns, 'I am breathing out short.' He trains himself, 'I will breathe in sensitive to the entire body.' He trains himself, 'I will breathe out sensitive to the entire body.' He trains himself, 'I will breathe in calming bodily fabrication.' He trains himself, 'I will breathe out calming bodily fabrication.'

While inhaling and exhaling, the meditator practises:

• training the mind to be sensitive to one or more of: the entire body, rapture, pleasure, the mind itself, and mental processes
• training the mind to be focused on one or more of: inconstancy, dispassion, cessation, and relinquishment
• steadying, satisfying, or releasing the mind.

If this practice is pursued and well developed, it is said by the Buddha to bring great benefit, aiding the development of mindfulness as one of the factors of awakening:

On whatever occasion the monk remains focused on the body in & of itself — ardent, alert, & mindful — putting aside greed & distress with reference to the world, on that occasion his mindfulness is steady & without lapse. When his mindfulness is steady & without lapse, then mindfulness as a factor for awakening becomes aroused. He develops it, and for him it goes to the culmination of its development.

Post-canonical development

Main article: Ganana

A popular post-canonical method still used today, follows four stages:

• repeatedly counting exhalations in cycles of 10
• repeatedly counting inhalations in cycles of 10
• focusing on the breath without counting
• focusing only on the spot where the breath enters and leaves the nostrils (i.e., the nostril and upper lip area).

Counting the breath is attributed by the Theravada tradition to Buddhaghosa's commentary the Visuddhimagga, but Vasubandhu's Abhidharmakośakārikā also teaches the counting of breaths to ten. The dhyāna sutras, based on Sarvastivada practices, and translated into Chinese by An Shigao, also recommends counting the breath, and forms the basis of Zen practices. In the dhyana sutras his is organized into a teaching called "the six aspects" or "the six means" in which, according to Florin Deleanu:

The practice starts with "counting" (ganana), which consists in counting breathing from one to ten. When this is accomplished without any counting failure (dosha), the practitioner advances to the second step, i.e., "pursuing" (anugama), which means intently following the inhalation as it enters the body and moves from the throat, through the heart, the navel, the kidneys, the thighs to the toes and then the reverse movement of the exhalation until it leaves the body. Next comes "concentration" (sthapana) which denotes focusing one's attention on some part of the body from the tip of the nose to the big toe. In the fourth step, called" observation" (upalaksana), the practitioner discerns that the air breathed in and out as well as form (rupa), mind (citta), and mental functions (caitta) ultimately consists of the four great elements. He thus analyzes all the five aggregates. Next follows "the turning away" (vivarta) which consists of changing the object of observation from the air breathed in and out to "the wholesome roots" of purity (kusalamula) and ultimately to "the highest mundane dharma". The last step is called "purification" (parisuddhi) and it marks entering the stage of "realization of the Way", which in Abhidharma literature denotes the stage of "the stream entry" (Sotāpanna) that will inevitably lead the adept to Nirvana in no more than seven lives.

Modern sources

Traditional anāpānasati teaches to observe inhalation and exhalation by focusing on the air coming in and out the nostrils, but followers of the Burmese Vipassana movement instead recommend focusing on the abdomen's movement during the act of breathing. Other Buddhist schools also teach that as an alternative point of focus.

According to John Dunne, for the practice to be successful, one should dedicate the practice, and set out the goal of the meditation session. According to Philip Kapleau, in Zen practice one may decide to either practice anāpānasati while seated or standing or lying down or walking, or while alternating seated, standing, lying down, and walking meditation. Then one may concentrate on the breath going through one's nose: the pressure in the nostrils on each inhalation, and the feeling of the breath moving along the upper lip on each exhalation. Other times practitioners are advised to attend to the breath at the tanden, a point slightly below the navel and beneath the surface of the body. Practitioners may choose to count each inhalation, "1, 2, 3,..." and so on, up to 10, and then begin from 1 again. Alternatively people sometimes also count the exhalation: "1, 2, 3,...", on both the inhalation and exhalation. If the count is lost then one should start again from the beginning.

The type of practice recommended in The Three Pillars of Zen is for one to count "1, 2, 3,..." on the inhalation for a while, then to eventually switch to counting on the exhalation, then eventually, once one has more consistent success in keeping track of the count, to begin to pay attention to the breath without counting. There are practitioners who count the breath all their lives as well. Beginning students are often advised to keep a brief daily practice of around 10 or 15 minutes a day. Also, a teacher or guide of some sort is often considered to be essential in Buddhist practice, as well as the sangha, or community of Buddhists, for support.

When one becomes distracted from the breath, which happens to both beginning and adept practitioners, either by a thought or something else, then one simply returns ones attention back to the breath. Philippe Goldin has said that important "learning" occurs at the moment when practitioners turn their attention back to the object of focus, the breath.

Active breathing, passive breathing

See also: Pranayama

Anapanasati is most commonly practiced with attention centered on the breath, without any effort to change the breathing.

In the throat singing prevalent amongst the Buddhist monks of Tibet and Mongolia the long and slow outbreath during chanting is the core of the practice. The sound of the chant also serves to focus the mind in one-pointed concentration samadhi, while the sense of self dissolves as awareness becomes absorbed into a realm of pure sound.

In some Japanese Zen meditation, the emphasis is upon maintaining "strength in the abdominal area" (dantian or "tanden") and slow deep breathing during the long outbreath, again to assist the attainment of a mental state of one-pointed concentration. There is also a "bamboo method," during which time one inhales and exhales in punctuated bits, as if running one's hand along the stalk of a bamboo tree.

Pranayama, or Yogic breath control, is very popular in traditional and modern forms of Yoga.

Scientifically demonstrated benefits

See also: Research on meditation

The practice of focusing one's attention changes the brain in ways to improve that ability over time; the brain grows in response to meditation. Meditation can be thought of as mental training, similar to learning to ride a bike or play a piano.

Meditators experienced in focused attention meditation (anapanasati is a type of focused attention meditation) showed a decrease in habitual responding a 20-minute Stroop test, which, as suggested by Richard Davidson and colleagues, may illustrate a lessening of emotionally reactive and automatic responding behavior. It has been scientifically demonstrated that ānāpānasati enhances connectivity in the brain.

In the Theravada tradition

Abbidhamma

The Abbidhamma literature discerns sixteen stages – or contemplations – of anapanasati. These are divided into four tetrads (i.e., sets or groups of four). The first four steps involve focusing the mind on breathing, which is the 'body-conditioner' (Pali: kāya-sankhāra). The second tetrad involves focusing on the feelings (vedanā), which are the 'mind-conditioner' (Pali: citta-sankhāra). The third tetrad involves focusing on the mind itself (Pali: citta), and the fourth on 'mental qualities' (Pali: dhamma). (Compare right mindfulness and satipatthana.)

Any anapanasati meditation session should progress through the stages in order, beginning at the first, whether the practitioner has performed all stages in a previous session or not.

Satipaṭṭhāna	Ānāpānasati	Tetrads
1. Contemplation of the body	1. Breathing long (Knowing Breath)	First Tetrad
	2. Breathing short (Knowing Breath)
	3. Experiencing the whole body
	4. Tranquillising the bodily activities
2. Contemplation of feelings	5. Experiencing rapture	Second Tetrad
	6. Experiencing bliss
	7. Experiencing mental activities
	8. Tranquillising mental activities
3. Contemplation of the mind	9. Experiencing the mind	Third Tetrad
	10. Gladdening the mind
	11. Centering the mind in samadhi
	12. Releasing the mind
4. Contemplation of Dhammas	13. Contemplating impermanence	Fourth Tetrad
	14. Contemplating fading of lust
	15. Contemplating cessation
	16. Contemplating relinquishment

Contemporary interpretations

According to several teachers in Theravada Buddhism, anapanasati alone will lead to the removal of all one's defilements (kilesa) and eventually to enlightenment. According to Roger Bischof, the Ven. Webu Sayadaw said of anapanasati: "This is a shortcut to Nirvana, anyone can use it. It stands up to investigation and is in accordance with the teachings of the Buddha as conserved in the scriptures. It is the straight path to Nirvana."

Anapanasati can also be practised with other traditional meditation subjects including the four frames of reference and mettā bhāvanā, as is done in modern Theravadan Buddhism.

In the Chinese tradition

Buddhacinga, a monk who came to China and widely propagated ānāpānasmṛti methods.

In the second century, the Buddhist monk An Shigao came from Northwest India to China and became one of the first translators of Buddhist scriptures into Chinese. He translated a version of the Ānāpānasmṛti Sūtra between 148 and 170 CE. Though once believed to have been lost, the original translation was rediscovered at Amanosan Kongoji, Osaka, Japan, by Professor Ochiai Toshinori in 1999. Its commentary, on the other hand, is a significantly longer text than what appears in the Ekottara Āgama, and is entitled, "The Great Ānāpānasmṛti Sūtra" (Ch. 大安般守意經) (Taishō Tripiṭaka 602).

At a later date, Buddhacinga, more commonly known as Fotudeng (佛圖澄) (231-349 CE), came from Central Asia to China in 310 and propagated Buddhism widely. He is said to have demonstrated many spiritual powers, and was able to convert the warlords in this region of China over to Buddhism. He is well known for teaching methods of meditation, and especially ānāpānasmṛti. Fotudeng widely taught ānāpānasmṛti through methods of counting breaths, so as to temper to the breathing, simultaneously focusing the mind into a state of peaceful meditative concentration. By teaching meditation methods as well as doctrine, Fotudeng popularized Buddhism quickly. According to Nan Huaijin, "Besides all its theoretical accounts of emptiness and existence, Buddhism also offered methods for genuine realization of spiritual powers and meditative concentration that could be relied upon. This is the reason that Buddhism began to develop so vigorously in China with Fotudeng."

As more monks such as Kumārajīva, Dharmanandi, Gautama Saṃghadeva, and Buddhabhadra came to the East, translations of meditation texts did as well, which often taught various methods of ānāpānasmṛti that were being used in India. These became integrated in various Buddhist traditions, as well as into non-Buddhist traditions such as Daoism.

In the sixth century, the Tiantai school was formed, teaching the One Vehicle (Skt. Ekayāna), the vehicle of attaining Buddhahood, as the main principle, and three forms of śamatha-vipaśyanā correlated with the meditative perspectives of emptiness, provisional existence, and the mean, as the method of cultivating realization. The Tiantai school places emphasis on ānāpānasmṛti in accordance with the principles of śamatha and vipaśyanā. In China, the Tiantai understanding of meditation has had the reputation of being the most systematic and comprehensive of all. The founder of the Tiantai school, Zhiyi, wrote many commentaries and treatises on meditation. Of these texts, Zhiyi's Concise Śamatha-vipaśyanā (小止観 Xiǎo Zhǐguān), his Mahāśamatha Vipaśyanā (摩訶止観 Móhē Zhǐguān), and his Six Subtle Dharma Gates (六妙法門 Liù Miào Fǎmén) are the most widely read in China. Zhiyi classifies breathing into four main categories: panting (喘 "chuǎn"), unhurried breathing (風 "fēng"), deep and quiet breathing (氣 "qì"), and stillness or rest (息 "xi"). Zhiyi holds that the first three kinds of breathing are incorrect, while the fourth is correct, and that the breathing should reach stillness and rest. Venerable Hsuan Hua, who taught Chan and Pure Land Buddhism, also taught that the external breathing reaches a state of stillness in correct meditation:

A practitioner with sufficient skill does not breathe externally. That external breathing has stopped, but the internal breathing functions. With internal breathing there is no exhalation through the nose or mouth, but all pores on the body are breathing. A person who is breathing internally appears to be dead, but actually he has not died. He does not breathe externally, but the internal breathing has come alive.

In the Indo-Tibetan tradition

In the Tibetan Buddhist lineage, ānāpānasmṛti is done to calm the mind in order to prepare one for various other practices.

Two of the most important Mahāyāna philosophers, Asaṅga and Vasubandhu, in the Śrāvakabhūmi chapter of the Yogācārabhūmi-śāstra and the Abhidharma-kośa, respectively, make it clear that they consider ānāpānasmṛti a profound practice leading to vipaśyanā (in accordance with the teachings of the Buddha in the Sutra pitika). However, as scholar Leah Zahler has demonstrated, "the practice traditions related to Vasubandhu's or Asaṅga's presentations of breath meditation were probably not transmitted to Tibet." Asaṅga correlates the sixteen stages of ānāpānasmṛti with the four smṛtyupasthānas in the same way that the Ānāpānasmṛti Sutra does, but because he does not make this explicit the point was lost on later Tibetan commentators.

As a result, the largest Tibetan lineage, the Gelug, came to view ānāpānasmṛti as a mere preparatory practice useful for settling the mind but nothing more. Zahler writes:

The practice tradition suggested by the Treasury itself--and also by Asaṅga's Grounds of Hearers--is one in which mindfulness of breathing becomes a basis for inductive reasoning on such topics as the five aggregates; as a result of such inductive reasoning, the meditator progresses through the Hearer paths of preparation, seeing, and meditation. It seems at least possible that both Vasubandhu and Asaṅga presented their respective versions of such a method, analogous to but different from modern Theravāda insight meditation, and that Gelukpa scholars were unable to reconstruct it in the absence of a practice tradition because of the great difference between this type of inductive meditative reasoning based on observation and the types of meditative reasoning using consequences (thal 'gyur, prasaanga) or syllogisms (sbyor ba, prayoga) with which Gelukpas were familiar. Thus, although Gelukpa scholars give detailed interpretations of the systems of breath meditation set forth in Vasubandu's and Asaṅga's texts, they may not fully account for the higher stages of breath meditation set forth in those texts. . . it appears that neither the Gelukpa textbook writers nor modern scholars such as Lati Rinpoche and Gendun Lodro were in a position to conclude that the first moment of the fifth stage of Vasubandhu's system of breath meditation coincides with the attainment of special insight and that, therefore, the first four stages must be a method for cultivating special insight.

Zahler continues,

[I]t appears . .that a meditative tradition consisting of analysis based on observation—inductive reasoning within meditation—was not transmitted to Tibet; what Gelukpa writers call analytical meditation is syllogistic reasoning within meditation. Thus, Jamyang Shaypa fails to recognize the possibility of an 'analytical meditation' based on observation, even when he cites passages on breath meditation from Vasubandhu's Treasury of Manifest Knowledge and, especially, Asaṅga's Grounds of Hearers that appear to describe it.

Stephen Batchelor, who for years was monk in the Gelukpa lineage, experienced this firsthand. He writes, "such systematic practice of mindfulness was not preserved in the Tibetan traditions. The Gelugpa lamas know about such methods and can point to long descriptions of mindfulness in their Abhidharma works, but the living application of the practice has largely been lost. (Only in dzog-chen, with the idea of 'awareness' [rig pa] do we find something similar.) For many Tibetans the very term 'mindfulness' (sati in Pali, rendered in Tibetan by dran pa) has come to be understood almost exclusively as 'memory' or 'recollection.'"

As Batchelor noted, however, in other traditions, particularly the Kagyu and Nyingma, mindfulness based on ānāpānasmṛti practice is considered to be quite profound means of calming the mind to prepare it for the higher practices of Dzogchen and Mahamudra. For the Kagyupa, in the context of mahāmudrā, ānāpānasmṛti is thought to be the ideal way for the meditator to transition into taking the mind itself as the object of meditation and generating vipaśyanā on that basis. The prominent contemporary Kagyu/Nyingma master Chogyam Trungpa, echoing the Kagyu Mahāmudrā view, wrote, "your breathing is the closest you can come to a picture of your mind. It is the portrait of your mind in some sense. . .The traditional recommendation in the lineage of meditators that developed in the Kagyu-Nyingma tradition is based on the idea of mixing mind and breath." The Gelukpa allow that it is possible to take the mind itself as the object of meditation, however, Zahler reports, the Gelukpa discourage it with "what seems to be thinly disguised sectarian polemics against the Nyingma Great Completeness [Dzogchen] and Kagyu Great Seal [mahāmudrā] meditations."

In the Pañcakrama tantric tradition ascribed to (the Vajrayana) Nagarjuna, ānāpānasmṛti counting breaths is said to be sufficient to provoke an experience of vipaśyanā (although it occurs in the context of "formal tantric practice of the completion stage in highest yogatantra").

Ice giant

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Ice_giant

Uranus photographed by Voyager 2 in January 1986

 

Neptune photographed by Voyager 2 in August 1989

An ice giant is a giant planet composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. There are two ice giants in the Solar System: Uranus and Neptune.

In astrophysics and planetary science the term "ice" refers to volatile chemical compounds with freezing points above about 100 K, such as water, ammonia, or methane, with freezing points of 273 K (0°C), 195 K (−78°C), and 91 K (−182°C), respectively (see Volatiles). In the 1990s, it was determined that Uranus and Neptune were a distinct class of giant planet, separate from the other giant planets, Jupiter and Saturn, which are gas giants predominantly composed of hydrogen and helium.

As such, Neptune and Uranus are now referred to as ice giants. Lacking well-defined solid surfaces, they are primarily composed of gases and liquids. Their constituent compounds were solids when they were primarily incorporated into the planets during their formation, either directly in the form of ice or trapped in water ice. Today, very little of the water in Uranus and Neptune remains in the form of ice. Instead, water primarily exists as supercritical fluid at the temperatures and pressures within them. Uranus and Neptune consist of only about 20% hydrogen and helium by mass, compared to the Solar System's gas giants, Jupiter and Saturn, which are more than 90% hydrogen and helium by mass.

Terminology

In 1952, science fiction writer James Blish coined the term gas giant and it was used to refer to the large non-terrestrial planets of the Solar System. However, since the late 1940s the compositions of Uranus and Neptune have been understood to be significantly different from those of Jupiter and Saturn. They are primarily composed of elements heavier than hydrogen and helium, forming a separate type of giant planet altogether. Because during their formation Uranus and Neptune incorporated their material as either ice or gas trapped in water ice, the term ice giant came into use. In the early 1970s, the terminology became popular in the science fiction community, e.g., Bova (1971), but the earliest scientific usage of the terminology was likely by Dunne & Burgess (1978) in a NASA report.

Formation

Modelling the formation of terrestrial and gas giants is relatively straightforward and uncontroversial. The terrestrial planets of the Solar System are widely understood to have formed through collisional accumulation of planetesimals within the protoplanetary disk. The gas giants—Jupiter, Saturn, and their extrasolar counterpart planets—are thought to have formed solid cores of around 10 Earth masses (M_Earth) through the same process, while accreting gaseous envelopes from the surrounding solar nebula over the course of a few to several million years (Ma), although alternative models of core formation based on pebble accretion have recently been proposed. Some extrasolar giant planets may instead have formed via gravitational disk instabilities.

The formation of Uranus and Neptune through a similar process of core accretion is far more problematic. The escape velocity for the small protoplanets about 20 astronomical units (AU) from the center of the Solar System would have been comparable to their relative velocities. Such bodies crossing the orbits of Saturn or Jupiter would have been liable to be sent on hyperbolic trajectories ejecting them from the system. Such bodies, being swept up by the gas giants, would also have been likely to just be accreted into larger planets or thrown into cometary orbits.

Despite the trouble modelling their formation, many ice giant candidates have been observed orbiting other stars since 2004. This indicates that they may be common in the Milky Way.

Migration

Considering the orbital challenges of protoplanets 20 AU or more from the centre of the Solar System would experience, a simple solution is that the ice giants formed between the orbits of Jupiter and Saturn before being gravitationally scattered outward to their now more distant orbits.

Disk instability

Gravitational instability of the protoplanetary disk could also produce several gas giant protoplanets out to distances of up to 30 AU. Regions of slightly higher density in the disk could lead to the formation of clumps that eventually collapse to planetary densities. A disk with even marginal gravitational instability could yield protoplanets between 10 and 30 AU in over one thousand years (ka). This is much shorter than the 100,000 to 1,000,000 years required to produce protoplanets through core accretion of the cloud and could make it viable in even the shortest-lived disks, which exist for only a few million years.

A problem with this model is determining what kept the disk stable before the instability. There are several possible mechanisms allowing gravitational instability to occur during disk evolution. A close encounter with another protostar could provide a gravitational kick to an otherwise stable disk. A disk evolving magnetically is likely to have magnetic dead zones, due to varying degrees of ionization, where mass moved by magnetic forces could pile up, eventually becoming marginally gravitationally unstable. A protoplanetary disk may simply accrete matter slowly, causing relatively short periods of marginal gravitational instability and bursts of mass collection, followed by periods where the surface density drops below what is required to sustain the instability.

Photoevaporation

Observations of photoevaporation of protoplanetary disks in the Orion Trapezium Cluster by extreme ultraviolet (EUV) radiation emitted by θ¹ Orionis C suggests another possible mechanism for the formation of ice giants. Multiple-Jupiter-mass gas-giant protoplanets could have rapidly formed due to disk instability before having most of their hydrogen envelopes stripped off by intense EUV radiation from a nearby massive star.

In the Carina Nebula, EUV fluxes are approximately 100 times higher than in Trapezium's Orion Nebula. Protoplanetary disks are present in both nebulae. Higher EUV fluxes make this an even more likely possibility for ice-giant formation. The stronger EUV would increase the removal of the gas envelopes from protoplanets before they could collapse sufficiently to resist further loss.

Characteristics

These cut-aways illustrate interior models of the giant planets. The planetary cores of gas giants Jupiter and Saturn are overlaid by a deep layer of metallic hydrogen, whereas the mantles of the ice giants Uranus and Neptune are composed of heavier elements.

The ice giants represent one of two fundamentally different categories of giant planets present in the Solar System, the other group being the more-familiar gas giants, which are composed of more than 90% hydrogen and helium (by mass). Their hydrogen is thought to extend all the way down to their small rocky cores, where hydrogen molecular ion transitions to metallic hydrogen under the extreme pressures of hundreds of gigapascals (GPa).

The ice giants are primarily composed of heavier elements. Based on the abundance of elements in the universe, oxygen, carbon, nitrogen, and sulfur are most likely. Although the ice giants also have hydrogen envelopes, these are much smaller. They account for less than 20% of their mass. Their hydrogen also never reaches the depths necessary for the pressure to create metallic hydrogen. These envelopes nevertheless limit observation of the ice giants' interiors, and thereby the information on their composition and evolution.

Although Uranus and Neptune are referred to as ice giant planets, it is thought that there is a supercritical water-ammonia ocean beneath their clouds, which accounts for about two-thirds of their total mass.

Atmosphere and weather

The gaseous outer layers of the ice giants have several similarities to those of the gas giants. These include long-lived, high-speed equatorial winds, polar vortices, large-scale circulation patterns, and complex chemical processes driven by ultraviolet radiation from above and mixing with the lower atmosphere.

Studying the ice giants' atmospheric patterns also gives insights into atmospheric physics. Their compositions promote different chemical processes and they receive far less sunlight in their distant orbits than any other planets in the Solar System (increasing the relevance of internal heating on weather patterns).

The largest visible feature on Neptune is the recurring Great Dark Spot. It forms and dissipates every few years, as opposed to the similarly sized Great Red Spot of Jupiter, which has persisted for centuries. Of all known giant planets in the Solar System, Neptune emits the most internal heat per unit of absorbed sunlight, a ratio of approximately 2.6. Saturn, the next-highest emitter, only has a ratio of about 1.8. Uranus emits the least heat, one-tenth as much as Neptune. It is suspected that this may be related to its extreme 98˚ axial tilt. This causes its seasonal patterns to be very different from those of any other planet in the Solar System.

There are still no complete models explaining the atmospheric features observed in the ice giants. Understanding these features will help elucidate how the atmospheres of giant planets in general function. Consequently, such insights could help scientists better predict the atmospheric structure and behaviour of giant exoplanets discovered to be very close to their host stars (pegasean planets) and exoplanets with masses and radii between that of the giant and terrestrial planets found in the Solar System.

Interior

Because of their large sizes and low thermal conductivities, the planetary interior pressures range up to several hundred GPa and temperatures of several thousand kelvins (K).

In March 2012, it was found that the compressibility of water used in ice-giant models could be off by one-third. This value is important for modeling ice giants, and has a ripple effect on understanding them.

Magnetic fields

The magnetic fields of Uranus and Neptune are both unusually displaced and tilted. Their field strengths are intermediate between those of the gas giants and those of the terrestrial planets, being 50 and 25 times that of Earth's, respectively. The equatorial magnetic field strengths of Uranus and Neptune are respectively 75 percent and 45 percent of Earth's 0.305 gauss. Their magnetic fields are believed to originate in an ionized convecting fluid-ice mantle.

Spacecraft visitation

Past

• Voyager 2 (Uranus and Neptune)

Proposals

• MUSE (proposed in 2012; considered by NASA in 2014 and ESA in 2016)
• NASA Uranus orbiter and probe (proposed in 2011; considered by NASA in 2017)
• OCEANUS (proposed in 2017)
• ODINUS (proposed in 2013)
• Outer Solar System (proposed in 2012)
• Triton Hopper (proposed in 2015; under consideration by NASA as of 2018)
• Uranus Pathfinder (proposed in 2010)
• Neptune Odyssey (proposed in 2022)