The observable universe is a spherical region of the Universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because electromagnetic radiation from these objects has had time to reach the Solar System and Earth since the beginning of the cosmological expansion. There are at least 2 trillion galaxies in the observable universe. Assuming the Universe is isotropic, the distance to the edge of the observable universe is the same in every direction; that is, the observable universe has a spherical volume centered on the observer. Every location in the Universe has its own observable universe, which may or may not overlap with the one centered on Earth; the word observable in this sense does not refer to the capability of modern technology to detect light or other information from an object, or whether there is anything to be detected. It refers to the physical limit created by the speed of light itself; because no signals can travel faster than light, any object farther away from us than light could travel in the age of the Universe cannot be detected, as the signals could not have reached us yet.
Sometimes astrophysicists distinguish between the visible universe, which includes only signals emitted since recombination —and the observable universe, which includes signals since the beginning of the cosmological expansion. According to calculations, the current comoving distance—proper distance, which takes into account that the universe has expanded since the light was emitted—to particles from which the cosmic microwave background radiation was emitted, which represent the radius of the visible universe, is about 14.0 billion parsecs, while the comoving distance to the edge of the observable universe is about 14.3 billion parsecs, about 2% larger. The radius of the observable universe is therefore estimated to be about 46.5 billion light-years and its diameter about 28.5 gigaparsecs. The total mass of ordinary matter in the universe can be calculated using the critical density and the diameter of the observable universe to be about 1.5 × 1053 kg. In November 2018, astronomers reported that the extragalactic background light amounted to 4 × 1084 photons.
Since the expansion of the universe is known to accelerate and will become exponential in the future, the light emitted from all distant objects, past some time dependent on their current redshift, will never reach the Earth. In the future all observable objects will freeze in time while emitting progressively redder and fainter light. For instance, objects with the current redshift z from 5 to 10 will remain observable for no more than 4–6 billion years. In addition, light emitted by objects situated beyond a certain comoving distance will never reach Earth; some parts of the universe are too far away for the light emitted since the Big Bang to have had enough time to reach Earth or its scientific space-based instruments, so lie outside the observable universe. In the future, light from distant galaxies will have had more time to travel, so additional regions will become observable. However, due to Hubble's law, regions sufficiently distant from the Earth are expanding away from it faster than the speed of light and furthermore the expansion rate appears to be accelerating due to dark energy.
Assuming dark energy remains constant, so that the expansion rate of the universe continues to accelerate, there is a "future visibility limit" beyond which objects will never enter our observable universe at any time in the infinite future, because light emitted by objects outside that limit would never reach the Earth. This future visibility limit is calculated at a comoving distance of 19 billion parsecs, assuming the universe will keep expanding forever, which implies the number of galaxies that we can theoretically observe in the infinite future is only larger than the number observable by a factor of 2.36. Though in principle more galaxies will become observable in the future, in practice an increasing number of galaxies will become redshifted due to ongoing expansion, so much so that they will seem to disappear from view and become invisible. An additional subtlety is that a galaxy at a given comoving distance is defined to lie within the "observable universe" if we can receive signals emitted by the galaxy at any age in its past history, but because of the universe's expansion, there may be some age at which a signal sent from the same galaxy can never reach the Earth at any point in the infinite future (so, for example, we might never see what the galaxy looked like 10 billion years after the Bi
A black hole is a region of spacetime exhibiting such strong gravitational effects that nothing—not particles and electromagnetic radiation such as light—can escape from inside it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole; the boundary of the region from which no escape is possible is called the event horizon. Although the event horizon has an enormous effect on the fate and circumstances of an object crossing it, no locally detectable features appear to be observed. In many ways, a black hole acts like an ideal black body. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass; this temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it impossible to observe. Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace.
The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was first published by David Finkelstein in 1958. Black holes were long considered a mathematical curiosity; the discovery of neutron stars by Jocelyn Bell Burnell in 1967 sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality. Black holes of stellar mass are expected to form when massive stars collapse at the end of their life cycle. After a black hole has formed, it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form. There is general consensus. Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as visible light.
Matter that falls onto a black hole can form an external accretion disk heated by friction, forming some of the brightest objects in the universe. If there are other stars orbiting a black hole, their orbits can be used to determine the black hole's mass and location; such observations can be used to exclude possible alternatives such as neutron stars. In this way, astronomers have identified numerous stellar black hole candidates in binary systems, established that the radio source known as Sagittarius A*, at the core of the Milky Way galaxy, contains a supermassive black hole of about 4.3 million solar masses. On 11 February 2016, the LIGO collaboration announced the first direct detection of gravitational waves, which represented the first observation of a black hole merger; as of December 2018, eleven gravitational wave events have been observed that originated from ten merging black holes. On 10 April 2019, the first direct image of a black hole and its vicinity was published, following observations made by the Event Horizon Telescope in 2017 of the supermassive black hole in Messier 87's galactic centre.
Larry Kimura, a Hawaiian language professor at the University of Hawaii at Hilo, named the hole Pōwehi—a Hawaiian phrase referring to an "embellished dark source of unending creation." The idea of a body so massive that light could not escape was proposed by astronomical pioneer and English clergyman John Michell in a letter published in November 1784. Michell's simplistic calculations assumed that such a body might have the same density as the Sun, concluded that such a body would form when a star's diameter exceeds the Sun's by a factor of 500, the surface escape velocity exceeds the usual speed of light. Michell noted that such supermassive but non-radiating bodies might be detectable through their gravitational effects on nearby visible bodies. Scholars of the time were excited by the proposal that giant but invisible stars might be hiding in plain view, but enthusiasm dampened when the wavelike nature of light became apparent in the early nineteenth century. If light were a wave rather than a "corpuscle", it became unclear what, if any, influence gravity would have on escaping light waves.
Modern relativity discredits Michell's notion of a light ray shooting directly from the surface of a supermassive star, being slowed down by the star's gravity and free-falling back to the star's surface. In 1915, Albert Einstein developed his theory of general relativity, having earlier shown that gravity does influence light's motion. Only a few months Karl Schwarzschild found a solution to the Einstein field equations, which describes the gravitational field of a point mass and a spherical mass. A few months after Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution for the point mass and wrote more extensively about its properties; this solution had a peculiar behaviour at what is now called the Schwarzschild radius, where it became singular, meaning that some of the terms in the Einstein equations became infinite. The nature of this surface was not quite understood at the time. In 1924, Arthur Eddington showed that the singularity disappeared after a change of coordinates, although it took until 1933 for Georges Lemaître to realize that this meant the singularity at the Schwarzschild radius was a non-physical coordinate singularity.
Arthur Eddington did however comment on the possibility of a star with mass c
A protoplanetary disk is a rotating circumstellar disk of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may be considered an accretion disk for the star itself, because gases or other material may be falling from the inner edge of the disk onto the surface of the star; this process should not be confused with the accretion process thought to build up the planets themselves. Externally illuminated photo-evaporating protoplanetary disks are called proplyds. In July 2018, the first confirmed image of such a disk, containing a nascent exoplanet, named PDS 70b, was reported. Protostars form from molecular clouds consisting of molecular hydrogen; when a portion of a molecular cloud reaches a critical size, mass, or density, it begins to collapse under its own gravity. As this collapsing cloud, called a solar nebula, becomes denser, random gas motions present in the cloud average out in favor of the direction of the nebula's net angular momentum.
Conservation of angular momentum causes the rotation to increase. This rotation causes the cloud to flatten out—much like forming a flat pizza out of dough—and take the form of a disk; this occurs because centripetal acceleration from the orbital motion resists the gravitational pull of the star only in the radial direction, but the cloud remains free to collapse in the vertical direction. The outcome is the formation of a thin disc supported by gas pressure in the vertical direction; the initial collapse takes about 100,000 years. After that time the star reaches a surface temperature similar to that of a main sequence star of the same mass and becomes visible, it is now a T Tauri star. Accretion of gas onto the star continues for another 10 million years, before the disk disappears being blown away by the young star's solar wind, or simply ceasing to emit radiation after accretion has ended; the oldest protoplanetary disk yet discovered is 25 million years old. Protoplanetary disks around T Tauri stars differ from the disks surrounding the primary components of close binary systems with respect to their size and temperature.
Protoplanetary disks have radii up to 1000 AU, only their innermost parts reach temperatures above 1000 K. They are often accompanied by jets. Protoplanetary disks have been observed around several young stars in our galaxy. Recent observations by the Hubble Space Telescope have shown proplyds and planetary disks to be forming within the Orion Nebula. Protoplanetary disks are thought to be thin structures, with a typical vertical height much smaller than the radius, a typical mass much smaller than the central young star; the mass of a typical proto-planetary disk is dominated by its gas, the presence of dust grains has a major role in its evolution. Dust grains shield the mid-plane of the disk from energetic radiation from outer space that creates a dead zone in which the MRI no longer operates, it is believed that these disks consist of a turbulent envelope of plasma called the active zone, that encases an extensive region of quiescent gas called the dead zone. The dead zone located at the mid-plane can slow down the flow of matter through the disk which prohibits achieving a steady state.
The nebular hypothesis of solar system formation describes how protoplanetary disks are thought to evolve into planetary systems. Electrostatic and gravitational interactions may cause the dust and ice grains in the disk to accrete into planetesimals; this process competes against the stellar wind, which drives the gas out of the system, gravity and internal stresses, which pulls material into the central T Tauri star. Planetesimals constitute the building blocks of both giant planets; some of the moons of Jupiter and Uranus are believed to have formed from smaller, circumplanetary analogs of the protoplanetary disks. The formation of planets and moons in geometrically thin, gas- and dust-rich disks is the reason why the planets are arranged in an ecliptic plane. Tens of millions of years after the formation of the Solar System, the inner few AU of the Solar System contained dozens of moon- to Mars-sized bodies that were accreting and consolidating into the terrestrial planets that we now see.
The Earth's moon formed after a Mars-sized protoplanet obliquely impacted the proto-Earth ~30 million years after the formation of the Solar System. Gas-poor disks of circumstellar dust have been found around many nearby stars—most of which have ages in the range of ~10 million years to billions of years; these systems are referred to as "debris disks". Given the older ages of these stars, the short lifetimes of micrometer-sized dust grains around stars due to Poynting Robertson drag and radiation pressure, it is thought that this dust is from the collisions of planetesimals. Hence the debris disks around these examples are not "protoplanetary", but represent a stage of disk evolution where extrasolar analogs of the asteroid belt and Kuiper belt are home to dust-generating collisions between planetesimals. Based on recent computer model studies, the complex organic molecules necessary for life may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of the Earth.
According to the computer studies, this same process may occur around other stars that acquire planets.. Williams, J. P.. A.. "Protoplanetary Disks and Their Evolution". Annual Review of Astronomy and Astroph
In astronomy, the term "compact star" refers collectively to white dwarfs, neutron stars, black holes. It would grow to include exotic stars. Most compact stars are the endpoints of stellar evolution, thus referred to as stellar remnants, the form of the remnant depending on the mass of the star when it formed. All of these objects have a high mass relative to their radius, giving them a high density; the term compact star is used when the exact nature of the star is not known, but evidence suggests that it is massive and has a small radius, thus implying one of the above-mentioned categories. A compact star, not a black hole may be called a degenerate star; the usual endpoint of stellar evolution is the formation of a compact star. Most stars will come to a point in their evolution when the outward radiation pressure from the nuclear fusions in its interior can no longer resist the ever-present gravitational forces; when this happens, the star collapses under its own weight and undergoes the process of stellar death.
For most stars, this will result in the formation of a dense and compact stellar remnant known as a compact star. Compact stars have no internal energy production, but will—with the exception of black holes—usually radiate for millions of years with excess heat left from the collapse itself. According to the most recent understanding, compact stars could form during the phase separations of the early Universe following the Big Bang. Primordial origins of known compact objects have not been determined with certainty. Although compact stars may radiate, thus cool off and lose energy, they do not depend on high temperatures to maintain their structure, as ordinary stars do. Barring external disturbances and proton decay, they can persist forever. Black holes are however believed to evaporate from Hawking radiation after trillions of years. According to our current standard models of physical cosmology, all stars will evolve into cool and dark compact stars, by the time the Universe enters the so-called degenerate era in a distant future.
The somewhat wider definition of compact objects includes smaller solid objects such as planets and comets. There is a remarkable variety of stars and other clumps of hot matter, but all matter in the Universe must end as some form of compact stellar or substellar object, according to the theory of thermodynamics; the stars called white or degenerate dwarfs are made up of degenerate matter. White dwarfs arise from the cores of main-sequence stars and are therefore hot when they are formed; as they cool they will redden and dim until they become dark black dwarfs. White dwarfs were observed in the 19th century, but the high densities and pressures they contain were not explained until the 1920s; the equation of state for degenerate matter is "soft", meaning that adding more mass will result in a smaller object. Continuing to add mass to what is now a white dwarf, the object shrinks and the central density becomes larger, with higher degenerate-electron energies; the star's radius has now shrunk to only a few thousand kilometers, the mass is approaching the theoretical upper limit of the mass of a white dwarf, the Chandrasekhar limit, about 1.4 times the mass of the Sun.
If we were to take matter from the center of our white dwarf and start to compress it, we would first see electrons forced to combine with nuclei, changing their protons to neutrons by inverse beta decay. The equilibrium would shift towards heavier, neutron-richer nuclei that are not stable at everyday densities; as the density increases, these nuclei become less well-bound. At a critical density of about 4×1014 kg/m3), called the neutron drip line, the atomic nucleus would tend to fall apart into protons and neutrons. We would reach a point where the matter is on the order of the density of an atomic nucleus. At this point the matter is chiefly free neutrons, with a small amount of electrons. In certain binary stars containing a white dwarf, mass is transferred from the companion star onto the white dwarf pushing it over the Chandrasekhar limit. Electrons react with protons to form neutrons and thus no longer supply the necessary pressure to resist gravity, causing the star to collapse. If the center of the star is composed of carbon and oxygen such a gravitational collapse will ignite runaway fusion of the carbon and oxygen, resulting in a Type Ia supernova that blows apart the star before the collapse can become irreversible.
If the center is composed of magnesium or heavier elements, the collapse continues. As the density further increases, the remaining electrons react with the protons to form more neutrons; the collapse continues. A new equilibrium is possible after the star shrinks by three orders of magnitude, to a radius between 10 and 20 km; this is a neutron star. Although the first neutron star was not observed until 1967 when the first radio pulsar was discovered, neutron stars were proposed by Baade and Zwicky in 1933, only one year after the neutron was discovered in 1932, they realized that because neutron stars are so dense, the collapse of an ordinary star to a neutron star would liberate a large amount of gravitational potential energy, providing a possible explanation for supernovae. This is the explanation for supernovae of types Ib, Ic, II; such supernovae occur when the iron core of a massive star exceeds the Chandrasekhar limit and collapses to a neutron star. Li
Star clusters are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds or thousands of old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars contain fewer than a few hundred members, are very young. Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space though they are no longer gravitationally bound. Star clusters visible to the naked eye include the Pleiades and the Beehive Cluster. Globular clusters are spherical groupings of from 10,000 to several million stars packed into regions of from 10 to 30 light years across, they consist of old Population II stars—just a few hundred million years younger than the universe itself—which are yellow and red, with masses less than two solar masses. Such stars predominate within clusters because hotter and more massive stars have exploded as supernovae, or evolved through planetary nebula phases to end as white dwarfs.
Yet a few rare blue stars exist in globulars, thought to be formed by stellar mergers in their dense inner regions. In our galaxy, globular clusters are distributed spherically in the galactic halo, around the galactic centre, orbiting the centre in elliptical orbits. In 1917, the astronomer Harlow Shapley made the first reliable estimate the Sun's distance from the galactic centre based on the distribution of globular clusters; until the mid-1990s, globular clusters were the cause of a great mystery in astronomy, as theories of stellar evolution gave ages for the oldest members of globular clusters that were greater than the estimated age of the universe. However improved distance measurements to globular clusters using the Hipparcos satellite and accurate measurements of the Hubble constant resolved the paradox, giving an age for the universe of about 13 billion years and an age for the oldest stars of a few hundred million years less. Our galaxy has about 150 globular clusters, some of which may have been captured from small galaxies disrupted by the Milky Way, as seems to be the case for the globular cluster M79.
Some galaxies are much richer in globulars: the giant elliptical galaxy M87 contains over a thousand. A few of the brightest globular clusters are visible to the naked eye, with the brightest, Omega Centauri, having been known since antiquity and catalogued as a star before the telescopic age; the brightest globular cluster in the northern hemisphere is Messier 13 in the constellation of Hercules. Super star clusters are large regions of recent star formation, are thought to be the precursors of globular clusters. Examples include Westerlund 1 in the Milky Way. Open clusters are different from globular clusters. Unlike the spherically distributed globulars, they are confined to the galactic plane, are always found within spiral arms, they are young objects, up to a few tens of millions of years old, with a few rare exceptions as old as a few billion years, such as Messier 67 for example. They form from H II regions such as the Orion Nebula. Open clusters contain up to a few hundred members, within a region up to about 30 light-years across.
Being much less densely populated than globular clusters, they are much less gravitationally bound, over time, are disrupted by the gravity of giant molecular clouds and other clusters. Close encounters between cluster members can result in the ejection of stars, a process known as'evaporation'; the most prominent open clusters are the Hyades in Taurus. The Double Cluster of h+Chi Persei can be prominent under dark skies. Open clusters are dominated by hot young blue stars, because although such stars are short-lived in stellar terms, only lasting a few tens of millions of years, open clusters tend to have dispersed before these stars die. Establishing precise distances to open clusters enables the calibration of the period-luminosity relationship shown by Cepheids variable stars, which are used as standard candles. Cepheids are luminous and can be used to establish both the distances to remote galaxies and the expansion rate of the Universe. Indeed, the open cluster NGC 7790 hosts three classical Cepheids which are critical for such efforts.
Embedded clusters are groups of young stars that are or encased in an Interstellar dust or gas, impervious to optical observations. Embedded clusters form in molecular clouds, when the clouds begin to form stars. There is ongoing star formation in these clusters, so embedded clusters may be home to various types of young stellar objects including protostars and pre-main-sequence stars. An example of an embedded cluster is the Trapezium cluster in the Orion Nebula. In ρ Ophiuchi cloud core region there is an embedded cluster; the embedded cluster phase may last for several million years, after which gas in the cloud is depleted by star formation or dispersed through radiation pressure, stellar winds and outflows, or supernova explosions. In general less than 30% of cloud mass is converted to stars before the cloud is dispersed, but this fraction may be higher in dense parts of the cloud. With the loss of mass in the cloud, the energy of the system is altered leading to the disruption of a star cluster.
Most young embedded clusters disperse shortly after the end of star formation. The open clusters fou
A lenticular galaxy is a type of galaxy intermediate between an elliptical and a spiral galaxy in galaxy morphological classification schemes. They contain large-scale discs but they do not have large-scale spiral arms. Lenticular galaxies are disc galaxies that have used up or lost most of their interstellar matter and therefore have little ongoing star formation, they may, retain significant dust in their disks. As a result, they consist of aging stars. Despite the morphological differences and elliptical galaxies share common properties like spectral features and scaling relations. Both can be considered early-type galaxies that are passively evolving, at least in the local part of the Universe. Connecting the E galaxies with the S0 galaxies are the ES galaxies with intermediate-scale discs. Lenticular galaxies are unique in that they have a visible disk component as well as a prominent bulge component, they have much higher bulge-to-disk ratios than typical spirals and do not have the canonical spiral arm structure of late-type galaxies, yet may exhibit a central bar.
This bulge dominance can be seen in the axis ratio distribution of a lenticular galaxy sample. The distribution for lenticular galaxies rises in the range 0.25 to 0.85 whereas the distribution for spirals is flat in that same range. Larger axial ratios can be explained by observing face-on disk galaxies or by having a sample of spheroidal galaxies. Imagine looking at two disk galaxies edge-on, one with a bulge and one without a bulge; the galaxy with a prominent bulge will have a larger edge-on axial ratio compared to the galaxy without a bulge based on the definition of axial ratio. Thus a sample of disk galaxies with prominent spheroidal components will have more galaxies at larger axial ratios; the fact that the lenticular galaxy distribution rises with increasing observed axial ratio implies that lenticulars are dominated by a central bulge component. Lenticular galaxies are considered to be a poorly understood transition state between spiral and elliptical galaxies, which results in their intermediate placement on the Hubble sequence.
This results from lenticulars having bulge components. The disk component is featureless, which precludes a classification system similar to spiral galaxies; as the bulge component is spherical, elliptical galaxy classifications are unsuitable. Lenticular galaxies are thus divided into subclasses based upon either the amount of dust present or the prominence of a central bar; the classes of lenticular galaxies with no bar are S01, S02, S03 where the subscripted numbers indicate the amount of dust absorption in the disk component. The surface brightness profiles of lenticular galaxies are well described by the sum of a Sérsic model for the spheroidal component plus an exponentially declining model for the disk, a third component for the bar. Sometimes there is an observed truncation in the surface brightness profiles of lenticular galaxies at ~ 4 disk scalelengths; these features are consistent with the general structure of spiral galaxies. However, the bulge component of lenticulars is more related to elliptical galaxies in terms of morphological classification.
This spheroidal region, which dominates the inner structure of lenticular galaxies, has a steeper surface brightness profile than the disk component. Lenticular galaxy samples are distinguishable from the diskless elliptical galaxy population through analysis of their surface brightness profiles. Like spiral galaxies, lenticular galaxies can possess a central bar structure. While the classification system for normal lenticulars depends on dust content, barred lenticular galaxies are classified by the prominence of the central bar. SB01 galaxies have the least defined bar structure and are only classified as having enhanced surface brightness along opposite sides of the central bulge; the prominence of the bar increases with index number, thus SB03 galaxies have well defined bars that can extend through the transition region between the bulge and disk. The properties of bars in lenticular galaxies have not been researched in great detail. Understanding these properties, as well as understanding the formation mechanism for bars, would help clarify the formation or evolution history of lenticular galaxies.
In many respects the composition of lenticular galaxies is like that of ellipticals. For example, they both consist of predominately older, hence redder, stars. All of their stars are thought to be older than about a billion years, in agreement with their offset from the Tully–Fisher relation. In addition to these general stellar attributes, globular clusters are found more in lenticular galaxies than in spiral galaxies of similar mass and luminosity, they have little to no molecular gas and no significant hydrogen α or 21-cm emission. Unlike ellipticals, they may still possess significant dust. Lenticular galaxies share kinematic properties with elliptical galaxies; this is due to the significant disk nature of lenticulars. The bulge component is similar to elliptical galaxies in that it is pressure supported by a central velocity dispersion; this situation is analogous to a balloon, where the motions of the air particles are dominated by random motions. However, the kinematics of lenticular galaxies are dominated
A globular cluster is a spherical collection of stars that orbit a galactic core, as a satellite. Globular clusters are tightly bound by gravity, which gives them their spherical shapes, high stellar densities toward their centers; the name of this category of star cluster is derived from globulus -- a small sphere. A globular cluster is sometimes known, more as a globular. Globular clusters are found in the halo of a galaxy and contain more stars, are much older, than the less dense, open clusters which are found in the disk of a galaxy. Globular clusters are common. Larger galaxies can have more: The Andromeda Galaxy, for instance, may have as many as 500; some giant elliptical galaxies, such as M87, have as many as 13,000 globular clusters. Every galaxy of sufficient mass in the Local Group has an associated group of globular clusters, every large galaxy surveyed, has been found to possess a system of globular clusters; the Sagittarius Dwarf galaxy, the disputed Canis Major Dwarf galaxy appear to be in the process of donating their associated globular clusters to the Milky Way.
This demonstrates. Although it appears that globular clusters contain some of the first stars to be produced in the galaxy, their origins and their role in galactic evolution are still unclear, it does appear clear that globular clusters are different from dwarf elliptical galaxies and were formed as part of the star formation of the parent galaxy, rather than as a separate galaxy. The first known globular cluster, now called M22, was discovered in 1665 by Abraham Ihle, a German amateur astronomer. However, given the small aperture of early telescopes, individual stars within a globular cluster were not resolved until Charles Messier observed M4 in 1764; the first eight globular clusters discovered are shown in the table. Subsequently, Abbé Lacaille would list NGC 104, NGC 4833, M55, M69, NGC 6397 in his 1751–52 catalogue; the M before a number refers to Charles Messier's catalogue, while NGC is from the New General Catalogue by John Dreyer. When William Herschel began his comprehensive survey of the sky using large telescopes in 1782 there were 34 known globular clusters.
Herschel discovered another 36 himself and was the first to resolve all of them into stars. He coined the term "globular cluster" in his Catalogue of a Second Thousand New Nebulae and Clusters of Stars published in 1789; the number of globular clusters discovered continued to increase, reaching 83 in 1915, 93 in 1930 and 97 by 1947. A total of 152 globular clusters have now been discovered in the Milky Way galaxy, out of an estimated total of 180 ± 20; these additional, undiscovered globular clusters are believed to be hidden behind the gas and dust of the Milky Way. Beginning in 1914, Harlow Shapley began a series of studies of globular clusters, published in about 40 scientific papers, he examined the RR Lyrae variables in the clusters and used their period–luminosity relationship for distance estimates. It was found that RR Lyrae variables are fainter than Cepheid variables, which caused Shapley to overestimate the distances of the clusters. Of the globular clusters within the Milky Way, the majority are found in a halo around the galactic core, the large majority are located in the celestial sky centered on the core.
In 1918, this asymmetrical distribution was used by Shapley to make a determination of the overall dimensions of the galaxy. By assuming a spherical distribution of globular clusters around the galaxy's center, he used the positions of the clusters to estimate the position of the Sun relative to the galactic center. While his distance estimate was in significant error, it did demonstrate that the dimensions of the galaxy were much greater than had been thought, his error was due to interstellar dust in the Milky Way, which absorbs and diminishes the amount of light from distant objects, such as globular clusters, that reaches the Earth, thus making them appear to be more distant than they are. Shapley's measurements indicated that the Sun is far from the center of the galaxy contrary to what had been inferred from the nearly distribution of ordinary stars. In reality, most ordinary stars lie within the galaxy's disk and those stars that lie in the direction of the galactic centre and beyond are thus obscured by gas and dust, whereas globular clusters lie outside the disk and can be seen at much further distances.
Shapley was subsequently assisted in his studies of clusters by Henrietta Swope and Helen Battles Sawyer. In 1927–29, Shapley and Sawyer categorized clusters according to the degree of concentration each system has toward its core; the most concentrated clusters were identified as Class I, with successively diminishing concentrations ranging to Class XII. This became known as the Shapley–Sawyer Concentration Class In 2015, a new type of globular cluster was proposed on the basis of observational data, the dark globular clusters; the formation of globular clusters remains a poorly understood phenomenon and it remains uncertain whether the stars in a globular cluster form in a single generation or are spawned across multiple generations over a period of several hundred million years. In many globular clusters, most of the stars are at approxima