140-667: Messier 30 (also known as M30 , NGC 7099 , or the Jellyfish Cluster ) is a globular cluster of stars in the southeast of the southern constellation of Capricornus , at about the declination of the Sun when the latter is at December solstice . It was discovered by the French astronomer Charles Messier in 1764, who described it as a circular nebula without a star. In the New General Catalogue , compiled during
280-516: A Plummer model . The simulation becomes more difficult when the effects of binaries and the interaction with external gravitation forces (such as from the Milky Way galaxy) must also be included. In 2010 a low-density globular cluster's lifetime evolution was able to be directly computed, star-by-star. Completed N-body simulations have shown that stars can follow unusual paths through the cluster, often forming loops and falling more directly toward
420-464: A bimodal population, for example. During their youth, these LMC clusters may have encountered giant molecular clouds that triggered a second round of star formation. This star-forming period is relatively brief, compared with the age of many globular clusters. It has been proposed that this multiplicity in stellar populations could have a dynamical origin. In the Antennae Galaxy , for example,
560-452: A chain reaction of star-building that spreads throughout the gaseous region. Only when the available gas is nearly consumed or dispersed does the activity end. Starbursts are often associated with merging or interacting galaxies. The prototype example of such a starburst-forming interaction is M82 , which experienced a close encounter with the larger M81 . Irregular galaxies often exhibit spaced knots of starburst activity. A radio galaxy
700-492: A cluster from being visually separated until Charles Messier observed M 4 in 1764. 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 and was the first to resolve virtually all of them into stars. He coined the term globular cluster in his Catalogue of a Second Thousand New Nebulae and Clusters of Stars (1789). In 1914, Harlow Shapley began
840-518: A cluster of thousands of stars can be enormous. A more efficient method of simulating the N-body dynamics of a globular cluster is done by subdivision into small volumes and velocity ranges, and using probabilities to describe the locations of the stars. Their motions are described by means of the Fokker–Planck equation , often using a model describing the mass density as a function of radius, such as
980-441: A cluster's adolescence, core collapse begins with stars nearest the core. Interactions between binary star systems prevents further collapse as the cluster approaches middle age. The central binaries are either disrupted or ejected, resulting in a tighter concentration at the core. The interaction of stars in the collapsed core region causes tight binary systems to form. As other stars interact with these tight binaries they increase
1120-483: A compressed core about one arcminute wide that has further star density within. It is longest observable (opposed to the Sun) in the first half of August. M30 is centered 27,100 light-years away from Earth with a roughly 2.5% margin of error, and is about 93 light-years across. The estimated age is roughly 12.9 billion years and it forms a mass of about 160,000 times the mass of the Sun ( M ☉ ). The cluster
1260-521: A concentration of mass at its core of about a million times the Sun's mass per cubic parsec . This makes it one of the highest density regions in the Milky Way galaxy. Stars in such close proximity will experience a high rate of interactions that can create binary star systems, as well as a type of star called a blue straggler that is formed by mass transfer. A process of mass segregation may have caused
1400-627: A debate took place between Harlow Shapley and Heber Curtis , the Great Debate , concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift. In 1922, the Estonian astronomer Ernst Öpik gave
1540-536: A distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object. Using the new 100-inch Mt. Wilson telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables , thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of
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#17327764152161680-418: A function of their radius (or distance from their cores) falls off more slowly than their smaller counterparts. The formation of these cD galaxies remains an active area of research, but the leading model is that they are the result of the mergers of smaller galaxies in the environments of dense clusters, or even those outside of clusters with random overdensities. These processes are the mechanisms that drive
1820-448: A globular cluster and a dwarf spheroidal galaxy . The formation of these extended clusters is likely related to accretion. It is unclear why the Milky Way lacks such clusters; Andromeda is unlikely to be the sole galaxy with them, but their presence in other galaxies remains unknown. Galaxy A galaxy is a system of stars , stellar remnants , interstellar gas , dust , and dark matter bound together by gravity . The word
1960-411: A globular cluster are similar to those in the bulge of a spiral galaxy but confined to a spheroid in which half the light is emitted within a radius of only a few to a few tens of parsecs . They are free of gas and dust, and it is presumed that all the gas and dust was long ago either turned into stars or blown out of the cluster by the massive first-generation stars. Globular clusters can contain
2100-440: A globular cluster must be either to accrete stars at its core, causing its steady contraction, or gradual shedding of stars from its outer layers. Binary stars form a significant portion of stellar systems, with up to half of all field stars and open cluster stars occurring in binary systems. The present-day binary fraction in globular clusters is difficult to measure, and any information about their initial binary fraction
2240-420: A half-mass radius of only 1.12 arc minutes. The tidal radius, or Hill sphere , is the distance from the center of the globular cluster at which the external gravitation of the galaxy has more influence over the stars in the cluster than does the cluster itself. This is the distance at which the individual stars belonging to a cluster can be separated away by the galaxy. The tidal radius of M3, for example,
2380-441: A high density of stars; on average about 0.4 stars per cubic parsec, increasing to 100 or 1000 stars/pc in the core of the cluster. In comparison, the stellar density around the Sun is roughly 0.1 stars/pc . The typical distance between stars in a globular cluster is about one light year, but at its core the separation between stars averages about a third of a light year – thirteen times closer than
2520-462: A huge number of stars held together by gravitational forces, akin to the Solar System but on a much larger scale, and that the resulting disk of stars could be seen as a band on the sky from a perspective inside it. In his 1755 treatise, Immanuel Kant elaborated on Wright's idea about the Milky Way's structure. The first project to describe the shape of the Milky Way and the position of
2660-429: A low portion of open clusters and a reduced rate of new star formation. Instead, they are dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions. The stars contain low abundances of heavy elements because star formation ceases after the initial burst. In this sense they have some similarity to the much smaller globular clusters . The largest galaxies are
2800-781: A lower metallicity. The Dutch astronomer Pieter Oosterhoff observed two special populations of globular clusters, which became known as Oosterhoff groups . The second group has a slightly longer period of RR Lyrae variable stars. While both groups have a low proportion of metallic elements as measured by spectroscopy , the metal spectral lines in the stars of Oosterhoff type I (Oo I) cluster are not quite as weak as those in type II (Oo II), and so type I stars are referred to as metal-rich (e.g. Terzan 7 ), while type II stars are metal-poor (e.g. ESO 280-SC06 ). These two distinct populations have been observed in many galaxies, especially massive elliptical galaxies. Both groups are nearly as old as
2940-448: A lower proportion of heavier elements. Astronomers refer to these heavier elements as metals (distinct from the material concept) and to the proportions of these elements as the metallicity. Produced by stellar nucleosynthesis , the metals are recycled into the interstellar medium and enter a new generation of stars. The proportion of metals can thus be an indication of the age of a star in simple models, with older stars typically having
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#17327764152163080-500: A more compact volume. When this gravothermal instability occurs, the central region of the cluster becomes densely crowded with stars, and the surface brightness of the cluster forms a power-law cusp. A massive black hole at the core could also result in a luminosity cusp. Over a long time, this leads to a concentration of massive stars near the core, a phenomenon called mass segregation . The dynamical heating effect of binary star systems works to prevent an initial core collapse of
3220-403: A mortal woman, the infant Heracles , on Hera 's breast while she is asleep so the baby will drink her divine milk and thus become immortal. Hera wakes up while breastfeeding and then realises she is nursing an unknown baby: she pushes the baby away, some of her milk spills, and it produces the band of light known as the Milky Way. In the astronomical literature, the capitalised word "Galaxy"
3360-560: A part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxy , began resolving them into huge conglomerations of stars, but based simply on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way. For this reason they were popularly called island universes , but this term quickly fell into disuse, as
3500-417: A pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms rotate around the center, but they do so with constant angular velocity . The spiral arms are thought to be areas of high-density matter, or " density waves ". As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of
3640-893: A plane in the outer part of the galaxy's halo. This observation supports the view that type II clusters were captured from a satellite galaxy, rather than being the oldest members of the Milky Way's globular cluster system as was previously thought. The difference between the two cluster types would then be explained by a time delay between when the two galaxies formed their cluster systems. Close interactions and near-collisions of stars occur relatively often in globular clusters because of their high star density. These chance encounters give rise to some exotic classes of stars – such as blue stragglers , millisecond pulsars , and low-mass X-ray binaries – which are much more common in globular clusters. How blue stragglers form remains unclear, but most models attribute them to interactions between stars, such as stellar mergers ,
3780-400: A plane, the majority of mass in spiral galaxies exists in a roughly spherical halo of dark matter which extends beyond the visible component, as demonstrated by the universal rotation curve concept. Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the bulge are relatively bright arms. In
3920-438: A reserve of cold gas that forms giant molecular clouds . Some galaxies have been observed to form stars at an exceptional rate, which is known as a starburst . If they continue to do so, they would consume their reserve of gas in a time span less than the galaxy's lifespan. Hence starburst activity usually lasts only about ten million years, a relatively brief period in a galaxy's history. Starburst galaxies were more common during
4060-435: A rotating bar structure in the center of this galaxy. With improved radio telescopes , hydrogen gas could also be traced in other galaxies. In the 1970s, Vera Rubin uncovered a discrepancy between observed galactic rotation speed and that predicted by the visible mass of stars and gas. Today, the galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter . Beginning in
4200-457: A roughly diagonal line sloping from hot, luminous stars in the upper left to cool, faint stars in the lower right. This line is known as the main sequence and represents the primary stage of stellar evolution . The diagram also includes stars in later evolutionary stages such as the cool but luminous red giants . Constructing an H–R diagram requires knowing the distance to the observed stars to convert apparent into absolute magnitude. Because all
4340-399: A series of studies of globular clusters, published across about forty scientific papers. He examined the clusters' RR Lyrae variables (stars which he assumed were Cepheid variables ) and used their luminosity and period of variability to estimate the distances to the clusters. RR Lyrae variables were later found to be fainter than Cepheid variables, causing Shapley to overestimate
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4480-488: A single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered. Most of the information we have about dwarf galaxies come from observations of the local group , containing two spiral galaxies, the Milky Way and Andromeda, and many dwarf galaxies. These dwarf galaxies are classified as either irregular or dwarf elliptical / dwarf spheroidal galaxies . A study of 27 Milky Way neighbors found that in all dwarf galaxies,
4620-419: A smaller companion galaxy—that as the two galaxy centers approach, they start to oscillate around a center point, and the oscillation creates gravitational ripples forming the shells of stars, similar to ripples spreading on water. For example, galaxy NGC 3923 has over 20 shells. Spiral galaxies resemble spiraling pinwheels . Though the stars and other visible material contained in such a galaxy lie mostly on
4760-462: A stable, compact formation. Globular clusters are similar in form to dwarf spheroidal galaxies , and though globular clusters were long held to be the more luminous of the two, discoveries of outliers had made the distinction between the two less clear by the early 21st century. Their name is derived from Latin globulus (small sphere). Globular clusters are occasionally known simply as "globulars". Although one globular cluster, Omega Centauri ,
4900-608: A star needs to cross the cluster and the number of stellar masses. The relaxation time varies by cluster, but a typical value is on the order of one billion years. Although globular clusters are generally spherical in form, ellipticity can form via tidal interactions. Clusters within the Milky Way and the Andromeda Galaxy are typically oblate spheroids in shape, while those in the Large Magellanic Cloud are more elliptical. Astronomers characterize
5040-528: Is 3C 236 , with lobes 15 million light-years across. It should however be noted that radio emissions are not always considered part of the main galaxy itself. A giant radio galaxy is a special class of objects characterized by the presence of radio lobes generated by relativistic jets powered by the central galaxy's supermassive black hole . Giant radio galaxies are different from ordinary radio galaxies in that they can extend to much larger scales, reaching upwards to several megaparsecs across, far larger than
5180-516: Is a galaxy with giant regions of radio emission extending well beyond its visible structure. These energetic radio lobes are powered by jets from its active galactic nucleus . Radio galaxies are classified according to their Fanaroff–Riley classification . The FR I class have lower radio luminosity and exhibit structures which are more elongated; the FR II class are higher radio luminosity. The correlation of radio luminosity and structure suggests that
5320-437: Is a graph of a large sample of stars plotting their absolute magnitude (their luminosity , or brightness measured from a standard distance), as a function of their color index . The color index, roughly speaking, measures the color of the star; positive color indices indicate a reddish star with a cool surface temperature, while negative values indicate a bluer star with a hotter surface. Stars on an H–R diagram mostly lie along
5460-495: Is a spiral galaxy having the number 109 in the catalogue of Messier. It also has the designations NGC 3992, UGC 6937, CGCG 269–023, MCG +09-20-044, and PGC 37617 (or LEDA 37617), among others. Millions of fainter galaxies are known by their identifiers in sky surveys such as the Sloan Digital Sky Survey . Greek philosopher Democritus (450–370 BCE) proposed that the bright band on the night sky known as
5600-399: Is about forty arc minutes, or about 113 pc. In most Milky Way clusters, the surface brightness of a globular cluster as a function of decreasing distance to the core first increases, then levels off at a distance typically 1–2 parsecs from the core. About 20% of the globular clusters have undergone a process termed "core collapse". The luminosity in such a cluster increases steadily all
5740-562: Is also used to observe distant, red-shifted galaxies that were formed much earlier. Water vapor and carbon dioxide absorb a number of useful portions of the infrared spectrum, so high-altitude or space-based telescopes are used for infrared astronomy . The first non-visual study of galaxies, particularly active galaxies, was made using radio frequencies . The Earth's atmosphere is nearly transparent to radio between 5 MHz and 30 GHz. The ionosphere blocks signals below this range. Large radio interferometers have been used to map
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5880-531: Is called the Galactic Center . The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter . Consequently, these galaxies also have
6020-451: Is derived from the Greek galaxias ( γαλαξίας ), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System . Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a thousand stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass . Most of
6160-469: Is directly a function of the cluster's age; an age scale can be plotted on an axis parallel to the magnitude. The morphology and luminosity of globular cluster stars in H–R diagrams are influenced by numerous parameters, many of which are still actively researched. Recent observations have overturned the historical paradigm that all globular clusters consist of stars born at exactly the same time, or sharing exactly
6300-618: Is filled with a tenuous gas (the intergalactic medium ) with an average density of less than one atom per cubic metre. Most galaxies are gravitationally organised into groups , clusters and superclusters . The Milky Way is part of the Local Group , which it dominates along with the Andromeda Galaxy . The group is part of the Virgo Supercluster . At the largest scale , these associations are generally arranged into sheets and filaments surrounded by immense voids . Both
6440-429: Is following a retrograde orbit (against the general flow) through the inner galactic halo, suggesting that it was acquired from a satellite galaxy rather than forming within the Milky Way . It is in this epoch 22.2 kly (6.8 kpc), from the center of the galaxy, compared to an estimated 26 kly (8.0 kpc) for the Sun . The cluster has passed through a dynamic process called core collapse and now has
6580-461: Is given by the Hubble sequence . Since the Hubble sequence is entirely based upon visual morphological type (shape), it may miss certain important characteristics of galaxies such as star formation rate in starburst galaxies and activity in the cores of active galaxies . Many galaxies are thought to contain a supermassive black hole at their center. This includes the Milky Way, whose core region
6720-414: Is lost by subsequent dynamical evolution. Numerical simulations of globular clusters have demonstrated that binaries can hinder and even reverse the process of core collapse in globular clusters. When a star in a cluster has a gravitational encounter with a binary system, a possible result is that the binary becomes more tightly bound and kinetic energy is added to the solitary star. When the massive stars in
6860-668: Is often used to refer to the Milky Way galaxy, to distinguish it from the other galaxies in the observable universe . The English term Milky Way can be traced back to a story by Geoffrey Chaucer c. 1380 : See yonder, lo, the Galaxyë Which men clepeth the Milky Wey , For hit is whyt. Galaxies were initially discovered telescopically and were known as spiral nebulae . Most 18th- to 19th-century astronomers considered them as either unresolved star clusters or anagalactic nebulae , and were just thought of as
7000-485: Is poorly understood. Globular clusters have traditionally been described as a simple star population formed from a single giant molecular cloud , and thus with roughly uniform age and metallicity (proportion of heavy elements in their composition). Modern observations show that nearly all globular clusters contain multiple populations; the globular clusters in the Large Magellanic Cloud (LMC) exhibit
7140-417: Is sometimes referred to as a flocculent spiral galaxy ; in contrast to the grand design spiral galaxy that has prominent and well-defined spiral arms. The speed in which a galaxy rotates is thought to correlate with the flatness of the disc as some spiral galaxies have thick bulges, while others are thin and dense. In spiral galaxies, the spiral arms do have the shape of approximate logarithmic spirals ,
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#17327764152167280-540: Is the distance at which the apparent surface luminosity has dropped by half. A comparable quantity is the half-light radius, or the distance from the core containing half the total luminosity of the cluster; the half-light radius is typically larger than the core radius. Most globular clusters have a half-light radius of less than ten parsecs (pc), although some globular clusters have very large radii, like NGC 2419 (r h = 18 pc) and Palomar 14 (r h = 25 pc). The half-light radius includes stars in
7420-526: The Galactic Center . He correctly concluded that the Milky Way's center is in the Sagittarius constellation and not near the Earth. He overestimated the distance, finding typical globular cluster distances of 10–30 kiloparsecs (33,000–98,000 ly); the modern distance to the Galactic Center is roughly 8.5 kiloparsecs (28,000 ly). Shapley's measurements indicated the Sun is relatively far from
7560-423: The Local Group has an associated system of globular clusters, as does almost every large galaxy surveyed. Some giant elliptical galaxies (particularly those at the centers of galaxy clusters ), such as M 87 , have as many as 13,000 globular clusters. Shapley was later assisted in his studies of clusters by Henrietta Swope and Helen Sawyer Hogg . In 1927–1929, Shapley and Sawyer categorized clusters by
7700-486: The Mayall ;II cluster of the Andromeda Galaxy. Both X-ray and radio emissions from Mayall II appear consistent with an intermediate-mass black hole; however, these claimed detections are controversial. The heaviest objects in globular clusters are expected to migrate to the cluster center due to mass segregation . One research group pointed out that the mass-to-light ratio should rise sharply towards
7840-583: The University of Nottingham , used 20 years of Hubble images to estimate that the observable universe contained at least two trillion ( 2 × 10 ) galaxies. However, later observations with the New Horizons space probe from outside the zodiacal light reduced this to roughly 200 billion ( 2 × 10 ). Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance
7980-403: The disks of spiral galaxies. The Milky Way has more than 150 known globulars , and there may be many more. Both the origin of globular clusters and their role in galactic evolution are unclear. Some are among the oldest objects in their galaxies and even the universe , constraining estimates of the universe's age . Star clusters were formerly thought to consist of stars that all formed at
8120-518: The galactic bulge or hidden by the gas and dust of the Milky Way. For example, most of the Palomar Globular Clusters have only been discovered in the 1950s, with some located relatively close-by yet obscured by dust, while others reside in the very far reaches of the Milky Way halo. The Andromeda Galaxy , which is comparable in size to the Milky Way, may have as many as five hundred globulars. Every galaxy of sufficient mass in
8260-403: The giant star stage. As the cluster ages, stars of successively lower masses will do the same. Therefore, the age of a single-population cluster can be measured by looking for those stars just beginning to enter the giant star stage, which form a "knee" in the H–R diagram called the main-sequence turnoff , bending to the upper right from the main-sequence line. The absolute magnitude at this bend
8400-453: The observable universe . Most galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3,000 to 300,000 light years ) and are separated by distances in the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 26,800 parsecs (87,400 ly) and is separated from the Andromeda Galaxy , its nearest large neighbour, by just over 750,000 parsecs (2.5 million ly). The space between galaxies
8540-603: The spectra invisible to humans (radio telescopes, infrared cameras, and x-ray telescopes ) allows detection of other galaxies that are not detected by Hubble. Particularly, surveys in the Zone of Avoidance (the region of sky blocked at visible-light wavelengths by the Milky Way) have revealed a number of new galaxies. A 2016 study published in The Astrophysical Journal , led by Christopher Conselice of
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#17327764152168680-442: The supermassive black holes at their centers. The mass of these supposed intermediate-mass black holes is proportional to the mass of their surrounding clusters, following a pattern previously discovered between supermassive black holes and their surrounding galaxies. Hertzsprung–Russell diagrams (H–R diagrams) of globular clusters allow astronomers to determine many of the properties of their populations of stars. An H–R diagram
8820-519: The type-cD galaxies . First described in 1964 by a paper by Thomas A. Matthews and others, they are a subtype of the more general class of D galaxies, which are giant elliptical galaxies, except that they are much larger. They are popularly known as the supergiant elliptical galaxies and constitute the largest and most luminous galaxies known. These galaxies feature a central elliptical nucleus with an extensive, faint halo of stars extending to megaparsec scales. The profile of their surface brightnesses as
8960-512: The "Great Andromeda Nebula", as the Andromeda Galaxy, Messier object M31 , was then known. Searching the photographic record, he found 11 more novae . Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within this galaxy. As a result, he was able to come up with a distance estimate of 150,000 parsecs . He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies. In 1920
9100-403: The 1880s, it was described as a "remarkable globular, bright, large, slightly oval." It can be easily viewed with a pair of 10×50 binoculars, forming a patch of hazy light some 4 arcminutes wide that is slightly elongated along the east–west axis. With a larger instrument, individual stars can be resolved and the cluster will cover an angle of up to 12 arcminutes across graduating into
9240-478: The 1970s. The required resolution for this task is exacting; it is only with the Hubble Space Telescope (HST) that the first claimed discoveries were made, in 2002 and 2003. Based on HST observations, other researchers suggested the existence of a 4,000 M ☉ (solar masses) intermediate-mass black hole in the globular cluster M15 and a 20,000 M ☉ black hole in
9380-451: The 1990s, the Hubble Space Telescope yielded improved observations. Among other things, its data helped establish that the missing dark matter in this galaxy could not consist solely of inherently faint and small stars. The Hubble Deep Field , an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion ( 1.25 × 10 ) galaxies in the observable universe. Improved technology in detecting
9520-549: The 20th century the distribution of globular clusters in the sky was some of the first evidence that the Sun is far from the center of the Milky Way . Globular clusters are found in nearly all galaxies . In spiral galaxies like the Milky Way, they are mostly found in the outer spheroidal part of the galaxy – the galactic halo . They are the largest and most massive type of star cluster , tending to be older, denser, and composed of lower abundances of heavy elements than open clusters , which are generally found in
9660-465: The Andromeda Galaxy's halo, similar to the globular cluster. The three new-found clusters have a similar star count to globular clusters and share other characteristics, such as stellar populations and metallicity, but are distinguished by their larger size – several hundred light years across – and some hundred times lower density. Their stars are separated by larger distances; parametrically, these clusters lie somewhere between
9800-549: The Hubble Space Telescope has observed clusters of clusters – regions in the galaxy that span hundreds of parsecs, in which many of the clusters will eventually collide and merge. Their overall range of ages and (possibly) metallicities could lead to clusters with a bimodal, or even multiple, distribution of populations. Observations of globular clusters show that their stars primarily come from regions of more efficient star formation, and from where
9940-432: The Hubble classification scheme, spiral galaxies are listed as type S , followed by a letter ( a , b , or c ) which indicates the degree of tightness of the spiral arms and the size of the central bulge. An Sa galaxy has tightly wound, poorly defined arms and possesses a relatively large core region. At the other extreme, an Sc galaxy has open, well-defined arms and a small core region. A galaxy with poorly defined arms
10080-655: The IC ( Index Catalogue ), the CGCG ( Catalogue of Galaxies and of Clusters of Galaxies ), the MCG ( Morphological Catalogue of Galaxies ), the UGC ( Uppsala General Catalogue of Galaxies), and the PGC ( Catalogue of Principal Galaxies , also known as LEDA). All the well-known galaxies appear in one or more of these catalogues but each time under a different number. For example, Messier 109 (or "M109")
10220-533: The Large Magellanic Cloud in his Book of Fixed Stars , referring to "Al Bakr of the southern Arabs", since at a declination of about 70° south it was not visible where he lived. It was not well known to Europeans until Magellan 's voyage in the 16th century. The Andromeda Galaxy was later independently noted by Simon Marius in 1612. In 1734, philosopher Emanuel Swedenborg in his Principia speculated that there might be other galaxies outside that were formed into galactic clusters that were minuscule parts of
10360-536: The Local Group and the Virgo Supercluster are contained in a much larger cosmic structure named Laniakea . The word galaxy was borrowed via French and Medieval Latin from the Greek term for the Milky Way, galaxías (kúklos) γαλαξίας ( κύκλος ) 'milky (circle)', named after its appearance as a milky band of light in the sky. In Greek mythology , Zeus places his son, born by
10500-497: The Milky Way galaxy emerged. A few galaxies outside the Milky Way are visible on a dark night to the unaided eye , including the Andromeda Galaxy , Large Magellanic Cloud , Small Magellanic Cloud , and the Triangulum Galaxy . In the 10th century, Persian astronomer Abd al-Rahman al-Sufi made the earliest recorded identification of the Andromeda Galaxy, describing it as a "small cloud". In 964, he probably mentioned
10640-461: The Milky Way might consist of distant stars. Aristotle (384–322 BCE), however, believed the Milky Way was caused by "the ignition of the fiery exhalation of some stars that were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere , in the region of the World that is continuous with the heavenly motions ." Neoplatonist philosopher Olympiodorus
10780-431: The Milky Way was a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways. Toward the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest celestial objects having nebulous appearance. Subsequently, William Herschel assembled a catalog of 5,000 nebulae. In 1845, Lord Rosse examined the nebulae catalogued by Herschel and observed
10920-487: The Milky Way, may be the precursors of globular clusters. Many of the Milky Way's globular clusters have a retrograde orbit (meaning that they revolve around the galaxy in the reverse of the direction the galaxy is rotating), including the most massive, Omega Centauri. Its retrograde orbit suggests it may be a remnant of a dwarf galaxy captured by the Milky Way. Globular clusters are generally composed of hundreds of thousands of low-metal , old stars. The stars found in
11060-481: The Milky Way. Despite the prominence of large elliptical and spiral galaxies, most galaxies are dwarf galaxies. They are relatively small when compared with other galactic formations, being about one hundredth the size of the Milky Way, with only a few billion stars. Blue compact dwarf galaxies contains large clusters of young, hot, massive stars . Ultra-compact dwarf galaxies have been discovered that are only 100 parsecs across. Many dwarf galaxies may orbit
11200-402: The Milky Way. In 1926 Hubble produced a classification of galactic morphology that is used to this day. Advances in astronomy have always been driven by technology. After centuries of success in optical astronomy , infrared astronomy was attempted in the 1830s, but only blossomed in the early 1900s. Radio astronomy was born in the 1930s, and matured by the 1950s and 1960s. The problem
11340-429: The Sun is to its nearest neighbor, Proxima Centauri . Globular clusters are thought to be unfavorable locations for planetary systems. Planetary orbits are dynamically unstable within the cores of dense clusters because of the gravitational perturbations of passing stars. A planet orbiting at one astronomical unit around a star that is within the core of a dense cluster, such as 47 Tucanae , would survive only on
11480-488: The Sun was undertaken by William Herschel in 1785 by counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with the Solar System close to the center . Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the center. A different method by Harlow Shapley based on
11620-506: The Younger ( c. 495 –570 CE) was critical of this view, arguing that if the Milky Way was sublunary (situated between Earth and the Moon) it should appear different at different times and places on Earth, and that it should have parallax , which it did not. In his view, the Milky Way was celestial. According to Mohani Mohamed, Arabian astronomer Ibn al-Haytham (965–1037) made
11760-407: The active jets emitted from active nuclei. Ultraviolet and X-ray telescopes can observe highly energetic galactic phenomena. Ultraviolet flares are sometimes observed when a star in a distant galaxy is torn apart from the tidal forces of a nearby black hole. The distribution of hot gas in galactic clusters can be mapped by X-rays. The existence of supermassive black holes at the cores of galaxies
11900-412: The cataloguing of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the centre. Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane ; but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters , the present picture of
12040-514: The center of the cluster, even without a black hole, in both M15 and Mayall II. Observations from 2018 find no evidence for an intermediate-mass black hole in any globular cluster, including M15, but cannot definitively rule out one with a mass of 500–1000 M ☉ . Finally, in 2023, an analysis of HST and the Gaia spacecraft data from the closest globular cluster, Messier 4 , revealed an excess mass of roughly 800 M ☉ in
12180-532: The center of the galaxy, contrary to what had been inferred from the observed uniform distribution of ordinary stars. In reality most ordinary stars lie within the galaxy's disk and are thus obscured by gas and dust in the disk, whereas globular clusters lie outside the disk and can be seen at much greater distances. The count of known globular clusters in the Milky Way has continued to increase, reaching 83 in 1915, 93 in 1930, 97 by 1947, and 157 in 2010. Additional, undiscovered globular clusters are believed to be in
12320-482: The center of this cluster, which appears to not be extended. This could thus be considered as kinematic evidence for an intermediate-mass black hole (even if an unusually compact cluster of compact objects like white dwarfs , neutron stars or stellar-mass black holes cannot be completely discounted). The confirmation of intermediate-mass black holes in globular clusters would have important ramifications for theories of galaxy development as being possible sources for
12460-749: The central mass is approximately 10 million solar masses , regardless of whether it has thousands or millions of stars. This suggests that galaxies are largely formed by dark matter , and that the minimum size may indicate a form of warm dark matter incapable of gravitational coalescence on a smaller scale. Interactions between galaxies are relatively frequent, and they can play an important role in galactic evolution . Near misses between galaxies result in warping distortions due to tidal interactions , and may cause some exchange of gas and dust. Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge. The stars of interacting galaxies usually do not collide, but
12600-452: The central region to gain a greater proportion of higher mass stars, creating a color gradient with increasing blueness toward the middle of the cluster. Globular cluster A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity , with a higher concentration of stars towards its center. It can contain anywhere from tens of thousands to many millions of member stars, all orbiting in
12740-421: The cluster are sped up by this process, it reduces the contraction at the core and limits core collapse. Cluster classification is not always definitive; objects have been found that can be classified in more than one category. For example, BH 176 in the southern part of the Milky Way has properties of both an open and a globular cluster. In 2005 astronomers discovered a new, "extended" type of star cluster in
12880-493: The cluster's core, while lighter stars pick up speed and tend to spend more time at the cluster's periphery. The cluster 47 Tucanae , made up of about one million stars, is one of the densest globular clusters in the Southern Hemisphere. This cluster was subjected to an intensive photographic survey that obtained precise velocities for nearly fifteen thousand stars in this cluster. The overall luminosities of
13020-440: The cluster. When a star passes near a binary system, the orbit of the latter pair tends to contract, releasing energy. Only after this primordial supply of energy is exhausted can a deeper core collapse proceed. In contrast, the effect of tidal shocks as a globular cluster repeatedly passes through the plane of a spiral galaxy tends to significantly accelerate core collapse. Core collapse may be divided into three phases. During
13160-480: The composition of the formational gas and dust affects stellar evolution; the stars' evolutionary tracks vary depending on the abundance of heavy elements. Data obtained from these studies are then used to study the evolution of the Milky Way as a whole. In contrast to open clusters, most globular clusters remain gravitationally bound together for time periods comparable to the lifespans of most of their stars. Strong tidal interactions with other large masses result in
13300-593: The coolest white dwarfs, often giving results as old as 12.7 billion years. In comparison, open clusters are rarely older than about half a billion years. The ages of globular clusters place a lower bound on the age of the entire universe, presenting a significant constraint in cosmology . Astronomers were historically faced with age estimates of clusters older than their cosmological models would allow, but better measurements of cosmological parameters, through deep sky surveys and satellites, appear to have resolved this issue. Studying globular clusters sheds light on how
13440-424: The core than would a single star orbiting a central mass. Additionally, some stars gain sufficient energy to escape the cluster due to gravitational interactions that result in a sufficient increase in velocity. Over long periods of time this process leads to the dissipation of the cluster, a process termed evaporation. The typical time scale for the evaporation of a globular cluster is 10 years. The ultimate fate of
13580-442: The core, or else due to a tidal interaction with another galaxy. Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms. Our own galaxy, the Milky Way , is a large disk-shaped barred-spiral galaxy about 30 kiloparsecs in diameter and a kiloparsec thick. It contains about two hundred billion (2×10 ) stars and has a total mass of about six hundred billion (6×10 ) times
13720-399: The core, then merges into the spiral arm structure. In the Hubble classification scheme, these are designated by an SB , followed by a lower-case letter ( a , b or c ) which indicates the form of the spiral arms (in the same manner as the categorization of normal spiral galaxies). Bars are thought to be temporary structures that can occur as a result of a density wave radiating outward from
13860-441: The cores of dwarf galaxies that have been consumed by larger galaxies. About a quarter of the globular cluster population in the Milky Way may have been accreted this way, as with more than 60% of the globular clusters in the outer halo of Andromeda. Globular clusters normally consist of Population II stars which, compared with Population I stars such as the Sun , have a higher proportion of hydrogen and helium and
14000-417: The cores of globular clusters are so dense that observations see multiple stars as a single target. The brightness measured for that seemingly single star is thus incorrect – too bright, given that multiple stars contributed. In turn, the computed distance is incorrect, so the blending effect can introduce a systematic uncertainty into the cosmic distance ladder and may bias the estimated age of
14140-597: The degree of concentration of stars toward each core. Their system, known as the Shapley–Sawyer Concentration Class , identifies the most concentrated clusters as Class I and ranges to the most diffuse Class XII. Astronomers from the Pontifical Catholic University of Chile proposed a new type of globular cluster on the basis of observational data in 2015: Dark globular clusters . The formation of globular clusters
14280-419: The dispersal of some stars, leaving behind "tidal tails" of stars removed from the cluster. After formation, the stars in the globular cluster begin to interact gravitationally with each other. The velocities of the stars steadily change, and the stars lose any history of their original velocity. The characteristic interval for this to occur is the relaxation time , related to the characteristic length of time
14420-488: The distance to other galaxies, under the assumption that globular clusters in remote galaxies behave similarly to those in the Milky Way. Computing the gravitational interactions between stars within a globular cluster requires solving the N-body problem . The naive computational cost for a dynamic simulation increases in proportion to N (where N is the number of objects), so the computing requirements to accurately simulate
14560-410: The distances. A large majority of the Milky Way's globular clusters are found in the halo around the galactic core. In 1918, Shapley used this strongly asymmetrical distribution to determine the overall dimensions of the galaxy. Assuming a roughly 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
14700-399: The energy at the core, causing the cluster to re-expand. As the average time for a core collapse is typically less than the age of the galaxy, many of a galaxy's globular clusters may have passed through a core collapse stage, then re-expanded. The HST has provided convincing observational evidence of this stellar mass-sorting process in globular clusters. Heavier stars slow down and crowd at
14840-524: The extent of their globular cluster systems. The mass of the SMBH in such a galaxy is often close to the combined mass of the galaxy's globular clusters. No known globular clusters display active star formation, consistent with the hypothesis that globular clusters are typically the oldest objects in their galaxy and were among the first collections of stars to form. Very large regions of star formation known as super star clusters , such as Westerlund 1 in
14980-424: The first attempt at observing and measuring the Milky Way's parallax, and he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere." Persian astronomer al-Biruni (973–1048) proposed the Milky Way galaxy was "a collection of countless fragments of the nature of nebulous stars." Andalusian astronomer Avempace ( d. 1138) proposed that it
15120-400: The formation of fossil groups or fossil clusters, where a large, relatively isolated, supergiant elliptical resides in the middle of the cluster and are surrounded by an extensive cloud of X-rays as the residue of these galactic collisions. Another older model posits the phenomenon of cooling flow , where the heated gases in clusters collapses towards their centers as they cool, forming stars in
15260-590: The galaxies' original morphology. If one of the galaxies is much more massive than the other, the result is known as cannibalism , where the more massive larger galaxy remains relatively undisturbed, and the smaller one is torn apart. The Milky Way galaxy is currently in the process of cannibalizing the Sagittarius Dwarf Elliptical Galaxy and the Canis Major Dwarf Galaxy . Stars are created within galaxies from
15400-448: The gas and dust within the two forms interacts, sometimes triggering star formation. A collision can severely distort the galaxies' shapes, forming bars, rings or tail-like structures. At the extreme of interactions are galactic mergers, where the galaxies' relative momentums are insufficient to allow them to pass through each other. Instead, they gradually merge to form a single, larger galaxy. Mergers can result in significant changes to
15540-500: The globular clusters within the Milky Way and the Andromeda Galaxy each have a roughly Gaussian distribution , with an average magnitude M v and a variance σ . This distribution of globular cluster luminosities is called the Globular Cluster Luminosity Function (GCLF). For the Milky Way, M v = −7.29 ± 0.13 , σ = 1.1 ± 0.1 . The GCLF has been used as a " standard candle " for measuring
15680-470: The higher density. (The velocity returns to normal after the stars depart on the other side of the arm.) This effect is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation, and therefore they harbor many bright and young stars. A majority of spiral galaxies, including the Milky Way galaxy, have a linear, bar-shaped band of stars that extends outward to either side of
15820-439: The interstellar medium is at a higher density, as compared to normal star-forming regions. Globular cluster formation is prevalent in starburst regions and in interacting galaxies . Some globular clusters likely formed in dwarf galaxies and were removed by tidal forces to join the Milky Way. In elliptical and lenticular galaxies there is a correlation between the mass of the supermassive black holes (SMBHs) at their centers and
15960-402: The main sequence) of the cluster's color–magnitude diagram to corresponding features in an H–R diagram of another set of stars, a method known as spectroscopic parallax or main-sequence fitting. Since globular clusters form at once from a single giant molecular cloud, a cluster's stars have roughly the same age and composition. A star's evolution is primarily determined by its initial mass, so
16100-466: The mass in a typical galaxy is in the form of dark matter , with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the centres of galaxies. Galaxies are categorised according to their visual morphology as elliptical , spiral , or irregular . The Milky Way is an example of a spiral galaxy. It is estimated that there are between 200 billion ( 2 × 10 ) to 2 trillion galaxies in
16240-421: The mass of the Sun. Recently, researchers described galaxies called super-luminous spirals. They are very large with an upward diameter of 437,000 light-years (compared to the Milky Way's 87,400 light-year diameter). With a mass of 340 billion solar masses, they generate a significant amount of ultraviolet and mid-infrared light. They are thought to have an increased star formation rate around 30 times faster than
16380-402: The morphology (shape) of a globular cluster by means of standard radii: the core radius ( r c ), the half-light radius ( r h ), and the tidal or Jacobi radius ( r t ). The radius can be expressed as a physical distance or as a subtended angle in the sky. Considering a radius around the core, the surface luminosity of the cluster steadily decreases with distance, and the core radius
16520-533: The order of a hundred million years. There is a planetary system orbiting a pulsar ( PSR B1620−26 ) that belongs to the globular cluster M4 , but these planets likely formed after the event that created the pulsar. Some globular clusters, like Omega Centauri in the Milky Way and Mayall II in the Andromeda Galaxy, are extraordinarily massive, measuring several million solar masses ( M ☉ ) and having multiple stellar populations. Both are evidence that supermassive globular clusters formed from
16660-401: The outer part of the cluster that happen to lie along the line of sight, so theorists also use the half-mass radius ( r m ) – the radius from the core that contains half the total mass of the cluster. A small half-mass radius, relative to the overall size, indicates a dense core. Messier 3 (M3), for example, has an overall visible dimension of about 18 arc minutes , but
16800-453: The positions of stars in a cluster's H–R or color–magnitude diagram mostly reflect their initial masses. A cluster's H–R diagram, therefore, appears quite different from H–R diagrams containing stars of a wide variety of ages. Almost all stars fall on a well-defined curve in globular cluster H–R diagrams, and that curve's shape indicates the age of the cluster. A more detailed H–R diagram often reveals multiple stellar populations as indicated by
16940-474: The presence of closely separated curves, each corresponding to a distinct population of stars with a slightly different age or composition. Observations with the Wide Field Camera 3 , installed in 2009 on the Hubble Space Telescope, made it possible to distinguish these slightly different curves. The most massive main-sequence stars have the highest luminosity and will be the first to evolve into
17080-474: The process, a phenomenon observed in clusters such as Perseus , and more recently in the Phoenix Cluster . A shell galaxy is a type of elliptical galaxy where the stars in its halo are arranged in concentric shells. About one-tenth of elliptical galaxies have a shell-like structure, which has never been observed in spiral galaxies. These structures are thought to develop when a larger galaxy absorbs
17220-431: The same chemical abundance. Some clusters feature multiple populations, slightly differing in composition and age; for example, high-precision imagery of cluster NGC 2808 discerned three close, but distinct, main sequences. Further, the placements of the cluster stars in an H–R diagram (including the brightnesses of distance indicators) can be influenced by observational biases. One such effect, called blending, arises when
17360-476: The same time from one star-forming nebula , but nearly all globular clusters contain stars that formed at different times, or that have differing compositions. Some clusters may have had multiple episodes of star formation, and some may be remnants of smaller galaxies captured by larger galaxies. The first known globular cluster, now called M 22 , was discovered in 1665 by Abraham Ihle , a German amateur astronomer. The cluster Omega Centauri , easily visible in
17500-439: The sources in these two types of galaxies may differ. Radio galaxies can also be classified as giant radio galaxies (GRGs), whose radio emissions can extend to scales of megaparsecs (3.26 million light-years). Alcyoneus is an FR II class low-excitation radio galaxy which has the largest observed radio emission, with lobed structures spanning 5 megaparsecs (16×10 ly ). For comparison, another similarly sized giant radio galaxy
17640-449: The southern sky with the naked eye, was known to ancient astronomers like Ptolemy as a star, but was reclassified as a nebula by Edmond Halley in 1677, then finally as a globular cluster in the early 19th century by John Herschel . The French astronomer Abbé Lacaille listed NGC 104 , NGC 4833 , M 55 , M 69 , and NGC 6397 in his 1751–1752 catalogue. The low resolution of early telescopes prevented individual stars in
17780-522: The sphere of the fixed stars." Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer Galileo Galilei used a telescope to study it and discovered it was composed of a huge number of faint stars. In 1750, English astronomer Thomas Wright , in his An Original Theory or New Hypothesis of the Universe , correctly speculated that it might be a rotating body of
17920-576: The spiral structure of Messier object M51 , now known as the Whirlpool Galaxy. In 1912, Vesto M. Slipher made spectrographic studies of the brightest spiral nebulae to determine their composition. Slipher discovered that the spiral nebulae have high Doppler shifts , indicating that they are moving at a rate exceeding the velocity of the stars he had measured. He found that the majority of these nebulae are moving away from us. In 1917, Heber Doust Curtis observed nova S Andromedae within
18060-413: The stars in a globular cluster have about the same distance from Earth, a color–magnitude diagram using their observed magnitudes looks like a shifted H–R diagram (because of the roughly constant difference between their apparent and absolute magnitudes). This shift is called the distance modulus and can be used to calculate the distance to the cluster. The modulus is determined by comparing features (like
18200-460: The transfer of material from one star to another, or even an encounter between two binary systems. The resulting star has a higher temperature than other stars in the cluster with comparable luminosity and thus differs from the main-sequence stars formed early in the cluster's existence. Some clusters have two distinct sequences of blue stragglers, one bluer than the other. Astronomers have searched for black holes within globular clusters since
18340-417: The universe and the Hubble constant . The blue stragglers appear on the H–R diagram as a series diverging from the main sequence in the direction of brighter, bluer stars. White dwarfs (the final remnants of some Sun-like stars), which are much fainter and somewhat hotter than the main-sequence stars, lie on the bottom-left of an H–R diagram. Globular clusters can be dated by looking at the temperatures of
18480-422: The universe itself and are of similar ages. Suggested scenarios to explain these subpopulations include violent gas-rich galaxy mergers, the accretion of dwarf galaxies, and multiple phases of star formation in a single galaxy. In the Milky Way, the metal-poor clusters are associated with the halo and the metal-rich clusters with the bulge. A large majority of the metal-poor clusters in the Milky Way are aligned on
18620-498: The universe that extended far beyond what could be seen. These views "are remarkably close to the present-day views of the cosmos." In 1745, Pierre Louis Maupertuis conjectured that some nebula -like objects were collections of stars with unique properties, including a glow exceeding the light its stars produced on their own, and repeated Johannes Hevelius 's view that the bright spots were massive and flattened due to their rotation. In 1750, Thomas Wright correctly speculated that
18760-442: The universe's early history, but still contribute an estimated 15% to total star production. Starburst galaxies are characterized by dusty concentrations of gas and the appearance of newly formed stars, including massive stars that ionize the surrounding clouds to create H II regions . These stars produce supernova explosions, creating expanding remnants that interact powerfully with the surrounding gas. These outbursts trigger
18900-412: The way to the core region. Models of globular clusters predict that core collapse occurs when the more massive stars in a globular cluster encounter their less massive counterparts. Over time, dynamic processes cause individual stars to migrate from the center of the cluster to the outside, resulting in a net loss of kinetic energy from the core region and leading the region's remaining stars to occupy
19040-724: The word universe implied the entirety of existence. Instead, they became known simply as galaxies. Millions of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxy , the Magellanic Clouds , the Whirlpool Galaxy , and the Sombrero Galaxy . Astronomers work with numbers from certain catalogues, such as the Messier catalogue , the NGC ( New General Catalogue ),
19180-416: Was composed of many stars that almost touched one another, and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects were near. In the 14th century, Syrian-born Ibn Qayyim al-Jawziyya proposed the Milky Way galaxy was "a myriad of tiny stars packed together in
19320-405: Was confirmed through X-ray astronomy. In 1944, Hendrik van de Hulst predicted that microwave radiation with wavelength of 21 cm would be detectable from interstellar atomic hydrogen gas; and in 1951 it was observed. This radiation is not affected by dust absorption, and so its Doppler shift can be used to map the motion of the gas in this galaxy. These observations led to the hypothesis of
19460-517: Was observed in antiquity and long thought to be a star, recognition of the clusters' true nature came with the advent of telescopes in the 17th century. In early telescopic observations, globular clusters appeared as fuzzy blobs, leading French astronomer Charles Messier to include many of them in his catalog of astronomical objects that he thought could be mistaken for comets . Using larger telescopes, 18th-century astronomers recognized that globular clusters are groups of many individual stars. Early in
19600-493: Was that air only allows visible light and radio waves to pass, with a little bit of near infrared. The first ultraviolet telescope was launched in 1968, and since then there's been major progress in all regions of the electromagnetic spectrum . The dust present in the interstellar medium is opaque to visual light. It is more transparent to far-infrared , which can be used to observe the interior regions of giant molecular clouds and galactic cores in great detail. Infrared
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