1.
Galaxy
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A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias, literally milky, Galaxies range in size from dwarfs with just a few billion stars to giants with one hundred trillion stars, each orbiting its galaxys center of mass. Galaxies are categorized according to their morphology as elliptical, spiral. Many galaxies are thought to have holes at their active centers. The Milky Ways central black hole, known as Sagittarius A*, has a four million times greater than the Sun. Recent estimates of the number of galaxies in the observable universe range from 200 billion to 2 trillion or more, most of the galaxies are 1,000 to 100,000 parsecs in diameter and separated by distances on the order of millions of parsecs. The space between galaxies is filled with a gas having an average density of less than one atom per cubic meter. The majority of galaxies are organized into groups, clusters. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Heras breast while she is asleep so that the baby will drink her divine milk and will thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing a baby, she pushes the baby away, some of her milk spills and. In the astronomical literature, the capitalized word Galaxy is often used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe. The English term Milky Way can be traced back to a story by Chaucer c. 1380, See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt. However, the word Universe was later understood to mean the entirety of existence, so this expression fell into disuse and the objects instead became known as galaxies. Tens of thousands 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, the IC, the CGCG, all of the well-known galaxies appear in one or more of these catalogues but each time under a different number. For example, Messier 109 is a galaxy having the number 109 in the catalogue of Messier, but also codes NGC3992, UGC6937, CGCG 269-023, MCG +09-20-044. One of the arguments to do so is that these impressive objects deserve better than uninspired codes, for instance, Bodifee and Berger propose the informal, descriptive name Callimorphus Ursae Majoris for the well-formed barred galaxy Messier 109 in Ursa Major
2.
Spiral galaxy
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A spiral galaxy is a type of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and these are surrounded by a much fainter halo of stars, many of which reside in globular clusters. Spiral galaxies are named for the structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them. Roughly two-thirds of all spirals are observed to have a component in the form of a bar-like structure, extending from the central bulge. Our own Milky Way has recently confirmed to be a barred spiral. The most convincing evidence for its existence comes from a recent survey, performed by the Spitzer Space Telescope, together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in the local Universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters, Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name, naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very loose arms, whereas Sa, either way, spiral arms contain many young, blue stars, which make the arms so bright. A bulge is a huge, tightly packed group of stars, the term commonly refers to the central group of stars found in most spiral galaxies. Using the Hubble classification, the bulge of Sa galaxies is usually composed of Population II stars, further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller and are composed of young, some bulges have similar properties to those of elliptical galaxies, others simply appear as higher density centers of disks, with properties similar to disk galaxies. Many bulges are thought to host a supermassive black hole at their centers, such black holes have never been directly observed, but many indirect proofs exist. In our own galaxy, for instance, the object called Sagittarius A* is believed to be a black hole. There is a correlation between the mass of the black hole and the velocity dispersion of the stars in the bulge. However, some stars inhabit a spheroidal halo or galactic spheroid, the orbital behaviour of these stars is disputed, but they may describe retrograde and/or highly inclined orbits, or not move in regular orbits at all. The galactic halo also contains many globular clusters, due to their irregular movement around the center of the galaxy—if they do so at all—these stars often display unusually high proper motion
3.
Elliptical galaxy
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An elliptical galaxy is a type of galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. Unlike flat spiral galaxies with organization and structure, they are more three-dimensional, without much structure and they are one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae, along with spiral and lenticular galaxies. Elliptical galaxies range in shape from spherical to highly flat. Originally Edwin Hubble hypothesized that elliptical galaxies evolved into spiral galaxies, stars found inside of elliptical galaxies are much older than stars found in spiral galaxies. Elliptical galaxies are believed to make up approximately 10%–15% of galaxies in the Virgo Supercluster and they are preferentially found close to the centers of galaxy clusters. Elliptical galaxies are also called early-type galaxies, due to their location in the Hubble sequence, elliptical galaxies are characterized by several properties that make them distinct from other classes of galaxy. They are spherical or ovoid masses of stars, starved of star-making gases, the smallest known elliptical galaxy is about one-tenth the size of the Milky Way. The motion of stars in galaxies is predominantly radial, unlike the disks of spiral galaxies. Large elliptical galaxies typically have a system of globular clusters. The dynamical properties of galaxies and the bulges of disk galaxies are similar, suggesting that they may be formed by the same physical processes. The luminosity profiles of both elliptical galaxies and bulges are well fit by Sersics law, every massive elliptical galaxy contains a supermassive black hole at its center. Observations of 46 elliptical galaxies,20 classical bulges, and 22 pseudobulges show that each contain a black hole at the center, elliptical galaxies are preferentially found in galaxy clusters and in compact groups of galaxies. The traditional portrait of elliptical galaxies paints them as galaxies where star formation finished after an initial burst at high-redshift, elliptical galaxies typically appear yellow-red, which is in contrast to the distinct blue tinge of most spiral galaxies. In spirals, this blue color emanates largely from the young, very little star formation is thought to occur in elliptical galaxies, because of their lack of gas compared to spiral or irregular galaxies. However, in recent years, evidence has shown that a proportion of these galaxies have residual gas reservoirs. Researchers with the Herschel Space Observatory have speculated that the black holes in elliptical galaxies keep the gas from cooling enough for star formation. Elliptical galaxies vary greatly in size and mass with diameters ranging from 3000 lightyears to more than 700,000 lightyears. This range is broader for this galaxy type than for any other
4.
Hubble sequence
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The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926. It is often known colloquially as the Hubble tuning fork diagram because of the shape in which it is traditionally represented, hubble’s scheme divides regular galaxies into 3 broad classes – ellipticals, lenticulars and spirals – based on their visual appearance. A fourth class contains galaxies with an irregular appearance, to this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy. On the left lie the ellipticals, elliptical galaxies have smooth, featureless light distributions and appear as ellipses in photographic images. They are denoted by the letter E, followed by an integer n representing their degree of ellipticity in the sky, the ellipticity increases from left to right on the Hubble diagram, with near-circular galaxies situated on the very left of the diagram. It is important to note that the ellipticity of a galaxy on the sky is only related to the true 3-dimensional shape. Observationally, the most flattened elliptical galaxies have ellipticities e =0.7 and this is consistent with their being truly ellipsoidal structures rather than disks viewed at a range of angles. Examples of elliptical galaxies, M49, M59, M60, M87, on the right of the Hubble sequence diagram are two parallel branches encompassing the spiral galaxies. A spiral galaxy consists of a disk, with stars forming a spiral structure. Roughly half of all spirals are observed to have a bar-like structure, extending from the central bulge. In the tuning-fork diagram, the regular spirals occupy the upper branch and are denoted by the letter S, while the lower branch contains the barred spirals, both type of spirals are further subdivided according to the detailed appearance of their spiral structures. The basic spiral types can be extended to enable finer distinctions of appearance, for example, spiral galaxies whose appearance is intermediate between two of the above classes are often identified by appending two lower-case letters to the main galaxy type. Our own Milky Way is generally classed as SBb, making it a barred spiral with well-defined arms, however, this classification is somewhat uncertain since we can only infer how our galaxy would appear to an outside observer. Examples of regular galaxies, M31, M74, M81, M104, M51a, NGC300. Examples of barred spiral galaxies, M91, M95, NGC1097, NGC1300, NGC1672, NGC2536, NGC2903. At the centre of the Hubble tuning fork, where the two branches and the elliptical branch join, lies an intermediate class of galaxies known as lenticulars. These galaxies consist of a central bulge, similar in appearance to an elliptical galaxy, surrounded by an extended. Unlike spiral galaxies, the disks of galaxies have no visible spiral structure and are not actively forming stars in any significant quantity
5.
Bulge (astronomy)
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In astronomy, a bulge is a tightly packed group of stars within a larger formation. The term almost exclusively refers to the group of stars found in most spiral galaxies. It is now thought there are at least two types of bulges, bulges that are like ellipticals and bulges that are like spiral galaxies. Bulges that have similar to those of elliptical galaxies are often called classical bulges due to their similarity to the historic view of bulges. These bulges are composed primarily of stars that are older, Population II stars and these stars are also in orbits that are essentially random compared to the plane of the galaxy, giving the bulge a distinct spherical form. Due to the lack of dust and gasses, bulges tend to have almost no star formation, the distribution of light is described by de Vaucouleurs law. Classical bulges are thought to be the result of collisions of smaller structures and these disrupt the paths of stars, resulting in the randomness of bulge orbits. Following such a merger, gas clouds are more likely to be converted into stars, many bulges have properties more similar to those of the central regions of spiral galaxies than elliptical galaxies. They are often referred to as pseudobulges or disky-bulges and these bulges have stars that are not orbiting randomly, but rather orbit in an ordered fashion in the same plane as the stars in the outer disk. This contrasts greatly with elliptical galaxies, subsequent studies show that the bulges of many galaxies are not devoid of dust, but rather show a varied and complex structure. This structure often looks similar to a galaxy, but is much smaller. Giant spiral galaxies are typically 2–100 times the size of those spirals that exist in bulges, where they exist, these central spirals dominate the light of the bulge in which they reside. Typically the rate at which new stars are formed in pseudobulges is similar to the rate at which stars form in disk galaxies, sometimes bulges contain nuclear rings that are forming stars at much higher rate than is typically found in outer disks, as shown in NGC4314. Properties such as structure and young stars suggest that some bulges did not form through the same process that made elliptical galaxies. Yet the theories for the formation of pseudobulges are less certain than those for classical bulges, pseudobulges may be the result of extremely gas-rich mergers that happened more recently than those mergers that formed classical bulges. However, it is difficult for disks to survive the merging process, many astronomers suggest that bulges that appear similar to disks form outside of the disk, and are not the product of a merging process. When left alone, disk galaxies can rearrange their stars and gas, the products of this process are often observed in such galaxies, both spiral disks and galactic bars can result from secular evolution of galaxy disks. Secular evolution is expected to send gas and stars to the center of a galaxy
6.
Spiral arm
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A spiral galaxy is a type of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and these are surrounded by a much fainter halo of stars, many of which reside in globular clusters. Spiral galaxies are named for the structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them. Roughly two-thirds of all spirals are observed to have a component in the form of a bar-like structure, extending from the central bulge. Our own Milky Way has recently confirmed to be a barred spiral. The most convincing evidence for its existence comes from a recent survey, performed by the Spitzer Space Telescope, together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in the local Universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters, Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name, naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very loose arms, whereas Sa, either way, spiral arms contain many young, blue stars, which make the arms so bright. A bulge is a huge, tightly packed group of stars, the term commonly refers to the central group of stars found in most spiral galaxies. Using the Hubble classification, the bulge of Sa galaxies is usually composed of Population II stars, further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller and are composed of young, some bulges have similar properties to those of elliptical galaxies, others simply appear as higher density centers of disks, with properties similar to disk galaxies. Many bulges are thought to host a supermassive black hole at their centers, such black holes have never been directly observed, but many indirect proofs exist. In our own galaxy, for instance, the object called Sagittarius A* is believed to be a black hole. There is a correlation between the mass of the black hole and the velocity dispersion of the stars in the bulge. However, some stars inhabit a spheroidal halo or galactic spheroid, the orbital behaviour of these stars is disputed, but they may describe retrograde and/or highly inclined orbits, or not move in regular orbits at all. The galactic halo also contains many globular clusters, due to their irregular movement around the center of the galaxy—if they do so at all—these stars often display unusually high proper motion
7.
Structure
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Structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as biological organisms, abstract structures include data structures in computer science and musical form. Types of structure include a hierarchy, a network featuring many-to-many links, buildings, aircraft, skeletons, anthills, beaver dams and salt domes are all examples of load-bearing structures. The results of construction are divided into buildings and non-building structures, the effects of loads on physical structures are determined through structural analysis, which is one of the tasks of structural engineering. The structural elements can be classified as one-dimensional, two-dimensional, or three-dimensional, the latter was the main option available to early structures such as Chichen Itza. Two-dimensional elements with a third dimension have little of either. The structure elements are combined in structural systems, the majority of everyday load-bearing structures are section-active structures like frames, which are primarily composed of one-dimensional structures. In biology, structures exist at all levels of organization, ranging hierarchically from the atomic and molecular to the cellular, tissue, organ, organismic, population, usually, a higher-level structure is composed of multiple copies of a lower-level structure. Structural biology is concerned with the structure of macromolecules, particularly proteins. The function of molecules is determined by their shape as well as their composition. Protein structure has a four-level hierarchy, the primary structure is the sequence of amino acids that make it up. It has a backbone made up of a repeated sequence of a nitrogen. The secondary structure consists of repeated patterns determined by hydrogen bonding, the two basic types are the α-helix and the β-pleated sheet. The tertiary structure is a back and forth bending of the chain. Chemical structure refers to both molecular geometry and electronic structure, the structure can be represented by a variety of diagrams called structural formulas. Lewis structures use a dot notation to represent the valence electrons for an atom, bonds between atoms can be represented by lines with one line for each pair of electrons that is shared. In a simplified version of such a diagram, called a skeletal formula, only carbon-carbon bonds, atoms in a crystal have a structure that involves repetition of a basic unit called a unit cell. The atoms can be modeled as points on a lattice, and one can explore the effect of symmetry operations that include rotations about a point, reflections about a symmetry planes, and translations
8.
Magellanic spiral
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Magellanic spiral galaxies are dwarf galaxies which are classified as the type Sm. They are galaxies with one single spiral arm, and are named after their prototype, the Large Magellanic Cloud and they can be considered to be intermediate between dwarf spiral galaxies and irregular galaxies. SAm galaxies are a type of unbarred spiral galaxy, while SBm are a type of barred spiral galaxy, sABm are a type of intermediate spiral galaxy. Type Sm and Im galaxies have also been categorized as irregular galaxies with some structure, Sm galaxies are typically disrupted and asymmetric. DSm galaxies are spiral galaxies or dwarf irregular galaxies, depending on categorization scheme. The Magellanic spiral classification was introduced by Gerard de Vaucouleurs, along with Magellanic irregular, Large Magellanic Cloud Small Magellanic Cloud NGC1311 NGC4618 NGC4236 NGC55 NGC4214 NGC3109 NGC4625 NGC5713 NGC5204 NGC2552 Galaxy morphological classification
9.
Metallicity
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In astronomy and physical cosmology, the metallicity or Z is the fraction of mass of a star or other kind of astronomical object that is not in hydrogen or helium. Most of the matter in the universe is in the form of hydrogen and helium, so astronomers use the word metals as a convenient short term for all elements except hydrogen. This usage is distinct from the physical definition of a solid metal. In cosmological terms, the universe is chemically evolving, according to the Big Bang Theory, the early universe first consisted of hydrogen and helium, with trace amounts of lithium and beryllium, but no heavier elements. It is believed that older generations of stars generally have lower metallicities than those of younger generations and these became commonly known as Population I and Population II stars. A third stellar population was introduced in 1978, known as Population III stars and these extremely metal-poor stars were theorised to have been the first-born stars created in the universe. Measurements have demonstrated the connection between a stars metallicity and gas giant planets, like Jupiter and Saturn, the more metals in a star and thus its planetary system and proplyd, the more likely the system may have gas giant planets and rocky planets. Current models show that the metallicity along with the planetary system temperature and distance from the star are key to planet. Metallicity also affects a stars color temperature, metal poor stars are bluer and metal rich stars are redder. The Sun, with 8 planets and 5 planetesimals, is used as the reference, other stars are noted with a positive or negative value. A star with a =0.0 has the iron abundance as the Sun. A star with =−1.0 has one tenth heavy elements of found in the Sun. At =+1, the element abundance is 10 times the Suns value. The survey of population of stars shows that older stars have less metallicity. Stellar composition, as determined by spectroscopy, is simply defined by the parameters X, Y and Z. Here X is the percentage of hydrogen, Y is the fractional percentage of helium. It is simply defined as, X + Y + Z =1.00 In most stars, nebulae and other sources, hydrogen. The hydrogen mass fraction is generally expressed as X ≡ m H M where M is the mass of the system
10.
Hubble Ultra-Deep Field
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The Hubble Ultra-Deep Field is an image of a small region of space in the constellation Fornax, containing an estimated 10,000 galaxies. It is composited from Hubble Space Telescope data accumulated over a period from September 24,2003, looking back approximately 13 billion years it has been used to search for galaxies that existed at that time. The HUDF image was taken in a section of the sky with a low density of stars in the near-field, allowing much better viewing of dimmer. In August and September 2009, the Hubbles Deep Field was expanded using the channel of the recently attached Wide Field Camera 3. When combined with existing HUDF data, astronomers were able to identify a new list of very distant galaxies. Located southwest of Orion in the southern-hemisphere constellation Fornax, the image is 2.4 arcminutes to an edge. The image is oriented so that the left corner points toward north on the celestial sphere. On September 25,2012, NASA released a further refined version of the Ultra-Deep Field dubbed the eXtreme Deep Field. The XDF reveals galaxies that span back 13.2 billion years in time, on June 3,2014, NASA released the Hubble Ultra-Deep Field image composed of, for the first time, the full range of ultraviolet to near-infrared light. In the years since the original Hubble Deep Field, the Hubble Deep Field South, a workshop on how to best carry out surveys with the ACS was held at STScI in late 2002. At the workshop Massimo Stiavelli advocated an Ultra Deep Field as a way to study the objects responsible for the reionization of the Universe, unlike the Deep Fields, the HUDF does not lie in Hubbles Continuous Viewing Zone. As with the fields, this one was required to contain very little emission from our galaxy, with little Zodiacal dust.8 galaxy. The coordinates of the field are right ascension 3h 32m 39. 0s, the field is 200 arcseconds to a side, with a total area of 11 square arcminutes, and lies in the constellation of Fornax. Four filters were used on the ACS, centered on 435,606,775 and 850 nm and these wavelength ranges match those used by the GOODS sample, allowing direct comparison between the two. As with the Deep Fields, the HUDF used Directors Discretionary Time, the observations were done in two sessions, from September 23 to October 28,2003, and December 4,2003, to January 15,2004. The total exposure time is just under 1 million seconds, from 400 orbits, in total,800 ACS exposures were taken over the course of 11.3 days,2 every orbit, and NICMOS observed for 4.5 days. To observe the sky to the same sensitivity, the HST would need to observe continuously for a million years. The sensitivity of the ACS limits its capability of detecting galaxies at high redshift to about 6
11.
Magellanic Clouds
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The Magellanic Clouds are two irregular dwarf galaxies visible from the southern hemisphere, they are members of the Local Group and are orbiting the Milky Way galaxy. Because they both show signs of a bar structure, they are reclassified as Magellanic spiral galaxies. Collectively they were known to Māori of New Zealand as Nga Patari-Kaihau or as Te Reporepo and were used as predictors of winds. The Magellanic Clouds have been known since the first millennium in Western Asia, the first preserved mention of the Large Magellanic Cloud is by the Persian astronomer Al Sufi. In Europe, the Clouds were first observed by Italian explorers Peter Martyr dAnghiera, subsequently, they were reported by Antonio Pigafetta, who accompanied the expedition of Ferdinand Magellan on its circumnavigation of the world in 1519–1522. However, naming the clouds after Magellan did not become widespread until much later, in Bayers Uranometria they are designated as nubecula major and nubecula minor. In the 1756 star map of the French astronomer Lacaille, they are designated as le Grand Nuage and le Petit Nuage. In Sri Lanka, from ancient times, these clouds have been referred to as the Maha Mera Paruwathaya meaning the great mountain, roughly 21° apart in the night sky, the true distance between them is roughly 75,000 light-years. Until the discovery of the Sagittarius Dwarf Elliptical Galaxy in 1994, the LMC lies about 160,000 light years away, while the SMC is around 200,000. The LMC is about twice the diameter of the SMC, for comparison, the Milky Way is about 100,000 ly across. Observation and theoretical evidence suggest that the Magellanic Clouds have both been greatly distorted by tidal interaction with the Milky Way as they close to it. Streams of neutral hydrogen connect them to the Milky Way and to each other and their gravity has affected the Milky Way as well, distorting the outer parts of the galactic disk. Aside from their different structure and lower mass, they differ from our galaxy in two major ways, first, they are gas-rich, a higher fraction of their mass is hydrogen and helium compared to the Milky Way. They are also more metal-poor than the Milky Way, the youngest stars in the LMC and SMC have a metallicity of 0.5 and 0.25 times solar, respectively. Both are noted for their nebulae and young populations, but as in our own galaxy their stars range from the very young to the very old. The Large Magellanic Cloud was host galaxy to a supernova, the brightest observed in four centuries. Measurements with the Hubble Space Telescope, announced in 2006, suggest the Magellanic Clouds may be moving too fast to be long term companions of the Milky Way. They suggest the reason for this is due to a past interaction with the LMC splitting the SMC, and they have dubbed this smaller remnant the Mini Magellanic Cloud
12.
Large Magellanic Cloud
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The Large Magellanic Cloud is a satellite galaxy of the Milky Way. The LMC has a diameter of about 14,000 light-years, the LMC is the fourth-largest galaxy in the Local Group, after the Andromeda Galaxy, the Milky Way, and the Triangulum Galaxy. The LMC is classified as a Magellanic spiral and it contains a very prominent bar in its center, suggesting that it may have been a barred dwarf spiral galaxy before its spiral arms were disrupted, likely by the Milky Ways gravity. The LMCs present irregular appearance is likely the result of interactions with both the Milky Way and the Small Magellanic Cloud. The first recorded mention of the Large Magellanic Cloud was by the Persian astronomer Abd al-Rahman al-Sufi Shirazi, the next recorded observation was in 1503–4 by Amerigo Vespucci in a letter about his third voyage. In this letter he mentions three Canopes, two bright and one obscure, bright refers to the two Magellanic Clouds, and obscure refers to the Coalsack, ferdinand Magellan sighted the LMC on his voyage in 1519, and his writings brought the LMC into common Western knowledge. The galaxy now bears his name, measurements with the Hubble Space Telescope, announced in 2006, suggest the Large and Small Magellanic Clouds may be moving too fast to be orbiting the Milky Way. The Large Magellanic Cloud is usually considered an irregular galaxy, however, it shows signs of a bar structure, and is often reclassified as a Magellanic-type dwarf spiral galaxy. The Large Magellanic Cloud has a prominent central bar and a spiral arm, the central bar seems to be warped so that the east and west ends are nearer the Milky Way than the middle. In 2014, measurements from the Hubble Space Telescope made it possible to determine that the LMC has a period of 250 million years. The LMC was long considered to be a galaxy that could be assumed to lie at a single distance from the Solar System. However, in 1986, Caldwell and Coulson found that field Cepheid variables in the northeast portion of the LMC lie closer to the Milky Way than Cepheids in the southwest portion. More recently, this inclined geometry for field stars in the LMC has been confirmed via observations of Cepheids, core helium-burning red clump stars, all three of these papers find an inclination of ~35°, where a face-on galaxy has an inclination of 0°. Further work on the structure of the LMC using the kinematics of stars showed that the LMCs disk is both thick and flared. These results were confirmed by Grocholski et al. who calculated distances to a number of clusters, the distance to the LMC has been calculated using a variety of standard candles, with Cepheid variables being one of the most popular. Cepheids have been shown to have a relationship between their absolute luminosity and the period over which their brightness varies, however, Cepheids appear to suffer from a metallicity effect, where Cepheids of different metallicities have different period–luminosity relations. Unfortunately, the Cepheids in the Milky Way typically used to calibrate the period–luminosity relation are more rich than those found in the LMC. Modern 8-meter-class optical telescopes have discovered eclipsing binaries throughout the Local Group, parameters of these systems can be measured without mass or compositional assumptions
13.
Barred spiral galaxy
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A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in approximately two-thirds of all spiral galaxies, bars generally affect both the motions of stars and interstellar gas within spiral galaxies and can affect spiral arms as well. The Milky Way Galaxy, where our own Solar System is located, is classified as a spiral galaxy. Edwin Hubble classified spiral galaxies of this type as SB in his Hubble sequence, sBa types feature tightly bound arms, while SBc types are at the other extreme and have loosely bound arms. SB0 is a lenticular galaxy. Among other types in Hubbles classifications for the galaxies are, spiral galaxy, elliptical galaxy, Barred galaxies are apparently predominant, with surveys showing that up to two-thirds of all spiral galaxies contain a bar. The current hypothesis is that the bar structure acts as a type of stellar nursery, the bar is thought to act as a mechanism that channels gas inwards from the spiral arms through orbital resonance, in effect funneling the flow to create new stars. This process is thought to explain why many barred spiral galaxies have active galactic nuclei. The creation of the bar is thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting further out, which creates a bar structure. Bars are thought to be temporary phenomena in the lives of spiral galaxies, past a certain size the accumulated mass of the bar compromises the stability of the overall bar structure. Barred spiral galaxies with high mass accumulated in their center tend to have short, since so many spiral galaxies have bar structures, it is likely that they are recurring phenomena in spiral galaxy development. The oscillating evolutionary cycle from spiral galaxy to barred spiral galaxy is thought to take on the average about two billion years, recent studies have confirmed the idea that bars are a sign of galaxies reaching full maturity as the formative years end. The sub-categories are based on how open or tight the arms of the spiral are, sBa types feature tightly bound arms. SBc types are at the extreme and have loosely bound arms. SBm describes somewhat irregular barred spirals, sB0 is a barred lenticular galaxy. Galaxy morphological classification Galaxy formation and evolution Lenticular galaxy Spiral galaxy Firehose instability Britt, Milky Way’s Central Structure Seen with Fresh Clarity. An article about the Spitzer Space Telescopes Milky Way discovery Devitt, galactic survey reveals a new look for the Milky Way
14.
Small Magellanic Cloud
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The Small Magellanic Cloud, or Nebucula Minor, is a dwarf galaxy near the Milky Way. It is classified as an irregular galaxy. It has a diameter of about 7,000 light-years, contains several hundred million stars, the SMC contains a central bar structure and it is speculated that it was once a barred spiral galaxy that was disrupted by the Milky Way to become somewhat irregular. At a distance of about 200,000 light-years, it is one of the Milky Ways nearest neighbors and it is also one of the most distant objects that can be seen with the naked eye. With a mean declination of approximately −73 degrees, it can only be viewed from the Southern Hemisphere, since it has a very low surface brightness, it is best viewed from a dark site away from city lights. It forms a pair with the Large Magellanic Cloud, which lies a further 20 degrees to the east, in the southern hemisphere, the Magellanic clouds have long been included in the lore of native inhabitants, including south sea islanders and indigenous Australians. Persian astronomer Al Sufi labelled the larger of the two clouds as Al Bakr, the White Ox, european sailors may have first noticed the clouds during the Middle Ages when they were used for navigation. Portuguese and Dutch sailors called them the Cape Clouds, a name that was retained for several centuries, during the circumnavigation of the Earth by Ferdinand Magellan in 1519–22, they were described by Antonio Pigafetta as dim clusters of stars. In Johann Bayers celestial atlas Uranometria, published in 1603, he named the smaller cloud, in Latin, Nubecula means a little cloud. Between 1834 and 1838, John Frederick William Herschel made observations of the skies with his 14-inch reflector from the Royal Observatory at the Cape of Good Hope. While observing the Nubecula Minor, he described it as a mass of light with an oval shape. Within the area of this cloud he catalogued a concentration of 37 nebulae, in 1891, Harvard College Observatory opened an observing station at Arequipa in Peru. Between 1893 and 1906, under the direction of Solon Bailey, henrietta Swan Leavitt, an astronomer at the Harvard College Observatory, used the plates from Arequipa to study the variations in relative luminosity of stars in the SMC. This important period-luminosity relation allowed the distance to any other variable to be estimated in terms of the distance to the SMC. Hence, once the distance to the SMC was known with greater accuracy, using this period-luminosity relation, in 1913 the distance to the SMC was first estimated by Ejnar Hertzsprung. First he measured thirteen nearby cepheid variables to find the magnitude of a variable with a period of one day. By comparing this to the periodicity of the variables as measured by Leavitt and this later proved to be a gross underestimate of the true distance, but it did demonstrate the potential usefulness of this technique. Announced in 2006, measurements with the Hubble Space Telescope suggest the Large, there is a bridge of gas connecting the Small Magellanic Cloud with the Large Magellanic Cloud, which is evidence of tidal interaction between the galaxies
15.
Galaxy morphological classification
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Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. The Hubble sequence is a classification scheme for galaxies invented by Edwin Hubble in 1926. It is often known colloquially as the “Hubble tuning-fork” because of the shape in which it is traditionally represented, Hubble’s scheme divides galaxies into three broad classes based on their visual appearance, Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in images. They are denoted by the letter E, followed by an integer n representing their degree of ellipticity on the sky. Spiral galaxies consist of a disk, with stars forming a spiral structure, and a central concentration of stars known as the bulge. They are given the symbol S, roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge. These barred spirals are given the symbol SB and these broad classes can be extended to enable finer distinctions of appearance and to encompass other types of galaxies, such as irregular galaxies, which have no obvious regular structure. The Hubble sequence is represented in the form of a two-pronged fork, with the ellipticals on the left. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the two meet the “handle”. To this day, the Hubble sequence is the most commonly used system for classifying galaxies, the de Vaucouleurs system for classifying galaxies is a widely used extension to the Hubble sequence, first described by Gérard de Vaucouleurs in 1959. In particular, he argued that rings and lenses are important structural components of spiral galaxies, the de Vaucouleurs system retains Hubble’s basic division of galaxies into ellipticals, lenticulars, spirals and irregulars. For example, a barred spiral galaxy with loosely wound arms. Visually, the de Vaucouleurs system can be represented as a version of Hubble’s tuning fork, with stage on the x-axis, family on the y-axis. De Vaucouleurs also assigned numerical values to each class of galaxy in his scheme, values of the numerical Hubble stage T run from −6 to +10, with negative numbers corresponding to early-type galaxies and positive numbers to late types. Elliptical galaxies are divided into three stages, compact ellipticals, normal ellipticals and late types, lenticulars are similarly subdivided into early, intermediate and late types. Irregular galaxies can be of type magellanic irregulars or compact, the use of numerical stages allows for more quantitative studies of galaxy morphology. Created by American astronomer William Wilson Morgan, together with Philip Keenan, Morgan developed the MK system for the classification of stars through their spectra. The Yerkes scheme uses the spectra of stars in the galaxy, the shape, real and apparent, thus, for example, the Andromeda Galaxy is classified as kS5. H
16.
Pavo (constellation)
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Pavo is a constellation in the southern sky with the Latin name for peacock. It is one of twelve constellations conceived by Petrus Plancius from the observations of Pieter Dirkszoon Keyser, Pavo first appeared on a 35-cm diameter celestial globe published in 1598 in Amsterdam by Plancius and Jodocus Hondius and was depicted in Johann Bayers star atlas Uranometria of 1603. French explorer and astronomer Nicolas-Louis de Lacaille gave its stars Bayer designations in 1756, the constellations Pavo, Grus, Phoenix and Tucana are collectively known as the Southern Birds. The constellations brightest member, Alpha Pavonis, is known as Peacock and appears as a 1. 91-magnitude blue-white star. Delta Pavonis is a nearby Sun-like star some 19.9 light-years distant, the constellation contains NGC6752, the third-brightest globular cluster in the sky, and the spiral galaxy NGC6744, which closely resembles the Milky Way but is twice as large. Pavo is the radiant of two meteor showers, the Delta Pavonids and August Pavonids. It first appeared on a 35-cm diameter celestial globe published in 1598 in Amsterdam by Plancius with Jodocus Hondius, the first depiction of this constellation in a celestial atlas was in German cartographer Johann Bayers Uranometria of 1603. De Houtman included it in his star catalogue the same year under the Dutch name De Pauww. Pavo and the nearby constellations Phoenix, Grus and Tucana are collectively called the Southern Birds, an alternate Latin name for the constellation was Junonia Avis. He was changed by the goddess Juno into a peacock and placed in the sky along with his ship, indeed, the peacock symboliz the starry firmament for the Greeks, and the goddess Hera was believed to drive through the heavens in a chariot drawn by peacocks. The peacock and the Argus nomenclature are also prominent in a different myth, in which Io, Zeus changed Io into a heifer to deceive his wife Hera and couple with her. Hera saw through Zeuss scheme and asked for the heifer as a gift, meanwhile, Zeus entreated Hermes to save Io, Hermes used music to lull Argus Panoptes to sleep, then slew him. Hera adorned the tail of a peacock—her favorite bird—with Arguss eyes in his honor, saturnia retrieved those eyes to set in place among the feathers of her bird and filled his tail with starry jewels. The Wardaman people of the Northern Territory in Australia saw the stars of Pavo, Pavo is bordered by Telescopium to the north, Apus and Ara to the west, Octans to the south, and Indus to the east and northeast. Covering 378 square degrees, it ranks 44th of the 88 modern constellations in size, the three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is Pav. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 10 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 18h 10. 4m and 21h 32. 4m, while the coordinates are between −56. 59° and −74. 98°. Some of the stars in the form an asterism known as the Saucepan in Australia when they are used for navigation
17.
Lynx (constellation)
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Lynx, named after the animal, is a constellation in the northern sky that was introduced in the 17th century by Johannes Hevelius. It is a faint constellation with its brightest stars forming a zigzag line, the orange giant Alpha Lyncis is the brightest star in the constellation, while the semiregular variable star Y Lyncis is a target for amateur astronomers. Six star systems have found to contain planets. Those of 6 Lyncis and HD75898 were discovered by the Doppler method, polish astronomer Johannes Hevelius formed the constellation in the 17th century from 19 faint stars that he observed with the unaided eye between the constellations Ursa Major and Auriga. Naming it Lynx because of its faintness, he challenged future stargazers to see it, Hevelius gave it the alternate name of Tiger in his catalogue as well, but kept the former name only in his atlas. English astronomer John Flamsteed adopted the constellation in his catalogue, published in 1712, Lynx is bordered by Camelopardalis to the north, Auriga to the west, Gemini to the southwest, Cancer to the south, Leo to the east and Ursa Major to the northeast. Covering 545.4 square degrees and 1. 322% of the sky, it ranks 28th of the 88 constellations in size. The three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is Lyn, the official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 20 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 06h 16m 13. 76s and 09h 42m 50. 22s, while the coordinates are between +32. 97° and +61. 96°. On dark nights, the stars can be seen as a crooked line extending roughly between Camelopardalis and Leo, and north of the bright star Castor. Lynx is most readily observed from the winter to late summer to northern hemisphere observers. The whole constellation is visible to observers north of latitude 28°S, english astronomer Francis Baily gave a single star a Bayer designation—Alpha Lyncis—while Flamsteed numbered 44 stars, though several lie across the boundary in Ursa Major. Overall, there are 97 stars within the constellations borders brighter than or equal to apparent magnitude 6.5, the brightest star in this constellation is Alpha Lyncis, with an apparent magnitude of 3.14. It is a giant of spectral type K7III located 203 ±2 light-years distant from Earth. Around twice as massive as the Sun, it has exhausted the hydrogen at its core and has evolved away from the main sequence, the star has swollen to about 55 times the Suns radius and it is emitting roughly 673 times the luminosity of the Sun. The stellar atmosphere has cooled, giving it a surface temperature of 3,880 K, the only star with a proper name is Alsciaukat, also known as 31 Lyncis, located 380 ±10 light-years from Earth. This star is also a giant with around twice the Suns mass that has swollen. It is anywhere from 59 to 75 times as wide as the Sun, Alsciaukat is also a variable star, ranging in brightness by 0.05 magnitude over 25 to 30 days from its baseline magnitude of 4.25
18.
Ursa Major
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Ursa Major is a constellation in the northern celestial hemisphere. One of the 48 constellations listed by Ptolemy, it one of the 88 modern constellations. It can be throughout the year in most of the northern hemisphere. The Big Dipper and the constellation as a whole have mythological significance in world cultures. Ursa Major occupies an area in the northern celestial hemisphere. Its also the namesake of its family, which includes all the constellations it borders except for Leo. The three-letter abbreviation for the constellation, as adopted by the IAU in 1922, is UMa, the Big Dipper is an asterism within Ursa Major composed of seven bright stars that together comprise one of the best-known patterns in the sky. β Ursae Majoris, called Merak, with a magnitude of 2.37, γ Ursae Majoris, known as either Phecda or Phad, with a magnitude of 2.44. δ Ursae Majoris, or Megrez, meaning root of the tail, Alioth is the brightest star of Ursa Major and the 33rd-brightest in the sky, with a magnitude of 1.76. It is also the brightest of the peculiar A stars, magnetic stars whose chemical elements are either depleted or enhanced, ζ Ursae Majoris, Mizar, the second star in from the end of the handle of the Big Dipper, and the constellations fourth-brightest star. Mizar, which means girdle, forms a double star, with its optical companion Alcor. The ability to resolve the two stars with the eye is often quoted as a test of eyesight, although even people with quite poor eyesight can see the two stars. η Ursae Majoris, known as either Alkaid or Benetnash, both meaning the end of the tail, with a magnitude of 1.85, Alkaid is the third-brightest star of Ursa Major. Except for Dubhe and Alkaid, the stars of the Big Dipper all have proper motions heading toward a point in Sagittarius. A few other such stars have been identified, and together they are called the Ursa Major Moving Group, the stars Merak and Dubhe are known as the pointer stars because they are helpful for finding Polaris, also known as the North Star or Pole Star. By visually tracing a line from Merak through Dubhe and continuing, ones eye will land on Polaris, W Ursae Majoris is the prototype of a class of contact binary variable stars, and ranges between 7. 75m and 8. 48m. 47 Ursae Majoris is a Sun-like star with a three-planet system,47 Ursae Majoris b, discovered in 1996, orbits every 1078 days and is 2.53 times the mass of Jupiter. 47 Ursae Majoris c, discovered in 2001, orbits every 2391 days and is 0.54 times the mass of Jupiter
19.
Dwarf galaxy
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A dwarf galaxy is a small galaxy composed of about 100 million up to several billion stars, a small number compared to the Milky Ways 200–400 billion stars. The Large Magellanic Cloud, which orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy. Dwarf galaxies formation and activity are thought to be influenced by interactions with larger galaxies. Astronomers identify numerous types of galaxies, based on their shape. Current theory states that most galaxies, including galaxies, form in association with dark matter. However, NASAs Galaxy Evolution Explorer space probe identified new dwarf galaxies forming out of gases with low metallicity and these galaxies were located in the Leo Ring, a cloud of hydrogen and helium around two massive galaxies in the constellation Leo. Because of their size, dwarf galaxies have been observed being pulled toward and ripped by neighbouring spiral galaxies. There are many galaxies in the Local Group, these small galaxies frequently orbit larger galaxies, such as the Milky Way, the Andromeda Galaxy. A2007 paper has suggested that dwarf galaxies were created by galactic tides during the early evolutions of the Milky Way. Tidal dwarf galaxies are produced when galaxies collide and their gravitational masses interact, streams of galactic material are pulled away from the parent galaxies and the halos of dark matter that surround them. These stars, the brightest of which are blue, cause the galaxy itself to appear blue in colour, most BCD galaxies are also classified as dwarf irregular galaxies or as dwarf lenticular galaxies. Because they are composed of clusters, BCD galaxies lack a uniform shape. They consume gas intensely, which causes their stars to become violent when forming. BCD galaxies cool in the process of forming new stars, the galaxies stars are all formed at different time periods, so the galaxies have time to cool and to build up matter to form new stars. As time passes, this star formation changes the shape of the galaxies, nearby examples include NGC1705, NGC2915, NGC3353 and UGCA281. Ultra-compact dwarf galaxies are a class of compact galaxies with very high stellar populations. They are thought to be on the order of 200 light years across, UCDs have been found in the Virgo Cluster, Fornax Cluster, Abell 1689, and the Coma Cluster, amongst others. In particular, a large sample of ~100 UCDs has been found in the core region of the Virgo cluster by the Next Generation Virgo Cluster Survey team
20.
Peculiar galaxy
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A peculiar galaxy is a galaxy which is unusual in its size, shape, or composition. They can be irregular in shape due to the immense gravitational forces which act on them during encounters with other galaxies. They are the result of recent mergers between two or more normal galaxies, specifically, Peculiar galaxies may be from galaxy mergers and collisions that happened in the recent past. Peculiar galaxies are of size to regular sized Spiral and Elliptical galaxies. In addition, peculiar galaxies can be used by astronomers in order to determine the structure of regular galaxies, astronomers have identified two types of peculiar galaxies, interacting galaxies and active galactic nuclei. Peculiar galaxies are designated by p or pec in some catalogs and they are snapshots of galaxy mergers in the first millions of years after the collision. Thats when the galaxy is in an active state and still maintains some common features of the host galaxies. They constitute between 5% and 10% of the galaxy population, although most ‘normal’ galaxies will show peculiar features if examined carefully. Peculiar galaxies show a diversity of form. Many peculiar galaxies are experiencing episodes of enhanced star formation, called starbursts and they also show a greater tendency to host ACTIVE GALACTIC NUCLEI compared with the normal galaxy population Peculiar galaxies have been mapped by Halton Arp in his 1966 Atlas of Peculiar Galaxies
21.
International Standard Book Number
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The International Standard Book Number is a unique numeric commercial book identifier. An ISBN is assigned to each edition and variation of a book, for example, an e-book, a paperback and a hardcover edition of the same book would each have a different ISBN. The ISBN is 13 digits long if assigned on or after 1 January 2007, the method of assigning an ISBN is nation-based and varies from country to country, often depending on how large the publishing industry is within a country. The initial ISBN configuration of recognition was generated in 1967 based upon the 9-digit Standard Book Numbering created in 1966, the 10-digit ISBN format was developed by the International Organization for Standardization and was published in 1970 as international standard ISO2108. Occasionally, a book may appear without a printed ISBN if it is printed privately or the author does not follow the usual ISBN procedure, however, this can be rectified later. Another identifier, the International Standard Serial Number, identifies periodical publications such as magazines, the ISBN configuration of recognition was generated in 1967 in the United Kingdom by David Whitaker and in 1968 in the US by Emery Koltay. The 10-digit ISBN format was developed by the International Organization for Standardization and was published in 1970 as international standard ISO2108, the United Kingdom continued to use the 9-digit SBN code until 1974. The ISO on-line facility only refers back to 1978, an SBN may be converted to an ISBN by prefixing the digit 0. For example, the edition of Mr. J. G. Reeder Returns, published by Hodder in 1965, has SBN340013818 -340 indicating the publisher,01381 their serial number. This can be converted to ISBN 0-340-01381-8, the check digit does not need to be re-calculated, since 1 January 2007, ISBNs have contained 13 digits, a format that is compatible with Bookland European Article Number EAN-13s. An ISBN is assigned to each edition and variation of a book, for example, an ebook, a paperback, and a hardcover edition of the same book would each have a different ISBN. The ISBN is 13 digits long if assigned on or after 1 January 2007, a 13-digit ISBN can be separated into its parts, and when this is done it is customary to separate the parts with hyphens or spaces. Separating the parts of a 10-digit ISBN is also done with either hyphens or spaces, figuring out how to correctly separate a given ISBN number is complicated, because most of the parts do not use a fixed number of digits. ISBN issuance is country-specific, in that ISBNs are issued by the ISBN registration agency that is responsible for country or territory regardless of the publication language. Some ISBN registration agencies are based in national libraries or within ministries of culture, in other cases, the ISBN registration service is provided by organisations such as bibliographic data providers that are not government funded. In Canada, ISBNs are issued at no cost with the purpose of encouraging Canadian culture. In the United Kingdom, United States, and some countries, where the service is provided by non-government-funded organisations. Australia, ISBNs are issued by the library services agency Thorpe-Bowker
22.
Lenticular galaxy
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A lenticular galaxy is a type of galaxy intermediate between an elliptical and a spiral galaxy in galaxy morphological classification schemes. Lenticular galaxies are galaxies that have used up or lost most of their interstellar matter. They may, however, retain significant dust in their disks, as a result, they consist mainly of aging stars. Because of their spiral arms, if they are inclined face-on it is often difficult to distinguish between them and elliptical galaxies. Despite the morphological differences, lenticular and elliptical galaxies share common properties like spectral features, both can be considered early-type galaxies that are passively evolving, at least in the local part of the Universe. Lenticular galaxies are unique in that they have a visible disk component as well as a prominent bulge component and 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 ratio distribution of a lenticular galaxy sample. The distribution for lenticular galaxies rises steadily in the range 0.25 to 0.85 whereas the distribution for spirals is essentially 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 galaxies with prominent spheroidal components will have more galaxies at larger axial ratios. The fact that the galaxy distribution rises with increasing observed axial ratio implies that lenticulars are dominated by a central bulge component. Lenticular galaxies are often considered to be a poorly understood transition state between spiral and elliptical galaxies, which results in their placement on the Hubble sequence. This results from lenticulars having both prominent disk and bulge components, the disk component is usually featureless, which precludes a classification system similar to spiral galaxies. As the bulge component is spherical, elliptical galaxy classifications are also unsuitable. Lenticular galaxies are divided into subclasses based upon either the amount of dust present or the prominence of a central bar. While ellipticals and spirals tend to have somewhat well-defined Sérsic profiles, the disk component of lenticular galaxies often has a very flat surface brightness distribution, especially in the outermost regions of the disk. Also, there is typically an observed truncation in the brightness of lenticular galaxies at ~4 scale radii of the disk. These features are consistent with the structure of spiral galaxies
23.
Flocculent spiral galaxy
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A flocculent spiral galaxy is a type of spiral galaxy. Unlike the well-defined spiral architecture of a grand design spiral galaxy, flocculent galaxies are patchy, approximately 30% of spirals are flocculent, 10% are grand design, and the rest are referred to as multi-armed. The multiple-arm type is sometimes grouped into the flocculent category, the prototypical flocculent spiral is NGC2841. PDF A Near-Infrared Atlas of Spiral Galaxies, Debra Meloy Elmegreen,1981, doi,10. 1086/190757, Bibcode, 1981ApJS.47. 229E COSMOS astronomy encyclopedia - Flocculent Spiral
24.
Grand design spiral galaxy
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A grand design spiral galaxy is a type of spiral galaxy with prominent and well-defined spiral arms, as opposed to multi-arm and flocculent spirals which have subtler structural features. The spiral arms of a grand design galaxy extend clearly around the galaxy through many radians, as of 2002, approximately 10 percent of all currently known spiral galaxies are classified as grand design type spirals, including M81, M51, M74, M100, and M101. Density wave theory is the explanation for the well-defined structure of grand design spirals. According to this theory, the arms are created inside density waves that turn around the galaxy at different speeds from the stars in the galaxys disk. Stars are clumped in these regions due to gravitational attraction towards the dense material. This causes material to clump in the dense regions, at the center of these galaxies are supermassive black holes
25.
Intermediate spiral galaxy
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An intermediate spiral galaxy is a galaxy that is in between the classifications of a barred spiral galaxy and an unbarred spiral galaxy. It is designated as SAB in the galaxy morphological classification scheme, by definition, a galaxy is a congregation of stars held together by gravity. The first intermediate spiral galaxy discovered is the Milky Way, by Galileo and he was the first person with a telescope powerful enough to make such a discovery. Before Galileo, it was thought that all objects in the sky were either the planets in the Solar System, moons, comets. Until the beginning on the century, astronomers did not know the size of the Universe. Nothing was resolved at the debate, neither side was able to provide evidence to prove their side correct over their opponent. In 1923, Edwin Hubble resolved the matter with a photograph that he took of the Andromeda Galaxy, what he found in his photograph was a very bright light source pulsing at a certain rate, a Cepheid variable, located outside the Milky Way. This can be used to determine the distance to it, Hubble proved that the Universe was full of galaxies, and disproved that the Milky Way was the extent of the Universe. There are many types of galaxies in the Universe, elliptical, barred spiral galaxies, they vary in shape and size, a galaxy starts out as a giant cloud of cold gas. The cloud of gas must be close to zero, if the gas cloud is too hot, the atoms will have too much kinetic energy. When a cloud reaches a mass of about 109−1011 times the mass of the Sun, for an intermediate spiral galaxy to form, the gas cloud must be rotating, and as the gas coalesces, the gas cloud will flatten out to form a disk shape. The mass remaining constant, to comply with the laws which govern the conservation of angular momentum. At the very center of the galaxy, the gas will condense so much under pressure that a supermassive black hole will form. All spiral galaxies have a hole at the center of their galaxy, called the galactic nucleus. The Central Bulge contains Population II stars, which are old stars, devoid of metals, for this reason the central bulge glows an orange/red color. The disk of the galaxy is full of star formation, and Population I stars, star formation takes place in the spiral arms of the galaxy. The rotation velocity of the galaxy is nearly uniform throughout the entire disk, the reason for this is due to an even bigger halo of postulated dark matter that extends beyond the size of the galaxy. This dark matter is to date made up of a substance which radiates no light
26.
Dwarf spiral galaxy
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A dwarf spiral galaxy is the dwarf version of a spiral galaxy. Dwarf galaxies are characterized as having low luminosities, small diameters, low surface brightnesses, the galaxies may be considered a subclass of low-surface-brightness galaxies. Dwarf spiral galaxies, particularly the dwarf counterparts of Sa-Sc type spiral galaxies, are quite rare, in contrast, dwarf elliptical galaxies, dwarf irregular galaxies, and the dwarf versions of Magellanic type galaxies are very common. It is suggested that dwarf spiral galaxies can transform into dwarf elliptical galaxies, NGC5474 NGC5204 NGC247 NGC6503 NGC3928 NGC625 NGC1051 NGC1311 NGC2188 IC4710 Sculptor Dwarf Galaxy Most identified dwarf spiral galaxies are located outside clusters. Strong gravitational interactions between galaxies and interactions between galaxies and intracluster gas are expected to destroy the disks of most dwarf spiral galaxies, nonetheless, dwarf galaxies with spiral-like structure have been identified within the Virgo Cluster and Coma Cluster
27.
Ring galaxy
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A ring galaxy is a galaxy with a circle-like appearance. Hoags Object, discovered by Art Hoag in 1950, is an example of a ring galaxy, the ring contains many massive, relatively young blue stars, which are extremely bright. The central region contains relatively little luminous matter, some astronomers believe that ring galaxies are formed when a smaller galaxy passes through the center of a larger galaxy. Because most of a galaxy consists of empty space, this collision rarely results in any actual collisions between stars, however, the gravitational disruptions caused by such an event could cause a wave of star formation to move through the larger galaxy. Other astronomers think that rings are formed around some galaxies when external accretion takes place, star formation would then take place in the accreted material because of the shocks and compressions of the accreted material. Interacting galaxy Cartwheel Galaxy AM 0644-741 Arp 147 Polar-ring galaxy Hoags Object at Astronomy Picture of the Day
28.
Polar-ring galaxy
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A polar-ring galaxy is a type of galaxy in which an outer ring of gas and stars rotates over the poles of the galaxy. These polar rings are thought to form when two galaxies gravitationally interact with each other, one possibility is that a material is tidally stripped from a passing galaxy to produce the polar ring seen in the polar-ring galaxy. The other possibility is that a smaller galaxy collides orthogonally with the plane of rotation of the larger galaxy, the best-known polar-ring galaxies are S0s, but from the physical point of view they are part of a wider category of galaxies, including several ellipticals. The first four S0 galaxies that were identified as polar-ring galaxies were NGC2685, NGC 4650A, A0136 -0801, while these galaxies have been extensively studied, many other polar-ring galaxies have since been identified. Polar-ring S0 galaxies may be found around 0. 5% of all nearby lenticular galaxies, the first polar-ring elliptical galaxies were identified in 1978. They were NGC5128, NGC5363, NGC1947 and Cygnus A, while the polar-ring S0 galaxies NGC2685 and NGC 4650A were at that time indicated as resulting from similar formation processes. In addition to the example, NGC5128, a very regular polar ring elliptical, is NGC5266 The SPRC is the Sloan Polar-Ring Catalogue. It was put together with images from the Sloan Digital Sky Survey. There are four sections, There are 6 galaxies in section A which are kinematically confirmed galaxies. There are 27 in section B, which are good candidates, in section C, there are 73 galaxies, which are possible candidates. And there are 51 galaxies in section D, which are related objects, There is a revised version of the SPRC, with 275 candidates. There are 70 of the best candidates,115 good candidates,53 related objects, and 37 face-on rings
29.
Active galactic nucleus
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Such excess non-stellar emission has been observed in the radio, microwaves, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy, the radiation from an AGN is believed to be a result of accretion of matter by a supermassive black hole at the center of its host galaxy. Numerous subclasses of AGN have been defined based on their observed characteristics, early photographic observations of nearby galaxies detected some characteristic signatures of AGN emission, although there was not yet a physical understanding of the nature of the AGN phenomenon. Further spectroscopic studies by astronomers including Vesto Slipher, Milton Humason, in 1943, Carl Seyfert published a paper in which he described observations of nearby galaxies having bright nuclei that were sources of unusually broad emission lines. Galaxies observed as part of study included NGC1068, NGC4151, NGC3516. Active galaxies such as these are known as Seyfert galaxies in honor of Seyferts pioneering work, the development of radio astronomy was a major catalyst to understanding AGN. Some of the earliest detected radio sources are nearby active elliptical galaxies such as Messier 87, the 3C radio survey led to further progress in discovery of new radio sources as well as identifying the visible-light sources associated with the radio emission. In photographic images, some of these objects were nearly point-like or quasi-stellar in appearance, a major breakthrough was the measurement of the redshift of the quasar 3C273 by Maarten Schmidt, published in 1963. Shortly afterward, optical spectra were used to measured the redshifts of a number of quasars including 3C48. The enormous luminosities of these quasars as well as their unusual spectral properties indicated that their power source could not be ordinary stars, Accretion of gas onto a supermassive black hole was suggested as the source of quasars power in papers by Edwin Salpeter and Yakov ZelDovich in 1964. Today, AGN are a topic of astrophysical research, both observational and theoretical. For a long time it has argued that an AGN must be powered by accretion of mass onto massive black holes. AGN are both compact and persistently extremely luminous, thus AGN-like characteristics are expected whenever a supply of material for accretion comes within the sphere of influence of the central black hole. In the standard model of AGN, cold material close to a black hole forms an accretion disc, dissipative processes in the accretion disc transport matter inwards and angular momentum outwards, while causing the accretion disc to heat up. The radiation from the accretion disc excites cold atomic material close to the black hole, a large fraction of the AGNs radiation may be obscured by interstellar gas and dust close to the accretion disc, but this will be re-radiated at some other waveband, most likely the infrared. Some accretion discs produce jets of twin, highly collimated, the direction of the jet ejection is determined either by the angular momentum axis of the accretion disc or the spin axis of the black hole. The jet production mechanism and indeed the jet composition on very small scales are not understood at present due to the resolution of astronomical instruments being too low, there exists a class of radiatively inefficient solutions to the equations that govern accretion. The most widely known of these is the Advection Dominated Accretion Flow, radiatively inefficient AGN would be expected to lack many of the characteristic features of standard AGN with an accretion disc
30.
Dark matter halo
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A dark matter halo is a hypothetical component of a galaxy that envelops the galactic disc and extends well beyond the edge of the visible galaxy. The halos mass dominates the total mass, since they consist of dark matter, halos cannot be observed directly, but their existence is inferred through their effects on the motions of stars and gas in galaxies. Dark matter halos play a key role in current models of galaxy formation and evolution, the presence of dark matter in the halo is inferred from its gravitational effect on a spiral galaxys rotation curve. Freeman noticed that the decline in velocity was not present in NGC300 nor M33. The DM Hypothesis has been reinforced by several studies, the formation of dark matter halos is believed to have played a major role in the early formation of galaxies. The current hypothesis for this is based on dark matter. These halos would continue to grow in mass, either through accretion of material from their immediate neighborhood, numerical simulations of CDM structure formation have been found to proceed as follows, A small volume with small perturbations initially expands with the expansion of the Universe. As time proceeds, small-scale perturbations grow and collapse to form small halos, at a later stage, these small halos merge to form a single virialized dark matter halo with an ellipsoidal shape, which reveals some substructure in the form of dark matter sub-halos. The use of CDM overcomes issues associated with the normal baryonic matter because it removes most of the thermal, the fact that the dark matter is cold compared to the baryonic matter allows the DM to form these initial, gravitationally bound clumps. Once these subhalos formed, their interaction with baryonic matter is enough to overcome the thermal energy. Simulations of this early galaxy formation matches the structure observed by galactic surveys as well as observation of the Cosmic Microwave Background. A commonly used model for galactic dark matter halos is the halo, ρ = ρ o −1 where ρ o denotes the finite central density. This provides a good fit to most rotation curve data, however, it cannot be a complete description, as the enclosed mass fails to converge to a finite value as the radius tends to infinity. The isothermal model is, at best, an approximation, many effects may cause deviations from the profile predicted by this simple model. The NFW profile is called universal because it works for a variety of halo masses, spanning four orders of magnitude. This profile has a gravitational potential even though the integrated mass still diverges logarithmically. It was later deduced that the density profile depends on the environment, NFW halos generally provide a worse description of galaxy data than does the pseudo-isothermal profile, leading to the cuspy halo problem. Higher resolution computer simulations are better described by the Einasto profile, the term d n is a function of n such that ρ e is the density at the radius r e that defines a volume containing half of the total mass
31.
Disc (galaxy)
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A disc is a component of disk galaxies, such as spiral galaxies, or lenticular galaxies. The galactic disc is the plane in which the spirals, bars, Galactic discs tend to have more gas and dust and younger stars than galactic bulges or galactic haloes. The galactic disc is composed of gas, dust and stars. The gas and dust component of the disc is called the gaseous disc. The star component of the disc is called the stellar disc. It has been noted that the velocity of stars in the disc of most disc galaxies is inconsistent with the amount of luminous matter calculated for the galaxy. A possible explanation for this problem is the dark matter. Galactic spheroid Galactic corona Tully–Fisher relation MOND Van Der Kruit, P. C, annual Review of Astronomy and Astrophysics
32.
Galactic plane
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The galactic plane is the plane in which the majority of a disk-shaped galaxys mass lies. The directions perpendicular to the galactic plane point to the galactic poles. Most often, in usage, the terms galactic plane and galactic poles are used to refer specifically to the plane and poles of the Milky Way. Some galaxies are irregular and do not have any well-defined disk, even in the case of a barred spiral galaxy like the Milky Way, defining the galactic plane is slightly imprecise and arbitrary since the stars are not perfectly coplanar. 282s, Dec 27° 07′42. 01″. This position is in Coma Berenices, near the bright star Arcturus, likewise, the zero of longitude of galactic coordinates was also defined in 1959 to be at position angle 123° from the north celestial pole. Thus the zero point on the galactic equator was at 17h 42m 26. 603s, −28° 55′00. 445″ or 17h 45m 37. 224s, −28° 56′10. 23″. The galactic center is located at position angle 31. 72° or 31. 40° east of north, galactic coordinate system Reid, M. J. Brunthaler, A. The Proper Motion of Sagittarius A*, the Mass of Sagittarius A*, The Astrophysical Journal,616, 872–884, arXiv, astro-ph/0408107, Bibcode, 2004ApJ.616. 872R, doi,10. 1086/424960. See appendix for the numbers listed above
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Interstellar medium
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In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and it fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of radiation, is the interstellar radiation field. The interstellar medium is composed of phases, distinguished by whether matter is ionic, atomic, or molecular. The interstellar medium is composed primarily of hydrogen followed by helium with trace amounts of carbon, oxygen, the thermal pressures of these phases are in rough equilibrium with one another. Magnetic fields and turbulent motions also provide pressure in the ISM, in all phases, the interstellar medium is extremely tenuous by terrestrial standards. In cool, dense regions of the ISM, matter is primarily in molecular form, in hot, diffuse regions of the ISM, matter is primarily ionized, and the density may be as low as 10−4 ions per cm3. Compare this with a density of roughly 1019 molecules per cm3 for air at sea level. By mass, 99% of the ISM is gas in any form, and 1% is dust. Of the gas in the ISM, by number 91% of atoms are hydrogen and 9% are helium, with 0. 1% being atoms of elements heavier than hydrogen or helium, by mass this amounts to 70% hydrogen, 28% helium, and 1. 5% heavier elements. The hydrogen and helium are primarily a result of primordial nucleosynthesis, the ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales. Stars form within the densest regions of the ISM, molecular clouds, and replenish the ISM with matter and energy through planetary nebulae, stellar winds, and supernovae. This interplay between stars and the ISM helps determine the rate at which a galaxy depletes its gaseous content, voyager 1 reached the ISM on August 25,2012, making it the first artificial object from Earth to do so. Interstellar plasma and dust will be studied until the end in 2025. Table 1 shows a breakdown of the properties of the components of the ISM of the Milky Way, field, Goldsmith & Habing put forward the static two phase equilibrium model to explain the observed properties of the ISM. Their modeled ISM consisted of a dense phase, consisting of clouds of neutral and molecular hydrogen. McKee & Ostriker added a third phase that represented the very hot gas which had been shock heated by supernovae. These phases are the temperatures where heating and cooling can reach a stable equilibrium and their paper formed the basis for further study over the past three decades
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Protogalaxy
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In physical cosmology, a protogalaxy, which could also be called a primeval galaxy, is a cloud of gas which is forming into a galaxy. The smaller clumps of gas in a form into stars. The term protogalaxy itself is generally accepted to mean Progenitors of the present day galaxies, however, the early stages of formation is not a clearly defined phrase. Or an over-dense region of dark matter in the early universe, destined to become gravitationally bound. It is thought that the universe began with a nearly uniform distribution of matter. The dark matter then began to clump together under gravitational attraction due to the initial density perturbation spectrum caused by quantum fluctuations and this derives from Heisenbergs uncertainty principle which shows that there can be tiny temporary changes in the amount of energy in empty space. The gravity of these denser clumps of matter then caused nearby matter to start falling into the denser region. This sort of process was observed and analysed by Nilsson et al. in 2006. This resulted in the formation of clouds of gas, predominantly hydrogen, and these clouds of gas and early stars, many times smaller than our galaxy, were the first protogalaxies. The established theory is that the groups of small protogalaxies were attracted together by gravity and collided and this follows the process of hierarchical assembly, which is an ongoing process where larger bodies are continually formed from the merging of smaller ones. Since there had no previous star formation to create other elements, protogalaxies would have been made up almost entirely of hydrogen. The hydrogen would bond to form H2 molecules, with some exceptions and this would change as star formation began and produced more elements through the process of nuclear fusion. Once a prototypical begins to form, all bound by its gravity begin to free fall towards it. The time taken for this free-fall to conclude can be approximated using the free-fall equations, most galaxies have completed this free-fall stage to become stable elliptical or disk galaxies, the disks taking longer to fully form. The formation of galaxy clusters takes much longer and is still in progress now and this stage is also where galaxies acquire most of their angular momentum. A protogalaxy acquires this due to influence from neighbouring dense clumps in the early universe, and the further the gas is away from the centre. The luminosity of protogalaxies comes from two sources, first and foremost is the radiation from nuclear fusion of Hydrogen into helium in early stars. This early burst of star formation is thought to have made a protogalaxys luminosity comparable to a present-day starburst galaxy or a quasar, the other is the release of excess gravitational binding energy
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