A mathematician is someone who uses an extensive knowledge of mathematics in his or her work to solve mathematical problems. Mathematics is concerned with numbers, quantity, space and change. One of the earliest known mathematicians was Thales of Miletus, he is credited with the first use of deductive reasoning applied to geometry, by deriving four corollaries to Thales' Theorem. The number of known mathematicians grew when Pythagoras of Samos established the Pythagorean School, whose doctrine it was that mathematics ruled the universe and whose motto was "All is number", it was the Pythagoreans who coined the term "mathematics", with whom the study of mathematics for its own sake begins. The first woman mathematician recorded by history was Hypatia of Alexandria, she succeeded her father as Librarian at the Great Library and wrote many works on applied mathematics. Because of a political dispute, the Christian community in Alexandria punished her, presuming she was involved, by stripping her naked and scraping off her skin with clamshells.
Science and mathematics in the Islamic world during the Middle Ages followed various models and modes of funding varied based on scholars. It was extensive patronage and strong intellectual policies implemented by specific rulers that allowed scientific knowledge to develop in many areas. Funding for translation of scientific texts in other languages was ongoing throughout the reign of certain caliphs, it turned out that certain scholars became experts in the works they translated and in turn received further support for continuing to develop certain sciences; as these sciences received wider attention from the elite, more scholars were invited and funded to study particular sciences. An example of a translator and mathematician who benefited from this type of support was al-Khawarizmi. A notable feature of many scholars working under Muslim rule in medieval times is that they were polymaths. Examples include the work on optics and astronomy of Ibn al-Haytham; the Renaissance brought an increased emphasis on science to Europe.
During this period of transition from a feudal and ecclesiastical culture to a predominantly secular one, many notable mathematicians had other occupations: Luca Pacioli. As time passed, many mathematicians gravitated towards universities. An emphasis on free thinking and experimentation had begun in Britain's oldest universities beginning in the seventeenth century at Oxford with the scientists Robert Hooke and Robert Boyle, at Cambridge where Isaac Newton was Lucasian Professor of Mathematics & Physics. Moving into the 19th century, the objective of universities all across Europe evolved from teaching the “regurgitation of knowledge” to “encourag productive thinking.” In 1810, Humboldt convinced the King of Prussia to build a university in Berlin based on Friedrich Schleiermacher’s liberal ideas. Thus and laboratories started to evolve. British universities of this period adopted some approaches familiar to the Italian and German universities, but as they enjoyed substantial freedoms and autonomy the changes there had begun with the Age of Enlightenment, the same influences that inspired Humboldt.
The Universities of Oxford and Cambridge emphasized the importance of research, arguably more authentically implementing Humboldt’s idea of a university than German universities, which were subject to state authority. Overall, science became the focus of universities in the 20th centuries. Students could conduct research in seminars or laboratories and began to produce doctoral theses with more scientific content. According to Humboldt, the mission of the University of Berlin was to pursue scientific knowledge; the German university system fostered professional, bureaucratically regulated scientific research performed in well-equipped laboratories, instead of the kind of research done by private and individual scholars in Great Britain and France. In fact, Rüegg asserts that the German system is responsible for the development of the modern research university because it focused on the idea of “freedom of scientific research and study.” Mathematicians cover a breadth of topics within mathematics in their undergraduate education, proceed to specialize in topics of their own choice at the graduate level.
In some universities, a qualifying exam serves to test both the breadth and depth of a student's understanding of mathematics. Mathematicians involved with solving problems with applications in real life are called applied mathematicians. Applied mathematicians are mathematical scientists who, with their specialized knowledge and professional methodology, approach many of the imposing problems presented in related scientific fields. With professional focus on a wide variety of problems, theoretical systems, localized constructs, applied mathematicians work in the study and formulation of mathematical models. Mathematicians and applied mathematicians are considered to be two of the STEM careers; the discipline of applied mathematics concerns
An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, moons and galaxies – in either observational or theoretical astronomy. Examples of topics or fields astronomers study include planetary science, solar astronomy, the origin or evolution of stars, or the formation of galaxies. Related but distinct subjects like physical cosmology. Astronomers fall under either of two main types: observational and theoretical. Observational astronomers analyze the data. In contrast, theoretical astronomers create and investigate models of things that cannot be observed; because it takes millions to billions of years for a system of stars or a galaxy to complete a life cycle, astronomers must observe snapshots of different systems at unique points in their evolution to determine how they form and die. They use these data to create models or simulations to theorize how different celestial objects work.
Further subcategories under these two main branches of astronomy include planetary astronomy, galactic astronomy, or physical cosmology. Astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using physical laws. Today, that distinction has disappeared and the terms "astronomer" and "astrophysicist" are interchangeable. Professional astronomers are educated individuals who have a Ph. D. in physics or astronomy and are employed by research institutions or universities. They spend the majority of their time working on research, although they quite have other duties such as teaching, building instruments, or aiding in the operation of an observatory; the number of professional astronomers in the United States is quite small. The American Astronomical Society, the major organization of professional astronomers in North America, has 7,000 members; this number includes scientists from other fields such as physics and engineering, whose research interests are related to astronomy.
The International Astronomical Union comprises 10,145 members from 70 different countries who are involved in astronomical research at the Ph. D. beyond. Contrary to the classical image of an old astronomer peering through a telescope through the dark hours of the night, it is far more common to use a charge-coupled device camera to record a long, deep exposure, allowing a more sensitive image to be created because the light is added over time. Before CCDs, photographic plates were a common method of observation. Modern astronomers spend little time at telescopes just a few weeks per year. Analysis of observed phenomena, along with making predictions as to the causes of what they observe, takes the majority of observational astronomers' time. Astronomers who serve as faculty spend much of their time teaching undergraduate and graduate classes. Most universities have outreach programs including public telescope time and sometimes planetariums as a public service to encourage interest in the field.
Those who become astronomers have a broad background in maths and computing in high school. Taking courses that teach how to research and present papers are invaluable. In college/university most astronomers get a Ph. D. in astronomy or physics. While there is a low number of professional astronomers, the field is popular among amateurs. Most cities have amateur astronomy clubs that meet on a regular basis and host star parties; the Astronomical Society of the Pacific is the largest general astronomical society in the world, comprising both professional and amateur astronomers as well as educators from 70 different nations. Like any hobby, most people who think of themselves as amateur astronomers may devote a few hours a month to stargazing and reading the latest developments in research. However, amateurs span the range from so-called "armchair astronomers" to the ambitious, who own science-grade telescopes and instruments with which they are able to make their own discoveries and assist professional astronomers in research.
List of astronomers List of women astronomers List of Muslim astronomers List of French astronomers List of Hungarian astronomers List of Russian astronomers and astrophysicists List of Slovenian astronomers Dallal, Ahmad. "Science and Technology". In Esposito, John; the Oxford History of Islam. Oxford University Press, New York. ISBN 0-300-15911-0. Kennedy, E. S.. "A Survey of Islamic Astronomical Tables. 46. Philadelphia: American Philosophical Society. Toomer, Gerald. "Al-Khwārizmī, Abu Jaʿfar Muḥammad ibn Mūsā". In Gillispie, Charles Coulston. Dictionary of Scientific Biography. 7. New York: Charles Scribner's Sons. ISBN 0-684-16962-2. American Astronomical Society European Astronomical Society International Astronomical Union Astronomical Society of the Pacific Space's astronomy news
Isfahan is a city in Iran. It is located 406 kilometres south of Tehran, is the capital of Isfahan Province. Isfahan has a population of 1.6 million, making it the third largest city in Iran after Tehran and Mashhad, but was once one of the largest cities in the world. Isfahan is an important city as it is located at the intersection of the two principal north–south and east–west routes that traverse Iran. Isfahan flourished from 1050 to 1722 in the 16th and 17th centuries under the Safavid dynasty when it became the capital of Persia for the second time in its history under Shah Abbas the Great. Today the city retains much of its past glory, it is famous for its Perso–Islamic architecture, grand boulevards, covered bridges, tiled mosques, minarets. Isfahan has many historical buildings, monuments and artefacts; the fame of Isfahan led to the Persian pun and proverb "Esfahān nesf-e- jahān ast": Isfahan is half the world. The Naghsh-e Jahan Square in Isfahan is one of the largest city squares in the world.
UNESCO has designated it a World Heritage Site. See also: Names of Isfahan"Isfahan" is derived from Middle Persian Spahān. Spahān is attested in various Middle Persian seals and inscriptions, including that of Zoroastrian Magi Kartir, is the Armenian name of the city; the present-day name is the Arabicized form of Ispahan. The region appears with the abbreviation GD on Sasanian numismatics. In Ptolemy's Geographia it appears as Aspadana, translating to "place of gathering for the army", it is believed. Human habitation of the Isfahan region can be traced back to the Palaeolithic period. Recent discoveries archaeologists have found artifacts dating back to the Palaeolithic, Neolithic and Iron ages. What was to become the city of Isfahan in historical periods emerged as a locality and settlement that developed over the course of the Elamite civilisation. Under Median rule, this commercial entrepôt began to show signs of a more sedentary urbanism growing into a noteworthy regional centre that benefited from the exceptionally fertile soil on the banks of the Zayandehrud River in a region called Aspandana or Ispandana.
Once Cyrus the Great had unified Persian and Median lands into the Achaemenid Empire, the religiously and ethnically diverse city of Isfahan became an early example of the king's fabled religious tolerance. It was Cyrus who, having just taken Babylon, made an edict in 538 BCE, declaring that the Jews in Babylon could return to Jerusalem. Now it seems that some of these freed Jews settled in Isfahan instead of returning to their homeland; the 10th-century Persian historian Ibn al-Faqih wrote:"When the Jews emigrated from Jerusalem, fleeing from Nebuchadnezzar, they carried with them a sample of the water and soil of Jerusalem. They did not settle down anywhere or in any city without examining the water and the soil of each place, they did all along. There they rested, found that both resembled Jerusalem. Thereupon they settled there, cultivated the soil, raised children and grandchildren, today the name of this settlement is Yahudia." The Parthians in the period 250–226 BCE continued the tradition of tolerance after the fall of the Achaemenids, fostering the Hellenistic dimension within Iranian culture and the political organisation introduced by Alexander the Great's invading armies.
Under the Parthians, Arsacid governors administered the provinces of the nation from Isfahan, the city's urban development accelerated to accommodate the needs of a capital city. The next empire to rule Persia, the Sassanids, presided over massive changes in their realm, instituting sweeping agricultural reform and reviving Iranian culture and the Zoroastrian religion. Both the city and region were called by the name Aspahan or Spahan; the city was governed by a group called the Espoohrans, who came from seven noble and important Iranian royal families. Extant foundations of some Sassanid-era bridges in Isfahan suggest that the Sasanian kings were fond of ambitious urban planning projects. While Isfahan's political importance declined during the period, many Sassanid princes would study statecraft in the city, its military role developed rapidly, its strategic location at the intersection of the ancient roads to Susa and Persepolis made it an ideal candidate to house a standing army, ready to march against Constantinople at any moment.
The words'Aspahan' and'Spahan' are derived from the Pahlavi or Middle Persian meaning'the place of the army'. Although many theories have been mentioned about the origin of Isfahan, in fact little is known of it before the rule of the Sasanian dynasty; the historical facts suggest that in the late 4th and early 5th centuries, Queen Shushandukht, the Jewish consort of Yazdegerd I settled a colony of Jews in Yahudiyyeh, a settlement 3 km northwest of the Zoroastrian city of Gabae (its Achaemid and Parthian name. The gradual population decrease of Gay and the simultaneous population increase of Yahudiyyeh and its suburbs
A planet is an astronomical body orbiting a star or stellar remnant, massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, has cleared its neighbouring region of planetesimals. The term planet is ancient, with ties to history, science and religion. Five planets in the Solar System are visible to the naked eye; these were regarded by many early cultures as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union adopted a resolution defining planets within the Solar System; this definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas and Vesta, Pluto, that were once considered planets by the scientific community, are no longer viewed as such.
The planets were thought by Ptolemy to orbit Earth in epicycle motions. Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. About the same time, by careful analysis of pre-telescopic observational data collected by Tycho Brahe, Johannes Kepler found the planets' orbits were elliptical rather than circular; as observational tools improved, astronomers saw that, like Earth, each of the planets rotated around an axis tilted with respect to its orbital pole, some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes and hydrology. Planets are divided into two main types: large low-density giant planets, smaller rocky terrestrials. There are eight planets in the Solar System.
In order of increasing distance from the Sun, they are the four terrestrials, Venus and Mars the four giant planets, Saturn and Neptune. Six of the planets are orbited by one or more natural satellites. Several thousands of planets around other stars have been discovered in the Milky Way; as of 1 April 2019, 4,023 known extrasolar planets in 3,005 planetary systems, ranging in size from just above the size of the Moon to gas giants about twice as large as Jupiter have been discovered, out of which more than 100 planets are the same size as Earth, nine of which are at the same relative distance from their star as Earth from the Sun, i.e. in the circumstellar habitable zone. On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.
Around one in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone. The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age; the concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy; the five classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the "fixed stars", which maintained a constant relative position in the sky. Ancient Greeks called these lights πλάνητες ἀστέρες or πλανῆται, from which today's word "planet" was derived. In ancient Greece, China and indeed all pre-modern civilizations, it was universally believed that Earth was the center of the Universe and that all the "planets" circled Earth.
The reasons for this perception were that stars and planets appeared to revolve around Earth each day and the common-sense perceptions that Earth was solid and stable and that it was not moving but at rest. The first civilization known to have a functional theory of the planets were the Babylonians, who lived in Mesopotamia in the first and second millennia BC; the oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa, a 7th-century BC copy of a list of observations of the motions of the planet Venus, that dates as early as the second millennium BC. The MUL. APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun and planets over the course of the year; the Babylonian astrologers laid the foundations of what would become Western astrology. The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC, comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.
Venus and the outer planets Mars and Saturn were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope in early modern times; the ancient Greeks did not attach as much significance to the planets as the Babylonians. The Pythagoreans, in the 6th and 5t
Kermanshah Province is one of the 31 provinces of Iran. The province was known from 1969 to 1986 from 1986 to 1995 as Bakhtaran. According to a 2014 segmentation by the Ministry of Interior it is center of Region 4, with the region's central secretariat located at the province's capital city, Kermanshah. A majority of people in Kermanshah Province are Shia, there are Sunni and Yarsanist minority groups. Major cities and towns in Kermanshah Province include Kermanshah, Eslamabad-e Gharb, Harsin, Sonqor, Ravansar, Gilan-e Gharb, Qasr-e Shirin, Sarpol-e Zahab. Kermanshah consists of 14 shahrestans: The province's capital is Kermanshah, located in the middle of the western part of Iran; the population of the city is 822,921. The city is extended toward south during last two decades; the builtup areas run alongside Sarab Valley. City's elevation average about 1350 meters above sea level; the distance between Kermanshah and Teheran is 525 km. It is the trade center of rich agricultural region that produces grain, vegetable and oilseeds, there are many industrial centers and sugar refineries, cement and flour factories, etc.
The airport is located in north east of the city, the distance from Tehran is 413 km by air. The province has a rich Paleolithic heritage. Many caves with Paleolithic remains have been excavated there; some of these cave sites are located in north of Kermanshah. The first known physical remains of Neanderthal man in Iran was discovered in Bisitun Cave. Do-Ashkaft, Kobeh and Mar Tarik are some of the Middle Paleolithic sites in the region. Kermanshah has many Neolithic sites, of which the most famous are Ganj Dareh and Asiab. At Ganj Dareh, the earliest evidence for goat domestication have been documented. In May 2009, based on a research conducted by the University of Hamedan and UCL, the head of Archeology Research Center of Iran's Cultural Heritage and Tourism Organization announced that the oldest prehistoric village in the Middle East dating back to 9800 B. C. was discovered in Sahneh, located in west of Kermanshah. The monuments found in Kermanshah show two glorious periods, the Achaemenid and Sassanid eras.
The mythical ruler of the Pishdadian is described as founding the city while Tahmores Divband built it. An alternative narrative is that the construction was by Bahram IV of the Sassanid dynasty during the 4th century CE. Kermanshah reached a peak during the reign of Hormizd IV and Khosrau I of Sassanids, before being demoted to a secondary royal residence; the city suffered major damage during the Arab invasions but recovered in the Safavid period to make great progress. Concurrent with the Afghan attack and the fall of Isfahan, Kermanshah was completely destroyed by the Ottoman invasion. During the Iran–Iraq War the province suffered heavy fighting. Most towns and cities were badly damaged and some like Sar-e Pol-e Zahab and Qhasr-e-Shirin were completely destroyed; the November 2017 Iran–Iraq earthquake killed more than 600 people. On December 28, 2017 Kermanshah became one of several Iranian provinces to break out into protests; the Supreme Leader of Iran has blamed western interference. Some female organizations such as The National Council of Resistance of Iran have taken partial credit for the organization of these protests.
As it is situated between two cold and warm regions enjoys a moderate climate. Kermanshah has a mountainous climate, it is moderately warm in summer. The annual rainfall is 500 mm; the average temperature in the hottest months is above 22 °C. Kermanshah University of Medical Sciences Razi University Islamic Azad University of Kermanshah http://www.en.iauksh.ac.ir Kermanshah lends its name to a type of Persian carpet named after the region. It has famous sweets made of rice, locally known as Nân berendji; the other famous Kermanshahi good is a special kind of oil, locally known as Rüne Dân and globally in Iran known as Roghan Kermanshahi. The Giveh of Kermanshah known as Klash is the highest quality Giveh. Various attractions exist that date from the pre-Islamic era, such as the Kohneh Bridge, to contemporary parks and museums; some of the more popular sites are: Bisotun: Darius the Great's inscription at Bisotun, which dates to 522 BCE, lies some 1300 meters high in the mountains, counts as one of the most famous sites in Near Eastern archeology.
The site is a UNESCO World Heritage Site, has been attracting visitors for centuries. The Behistun inscription is to Old Persian cuneiform what the Rosetta Stone is to Egyptian hieroglyphs: the trilingual inscription was crucial in the decipherment of the script; the relief above the inscription depicts Darius facing nine rebels. At the king's feet lies Gaumata; the location of this important historical document is not coincidental: Gaumata, a usurper, depicted as lying at Darius' feet, was a Medean and in Achaemenid times Behistun lay on the Medea-Parsa highway. Behistun is notable for three reliefs at the foot of the hill that date from the Parthian era. Among them is a Hellenistic-era depiction of the divinity Bahram as the Greek hero Hercules, who reclines with a goblet in his hand, a club at his feet and a lion-skin beneath him; because it lies on the route of an ancient highway, this life-size rock sculpture may reflect Bahram's status as patron divinity of travelers. Taq-e Bostan:The rock reliefs at Taq-e Bostan lie 6 kilometres northeast of Kermanshah, where a spring gushes from a mountain cliff and empties into a large
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three celestial objects is known as a syzygy. Apart from syzygy, the term eclipse is used when a spacecraft reaches a position where it can observe two celestial bodies so aligned. An eclipse is the result of either a transit; the term eclipse is most used to describe either a solar eclipse, when the Moon's shadow crosses the Earth's surface, or a lunar eclipse, when the Moon moves into the Earth's shadow. However, it can refer to such events beyond the Earth–Moon system: for example, a planet moving into the shadow cast by one of its moons, a moon passing into the shadow cast by its host planet, or a moon passing into the shadow of another moon. A binary star system can produce eclipses if the plane of the orbit of its constituent stars intersects the observer's position.
For the special cases of solar and lunar eclipses, these only happen during an "eclipse season", the two times of each year when the plane of the Earth's orbit around the Sun crosses with the plane of the Moon's orbit around the Earth. The type of solar eclipse that happens during each season depends on apparent sizes of the Sun and Moon. If the orbit of the Earth around the Sun, the Moon's orbit around the Earth were both in the same plane with each other eclipses would happen each and every month. There would be a lunar eclipse at every full moon, a solar eclipse at every new moon, and if both orbits were circular each solar eclipse would be the same type every month. It is because of non-circular differences that eclipses are not a common event. Lunar eclipses can be viewed from the entire nightside half of the Earth, but solar eclipses total eclipses occurring at any one particular point on the Earth's surface, are rare events that can be many decades apart. The term is derived from the ancient Greek noun ἔκλειψις, which means "the abandonment", "the downfall", or "the darkening of a heavenly body", derived from the verb ἐκλείπω which means "to abandon", "to darken", or "to cease to exist," a combination of prefix ἐκ-, from preposition ἐκ, "out," and of verb λείπω, "to be absent".
For any two objects in space, a line can be extended from the first through the second. The latter object will block some amount of light being emitted by the former, creating a region of shadow around the axis of the line; these objects are moving with respect to each other and their surroundings, so the resulting shadow will sweep through a region of space, only passing through any particular location in the region for a fixed interval of time. As viewed from such a location, this shadowing event is known as an eclipse; the cross-section of the objects involved in an astronomical eclipse are disk shaped. The region of an object's shadow during an eclipse is divided into three parts: The umbra, within which the object covers the light source. For the Sun, this light source is the photosphere; the antumbra, extending beyond the tip of the umbra, within which the object is in front of the light source but too small to cover it. The penumbra, within which the object is only in front of the light source.
A total eclipse occurs when the observer is within the umbra, an annular eclipse when the observer is within the antumbra, a partial eclipse when the observer is within the penumbra. During a lunar eclipse only the umbra and penumbra are applicable; this is because Earth's apparent diameter from the viewpoint of the Moon is nearly four times that of the Sun. The same terms may be used analogously in describing other eclipses, e.g. the antumbra of Deimos crossing Mars, or Phobos entering Mars's penumbra. The first contact occurs when the eclipsing object's disc first starts to impinge on the light source. For spherical bodies, when the occulting object is smaller than the star, the length of the umbra's cone-shaped shadow is given by: L = r ⋅ R o R s − R o where Rs is the radius of the star, Ro is the occulting object's radius, r is the distance from the star to the occulting object. For Earth, on average L is equal to 1.384×106 km, much larger than the Moon's semimajor axis of 3.844×105 km. Hence the umbral cone of the Earth can envelop the Moon during a lunar eclipse.
If the occulting object has an atmosphere, some of the luminosity of the star can be refracted into the volume of the umbra. This occurs, for example, during an eclipse of the Moon by the Earth—producing a faint, ruddy illumination of the Moon at totality. On Earth, the shadow cast during an eclipse moves approximately at 1 km per sec; this depends on the angle in which it is moving. An eclipse cycle takes place; this happens. A particular instance is the saros, which results in a repetition of a solar or lunar eclipse every 6,585.3 days, or a little over 18 years. Because this is not a
An astronomical object or celestial object is a occurring physical entity, association, or structures that exists in the observable universe. In astronomy, the terms object and body are used interchangeably. However, an astronomical body or celestial body is a single bound, contiguous entity, while an astronomical or celestial object is a complex, less cohesively bound structure, which may consist of multiple bodies or other objects with substructures. Examples of astronomical objects include planetary systems, star clusters and galaxies, while asteroids, moons and stars are astronomical bodies. A comet may be identified as both body and object: It is a body when referring to the frozen nucleus of ice and dust, an object when describing the entire comet with its diffuse coma and tail; the universe can be viewed as having a hierarchical structure. At the largest scales, the fundamental component of assembly is the galaxy. Galaxies are organized into groups and clusters within larger superclusters, that are strung along great filaments between nearly empty voids, forming a web that spans the observable universe.
The universe has a variety of morphologies, with irregular and disk-like shapes, depending on their formation and evolutionary histories, including interaction with other galaxies, which may lead to a merger. Disc galaxies encompass lenticular and spiral galaxies with features, such as spiral arms and a distinct halo. At the core, most galaxies have a supermassive black hole, which may result in an active galactic nucleus. Galaxies can have satellites in the form of dwarf galaxies and globular clusters; the constituents of a galaxy are formed out of gaseous matter that assembles through gravitational self-attraction in a hierarchical manner. At this level, the resulting fundamental components are the stars, which are assembled in clusters from the various condensing nebulae; the great variety of stellar forms are determined entirely by the mass and evolutionary state of these stars. Stars may be found in multi-star systems. A planetary system and various minor objects such as asteroids and debris, can form in a hierarchical process of accretion from the protoplanetary disks that surrounds newly formed stars.
The various distinctive types of stars are shown by the Hertzsprung–Russell diagram —a plot of absolute stellar luminosity versus surface temperature. Each star follows an evolutionary track across this diagram. If this track takes the star through a region containing an intrinsic variable type its physical properties can cause it to become a variable star. An example of this is the instability strip, a region of the H-R diagram that includes Delta Scuti, RR Lyrae and Cepheid variables. Depending on the initial mass of the star and the presence or absence of a companion, a star may spend the last part of its life as a compact object; the table below lists the general categories of bodies and objects by their structure. List of light sources List of Solar System objects List of Solar System objects by size Lists of astronomical objects SkyChart, Sky & Telescope at the Library of Congress Web Archives Monthly skymaps for every location on Earth