1.
Sun
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The Sun is the star at the center of the Solar System. It is a perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 109 times that of Earth, and its mass is about 330,000 times that of Earth, accounting for about 99. 86% of the total mass of the Solar System. About three quarters of the Suns mass consists of hydrogen, the rest is mostly helium, with smaller quantities of heavier elements, including oxygen, carbon, neon. The Sun is a G-type main-sequence star based on its spectral class and it formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into a disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core and it is thought that almost all stars form by this process. The Sun is roughly middle-aged, it has not changed dramatically for more than four billion years and it is calculated that the Sun will become sufficiently large enough to engulf the current orbits of Mercury, Venus, and probably Earth. The enormous effect of the Sun on Earth has been recognized since prehistoric times, the synodic rotation of Earth and its orbit around the Sun are the basis of the solar calendar, which is the predominant calendar in use today. The English proper name Sun developed from Old English sunne and may be related to south, all Germanic terms for the Sun stem from Proto-Germanic *sunnōn. The English weekday name Sunday stems from Old English and is ultimately a result of a Germanic interpretation of Latin dies solis, the Latin name for the Sun, Sol, is not common in general English language use, the adjectival form is the related word solar. The term sol is used by planetary astronomers to refer to the duration of a solar day on another planet. A mean Earth solar day is approximately 24 hours, whereas a mean Martian sol is 24 hours,39 minutes, and 35.244 seconds. From at least the 4th Dynasty of Ancient Egypt, the Sun was worshipped as the god Ra, portrayed as a falcon-headed divinity surmounted by the solar disk, and surrounded by a serpent. In the New Empire period, the Sun became identified with the dung beetle, in the form of the Sun disc Aten, the Sun had a brief resurgence during the Amarna Period when it again became the preeminent, if not only, divinity for the Pharaoh Akhenaton. The Sun is viewed as a goddess in Germanic paganism, Sól/Sunna, in ancient Roman culture, Sunday was the day of the Sun god. It was adopted as the Sabbath day by Christians who did not have a Jewish background, the symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions
2.
Stellar classification
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In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with absorption lines, each line indicates an ion of a certain chemical element, with the line strength indicating the abundance of that ion. The relative abundance of the different ions varies with the temperature of the photosphere, the spectral class of a star is a short code summarizing the ionization state, giving an objective measure of the photospheres temperature and density. Most stars are classified under the Morgan–Keenan system using the letters O, B, A, F, G, K, and M. Each letter class is subdivided using a numeric digit with 0 being hottest and 9 being coolest. The sequence has been expanded with classes for other stars and star-like objects that do not fit in the system, such as class D for white dwarfs. In the MK system, a luminosity class is added to the class using Roman numerals. This is based on the width of absorption lines in the stars spectrum. The full spectral class for the Sun is then G2V, indicating a main-sequence star with a temperature around 5,800 K, the conventional color description takes into account only the peak of the stellar spectrum. This means that the assignment of colors of the spectrum can be misleading. There are no green, indigo, or violet stars, likewise, the brown dwarfs do not literally appear brown. The modern classification system is known as the Morgan–Keenan classification, each star is assigned a spectral class from the older Harvard spectral classification and a luminosity class using Roman numerals as explained below, forming the stars spectral type. The spectral classes O through M, as well as more specialized classes discussed later, are subdivided by Arabic numerals. For example, A0 denotes the hottest stars in the A class, fractional numbers are allowed, for example, the star Mu Normae is classified as O9.7. The Sun is classified as G2, the conventional color descriptions are traditional in astronomy, and represent colors relative to the mean color of an A-class star, which is considered to be white. The apparent color descriptions are what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars. However, most stars in the sky, except the brightest ones, red supergiants are cooler and redder than dwarfs of the same spectral type, and stars with particular spectral features such as carbon stars may be far redder than any black body. O-, B-, and A-type stars are called early type
3.
Solar mass
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The solar mass is a standard unit of mass in astronomy, equal to approximately 1.99 ×1030 kilograms. It is used to indicate the masses of stars, as well as clusters, nebulae. It is equal to the mass of the Sun, about two kilograms, M☉ = ×1030 kg The above mass is about 332946 times the mass of Earth. Because Earth follows an orbit around the Sun, its solar mass can be computed from the equation for the orbital period of a small body orbiting a central mass. The value he obtained differs by only 1% from the modern value, the diurnal parallax of the Sun was accurately measured during the transits of Venus in 1761 and 1769, yielding a value of 9″. From the value of the parallax, one can determine the distance to the Sun from the geometry of Earth. The first person to estimate the mass of the Sun was Isaac Newton, in his work Principia, he estimated that the ratio of the mass of Earth to the Sun was about 1/28700. Later he determined that his value was based upon a faulty value for the solar parallax and he corrected his estimated ratio to 1/169282 in the third edition of the Principia. The current value for the parallax is smaller still, yielding an estimated mass ratio of 1/332946. As a unit of measurement, the solar mass came into use before the AU, the mass of the Sun has been decreasing since the time it formed. This occurs through two processes in nearly equal amounts, first, in the Suns core, hydrogen is converted into helium through nuclear fusion, in particular the p–p chain, and this reaction converts some mass into energy in the form of gamma ray photons. Most of this energy eventually radiates away from the Sun, second, high-energy protons and electrons in the atmosphere of the Sun are ejected directly into outer space as a solar wind. The original mass of the Sun at the time it reached the main sequence remains uncertain, the early Sun had much higher mass-loss rates than at present, and it may have lost anywhere from 1–7% of its natal mass over the course of its main-sequence lifetime. The Sun gains a small amount of mass through the impact of asteroids. However, as the Sun already contains 99. 86% of the Solar Systems total mass, M☉ G / c2 ≈1.48 km M☉ G / c3 ≈4.93 μs I. -J. A Bright Young Sun Consistent with Helioseismology and Warm Temperatures on Ancient Earth and Mars
4.
Effective temperature
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The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is used as an estimate of a bodys surface temperature when the bodys emissivity curve is not known. When the stars or planets net emissivity in the relevant wavelength band is less than unity, the net emissivity may be low due to surface or atmospheric properties, including greenhouse effect. Notice that the luminosity of a star is then L =4 π R2 σ T e f f 4. The definition of the radius is obviously not straightforward. More rigorously the effective temperature corresponds to the temperature at the radius that is defined by a value of the Rosseland optical depth within the stellar atmosphere. The effective temperature and the bolometric luminosity are the two fundamental physical parameters needed to place a star on the Hertzsprung–Russell diagram, both effective temperature and bolometric luminosity depend on the chemical composition of a star. The effective temperature of our Sun is around 5780 kelvin, stars have a decreasing temperature gradient, going from their central core up to the atmosphere. The core temperature of the temperature at the centre of the sun where nuclear reactions take place—is estimated to be 15,000,000 K. The effective temperature of a star indicates the amount of heat that the star radiates per unit of surface area, from the warmest surfaces to the coolest is the sequence of star types known as O, B, A, F, G, K, and M. The effective temperature of a planet can be calculated by equating the power received by the planet with the emitted by a blackbody of temperature T. Take the case of a planet at a distance D from the star and we also allow the planet to reflect some of the incoming radiation by incorporating a parameter called the albedo. An albedo of 1 means that all the radiation is reflected, the effective temperature for Jupiter from this calculation is 112 K and 51 Pegasi b is 1258 K. A better estimate of effective temperature for some planets, such as Jupiter, the actual temperature depends on albedo and atmosphere effects. The actual temperature from spectroscopic analysis for HD209458 b is 1130 K, the internal heating within Jupiter raises the effective temperature to about 152 K. The surface temperature of a planet can be estimated by modifying the effective-temperature calculation to account for emissivity and this area intercepts some of the power which is spread over the surface of a sphere of radius D. We also allow the planet to some of the incoming radiation by incorporating a parameter a called the albedo. An albedo of 1 means that all the radiation is reflected, there is also a factor ε, which is the emissivity and represents atmospheric effects
5.
Main sequence
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In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung, Stars on this band are known as main-sequence stars or dwarf stars. These are the most numerous true stars in the universe, after a star has formed, it generates thermal energy in the dense core region through nuclear fusion of hydrogen atoms into helium. During this stage of the lifetime, it is located along the main sequence at a position determined primarily by its mass. All main-sequence stars are in equilibrium, where outward thermal pressure from the hot core is balanced by the inward pressure of gravitational collapse from the overlying layers. The strong dependence of the rate of generation in the core on the temperature and pressure helps to sustain this balance. Energy generated at the core makes its way to the surface and is radiated away at the photosphere, the energy is carried by either radiation or convection, with the latter occurring in regions with steeper temperature gradients, higher opacity or both. The main sequence is divided into upper and lower parts. Stars below about 1.5 times the mass of the Sun primarily fuse hydrogen atoms together in a series of stages to form helium, a sequence called the proton–proton chain. Above this mass, in the main sequence, the nuclear fusion process mainly uses atoms of carbon. Below this mass, stars have cores that are entirely radiative with convective zones near the surface, with decreasing stellar mass, the proportion of the star forming a convective envelope steadily increases, whereas main-sequence stars below 0.4 M☉ undergo convection throughout their mass. When core convection does not occur, a helium-rich core develops surrounded by an layer of hydrogen. In general, the more massive a star is, the shorter its lifespan on the main sequence, after the hydrogen fuel at the core has been consumed, the star evolves away from the main sequence on the HR diagram. The behavior of a star now depends on its mass, with stars below 0.23 M☉ becoming white dwarfs directly, more massive stars can explode as a supernova, or collapse directly into a black hole. In the early part of the 20th century, information about the types and distances of stars became more readily available, the spectra of stars were shown to have distinctive features, which allowed them to be categorized. Annie Jump Cannon and Edward C, pickering at Harvard College Observatory developed a method of categorization that became known as the Harvard Classification Scheme, published in the Harvard Annals in 1901. In Potsdam in 1906, the Danish astronomer Ejnar Hertzsprung noticed that the reddest stars—classified as K and M in the Harvard scheme—could be divided into two distinct groups and these stars are either much brighter than the Sun, or much fainter. To distinguish these groups, he called them giant and dwarf stars, the following year he began studying star clusters, large groupings of stars that are co-located at approximately the same distance
6.
Star
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A star is a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun, many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the stars in the Universe, including all stars outside our galaxy, indeed, most are invisible from Earth even through the most powerful telescopes. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the stars lifetime, near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity, and many properties of a star by observing its motion through space, its luminosity. The total mass of a star is the factor that determines its evolution. Other characteristics of a star, including diameter and temperature, change over its life, while the environment affects its rotation. A plot of the temperature of stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram. Plotting a particular star on that allows the age and evolutionary state of that star to be determined. A stars life begins with the collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, the remainder of the stars interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The stars internal pressure prevents it from collapsing further under its own gravity, a star with mass greater than 0.4 times the Suns will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements at the core or in shells around the core, as the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars. Meanwhile, the core becomes a remnant, a white dwarf. Binary and multi-star systems consist of two or more stars that are bound and generally move around each other in stable orbits. When two such stars have a close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, historically, stars have been important to civilizations throughout the world
7.
Kelvin
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The kelvin is a unit of measure for temperature based upon an absolute scale. It is one of the seven units in the International System of Units and is assigned the unit symbol K. The kelvin is defined as the fraction 1⁄273.16 of the temperature of the triple point of water. In other words, it is defined such that the point of water is exactly 273.16 K. The Kelvin scale is named after the Belfast-born, Glasgow University engineer and physicist William Lord Kelvin, unlike the degree Fahrenheit and degree Celsius, the kelvin is not referred to or typeset as a degree. The kelvin is the unit of temperature measurement in the physical sciences, but is often used in conjunction with the Celsius degree. The definition implies that absolute zero is equivalent to −273.15 °C, Kelvin calculated that absolute zero was equivalent to −273 °C on the air thermometers of the time. This absolute scale is known today as the Kelvin thermodynamic temperature scale, when spelled out or spoken, the unit is pluralised using the same grammatical rules as for other SI units such as the volt or ohm. When reference is made to the Kelvin scale, the word kelvin—which is normally a noun—functions adjectivally to modify the noun scale and is capitalized, as with most other SI unit symbols there is a space between the numeric value and the kelvin symbol. Before the 13th CGPM in 1967–1968, the unit kelvin was called a degree and it was distinguished from the other scales with either the adjective suffix Kelvin or with absolute and its symbol was °K. The latter term, which was the official name from 1948 until 1954, was ambiguous since it could also be interpreted as referring to the Rankine scale. Before the 13th CGPM, the form was degrees absolute. The 13th CGPM changed the name to simply kelvin. Its measured value was 7002273160280000000♠0.01028 °C with an uncertainty of 60 µK, the use of SI prefixed forms of the degree Celsius to express a temperature interval has not been widely adopted. In 2005 the CIPM embarked on a program to redefine the kelvin using a more experimentally rigorous methodology, the current definition as of 2016 is unsatisfactory for temperatures below 20 K and above 7003130000000000000♠1300 K. In particular, the committee proposed redefining the kelvin such that Boltzmanns constant takes the exact value 6977138065049999999♠1. 3806505×10−23 J/K, from a scientific point of view, this will link temperature to the rest of SI and result in a stable definition that is independent of any particular substance. From a practical point of view, the redefinition will pass unnoticed, the kelvin is often used in the measure of the colour temperature of light sources. Colour temperature is based upon the principle that a black body radiator emits light whose colour depends on the temperature of the radiator, black bodies with temperatures below about 7003400000000000000♠4000 K appear reddish, whereas those above about 7003750000000000000♠7500 K appear bluish
8.
Chemical element
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A chemical element or element is a species of atoms having the same number of protons in their atomic nuclei. There are 118 elements that have identified, of which the first 94 occur naturally on Earth with the remaining 24 being synthetic elements. There are 80 elements that have at least one stable isotope and 38 that have exclusively radioactive isotopes, iron is the most abundant element making up Earth, while oxygen is the most common element in the Earths crust. Chemical elements constitute all of the matter of the universe. The two lightest elements, hydrogen and helium, were formed in the Big Bang and are the most common elements in the universe. The next three elements were formed mostly by cosmic ray spallation, and are rarer than those that follow. Formation of elements with from 6 to 26 protons occurred and continues to occur in main sequence stars via stellar nucleosynthesis, the high abundance of oxygen, silicon, and iron on Earth reflects their common production in such stars. The term element is used for atoms with a number of protons as well as for a pure chemical substance consisting of a single element. A single element can form multiple substances differing in their structure, when different elements are chemically combined, with the atoms held together by chemical bonds, they form chemical compounds. Only a minority of elements are found uncombined as relatively pure minerals, among the more common of such native elements are copper, silver, gold, carbon, and sulfur. All but a few of the most inert elements, such as gases and noble metals, are usually found on Earth in chemically combined form. While about 32 of the elements occur on Earth in native uncombined forms. For example, atmospheric air is primarily a mixture of nitrogen, oxygen, and argon, the history of the discovery and use of the elements began with primitive human societies that found native elements like carbon, sulfur, copper and gold. Later civilizations extracted elemental copper, tin, lead and iron from their ores by smelting, using charcoal, alchemists and chemists subsequently identified many more, almost all of the naturally occurring elements were known by 1900. Save for unstable radioactive elements with short half-lives, all of the elements are available industrially, almost all other elements found in nature were made by various natural methods of nucleosynthesis. On Earth, small amounts of new atoms are produced in nucleogenic reactions, or in cosmogenic processes. Of the 94 naturally occurring elements, those with atomic numbers 1 through 82 each have at least one stable isotope, Isotopes considered stable are those for which no radioactive decay has yet been observed. Elements with atomic numbers 83 through 94 are unstable to the point that radioactive decay of all isotopes can be detected, the very heaviest elements undergo radioactive decay with half-lives so short that they are not found in nature and must be synthesized
9.
Hydrogen
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Hydrogen is a chemical element with chemical symbol H and atomic number 1. With a standard weight of circa 1.008, hydrogen is the lightest element on the periodic table. Its monatomic form is the most abundant chemical substance in the Universe, non-remnant stars are mainly composed of hydrogen in the plasma state. The most common isotope of hydrogen, termed protium, has one proton, the universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, since hydrogen readily forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays an important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a charge when it is known as a hydride. The hydrogen cation is written as though composed of a bare proton, Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. Industrial production is mainly from steam reforming natural gas, and less often from more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing and ammonia production, mostly for the fertilizer market, Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks. Hydrogen gas is flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion is −286 kJ/mol,2 H2 + O2 →2 H2O +572 kJ Hydrogen gas forms explosive mixtures with air in concentrations from 4–74%, the explosive reactions may be triggered by spark, heat, or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C, the detection of a burning hydrogen leak may require a flame detector, such leaks can be very dangerous. Hydrogen flames in other conditions are blue, resembling blue natural gas flames, the destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a mixture of hydrogen to oxygen combined with carbon compounds from the airship skin. H2 reacts with every oxidizing element, the ground state energy level of the electron in a hydrogen atom is −13.6 eV, which is equivalent to an ultraviolet photon of roughly 91 nm wavelength. The energy levels of hydrogen can be calculated fairly accurately using the Bohr model of the atom, however, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity. The most complicated treatments allow for the effects of special relativity
10.
Helium
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Helium is a chemical element with symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and its boiling point is the lowest among all the elements. Its abundance is similar to figure in the Sun and in Jupiter. This is due to the high nuclear binding energy of helium-4 with respect to the next three elements after helium. This helium-4 binding energy also accounts for why it is a product of nuclear fusion and radioactive decay. Most helium in the universe is helium-4, and is believed to have formed during the Big Bang. Large amounts of new helium are being created by fusion of hydrogen in stars. Helium is named for the Greek god of the Sun, Helios and it was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by French astronomer Jules Janssen. Janssen is jointly credited with detecting the element along with Norman Lockyer, Janssen observed during the solar eclipse of 1868 while Lockyer observed from Britain. Lockyer was the first to propose that the line was due to a new element, the formal discovery of the element was made in 1895 by two Swedish chemists, Per Teodor Cleve and Nils Abraham Langlet, who found helium emanating from the uranium ore cleveite. In 1903, large reserves of helium were found in gas fields in parts of the United States. Liquid helium is used in cryogenics, particularly in the cooling of superconducting magnets, a well-known but minor use is as a lifting gas in balloons and airships. As with any gas whose density differs from that of air, inhaling a small volume of helium temporarily changes the timbre, on Earth it is relatively rare—5.2 ppm by volume in the atmosphere. Most terrestrial helium present today is created by the radioactive decay of heavy radioactive elements. Previously, terrestrial helium—a non-renewable resource, because once released into the atmosphere it readily escapes into space—was thought to be in short supply. The first evidence of helium was observed on August 18,1868, the line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India. This line was assumed to be sodium. He concluded that it was caused by an element in the Sun unknown on Earth, Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος
11.
Nuclear fusion
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In nuclear physics, nuclear fusion is a reaction in which two or more atomic nuclei come close enough to form one or more different atomic nuclei and subatomic particles. The difference in mass between the products and reactants is manifested as the release of large amounts of energy and this difference in mass arises due to the difference in atomic binding energy between the atomic nuclei before and after the reaction. Fusion is the process that powers active or main sequence stars, the fusion process that produces a nucleus lighter than iron-56 or nickel-62 will generally yield a net energy release. These elements have the smallest mass per nucleon and the largest binding energy per nucleon, the opposite is true for the reverse process, nuclear fission. This means that the elements, such as hydrogen and helium, are in general more fusable, while the heavier elements. The extreme astrophysical event of a supernova can produce energy to fuse nuclei into elements heavier than iron. During the remainder of that decade the steps of the cycle of nuclear fusion in stars were worked out by Hans Bethe. Research into fusion for military purposes began in the early 1940s as part of the Manhattan Project, fusion was accomplished in 1951 with the Greenhouse Item nuclear test. Nuclear fusion on a scale in an explosion was first carried out on November 1,1952. Research into developing controlled thermonuclear fusion for civil purposes also began in earnest in the 1950s, the protons are positively charged and repel each other but they nonetheless stick together, demonstrating the existence of another force referred to as nuclear attraction. This force, called the nuclear force, overcomes electric repulsion at very close range. The effect of force is not observed outside the nucleus. The same force also pulls the nucleons together allowing ordinary matter to exist, light nuclei, are sufficiently small and proton-poor allowing the nuclear force to overcome the repulsive Coulomb force. This is because the nucleus is small that all nucleons feel the short-range attractive force at least as strongly as they feel the infinite-range Coulomb repulsion. Building up these nuclei from lighter nuclei by fusion thus releases the energy from the net attraction of these particles. For larger nuclei, however, no energy is released, since the force is short-range. Thus, energy is no longer released when such nuclei are made by fusion, instead, fusion reactions create the light elements that power the stars and produce virtually all elements in a process called nucleosynthesis. The fusion of elements in stars releases energy and the mass that always accompanies it
12.
Solar System
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The Solar System is the gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly. Of those objects that orbit the Sun directly, the largest eight are the planets, with the remainder being significantly smaller objects, such as dwarf planets, of the objects that orbit the Sun indirectly, the moons, two are larger than the smallest planet, Mercury. The Solar System formed 4.6 billion years ago from the collapse of a giant interstellar molecular cloud. The vast majority of the mass is in the Sun. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being composed of rock. The four outer planets are giant planets, being more massive than the terrestrials. All planets have almost circular orbits that lie within a flat disc called the ecliptic. The Solar System also contains smaller objects, the asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptunes orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity. Such objects are categorized as dwarf planets, identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds. Six of the planets, at least four of the dwarf planets, each of the outer planets is encircled by planetary rings of dust and other small objects. The solar wind, a stream of charged particles flowing outwards from the Sun, the heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium, it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, the Solar System is located in the Orion Arm,26,000 light-years from the center of the Milky Way. For most of history, humanity did not recognize or understand the concept of the Solar System, the invention of the telescope led to the discovery of further planets and moons. The principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99. 86% of the known mass. The Suns four largest orbiting bodies, the giant planets, account for 99% of the mass, with Jupiter. The remaining objects of the Solar System together comprise less than 0. 002% of the Solar Systems total mass, most large objects in orbit around the Sun lie near the plane of Earths orbit, known as the ecliptic
13.
Apparent magnitude
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The apparent magnitude of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth. The brighter an object appears, the lower its magnitude value, the Sun, at apparent magnitude of −27, is the brightest object in the sky. It is adjusted to the value it would have in the absence of the atmosphere, furthermore, the magnitude scale is logarithmic, a difference of one in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry, apparent magnitudes are used to quantify the brightness of sources at ultraviolet, visible, and infrared wavelengths. An apparent magnitude is measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V filter band would be denoted either as mV or often simply as V, the scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes. The brightest stars in the sky were said to be of first magnitude, whereas the faintest were of sixth magnitude. Each grade of magnitude was considered twice the brightness of the following grade and this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest, and is generally believed to have originated with Hipparchus. This implies that a star of magnitude m is 2.512 times as bright as a star of magnitude m +1 and this figure, the fifth root of 100, became known as Pogsons Ratio. The zero point of Pogsons scale was defined by assigning Polaris a magnitude of exactly 2. However, with the advent of infrared astronomy it was revealed that Vegas radiation includes an Infrared excess presumably due to a disk consisting of dust at warm temperatures. At shorter wavelengths, there is negligible emission from dust at these temperatures, however, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the scale was extrapolated to all wavelengths on the basis of the black body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance for the zero magnitude point, with the modern magnitude systems, brightness over a very wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30, astronomers have developed other photometric zeropoint systems as alternatives to the Vega system. The AB magnitude zeropoint is defined such that an objects AB, the dimmer an object appears, the higher the numerical value given to its apparent magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of exactly 100. Since an increase of 5 magnitudes corresponds to a decrease in brightness by a factor of exactly 100, each magnitude increase implies a decrease in brightness by the factor 5√100 ≈2.512. Inverting the above formula, a magnitude difference m1 − m2 = Δm implies a brightness factor of F2 F1 =100 Δ m 5 =100.4 Δ m ≈2.512 Δ m
14.
Matter
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All the everyday objects that we can bump into, touch or squeeze are ultimately composed of atoms. This ordinary atomic matter is in turn made up of interacting subatomic particles—usually a nucleus of protons and neutrons, typically, science considers these composite particles matter because they have both rest mass and volume. By contrast, massless particles, such as photons, are not considered matter, however, not all particles with rest mass have a classical volume, since fundamental particles such as quarks and leptons are considered point particles with no effective size or volume. Nevertheless, quarks and leptons together make up ordinary matter, Matter exists in states, the classical solid, liquid, and gas, as well as the more exotic plasma, Bose–Einstein condensates, fermionic condensates, and quark–gluon plasma. For much of the history of the natural sciences people have contemplated the nature of matter. The idea that matter was built of discrete building blocks, the so-called particulate theory of matter, was first put forward by the Greek philosophers Leucippus and Democritus, Matter should not be confused with mass, as the two are not the same in modern physics. Matter is itself a physical substance of which systems may be composed, while mass is not a substance, while there are different views on what should be considered matter, the mass of a substance or system is the same irrespective of any such definition of matter. Another difference is that matter has an opposite called antimatter, antimatter has the same mass property as its normal matter counterpart. Different fields of use the term matter in different, and sometimes incompatible. Some of these ways are based on loose historical meanings, from a time there was no reason to distinguish mass from simply a quantity of matter. As such, there is no universally agreed scientific meaning of the word matter. Scientifically, the mass is well-defined, but matter can be defined in several ways. Sometimes in the field of matter is simply equated with particles that exhibit rest mass, such as quarks. However, in physics and chemistry, matter exhibits both wave-like and particle-like properties, the so-called wave–particle duality. A definition of based on its physical and chemical structure is. Such atomic matter is sometimes termed ordinary matter. As an example, deoxyribonucleic acid molecules are matter under this definition because they are made of atoms and this definition can extend to include charged atoms and molecules, so as to include plasmas and electrolytes, which are not obviously included in the atoms definition. Alternatively, one can adopt the protons, neutrons, and electrons definition, at a microscopic level, the constituent particles of matter such as protons, neutrons, and electrons obey the laws of quantum mechanics and exhibit wave–particle duality
15.
Energy
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In physics, energy is the property that must be transferred to an object in order to perform work on – or to heat – the object, and can be converted in form, but not created or destroyed. The SI unit of energy is the joule, which is the transferred to an object by the mechanical work of moving it a distance of 1 metre against a force of 1 newton. Mass and energy are closely related, for example, with a sensitive enough scale, one could measure an increase in mass after heating an object. Living organisms require available energy to stay alive, such as the humans get from food. Civilisation gets the energy it needs from energy resources such as fuels, nuclear fuel. The processes of Earths climate and ecosystem are driven by the radiant energy Earth receives from the sun, the total energy of a system can be subdivided and classified in various ways. It may also be convenient to distinguish gravitational energy, thermal energy, several types of energy, electric energy. Many of these overlap, for instance, thermal energy usually consists partly of kinetic. Some types of energy are a mix of both potential and kinetic energy. An example is energy which is the sum of kinetic. Whenever physical scientists discover that a phenomenon appears to violate the law of energy conservation. Heat and work are special cases in that they are not properties of systems, in general we cannot measure how much heat or work are present in an object, but rather only how much energy is transferred among objects in certain ways during the occurrence of a given process. Heat and work are measured as positive or negative depending on which side of the transfer we view them from, the distinctions between different kinds of energy is not always clear-cut. In contrast to the definition, energeia was a qualitative philosophical concept, broad enough to include ideas such as happiness. The modern analog of this property, kinetic energy, differs from vis viva only by a factor of two, in 1807, Thomas Young was possibly the first to use the term energy instead of vis viva, in its modern sense. Gustave-Gaspard Coriolis described kinetic energy in 1829 in its modern sense, the law of conservation of energy was also first postulated in the early 19th century, and applies to any isolated system. It was argued for years whether heat was a physical substance, dubbed the caloric, or merely a physical quantity. In 1845 James Prescott Joule discovered the link between mechanical work and the generation of heat and these developments led to the theory of conservation of energy, formalized largely by William Thomson as the field of thermodynamics
16.
Alpha Centauri
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Alpha Centauri is a star system in the Orion Arm of the Milky Way galaxy, and is the closest star system to the Solar System, being 4.37 light-years from the Sun. It consists of three stars, the pair Alpha Centauri A and Alpha Centauri B together with a small and faint red dwarf, Alpha Centauri C, that may be gravitationally bound to the other two. Alpha Centauri A has 1.1 times the mass and 1.519 times the luminosity of the Sun, while Alpha Centauri B is smaller and cooler, at 0.907 times the Suns mass and 0.445 times its visual luminosity. During the pairs 79. 91-year orbit about a common centre, Proxima Centauri is at the slightly smaller distance of 4.24 light-years from the Sun, making it the closest star to the Sun, even though it is not visible to the naked eye. The separation of Proxima from Alpha Centauri AB is about 15,000 astronomical units, α Centauri is the systems Bayer designation. It bore the traditional name Rigil Kentaurus, which is a latinisation of the Arabic name رجل القنطورس Rijl al-Qanṭūris, meaning Foot of the Centaur. Alpha Centauri C was discovered in 1915 by the Scottish astronomer Robert Innes, Director of the Union Observatory in Johannesburg, South Africa, the name is from Latin, meaning nearest of Centaurus. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalog, the WGSN states that in the case of multiple stars the name should be understood to be attributed to the brightest component by visual brightness. The WGSN approved the name Proxima Centauri for Alpha Centauri C on 21 August 2016 and they are now both so entered in the IAU Catalog of Star Names. Alpha Centauri is the given to what appears as a single star to the naked eye. At −0.27 apparent visual magnitude, it is fainter only than Sirius and Canopus, the next-brightest star in the night sky is Arcturus. Alpha Centauri is a system, with its two main stars being Alpha Centauri A and Alpha Centauri B, usually defined to identify them as the different components of the binary α Cen AB. A third companion—Proxima Centauri —has a distance greater than the observed separation between stars A and B and is probably gravitationally associated with the AB system. As viewed from Earth, it is located at a separation of 2. 2° from the two main stars. Proxima Centauri would appear to the eye as a separate star from α Cen AB if it were bright enough to be seen without a telescope. Alpha Centauri AB and Proxima Centauri form a double star. Direct evidence that Proxima Centauri has an orbit typical of binary stars has yet to be found. Together, the three make a triple star system, referred to by double-star observers as the triple star
17.
Tau Ceti
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Tau Ceti is a star in the constellation Cetus that is spectrally similar to the Sun, although it has only about 78% of the Suns mass. At a distance of just under 12 light-years from the Solar System, it is a nearby star. The star appears stable, with little variation, and is metal-deficient. Observations have detected more than ten times as much dust surrounding Tau Ceti as is present in the Solar System, since December 2012, there has been evidence of possibly five planets orbiting Tau Ceti, with two of these being potentially in the habitable zone. Because of its debris disk, any planet orbiting Tau Ceti would face far more impact events than Earth, despite this hurdle to habitability, its solar analog characteristics have led to widespread interest in the star. It can be seen with the eye as a third-magnitude star. As seen from Tau Ceti, the Sun would be a star in the northern hemisphere constellation Boötes. This star, along with η Cet, θ Cet, ζ Cet, and υ Cet, were Al Naʽāmāt, the Hen Ostriches. In Chinese, the Square Celestial Granary refers to an asterism consisting of τ Ceti, ι Ceti, η Ceti, ζ Ceti, consequently, τ Ceti itself is known as the Fifth Star of Square Celestial Granary. The proper motion of a star is its amount of movement across the celestial sphere, Tau Ceti is considered to be a high-proper-motion star, although it only has an annual traverse of just under two arc seconds. It will require about two years before the location of this star shifts by more than a degree. A high proper motion is an indicator of closeness to the Sun, Nearby stars can traverse an angle of arc across the sky more rapidly than the distant background stars and are good candidates for parallax studies. In the case of Tau Ceti, the parallax measurements indicate a distance of 11.9 ly and this makes it one of the closest star systems to the Sun, and the next-closest spectral class-G star after Alpha Centauri A. The radial velocity of a star is its motion toward or away from the Sun, unlike proper motion, a stars radial velocity cannot be directly observed, but must be determined through measurement of the spectrum. In the case of Tau Ceti, the velocity is about −17 km/s. The distance to Tau Ceti, along with its motion and radial velocity. The space velocity relative to the Sun is about 37 km/s and this result can then be used to compute an orbital path of Tau Ceti through the Milky Way. It has a mean distance of 9.7 kiloparsec
18.
51 Pegasi
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51 Pegasi, also named Helvetios, is a Sun-like star located 50.9 light-years from Earth in the constellation of Pegasus. It was the first main-sequence star found to have an exoplanet orbiting it, the star is of apparent magnitude 5.49, and so is visible with the naked eye under suitable viewing conditions. 51 Pegasi has a classification of G5V, indicating that it is a main-sequence star that is generating energy through the thermonuclear fusion of hydrogen at its core. The effective temperature of the chromosphere is about 5571 K, giving 51 Pegasi the characteristic yellow hue of a G-type star. It is estimated to be 6. 1–8.1 billion years old, somewhat older than the Sun, with a radius 24% larger, the star has a higher proportion of elements other than hydrogen/helium compared to the Sun, a quantity astronomers term a stars metallicity. Stars with higher metallicity such as this are likely to host planets. In 1996 astronomers Baliunas, Sokoloff, and Soon measured a rotational period of 37 days for 51 Pegasi, although the star was suspected of being variable during a 1981 study, subsequent observation showed there was almost no chromospheric activity between 1977 and 1989. Further examination between 1994 and 2007 showed a low or flat level of activity. This, along with its relatively low X-ray emission, suggests that the star may be in a Maunder minimum period during which a star produces a number of star spots. The star rotates at an inclination of 79+11 −30 degrees relative to Earth,51 Pegasi is the Flamsteed designation. In July 2014 the International Astronomical Union launched a process for giving proper names to certain exoplanets, the process involved public nomination and voting for the new names. In December 2015, the IAU announced the names were Helvetios for this star. The winning names were submitted by the Astronomische Gesellschaft Luzern. In 2016, the IAU organized a Working Group on Star Names to catalog and this star is now so entered in the IAU Catalog of Star Names. On October 6,1995, Swiss astronomers Michel Mayor and Didier Queloz announced the discovery of an exoplanet orbiting 51 Pegasi, the discovery was made with the radial velocity method on a telescope at Observatoire de Haute-Provence in France and using the ELODIE spectrograph. 51 Pegasi b is the first discovered planetary-mass companion of its parent star, at the time, this close distance was not compatible with theories of planet formation and resulted in discussions of planetary migration. It has been assumed that the shares the stars inclination of 79 degrees. However, several hot Jupiters are now known to be relative to the stellar axis
19.
Earth
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Earth, otherwise known as the World, or the Globe, is the third planet from the Sun and the only object in the Universe known to harbor life. It is the densest planet in the Solar System and the largest of the four terrestrial planets, according to radiometric dating and other sources of evidence, Earth formed about 4.54 billion years ago. Earths gravity interacts with objects in space, especially the Sun. During one orbit around the Sun, Earth rotates about its axis over 365 times, thus, Earths axis of rotation is tilted, producing seasonal variations on the planets surface. The gravitational interaction between the Earth and Moon causes ocean tides, stabilizes the Earths orientation on its axis, Earths lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earths surface is covered with water, mostly by its oceans, the remaining 29% is land consisting of continents and islands that together have many lakes, rivers and other sources of water that contribute to the hydrosphere. The majority of Earths polar regions are covered in ice, including the Antarctic ice sheet, Earths interior remains active with a solid iron inner core, a liquid outer core that generates the Earths magnetic field, and a convecting mantle that drives plate tectonics. Within the first billion years of Earths history, life appeared in the oceans and began to affect the Earths atmosphere and surface, some geological evidence indicates that life may have arisen as much as 4.1 billion years ago. Since then, the combination of Earths distance from the Sun, physical properties, in the history of the Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely, over 7.4 billion humans live on Earth and depend on its biosphere and minerals for their survival. Humans have developed diverse societies and cultures, politically, the world has about 200 sovereign states, the modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most often spelled eorðe. It has cognates in every Germanic language, and their proto-Germanic root has been reconstructed as *erþō, originally, earth was written in lowercase, and from early Middle English, its definite sense as the globe was expressed as the earth. By early Modern English, many nouns were capitalized, and the became the Earth. More recently, the name is simply given as Earth. House styles now vary, Oxford spelling recognizes the lowercase form as the most common, another convention capitalizes Earth when appearing as a name but writes it in lowercase when preceded by the. It almost always appears in lowercase in colloquial expressions such as what on earth are you doing, the oldest material found in the Solar System is dated to 4. 5672±0.0006 billion years ago. By 4. 54±0.04 Gya the primordial Earth had formed, the formation and evolution of Solar System bodies occurred along with the Sun
20.
Rayleigh scattering
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Rayleigh scattering does not change the state of material and is, hence, a parametric process. The particles may be atoms or molecules. It can occur when light travels through transparent solids and liquids, Rayleigh scattering results from the electric polarizability of the particles. The oscillating electric field of a light wave acts on the charges within a particle, the particle therefore becomes a small radiating dipole whose radiation we see as scattered light. Rayleigh scattering of sunlight in the atmosphere causes diffuse sky radiation, which is the reason for the color of the sky. The amount of scattering is proportional to the fourth power of the wavelength. This can lead to changes in the state of the molecules. Furthermore, the contribution has the same wavelengths dependency as the elastic part. Scattering by particles similar to, or larger than, the wavelength of light is treated by the Mie theory. Rayleigh scattering applies to particles that are small with respect to wavelengths of light, on the other hand, anomalous diffraction theory applies to optically soft but larger particles. The size of a particle is often parameterized by the ratio where r is its characteristic length. The wavelength dependence is characteristic of dipole scattering and the volume dependence will apply to any scattering mechanism, objects with x ≫1 act as geometric shapes, scattering light according to their projected area. At the intermediate x ≃1 of Mie scattering, interference effects develop through phase variations over the objects surface, Rayleigh scattering applies to the case when the scattering particle is very small and the whole surface re-radiates with the same phase. For example, the constituent of the atmosphere, nitrogen, has a Rayleigh cross section of 5. 1×10−31 m2 at a wavelength of 532 nm. This means that at atmospheric pressure, where there are about 2×1025 molecules per cubic meter, the strong wavelength dependence of the scattering means that shorter wavelengths are scattered more strongly than longer wavelengths. This results in the blue light coming from all regions of the sky. Rayleigh scattering is an approximation of the manner in which light scattering occurs within various media for which scattering particles have a small size. A portion of the beam of light coming from the sun scatters off molecules of gas, here, Rayleigh scattering primarily occurs through sunlights interaction with randomly located air molecules
21.
Giant star
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A giant star is a star with substantially larger radius and luminosity than a main-sequence star of the same surface temperature. They lie above the main sequence on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III, the terms giant and dwarf were coined for stars of quite different luminosity despite similar temperature or spectral type by Ejnar Hertzsprung about 1905. Giant stars have radii up to a few hundred times the Sun, stars still more luminous than giants are referred to as supergiants and hypergiants. A hot, luminous main-sequence star may also be referred to as a giant, a star becomes a giant star after all the hydrogen available for fusion at its core has been depleted and, as a result, leaves the main sequence. The behaviour of a star depends largely on its mass. For a star with a mass above about 0.25 solar masses, the portion of the star outside the shell expands and cools, but with only a small increase in luminosity, and the star becomes a subgiant. The inert helium core continues to grow and increase temperature as it accretes helium from the shell, instead, after just a few million years the core reaches the Schönberg–Chandrasekhar limit, rapidly collapses, and may become degenerate. This causes the layers to expand even further and generates a strong convective zone that brings heavy elements to the surface in a process called the first dredge-up. The core continues to gain mass, contract, and increase in temperature, if the stars mass, when on the main sequence, was below approximately 0.4 M☉, it will never reach the central temperatures necessary to fuse helium. It will therefore remain a hydrogen-fusing red giant until it runs out of hydrogen and this is entirely theoretical because no star of such low mass has been in existence long enough to evolve to that stage. In stars above about 0.4 M☉ the core eventually reaches 108 K and helium will begin to fuse to carbon and oxygen in the core by the triple-alpha process. §5.9. When the core is degenerate helium fusion begins explosively, but most of the energy goes into lifting the degeneracy, the energy generated by helium fusion reduces the pressure in the surrounding hydrogen-burning shell, which reduces its energy-generation rate. The overall luminosity of the star decreases, its outer envelope contracts again, when the core helium is exhausted, a star with up to about 8 M☉ has a carbon–oxygen core that becomes degenerate and starts helium burning in a shell. As with the collapse of the helium core, this starts convection in the outer layers, triggers a second dredge-up. This is the giant branch analogous to the red-giant branch but more luminous. They start core-helium burning before the core becomes degenerate and develop smoothly into red supergiants without an increase in luminosity. Stars in the 8-12 M☉ range have somewhat intermediate properties and have been called super-AGB stars and they largely follow the tracks of lighter stars through RGB, HB, and AGB phases, but are massive enough to initiate core carbon burning and even some neon burning. They form oxygen–magnesium–neon cores, which may collapse in an electron-capture supernova, O class main sequence stars are already highly luminous
22.
Milky Way
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The Milky Way is the galaxy that contains our Solar System. The descriptive milky is derived from the appearance from Earth of the galaxy – a band of light seen in the night sky formed from stars that cannot be distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610, until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies, the Milky Way is a barred spiral galaxy with a diameter between 100,000 light-years and 180,000 light-years. The Milky Way is estimated to contain 100–400 billion stars, there are probably at least 100 billion planets in the Milky Way. The Solar System is located within the disk, about 26,000 light-years from the Galactic Center, on the edge of one of the spiral-shaped concentrations of gas. The stars in the inner ≈10,000 light-years form a bulge, the very center is marked by an intense radio source, named Sagittarius A*, which is likely to be a supermassive black hole. Stars and gases at a range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been termed dark matter, the rotational period is about 240 million years at the position of the Sun. The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself, the Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which is a component of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster. The Milky Way can be seen as a band of white light some 30 degrees wide arcing across the sky. Dark regions within the band, such as the Great Rift, the area of the sky obscured by the Milky Way is called the Zone of Avoidance. The Milky Way has a low surface brightness. Its visibility can be reduced by background light such as light pollution or stray light from the Moon. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be seen and it should be visible when the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1
23.
Orange dwarf
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A K-type main-sequence star, also referred to as an orange dwarf or K dwarf, is a main-sequence star of spectral type K and luminosity class V. These stars are intermediate in size between red M-type main-sequence stars and yellow G-type main-sequence stars and they have masses between 0.45 and 0.8 times the mass of the Sun and surface temperatures between 3,900 and 5,200 K. Tables VII, VIII. Better-known examples include Alpha Centauri B and Epsilon Indi and these stars are of particular interest in the search for extraterrestrial life because they are stable on the main sequence for a very long time. This may create an opportunity for life to evolve on terrestrial planets orbiting such stars, K-type stars also emit less ultraviolet radiation than G-type stars like the Sun. K-type main-sequence stars are three to four times as abundant as G-type main-sequence stars, making planet searches easier. The revised Yerkes Atlas system listed 12 K-type dwarf spectral standard stars, the anchor points of the MK classification system among the K-type main-sequence dwarf stars, i. e. those standard stars that have remain unchanged over the years, are Epsilon Eridani, and 61 Cygni A. Other primary MK standard stars include 70 Ophiuchi A,107 Piscium, HD219134, TW Piscis Austrini, HD120467,61 Cygni B. Based on the set in some references, many authors consider the step between K7 V and M0 V to be a single subdivision, and one rarely encounters K8 or K9 classifications in the literature. Recent, however, HIP111288 and HIP3261 have been proposed as spectral standard stars, some of the nearest K-type stars known to have planets include Alpha Centauri B, Epsilon Eridani, HD192310, Gliese 86, and 54 Piscium. Solar analog Red dwarf Stellar classification, Class K Star count, survey of stars Habitability of K-type main-sequence star systems
24.
White dwarf
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A white dwarf, also called a degenerate dwarf, is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense, its mass is comparable to that of the Sun, a white dwarfs faint luminosity comes from the emission of stored thermal energy, no fusion takes place in a white dwarf wherein mass is converted to energy. The nearest known white dwarf is Sirius B, at 8.6 light years, there are currently thought to be eight white dwarfs among the hundred star systems nearest the Sun. The unusual faintness of white dwarfs was first recognized in 1910, the name white dwarf was coined by Willem Luyten in 1922. The universe has not existed long enough to experience a white dwarf releasing all of its energy as it will take many billions of years. If a red giant has insufficient mass to generate the temperatures, around 1 billion K, required to fuse carbon. After such a star sheds its outer layers and forms a planetary nebula, it will leave behind a core, usually, white dwarfs are composed of carbon and oxygen. If the mass of the progenitor is between 8 and 10.5 solar masses, the temperature will be sufficient to fuse carbon but not neon. Stars of very low mass will not be able to fuse helium, hence, the material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy. As a result, it support itself by the heat generated by fusion against gravitational collapse. A carbon-oxygen white dwarf that approaches this limit, typically by mass transfer from a companion star. A white dwarf is very hot when it forms, but because it has no source of energy, it will gradually radiate its energy and this means that its radiation, which initially has a high color temperature, will lessen and redden with time. Over a very time, a white dwarf will cool. The stars low temperature means it will no longer emit significant heat or light, and it will become a cold black dwarf. Because the length of time it takes for a dwarf to reach this state is calculated to be longer than the current age of the universe. The oldest white dwarfs still radiate at temperatures of a few thousand kelvins, the pair 40 Eridani B/C was discovered by William Herschel on 31 January 1783, p.73 it was again observed by Friedrich Georg Wilhelm Struve in 1825 and by Otto Wilhelm von Struve in 1851. In 1910, Henry Norris Russell, Edward Charles Pickering and Williamina Fleming discovered that, despite being a dim star,40 Eridani B was of spectral type A, or white. In 1939, Russell looked back on the discovery, p.1 I was visiting my friend and generous benefactor and this piece of apparently routine work proved very fruitful—it led to the discovery that all the stars of very faint absolute magnitude were of spectral class M
25.
Compact star
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In astronomy, the term compact star is used to refer collectively to white dwarfs, neutron stars, and black holes. It would grow to include exotic stars if such hypothetical dense bodies are confirmed, most compact stars are the endpoints of stellar evolution and are thus often referred to as stellar remnants, the form of the remnant depending primarily on the mass of the star when it formed. These objects are all small in volume for their mass, giving them a high density. A compact star that is not a hole may be called a degenerate star. The usual endpoint of stellar evolution is the formation of a compact star, most stars will eventually come to a point in their evolution, when the outward radiation pressure from the nuclear fusions in its interior can no longer resist the ever-present gravitational forces. When this happens, the star collapses under its own weight, for most stars, this will result in the formation of a very dense and compact stellar remnant, also known as a compact star. Compact stars have no internal energy production, but will—with the exception of black holes—usually radiate for millions of years with excess heat left from the collapse itself. According to the most recent understanding, compact stars could form during the phase separations of the early Universe following the Big Bang. Primordial origins of known objects have not been determined with certainty. Although compact stars may radiate, and thus cool off and lose energy, they do not depend on high temperatures to maintain their structure, barring external disturbances and proton decay, they can persist virtually forever. Black holes are generally believed to finally evaporate from Hawking radiation after trillions of years. The somewhat wider definition of compact objects often includes smaller solid objects such as planets, asteroids, the stars called white or degenerate dwarfs are made up mainly of degenerate matter, typically carbon and oxygen nuclei in a sea of degenerate electrons. White dwarfs arise from the cores of stars and are therefore very hot when they are formed. As they cool they will redden and dim until they become dark black dwarfs. White dwarfs were observed in the 19th century, but the high densities and pressures they contain were not explained until the 1920s. The equation of state for degenerate matter is soft, meaning that adding more mass will result in a smaller object, continuing to add mass to what is now a white dwarf, the object shrinks and the central density becomes even larger, with higher degenerate-electron energies. The equilibrium would shift towards heavier, neutron-richer nuclei that are not stable at everyday densities, as the density increases, these nuclei become still larger and less well-bound. At a critical density of about 4·1014 kg/m³, called the drip line
26.
Red giant
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A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large, the appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars. The most common red giants are stars on the red-giant branch that are still fusing hydrogen into helium in a shell surrounding an inert helium core. Red giants are stars that have exhausted the supply of hydrogen in their cores and have begun thermonuclear fusion of hydrogen in a shell surrounding the core and they have radii tens to hundreds of times larger than that of the Sun. However, their outer envelope is lower in temperature, giving them a reddish-orange hue, despite the lower energy density of their envelope, red giants are many times more luminous than the Sun because of their great size. Among the asymptotic-giant-branch stars belong the carbon stars of type C-N and late C-R, produced when carbon, the first dredge-up occurs during hydrogen shell burning on the red-giant branch, but does not produce a large carbon abundance at the surface. The second, and sometimes third, dredge up occurs during helium shell burning on the asymptotic-giant branch, the stellar limb of a red giant is not sharply-defined, contrary to their depiction in many illustrations. Rather, due to the low mass density of the envelope, such stars lack a well-defined photosphere. The coolest red giants have complex spectra, with lines, emission features. Red giants are evolved from main-sequence stars with masses in the range from about 0.3 M☉ to around 8 M☉, when a star initially forms from a collapsing molecular cloud in the interstellar medium, it contains primarily hydrogen and helium, with trace amounts of metals. These elements are all uniformly mixed throughout the star, the star reaches the main sequence when the core reaches a temperature high enough to begin fusing hydrogen and establishes hydrostatic equilibrium. Over its main sequence life, the star slowly converts the hydrogen in the core into helium, for the Sun, the main-sequence lifetime is approximately 10 billion years. More-massive stars burn disproportionately faster and so have a lifetime than less massive stars. When the star exhausts the hydrogen fuel in its core, nuclear reactions can no longer continue and this brings additional hydrogen into a zone where the temperature and pressure are adequate to cause fusion to resume in a shell around the core. The higher temperatures lead to increasing rates, enough to increase the stars luminosity by a factor of 1. The outer layers of the star then expand greatly, thus beginning the red-giant phase of the stars life, in actuality, though, the color usually is orange. At this time, the star is said to be ascending the red-giant branch of the Hertzsprung–Russell diagram, the outer layers carry the energy evolved from fusion to the surface by way of convection. This causes material exposed to nuclear burning in the interior to be brought to the stars surface for the first time in its history
27.
Aldebaran
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Aldebaran, designated Alpha Tauri, is an orange giant star located about 65 light years from the Sun in the zodiac constellation of Taurus. It is the brightest star in its constellation and usually the fourteenth-brightest star in the nighttime sky and it is likely that Aldebaran hosts a planet several times the size of Jupiter. The planetary exploration probe Pioneer 10 is currently heading in the direction of the star. Alpha Tauri is the stars Bayer designation, the name Aldebaran is Arabic and means the Follower, presumably because it rises near and soon after the Pleiades. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalog, the WGSNs first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN, which included Aldebaran for this star. It is now so entered in the IAU Catalog of Star Names, in Persia it was known as Tascheter. In the Middle Ages it was sometimes called Cor Tauri, john Gower refers to it as Aldeboran. In Chinese it is known as 畢宿五, in Hindu astronomy it is identified as the lunar mansion Rohini and as one of the twenty-seven daughters of Daksha and the wife of the god Chandra. This easily seen and striking star in its suggestive asterism is a subject for ancient. Mexican culture, For the Seris of northwestern Mexico, this star provides light for the seven women giving birth and it has three names, Hant Caalajc Ipápjö, Queeto, and Azoj Yeen oo Caap. The lunar month corresponding to October is called Queeto yaao Aldebarans path, aboriginal culture, in the Clarence River of northeastern New South Wales, this star is the Ancestor Karambal, who stole another mans wife. The womans husband tracked him down and burned the tree in which he was hiding and it is believed that he rose to the sky as smoke and became the star Aldebaran. On March 11, of 509 AD, an occultation of Aldebaran was observed in Athens. English astronomer Edmund Halley studied the timing of this event, and in 1718 concluded that Aldebaran must have changed position since that time and this, as well as observations of the changing positions of stars Sirius and Arcturus, led to the discovery of proper motion. Based on present day observations, the position of Aldebaran has shifted 7′ in the last 2000 years, note that 5,000 years ago the vernal equinox was close to Aldebaran. English astronomer William Herschel discovered a faint companion to Aldebaran in 1782 and this star was shown to be itself a close double star by S. W. Burnham in 1888, and he discovered an additional 14th magnitude companion at an angular separation of 31″. Follow on measurements of proper motion showed that Herschels companion was diverging from Aldebaran, however, the companion discovered by Burnham had almost exactly the same proper motion as Aldebaran, suggesting that the two formed a wide binary star system. Pickering at the Harvard College Observatory used a plate to capture fifty absorption lines in the spectrum of Aldebaran
28.
Planetary nebula
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A planetary nebula, often abbreviated as PN or plural PNe, is a kind of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from old red giant stars late in their lives. Herschels name for these objects was popularly adopted and has not been changed and they are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years. After most of the red giants atmosphere is dissipated, the radiation of the hot luminous core, called a planetary nebula nucleus. Absorbed ultraviolet light energises the shell of gas around the central star. Planetary nebulae likely play a role in the chemical evolution of the Milky Way by expelling elements to the interstellar medium from stars where those elements were created. Planetary nebulae are also observed in distant galaxies, yielding useful information about their chemical abundances. Starting from the 1990s, Hubble Space Telescope images have revealed many planetary nebulae to have extremely complex, about one-fifth are roughly spherical, but the majority are not spherically symmetric. The mechanisms that produce such a variety of shapes and features are not yet well understood. The first planetary nebula discovered was the Dumbbell Nebula in the constellation of Vulpecula and it was observed by Charles Messier in 1764 and listed as M27 in his catalogue of nebulous objects. To early observers with telescopes, M27 and subsequently discovered planetary nebulae resembled the giant planets like Uranus. William Herschel, discoverer of Uranus, eventually coined the term planetary nebula, the nature of planetary nebulae was unknown until the first spectroscopic observations were made in the mid-19th century. Using a prism to disperse their light, William Huggins was one of the earliest astronomers to study the spectra of astronomical objects. On August 29,1864, Huggins was the first to analyze the spectrum of a planetary nebula when he observed Cats Eye Nebula and his observations of stars showed that their spectra consisted of a continuum of radiation with many dark lines superimposed. He later found that many objects such as the Andromeda Nebula had spectra that were quite similar. Those nebulae were later shown to be collections of stars now called galaxies, however, when Huggins looked at the Cats Eye Nebula, he found a very different spectrum. Rather than a strong continuum with absorption lines superimposed, the Cats Eye Nebula, the brightest of these was at a wavelength of 500.7 nanometres, which did not correspond with a line of any known element. At first, it was hypothesized that the line might be due to an unknown element, a similar idea had led to the discovery of helium through analysis of the Suns spectrum in 1868. While helium was isolated on Earth soon after its discovery in the spectrum of the Sun, nebulium was not
29.
Beta CVn
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Beta Canum Venaticorum, also named Chara, is a G-type main-sequence star in the northern constellation of Canes Venatici. At an apparent visual magnitude of 4.26, it is the second-brightest star in this relatively faint constellation, based upon an annual parallax shift of 118.49 mas, this star is 27.53 light-years distant from the Sun. Along with the brighter star Cor Caroli, the form the southern dog in this constellation that represents hunting dogs. β Canum Venaticorum is the stars Bayer designation, the traditional name Chara was originally applied to the southern dog, but it later became used specifically to refer to Beta Canum Venaticorum. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalog, the WGSNs first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN, which included Chara for this star. Consequently, Beta Canum Venaticorum itself is known as 常陳四 Beta CVn has a classification of G0 V. Since 1943, the spectrum of this star has served as one of the anchor points by which other stars are classified. Beta CVn is considered to be slightly metal-poor, which means it has a lower portion of elements heavier than helium when compared to the Sun. In terms of mass, age and evolutionary status, however, as a result, it has been called a solar analog. It is about 3% more massive than the Sun, with a radius 12% larger than the Suns, the components of this stars space velocity are = km/s. In the past it was suggested that it may be a spectroscopic binary, however, further analysis of the data does not seem to bear that out. In addition, a 2005 search for a dwarf in orbit around this star failed to discover any such companion. In 2006, astronomer Margaret Turnbull labeled Beta CVn as the top stellar system candidate to search for life forms. Because of its properties, astrobiologists have listed it among the most astrobiologically interesting stars within 10 parsecs of the Sun. However, as of 2009, this star is not known to host planets and this chart compares the sun to Beta Canum Venaticorum. An exact solar twin would be a G2V star with a 5, 778K temperature, be 4.6 billion years old, with the correct metallicity, stars with an age of 4.6 billion years are at the most stable state. Proper metallicity and size are very important to low luminosity variation. List of star systems within 25–30 light-years Chara, astronomer announces shortlist of stellar candidates for habitable worlds
30.
Kappa1 Ceti
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Kappa1 Ceti is a yellow dwarf star approximately 30 light-years away in the equatorial constellation of Cetus. The star was discovered to have a rotation, roughly once every nine days. Though there are no extrasolar planets confirmed to be orbiting the star, the system is a candidate binary star, but has not been confirmed. The star should not be confused with the star Kappa2 Ceti, Kappa1 Ceti is a yellow dwarf star of the spectral type G5Ve. Since 1943, the spectrum of this star has served as one of the anchor points by which other stars are classified. The star has roughly the mass as the Sun, with 95% of the Suns radius. It is unclear whether the star is equal or is more enriched in heavier than hydrogen. Kappa1 Ceti is much younger than the Sun, and may only be around 800 million years old, the rapid rotation rate of this star, approximately once every nine days, is indicative of a relatively youthful body several hundred million years in age. Due to starspots, the star varies slightly over the approximately the same period, the variations in period are thought to be caused by differential rotation at various latitudes, similar to what happens on the surface of the Sun. The star spots on Kappa1 Ceti range in latitude from 10° to 75° The magnetic properties of this make it an excellent match for the Sun at a key point in the Earths past. In 1998, nine Solar twin stars were observed to have produced superflares, on average, none of these stars rotate particularly fast, have close binary companions, or are very young. Previously, such large flares had not been observed in solar-type main sequence stars, although they are common in a group of dim main-sequence, the space velocity components of this star are = km/s. It is not known to be a member of a group of stars. Using the radial velocity technique, the search for substellar companions has thus far failed to find a dwarf or extrasolar planet in the hot zone orbit around Kappa1 Ceti. Given the regular eruption of superflares, however, it is unlikely that Earth-type life could survive for long on any terrestrial planet. The distance from the star where an Earth-type planet would be stable is centered on 0.92 astronomical units from the star, at this distance, such a planet would have an orbital period of almost 324 days. List of star systems within 25–30 light-years Kaler, Jim, young sun-like star shows a magnetic field was critical for life on the early Earth
31.
70 Virginis
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70 Virginis is the Flamsteed designation of a yellow dwarf star approximately 59 light-years away in the constellation Virgo. It is rather unusually bright for its type and may be just starting to evolve into the subgiant phase. In 1996,70 Virginis was discovered to have a planet in orbit around it. There is also a dust disc with a temperature of 153 K located at a minimum distance of 3.4 AU from the star
32.
82 G. Eridani
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82 G. Eridani is a star about 20 light years away from Earth in the constellation Eridanus. It is a star with a stellar classification of G8. In the southern-sky catalog Uranometria Argentina,82 G. Eridani is the 82nd star listed in the constellation Eridanus. The Argentina catalog, compiled by the 19th-century astronomer Benjamin Gould, is a celestial hemisphere analog of the more famous Flamsteed catalog. 82 G. Eridani, like other stars near the Sun, has held on to its Gould designation, even while other more distant stars have not. This star is smaller and less massive than the Sun, making it marginally dimmer than the Sun in terms of luminosity. The projected equatorial rotation rate is 4.0 km/s, compared to 2 km/s for the Sun, like many other Population II stars,82 G. Eridani is somewhat metal-deficient, and is older than the Sun. It has a high orbital eccentricity of 0.40 about the galaxy. Estimates of the age of this star ranged from 6 to 12 billion years and this star is located in a region of low-density interstellar matter, so it is believed to have a large astropause that subtends an angle of 6″ across the sky. Relative to the Sun, this star is moving at a velocity of 101 km/s. On August 17,2011, European astronomers announced the discovery of three planets orbiting 82 G. Eridani, the mass range of these planets classifies them as super-Earths, objects with only a few times the Earths mass. These planets were discovered by precise measurements of the velocity of the star. None of the display a significant orbital eccentricity. However, their orbital periods are all 90 days or less, the equilibrium temperature for the most distant planet, based on an assumed Bond albedo of 0.3, would be about 388 K, significantly above the boiling point of water. The number of planets in the system is slightly uncertain, at the time of planet cs detection, it exerted the lowest gravitational perturbation. There was also a similarity noted between its orbital period and the period of the star. For these reasons the team were somewhat more cautious regarding the verity of its candidate planet status than for the other two. Continued observation of the star will be required to determine the nature of the planetary system
33.
61 Virginis
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61 Virginis is the Flamsteed designation of a G-type main-sequence star slightly less massive than the Sun, located about 27.9 light-years away in the constellation of Virgo. The composition of this star is identical to the Sun. 61 Virginis is the first well-established main-sequence star very similar to the Sun with a potential Super-Earth,61 Virginis is a fifth-magnitude G-type main-sequence star with a stellar classification of G7 V. It is faint but visible to the naked eye south and east of the bright star Spica in the constellation of Virgo. The designation 61 Virginis originated in the catalogue of English astronomer John Flamsteed. An 1835 account of Flamsteeds work by English astronomer Francis Baily noted that the star showed a proper motion and this made the star of interest for parallax studies, and by 1950 a mean annual value of 0. 006″ was obtained. The present day result, obtained data from the Hipparcos satellite, gives a parallax of 116.89 mas. This star is similar in properties to the Sun, with around 95% of the Suns mass, 98% of the radius. The abundance of elements is similar to the Sun, with the star having an estimated 95% of the Suns proportion of elements other than hydrogen. It is older than the Sun at around 6. 1–6.6 billion years of age, on average, there is only a low level of activity in the stellar chromosphere and it is a candidate for being in a Maunder minimum state. But the star was suspected as variable in 1988, and a burst of activity was observed between Julian days 54800 and 55220, the space velocity components of this star are U = –37.9, V = –35.3 and W = –24.7 km/s. 61 Vir is orbiting through the Milky Way galaxy at a distance of 6.9 kpc from the core and it is believed to be a member of the disk population. The ecliptic of the 61 Virginis system, as inferred from its dust disc, is inclined to the Solar system at 77°, the star itself is probably inclined at 72°. In 1988, a study surmised that 61 Virginis was a possible variable, a subsequent study, over eleven years, also failed to find any companion up to the mass of Jupiter and out to 3 AU. On 14 December 2009, scientists announced the discovery of three planets with masses between 5 and 25 times that of Earth orbiting 61 Virginis. The three planets all orbit very near the star, when compared to the orbits of the planets in the Solar System, the outermost of these three, d, has not yet been confirmed in the HARPS data. A survey with the Spitzer Space Telescope revealed an excess of infrared radiation at a wavelength of 160 μm and this indicated the presence of a debris disk in orbit around the star. This disk was resolved at 70 μm and it was then thought to correspond to an inner radius of 96 AU from the star and outer radius at 195 AU, it is now constrained 30 to over 100 AU
34.
47 Ursae Majoris
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47 Ursae Majoris, also named Chalawan, is a yellow dwarf star approximately 46 light-years from Earth in the constellation of Ursa Major. As of 2011, three planets are believed to orbit the star. With an apparent magnitude of +5.03, it is visible to the naked eye, like the Sun,47 Ursae Majoris is on the main sequence, converting hydrogen to helium in its core by nuclear fusion. Based on its activity, the star may be around 6,000 million years old. Other studies have yielded estimates of 4,400 and 7,000 million years for the star,47 Ursae Majoris is the Flamsteed designation. On their discoveries the planets were successively designated 47 Ursae Majoris b, c and d, in July 2014 the International Astronomical Union launched a process for giving proper names to certain exoplanets and their host stars. The process involved public nomination and voting for the new names, in December 2015, the IAU announced the winning names were Chalawan for this star and Taphao Thong and Taphao Kaew for two of the planets. The winning names were submitted by the Thai Astronomical Society, Thailand, Chalawan is a mythological crocodile king from a Thai folktale and Taphaothong and Taphaokaeo are two sisters associated with the tale. In 2016, the IAU organized a Working Group on Star Names to catalog and this star is now so entered in the IAU Catalog of Star Names. In 1996 an exoplanet was announced in orbit around 47 Ursae Majoris by Geoffrey Marcy, the discovery was made by observing the Doppler shift of the stars spectrum corresponding to changes in the stars radial velocity as the planets gravity pulled it around. The planet was the first long-period extrasolar planet discovered, unlike the majority of known such planets, it has a low-eccentricity orbit. The planet is at least 2.53 times the mass of Jupiter, if it were to be located in the Solar System, it would lie between the orbits of Mars and Jupiter. A second planet was announced in 2002 by Debra Fischer, Geoffrey Marcy, the discovery was made using the same radial velocity method. According to Fischer et al. the planet takes around 2,391 days or 6.55 years to complete an orbit and this configuration is similar to the configuration of Jupiter and Saturn in the Solar System, with the orbital ratio, and mass ratio roughly similar. Nevertheless, the parameters of the planet are still highly uncertain. Paper is cited as a reference in spite of the fact that it gives different parameters, in 2010, the discovery of a third planet was made by using the Bayesian Kepler Periodogram. Using this model of planetary system it was determined that it is 100,000 times more likely to have three planets than two planets. This discovery was announced by Debra Fischer and P. C and this 1.64 MJ planet has an orbital period of 14,002 days or 38.33 years and a semi-major axis of 11.6 AU with a moderate eccentricity of 0.16
35.
Mu Arae
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Mu Arae, often designated HD160691, also named Cervantes, is a main sequence G-type star approximately 50 light-years away from the Sun in the constellation of Ara. The star has a system with four known extrasolar planets. The systems innermost planet was the first hot Neptune or super-Earth to be discovered, μ Arae is the stars Bayer designation. HD160691 is the entry in the Henry Draper Catalogue, the established convention for extrasolar planets is that the planets receive designations consisting of the stars name followed by lower-case Roman letters starting from b, in order of discovery. This system was used by a led by Krzysztof Goździewski. On the other hand, a team led by Francesco Pepe proposed a modification of the designation system, both systems agree on the designation of the 640-day planet as b. The old system designates the 9-day planet as d, the 310-day planet as e, in July 2014 the International Astronomical Union launched a process for giving proper names to certain exoplanets and their host stars. The process involved public nomination and voting for the new names, in December 2015, the IAU announced the winning names were Cervantes for this star and Quijote, Dulcinea, Rocinante and Sancho, for its planets. The winning names were submitted by the Planetario de Pamplona. Miguel de Cervantes Saavedra was a famous Spanish writer and author of El Ingenioso Hidalgo Don Quixote de la Mancha, the planets are named after characters of that novel, Quijote was the lead character, Dulcinea his love interest, Rocinante his horse, and Sancho his squire. In 2016, the IAU organized a Working Group on Star Names to catalog and this star is now so entered in the IAU Catalog of Star Names. According to measurements made by the Hipparcos astrometric satellite, Mu Arae exhibits a parallax of 64.47 milliarcseconds as the Earth moves around the Sun. When combined with the distance from the Earth to the Sun. Seen from Earth it has an apparent magnitude of +5.12 and is visible to the naked eye, asteroseismic analysis of the star reveals it is approximately 10% more massive than the Sun and significantly older, at around 6.34 billion years. The radius of the star is 36% greater than that of the Sun, the star contains twice the abundance of iron relative to hydrogen of the Sun and is therefore described as metal-rich. Mu Arae is also more enriched than the Sun in the element helium, Mu Arae has a listed spectral type of G3IV–V. The G3 part means the star is similar to the Sun, the star may be entering the subgiant stage of its evolution as it starts to run out of hydrogen in its core. This is reflected in its luminosity class, between IV and V
36.
Brown dwarf
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Below this range are the sub-brown dwarfs, and above it are the lightest red dwarfs. Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth, unlike the stars in the main-sequence, brown dwarfs are not massive enough to sustain nuclear fusion of ordinary hydrogen to helium in their cores. They are, however, thought to fuse deuterium and to fuse lithium if their mass is above a threshold of 13 MJ and 65 MJ. It is also debated whether brown dwarfs would be defined by their formation processes rather than by their supposed nuclear fusion reactions. Stars are categorized by spectral class, with brown dwarfs designated as types M, L, T, despite their name, brown dwarfs are of different colors. Many brown dwarfs would likely appear magenta to the human eye, Brown dwarfs are not very luminous at visible wavelengths. Planets are known to orbit some brown dwarfs, 2M1207b, MOA-2007-BLG-192Lb, at a distance of about 6.5 light years, the nearest known brown dwarf is Luhman 16, a binary system of brown dwarfs discovered in 2013. DENIS-P J082303. 1-491201 b is listed as the known exoplanet in NASAs exoplanet archive. However, a) the term black dwarf was already in use to refer to a white dwarf, b) red dwarfs fuse hydrogen. Because of this, alternate names for these objects were proposed, in 1975, Jill Tarter suggested the term brown dwarf, using brown as an approximate color. The term black dwarf still refers to a dwarf that has cooled to the point that it no longer emits significant light. The first self-consistent calculation of the minimum mass confirmed a value between 0.08 and 0.07 solar masses for population I objects. The discovery of deuterium burning down to 0.012 solar masses, however, such objects were hard to find as they emit almost no visible light. Their strongest emissions are in the spectrum, and ground-based IR detectors were too imprecise at that time to readily identify any brown dwarfs. Since then, numerous searches by various methods have sought these objects, for many years, efforts to discover brown dwarfs were fruitless. In 1988, however, a faint companion to a known as GD165 was found in an infrared search of white dwarfs. The spectrum of the companion GD 165B was very red and enigmatic and it became clear that GD 165B would need to be classified as a much cooler object than the latest M dwarfs then known. GD 165B remained unique for almost a decade until the advent of the Two Micron All Sky Survey which discovered many objects with similar colors, today, GD 165B is recognized as the prototype of a class of objects now called L dwarfs
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K-type main-sequence star
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A K-type main-sequence star, also referred to as an orange dwarf or K dwarf, is a main-sequence star of spectral type K and luminosity class V. These stars are intermediate in size between red M-type main-sequence stars and yellow G-type main-sequence stars and they have masses between 0.45 and 0.8 times the mass of the Sun and surface temperatures between 3,900 and 5,200 K. Tables VII, VIII. Better-known examples include Alpha Centauri B and Epsilon Indi and these stars are of particular interest in the search for extraterrestrial life because they are stable on the main sequence for a very long time. This may create an opportunity for life to evolve on terrestrial planets orbiting such stars, K-type stars also emit less ultraviolet radiation than G-type stars like the Sun. K-type main-sequence stars are three to four times as abundant as G-type main-sequence stars, making planet searches easier. The revised Yerkes Atlas system listed 12 K-type dwarf spectral standard stars, the anchor points of the MK classification system among the K-type main-sequence dwarf stars, i. e. those standard stars that have remain unchanged over the years, are Epsilon Eridani, and 61 Cygni A. Other primary MK standard stars include 70 Ophiuchi A,107 Piscium, HD219134, TW Piscis Austrini, HD120467,61 Cygni B. Based on the set in some references, many authors consider the step between K7 V and M0 V to be a single subdivision, and one rarely encounters K8 or K9 classifications in the literature. Recent, however, HIP111288 and HIP3261 have been proposed as spectral standard stars, some of the nearest K-type stars known to have planets include Alpha Centauri B, Epsilon Eridani, HD192310, Gliese 86, and 54 Piscium. Solar analog Red dwarf Stellar classification, Class K Star count, survey of stars Habitability of K-type main-sequence star systems
Media related to Yellow dwarfs at Wikimedia Commons
Also known as G2V
