1.
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
2.
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
3.
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
4.
Subdwarf
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A subdwarf, sometimes denoted by sd, is a star with luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of stars of the same spectral type. On an Hertzsprung–Russell diagram subdwarfs appear to lie below the main sequence, the term subdwarf was coined by Gerard Kuiper in 1939, to refer to a series of stars with anomalous spectra that were previously labeled as intermediate white dwarfs. Like ordinary main-sequence stars, cool subdwarfs produce their energy from hydrogen fusion, the explanation of their underluminosity lies in their low metallicity, these stars are unenriched in elements heavier than helium. The lower metallicity decreases the opacity of their layers and decreases the radiation pressure. This lower opacity also allows them to emit a higher percentage of light for the same spectral type relative to a Population I star. Usually members of the Milky Ways halo, they frequently have high space velocities relative to the Sun and these stars represent a late stage in the evolution of some stars, caused when a red giant star loses its outer hydrogen layers before the core begins to fuse helium. The reasons why this premature mass loss occurs are unclear, single subdwarfs may be the result of a merger of two white dwarfs or gravitational influence from substellar companions. B-type subdwarfs, being more luminous white dwarfs, are a significant component in the hot star population of old stellar systems, such as globular clusters. Kapteyns Star Groombridge 1830 Mu Cassiopeiae 2MASS J05325346+8246465, a possible brown dwarf
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.
Subgiant
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A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as true giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution of a star, the term subgiant was first used in 1930 for class G and early K stars with absolute magnitudes between +2.5 and +4. Luminosity-class-IV stars are the subgiants, located between main-sequence stars and red giants, O class stars and stars cooler than K1 are rarely given subgiant luminosity classes. The subgiant branch is a stage in the evolution of low to intermediate mass stars, stars with a subgiant spectral type are not always on the evolutionary subgiant branch, and vice versa. For example, the stars FK Com and 31 Com both lie in the Hertzsprung Gap and are likely evolutionary subgiants, but both are often assigned giant luminosity classes, the spectral classification can be influenced by metallicity, rotation, unusual chemical peculiarities, etc. The initial stages of the subgiant branch in a star like the sun are prolonged with little indication of the internal changes. One approach to identifying evolutionary subgiants include chemical abundances such as Lithium which is diluted in subgiants, as the fraction of hydrogen remaining in the core of a main sequence star decreases, the core temperature increases and so the rate of fusion increases. This causes stars to evolve slowly to high luminosities as they age, once a main sequence star ceases to fuse hydrogen in its core, the core begins to collapse under its own weight. This causes it to increase in temperature and hydrogen fuses in a shell outside the core, low- and intermediate-mass stars expand and cool until at about 5,000 K they begin to increase in luminosity in a stage known as the red-giant branch. The transition from the sequence to the red giant branch is known as the subgiant branch. The shape and duration of the subgiant branch varies for stars of different masses, stars less massive than about 0.4 M☉ are convective throughout most of the star. These stars continue to fuse hydrogen in their cores until essentially the entire star has converted to helium. Stars of this mass have main-sequence lifetimes many times longer than the current age of the Universe, stars less massive than the Sun have non-convective cores with a strong temperature gradient from the centre outwards. When they exhaust hydrogen at the centre of the star, a shell of hydrogen outside the central core continues to fuse without interruption. The star is considered to be a subgiant at this point although there is little change visible from the exterior, the helium core mass is below the Schönberg–Chandrasekhar limit and it remains in thermal equilibrium with the fusing hydrogen shell. Its mass continues to increase and the star very slowly expands as the hydrogen shell migrates outwards, any increase in energy output from the shell goes into expanding the envelop of the star and the luminosity stays approximately constant. The subgiant branch for stars is short, horizontal, and heavily populated. After several billion years, the core becomes too massive to support its own weight
7.
Supergiant star
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Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute magnitudes between about −3 and −8 with temperatures spanning from about 3,500 K to over 20,000 K. The term supergiant, as applied to a star, does not have a concrete definition. The term giant star was first coined by Hertzsprung when it became apparent that the majority of stars fell into two regions of the Hertzsprung–Russell diagram. One region contained larger and more stars of spectral types A to M. Supergiant stars can be identified on the basis of their spectra, with distinctive lines sensitive to high luminosity, maury, had divided stars based on the widths of their spectral lines, with her class c identifying stars with the narrowest lines. Although it was not known at the time, these were the most luminous stars, in 1943 Morgan and Keenan formalised the definition of spectral luminosity classes, with class I referring to supergiant stars. The same system of MK luminosity classes is used today. Supergiants occur in every class from young blue class O supergiants to highly evolved red class M supergiants. Because they are enlarged compared to main-sequence and giant stars of the spectral type. Supergiants are also evolved stars with higher levels of heavy elements than main-sequence stars and this is the basis of the MK luminosity system which assigns stars to luminosity classes purely from observing their spectra. The MK system assigns stars to luminosity classes, Ib for supergiants, Ia for luminous supergiants, in reality there is very much of a continuum rather than well defined bands for these classifications, and classifications such as Iab are used for intermediate luminosity supergiants. Supergiant spectra are frequently annotated to indicate spectral peculiarities, for example B2 Iae or F5 Ipec, supergiants can also be defined as a specific phase in the evolutionary history of certain stars. Once these massive stars leave the main sequence their atmospheres inflate, stars initially under 10 M☉ will never form an iron core and in evolutionary terms do not become supergiants, although they can reach luminosities thousands of times the suns. They cannot fuse carbon and heavier elements after their core helium is exhausted, so they eventually just lose their outer layers and become a white dwarf. The phase where these stars have both hydrogen and helium burning shells is referred to as the giant branch, as stars gradually become more and more luminous class M stars. There are several categories of evolved star which are not supergiants in evolutionary terms, the dividing line becomes blurred at around 7–10 M☉ where stars start to undergo limited fusion of elements heavier than helium. Specialists studying these stars often refer to them as super AGB stars, others describe them as low-mass supergiants since they start to burn elements heavier than helium and can explode as supernovae
8.
Hypergiant
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A hypergiant is among the very rare kinds of stars that typically show tremendous luminosities and very high rates of mass loss by stellar winds. The term hypergiant is defined as luminosity class 0 in the MKK system, more commonly, hypergiants maybe classed as Ia-0 or Ia+, but red supergiants rarely receive these extra spectral classifications. Astronomers are mostly interested in these stars because they relate to understanding stellar evolution, especially with star formation, stability, in 1956, the astronomers Feast and Thackeray used the term super-supergiant for stars with an absolute magnitude brighter than MV = −7. The Keenan criterion is the one most commonly used by scientists today, observation of a highly luminous star is insufficient for it to be defined as a hypergiant. That requires the detection of the signatures of atmospheric instability. So it is possible for non-hypergiant supergiant stars to have the same or higher luminosity as a hypergiant of the same spectral class. Additionally, hypergiants are expected to have characteristic broadening and red-shifting of their spectral lines producing a shape known as a P Cygni profile. The use of emission is not helpful for defining the coolest hypergiants. Stars with a mass above about 25 M☉ quickly move away from the main sequence. They cool and enlarge at approximately constant luminosity to become a red supergiant, then contract, stars with an initial mass above about 40 M☉ are simply too luminous to develop a stable extended atmosphere and so they never cool sufficiently to become red supergiants. The most massive stars, especially rapidly rotating stars with enhanced convection and mixing, may skip these steps and these are thought to be the hypergiants, near the Eddington limit and rapidly losing mass. The yellow hypergiants are thought to be generally post-red supergiant stars that have already lost most of their atmospheres and hydrogen. Because of their masses, the lifetime of a hypergiant is very short in astronomical timescales. The time spent in some such as LBVs can be as short as a few thousand years. This means that the flux passing through the photosphere of a hypergiant may be nearly strong enough to lift off the photosphere. A good candidate for hosting a continuum-driven wind is Eta Carinae, with an estimated mass of around 130 solar masses and a luminosity four million times that of the Sun, astrophysicists speculate that Eta Carinae may occasionally exceed the Eddington limit. The last time might have been a series of outbursts observed in 1840–1860, as opposed to line-driven stellar winds, continuum driving does not require the presence of metallic atoms — atoms other than hydrogen and helium, which have few such lines — in the photosphere. This is important, since most massive stars also are very metal-poor, another theory to explain the massive outbursts of, for example, Eta Carinae is the idea of a deeply situated hydrodynamic explosion, blasting off parts of the stars outer layers
9.
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
10.
Luminosity
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In astronomy, luminosity is the total amount of energy emitted by a star, galaxy, or other astronomical object per unit time. It is related to the brightness, which is the luminosity of an object in a spectral region. In SI units luminosity is measured in joules per second or watts, values for luminosity are often given in the terms of the luminosity of the Sun, which has a total power output of 7026384600000000000♠3. 846×1026 W. The symbol for solar luminosity is L⊙. Luminosity can also be given in terms of magnitude, the absolute bolometric magnitude of an object is a logarithmic measure of its total energy emission. In astronomy, luminosity is the amount of energy a body radiates per unit of time. It is most frequently measured in two forms, visual and bolometric, although luminosities at other wavelengths are increasingly being used as instruments become available to measure them, a bolometer is the instrument used to measure radiant energy over a wide band by absorption and measurement of heating. When not qualified, the term luminosity means bolometric luminosity, which is measured either in the SI units, watts, a star also radiates neutrinos, which carry off some energy, contributing to the stars total luminosity. In practice bolometric magnitudes are measured by taking measurements at certain wavelengths, a stars luminosity can be determined from two stellar characteristics, size and effective temperature. The former is represented in terms of solar radii, R⊙, while the latter is represented in kelvins. To determine a stars radius, two metrics are needed, the stars angular diameter and its distance from Earth, often calculated using parallax. However, for most stars the angular diameter or parallax, or both, are far below our ability to measure with any certainty, an alternate way to measure stellar luminosity is to measure the stars apparent brightness and distance. Because luminosity is proportional to temperature to the power, the large variation in stellar temperatures produces an even vaster variation in stellar luminosity. Because the luminosity depends on a power of the stellar mass. The most luminous stars are young stars, no more than a few million years for the most extreme. In the Hertzsprung–Russell diagram, the x-axis represents temperature or spectral type while the y-axis represents luminosity or magnitude. The vast majority of stars are found along the sequence with blue Class 0 stars found at the top left of the chart while red Class M stars fall to the bottom right. Certain stars like Deneb and Betelgeuse are found above and to the right of the main sequence, blue and white supergiants are high luminosity stars somewhat cooler than the most luminous main sequence stars. A star like Deneb, for example, has a luminosity around 200,000 L⊙, a type of A2
11.
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
12.
Ejnar Hertzsprung
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Ejnar Hertzsprung was a Danish chemist and astronomer, born in Copenhagen, Denmark. In the period 1911–1913, together with Henry Norris Russell, he developed the Hertzsprung–Russell diagram, in this determination he made a mistake, possibly a slip of the pen, putting the stars 10 times too close. He used this relationship to estimate the distance to the Small Magellanic Cloud, from 1919 to 1946 Hertzsprung worked at Leiden Observatory in the Netherlands, from 1937 as director. Among his graduate students at Leiden was Gerard Kuiper, perhaps his greatest contribution to astronomy was the development of a classification system for stars to divide them by spectral type, stage in their development, and luminosity. The so-called Hertzsprung–Russell Diagram has been used ever since as a system to explain stellar types. He also discovered two asteroids, one of which is 1627 Ivar, an Amor asteroid and his wife Henrietta was a daughter of the Dutch astronomer Jacobus Kapteyn. Hertzsprung died in Roskilde in 1967, the asteroid 1693 Hertzsprung was named in his honour
13.
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
14.
European Southern Observatory
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The European Southern Observatory is a 16-nation intergovernmental research organization for ground-based astronomy. Created in 1962, ESO has provided astronomers with state-of-the-art research facilities, the organisation employs about 730 staff members and receives annual member state contributions of approximately €131 million. Its observatories are located in northern Chile, ESO has built and operated some of the largest and most technologically advanced telescopes. The Atacama Large Millimeter Array observes the universe in the millimetre and submillimetre wavelength ranges and it was completed in March 2013 in an international collaboration by Europe, North America, East Asia and Chile. Currently under construction is the European Extremely Large Telescope and it will use a 39. 3-metre-diameter segmented mirror, and become the worlds largest optical reflecting telescope when operational in 2024. ESOs observing facilities have made discoveries and produced several astronomical catalogues. Its findings include the discovery of the most distant gamma-ray burst, in 2004, the VLT allowed astronomers to obtain the first picture of an extrasolar planet orbiting a brown dwarf 173 light-years away. The idea that European astronomers should establish a large observatory was broached by Walter Baade. It was pursued by Oort, who gathered a group of astronomers in Leiden to consider it on June 21 that year, immediately thereafter, the subject was further discussed at the Groningen conference in the Netherlands. On January 26,1954, an ESO declaration was signed by astronomers from six European countries expressing the wish that a joint European observatory be established in the southern hemisphere. At the time, all reflector telescopes with an aperture of 2 metres or more were located in the northern hemisphere. The decision to build the observatory in the southern hemisphere resulted from the necessity of observing the southern sky, although it was initially planned to set up telescopes in South Africa, tests from 1955 to 1963 demonstrated that a site in the Andes was preferable. On November 15,1963 Chile was chosen as the site for ESOs observatory, the decision was preceded by the ESO Convention, signed 5 October 1962 by Belgium, Germany, France, the Netherlands and Sweden. Otto Heckmann was nominated as the organisations first director general on 1 November 1962, a preliminary proposal for a convention of astronomy organisations in these five countries was drafted in 1954. Although some amendments were made in the document, the convention proceeded slowly until 1960 when it was discussed during that years committee meeting. The new draft was examined in detail, and a member of CERN highlighted the need for a convention between governments. The convention and government involvement became pressing due to rising costs of site-testing expeditions. The final 1962 version was adopted from the CERN convention
15.
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
16.
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
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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
18.
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
19.
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
20.
Carbon
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Carbon is a chemical element with symbol C and atomic number 6. It is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds, three isotopes occur naturally, 12C and 13C being stable, while 14C is a radioactive isotope, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity, Carbon is the 15th most abundant element in the Earths crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. It is the second most abundant element in the body by mass after oxygen. The atoms of carbon can bond together in different ways, termed allotropes of carbon, the best known are graphite, diamond, and amorphous carbon. The physical properties of carbon vary widely with the allotropic form, for example, graphite is opaque and black while diamond is highly transparent. Graphite is soft enough to form a streak on paper, while diamond is the hardest naturally occurring material known, graphite is a good electrical conductor while diamond has a low electrical conductivity. Under normal conditions, diamond, carbon nanotubes, and graphene have the highest thermal conductivities of all known materials, all carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form. They are chemically resistant and require high temperature to react even with oxygen, the most common oxidation state of carbon in inorganic compounds is +4, while +2 is found in carbon monoxide and transition metal carbonyl complexes. The largest sources of carbon are limestones, dolomites and carbon dioxide, but significant quantities occur in organic deposits of coal, peat, oil. For this reason, carbon has often referred to as the king of the elements. The allotropes of carbon graphite, one of the softest known substances, and diamond. It bonds readily with other small atoms including other carbon atoms, Carbon is known to form almost ten million different compounds, a large majority of all chemical compounds. Carbon also has the highest sublimation point of all elements, although thermodynamically prone to oxidation, carbon resists oxidation more effectively than elements such as iron and copper that are weaker reducing agents at room temperature. Carbon is the element, with a ground-state electron configuration of 1s22s22p2. Its first four ionisation energies,1086.5,2352.6,4620.5 and 6222.7 kJ/mol, are higher than those of the heavier group 14 elements. Carbons covalent radii are normally taken as 77.2 pm,66.7 pm and 60.3 pm, although these may vary depending on coordination number, in general, covalent radius decreases with lower coordination number and higher bond order. Carbon compounds form the basis of all life on Earth
21.
Oxygen
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Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the group on the periodic table and is a highly reactive nonmetal. By mass, oxygen is the third-most abundant element in the universe, after hydrogen, at standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. This is an important part of the atmosphere and diatomic oxygen gas constitutes 20. 8% of the Earths atmosphere, additionally, as oxides the element makes up almost half of the Earths crust. Most of the mass of living organisms is oxygen as a component of water, conversely, oxygen is continuously replenished by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide. Oxygen is too reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone, strongly absorbs ultraviolet UVB radiation, but ozone is a pollutant near the surface where it is a by-product of smog. At low earth orbit altitudes, sufficient atomic oxygen is present to cause corrosion of spacecraft, the name oxygen was coined in 1777 by Antoine Lavoisier, whose experiments with oxygen helped to discredit the then-popular phlogiston theory of combustion and corrosion. One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle, Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries later Leonardo da Vinci built on Philos work by observing that a portion of air is consumed during combustion and respiration, Oxygen was discovered by the Polish alchemist Sendivogius, who considered it the philosophers stone. In the late 17th century, Robert Boyle proved that air is necessary for combustion, English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. From this he surmised that nitroaereus is consumed in both respiration and combustion, Mayow observed that antimony increased in weight when heated, and inferred that the nitroaereus must have combined with it. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract De respiratione. Robert Hooke, Ole Borch, Mikhail Lomonosov, and Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element. This may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, which was then the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, one part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. The fact that a substance like wood gains overall weight in burning was hidden by the buoyancy of the combustion products
22.
Triple-alpha process
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The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei are transformed into carbon. Helium accumulates in the core of stars as a result of the chain reaction. Further nuclear fusion reactions of helium with hydrogen or another alpha particle produce lithium-5, both products are highly unstable and decay, almost instantly, back into smaller nuclei, unless a third alpha particle fuses with a beryllium before that time to produce a stable carbon-12 nucleus. When a star out of hydrogen to fuse in its core, it begins to collapse until the central temperature rises to 108 K. This creates a situation in which stellar nucleosynthesis produces large amounts of carbon and oxygen but only a fraction of those elements are converted into neon. Both oxygen and carbon make up the ash of helium-4 burning, because the triple-alpha process is unlikely, it normally needs a long time to produce much carbon. Ordinarily, the probability of the alpha process is extremely small. However, the ground state has almost exactly the energy of two alpha particles. In the second step, 8Be + 4He has almost exactly the energy of a state of 12C. This resonance greatly increases the probability that an alpha particle will combine with beryllium-8 to form carbon. The existence of this resonance was predicted by Fred Hoyle before its actual observation, based on the necessity for it to exist. The anthropic principle has been cited to explain the fact that nuclear resonances are sensitively arranged to create large amounts of carbon. With further increases of temperature and density, fusion processes produce nuclides only up to nickel-56, the slow capture of neutrons, the s-process, produces about half of elements beyond iron. The other half are produced by neutron capture, the r-process. The triple-alpha steps are dependent on the temperature and density of the stellar material. The power released by the reaction is proportional to the temperature to the 40th power. This strong temperature dependence has consequences for the stage of stellar evolution. For lower mass stars, the helium accumulating in the core is prevented from further collapse only by degeneracy pressure
23.
Horizontal branch
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The horizontal branch is a stage of stellar evolution that immediately follows the red giant branch in stars whose masses are similar to the Suns. Horizontal-branch stars are powered by fusion in the core and by hydrogen fusion in a shell surrounding the core. The horizontal branch is so named because in low-metallicity star collections like globular clusters, in due course, the helium-enriched core becomes unable to sustain nuclear fusion of hydrogen and that fusion process migrates outward to a shell. The core becomes a region of degenerate matter that does not contribute to the generation of energy and it continues to grow and increase in temperature as the hydrogen fusion in the shell contributes more helium. Stars initially between about 2.3 M☉ and 8 M☉ have larger helium cores that do not become degenerate, instead their cores reach the Schoenberg-Chandrasekhar mass at which they are no longer in hydrostatic or thermal equilibrium. They then contract and heat up, which triggers helium fusion before the core becomes degenerate, if the star has more than about 0.5 solar masses, the core eventually reaches the temperature necessary for the fusion of helium into carbon through the triple-alpha process. The initiation of helium fusion begins across the region, which will cause an immediate temperature rise. Within a few seconds the core becomes non-degenerate and quickly expands, non-degenerate cores initiate fusion more smoothly, without a flash. The output of this event is absorbed by the layers of plasma above, the star now changes to a new equilibrium state, and its evolutionary path switches from the red giant branch onto the horizontal branch of the Hertzsprung–Russell diagram. This term means that the luminosity of the star will stay relatively stable while the temperature increases. More massive stars spend a time on the horizontal branch. The shape of the branch is due both to the movement of individual stars bluewards as they age, and to the temperature of stars with different masses when they reach the horizontal branch. There are further variations, both in luminosity and temperature, due to metallicity and helium content, the horizontal branch ends in a blue tail with hotter stars having lower luminosity, occasionally with a blue hook of extremely hot stars. It is also not horizontal when plotted by bolometric luminosity, with hotter horizontal branch stars being less luminous than cooler ones, the hottest horizontal-branch stars, referred to as extreme horizontal branch, have temperatures of 20, 000–30, 000K. This is far beyond what would be expected for a core helium burning star. These stars are born again with unusual properties, globular cluster CMDs generally show horizontal branches that have a prominent gap in the HB. This gap in the CMD incorrectly suggests that the cluster has no stars in this region of its CMD, the gap occurs at the instability strip, so many stars in this region pulsate. These pulsating horizontal-branch stars are known as RR Lyrae variable stars and it requires an extended observing program to establish the stars true apparent magnitude and color
24.
Asymptotic giant branch
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The asymptotic giant branch is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars late in their lives, observationally, an asymptotic-giant-branch star will appear as a bright red giant with a luminosity thousands of times greater than the Sun. When a star exhausts the supply of hydrogen by nuclear processes in its core. The star becomes a red giant, following a track towards the upper-right hand corner of the HR diagram, eventually, once the temperature in the core has reached approximately 3×108 K, helium burning begins. The onset of burning in the core halts the stars cooling and increase in luminosity. This is the branch or red clump, or a blue loop for stars more massive than about 2 M☉. After the completion of burning in the core, the star again moves to the right and upwards on the diagram. Stars at this stage of evolution are known as AGB stars. The AGB phase is divided into two parts, the early AGB and the thermally pulsing AGB, during the E-AGB phase, the main source of energy is helium fusion in a shell around a core consisting mostly of carbon and oxygen. During this phase, the star swells up to giant proportions to become a red giant again, the stars radius may become as large as one astronomical unit. After the helium shell runs out of fuel, the TP-AGB starts, now the star derives its energy from fusion of hydrogen in a thin shell, which restricts the inner helium shell to a very thin layer and prevents it fusing stably. However, over periods of 10,000 to 100,000 years, helium from the hydrogen shell burning builds up and eventually the helium shell ignites explosively, a process known as a helium shell flash. The luminosity of the shell flash peaks at thousands of times the luminosity of the star. The shell flash causes the star to expand and cool which shuts off the hydrogen shell burning, when the helium shell burning nears the base of the hydrogen shell, the increased temperature reignites hydrogen fusion and the cycle begins again. During the thermal pulses, which last only a few hundred years, material from the region may be mixed into the outer layers, changing the surface composition. Because of this dredge-up, AGB stars may show S-process elements in their spectra, all dredge-ups following thermal pulses are referred to as third dredge-ups, after the first dredge-up, which occurs on the red-giant branch, and the second dredge up, which occurs during the E-AGB. In some cases there may not be a second dredge-up but dredge-ups following thermal pulses will still be called a third dredge-up, thermal pulses increase rapidly in strength after the first few, so third dredge-ups are generally the deepest and most likely to circulate core material to the surface. AGB stars are typically long-period variables, and suffer loss in the form of a stellar wind
25.
Convection
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Convection is the movement of groups of molecules within fluids such as liquids or gases, and within rheids. Convection takes place through advection, diffusion or both, convection cannot take place in solids because neither bulk current flows nor significant diffusion can take place in solids. Diffusion of heat can take place in solids, but that is called heat conduction, convective heat transfer is one of the major types of heat transfer, and convection is also a major mode of mass transfer in fluids. In the context of heat and mass transfer, the convection is used to refer to the sum of advective and diffusive transfer. In common use the term convection may refer loosely to heat transfer by convection, as opposed to mass transfer by convection, sometimes convection is even used to refer specifically to free heat convection as opposed to forced heat convection. However, in mechanics the correct use of the word is the general sense, convection can be qualified in terms of being natural, forced, gravitational, granular, or thermomagnetic. It may also be said to be due to combustion, capillary action, or Marangoni, heat transfer by natural convection plays a role in the structure of Earths atmosphere, its oceans, and its mantle. Discrete convective cells in the atmosphere can be seen as clouds, natural convection also plays a role in stellar physics. The word convection may have different but related usages in different scientific or engineering contexts or applications. The broader sense is in fluid mechanics, where convection refers to the motion of fluid regardless of cause, however, in thermodynamics convection often refers specifically to heat transfer by convection. Additionally, convection includes fluid movement both by bulk motion and by the motion of individual particles, however, in some cases, convection is taken to mean only advective phenomena. Convection occurs on a scale in atmospheres, oceans, planetary mantles. Fluid movement during convection may be slow, or it may be obvious and rapid. On astronomical scales, convection of gas and dust is thought to occur in the disks of black holes. Convective heat transfer is a mechanism of heat transfer occurring because of bulk motion of fluids, heat is the entity of interest being advected, and diffused. Heat is transferred by convection in numerous examples of naturally occurring fluid flow, such as, wind, oceanic currents, convection is also used in engineering practices of homes, industrial processes, cooling of equipment, etc. The rate of heat transfer may be improved by the use of a heat sink. For instance, a typical computer CPU will have a fan to ensure its operating temperature is kept within tolerable limits
26.
Universe
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The Universe is all of time and space and its contents. It includes planets, moons, minor planets, stars, galaxies, the contents of intergalactic space, the size of the entire Universe is unknown. The earliest scientific models of the Universe were developed by ancient Greek and Indian philosophers and were geocentric, over the centuries, more precise astronomical observations led Nicolaus Copernicus to develop the heliocentric model with the Sun at the center of the Solar System. In developing the law of gravitation, Sir Isaac Newton built upon Copernicuss work as well as observations by Tycho Brahe. Further observational improvements led to the realization that our Solar System is located in the Milky Way galaxy and it is assumed that galaxies are distributed uniformly and the same in all directions, meaning that the Universe has neither an edge nor a center. Discoveries in the early 20th century have suggested that the Universe had a beginning, the majority of mass in the Universe appears to exist in an unknown form called dark matter. The Big Bang theory is the prevailing cosmological description of the development of the Universe, under this theory, space and time emerged together 13. 799±0.021 billion years ago with a fixed amount of energy and matter that has become less dense as the Universe has expanded. After the initial expansion, the Universe cooled, allowing the first subatomic particles to form, giant clouds later merged through gravity to form galaxies, stars, and everything else seen today. Some physicists have suggested various multiverse hypotheses, in which the Universe might be one among many universes that likewise exist, the Universe can be defined as everything that exists, everything that has existed, and everything that will exist. According to our current understanding, the Universe consists of spacetime, forms of energy, the Universe encompasses all of life, all of history, and some philosophers and scientists suggest that it even encompasses ideas such as mathematics and logic. The word universe derives from the Old French word univers, which in turn derives from the Latin word universum, the Latin word was used by Cicero and later Latin authors in many of the same senses as the modern English word is used. Another synonym was ὁ κόσμος ho kósmos, synonyms are also found in Latin authors and survive in modern languages, e. g. the German words Das All, Weltall, and Natur for Universe. The same synonyms are found in English, such as everything, the cosmos, the world, the prevailing model for the evolution of the Universe is the Big Bang theory. The Big Bang model states that the earliest state of the Universe was extremely hot and dense, the model is based on general relativity and on simplifying assumptions such as homogeneity and isotropy of space. The Big Bang model accounts for such as the correlation of distance and redshift of galaxies, the ratio of the number of hydrogen to helium atoms. The initial hot, dense state is called the Planck epoch, after the Planck epoch and inflation came the quark, hadron, and lepton epochs. Together, these epochs encompassed less than 10 seconds of time following the Big Bang, the observed abundance of the elements can be explained by combining the overall expansion of space with nuclear and atomic physics. As the Universe expands, the density of electromagnetic radiation decreases more quickly than does that of matter because the energy of a photon decreases with its wavelength
27.
Gamma Geminorum
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Gamma Geminorum, also named Alhena, is the third-brightest star in the constellation of Gemini. It has an apparent visual magnitude of 1.9, making it visible to the naked eye even in urban regions. Based upon parallax measurements with the Hipparcos satellite, it is located at a distance of roughly 109 light-years from the Sun, Alhena is an evolving star that is exhausting the supply of hydrogen at its core and has entered the subgiant stage. The spectrum matches a stellar classification of A0 IV, compared to the Sun it has 2.8 times the mass and 3.3 times the radius. It is radiating around 123 times the luminosity of the Sun from its outer envelope at a temperature of 9,260 K. This gives it a white hue typical of an A-class star and this is a spectroscopic binary system with a period of 12.6 years in a highly eccentric Keplerian orbit. γ Geminorum is the stars Bayer designation, the traditional name Alhena is derived from the Arabic الهنعة Al Hanah, the brand, whilst the alternate name Almeisan is from the Arabic المیسان Al Maisan, the shining one. Al Hanʽah was the name of star association consisting of this star, along with Mu Geminorum, Nu Geminorum, Eta Geminorum and they also were associated in Al Nuḥātai, the dual form of Al Nuḥāt, a Camels Hump. 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 Alhena for this star. In Chinese, 井宿, meaning Well, refers to an asterism consisting of γ Geminorum, μ Geminorum, ν Geminorum, ξ Geminorum, ε Geminorum,36 Geminorum, ζ Geminorum and λ Geminorum. Consequently, γ Geminorum itself is known as 井宿三 Alhena was the name of a Dutch ship that many people from an Italian cruise liner. In addition, the American attack cargo ship USS Alhena was named after the star
28.
Eta Bootis
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Eta Boötis, also named Muphrid, is a star in the constellation of Boötes. Since 1943, the spectrum of this star has served as one of the anchor points by which other stars are classified. η Boötis is the stars Bayer designation and it also bears the Flamsteed designation of 8 Boötis. It bore the traditional names Muphrid and Saak, Muphrid is from the Arabic مفرد الرامح mufrid ar-rāmiħ the one of the lancer. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalogue, the WGSN approved the name Muphrid for this star on 12 September 2016 and it is now so entered in the IAU Catalog of Star Names. In Chinese, 右攝提, meaning the Right Conductor, refers to an asterism consisting of Eta Boötis, Tau Boötis, consequently, Eta Boötis itself is known as 右攝提一. Eta Boötis is a subgiant that has begun the process of evolving from a main sequence star into a red giant and it has about 1.7 times the mass of the Sun and 2.7 times the Suns radius. The estimated age of this star is about 2.7 billion years, based on its spectra, it has a significant excess of elements heavier than helium. In fact the ratio of iron to hydrogen is considered close to the limit for dwarf stars in the galactic disk. The star is a spectroscopic binary with a reported period of 494 days. This measurement does not rule out a low mass companion of spectral class M7. Eta Boötis appears close to the prominent star Arcturus in Earths sky, the two stars are about 3.24 light years apart, and each would appear bright in the others sky. List of the nearest stars Eta Boötis in fiction STARS link The Constellations and Named Stars GJ534 CCDM J13547+1824 Image Eta Boötis Richard Allen Hinkley, Star Names, Their Lore and Meaning 104
29.
Pollux (star)
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Pollux, also designated Beta Geminorum, is an orange-hued evolved giant star approximately 34 light-years from the Sun in the northern constellation of Gemini. It is the closest giant star to the Sun, since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. In 2006, a planet was confirmed to be orbiting it. β Geminorum is the stars Bayer designation, the traditional name Pollux refers specifically to the twins Castor and Pollux in Greek and Roman mythology. 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 Pollux for this star. Castor and Pollux are the two heavenly twin stars giving the constellation Gemini its name, the stars, however, are quite different in detail. Castor is a sextuple system of hot, bluish-white A-type stars and dim red dwarfs, while Pollux is a single. In Percy Shelleys 1818 poem Homers Hymn To Castor And Pollux, following its discovery the planet was designated Pollux b. 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 name was Thestias for this planet. The winning name was based on that submitted by theSkyNet of Australia, namely Leda. At the request of the IAU, Thestias was substituted and this was because Leda was already attributed to an asteroid and to one of Jupiters satellites. In the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, this star was designated Muekher al Dzira, in Chinese, 北河, meaning North River, refers to an asterism consisting of Pollux, ρ Geminorum and Castor. Consequently, Pollux itself is known as 北河三 Parallax measurements made with the Hipparcos astrometry satellite place Pollux at a distance of about 33.78 light-years from the Sun. At an apparent visual magnitude of 1.1, Pollux is the brightest star in the constellation, the star is larger than the Sun, with about two times its mass and almost nine times its radius. Once an A-type main sequence star, Pollux has exhausted the hydrogen at its core, the effective temperature of this stars outer envelope is about 4666 K, which lies in the range that produces the characteristic orange hue of K-type stars. Pollux has a rotational velocity of 2.8 km·s−1. The abundance of other than hydrogen and helium, what astronomers term the stars metallicity, is somewhat uncertain
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Mintaka
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Mintaka, also designated Delta Orionis and 34 Orionis is a multiple star some 1,200 light years from the Sun in the constellation of Orion. Together with Alnitak and Alnilam, the three make up the belt of Orion, known by many names among ancient cultures. When Orion is close to the meridian, Mintaka is the right-most of the stars as seen by an observer in the Northern Hemisphere facing south. Delta Orionis is the stars Bayer designation,34 Orionis its Flamsteed designation, the name Mintaka itself is derived from an Arabic term for belt, منطقة or manṭaqa. 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 Mintaka for this star. It is now so entered in the IAU Catalog of Star Names, Mintaka is the westernmost of the three stars of Orions belt. It is easily visible to the eye, one of the brightest stars in the sky. Radial velocity measurements taken by Henri-Alexandre Deslandres at Paris Observatory showed that Mintaka had a radial velocity. Hartmann also noticed that the calcium K line at 393 and this was the first detection of the interstellar medium. Mintaka is a star system with a magnitude 7 star about 52 arcseconds away from the main component. At the primary eclipse, the apparent magnitude drops from 2.23 to 2.35, the seventh magnitude companion, HD36485, is an unusual B-type main sequence star and itself a spectroscopic binary with a faint A-type companion in a 30-day orbit. The 14th magnitude star is thought to be at the same distance, Mintaka is surrounded by a cluster of faint stars, possibly part of the cluster surrounding σ Ori. The bright central star of the system is δ Ori A and its components are generally referred to by researchers as Aa1, Aa2, and Ab, with HD36485 referred to as Mintaka C, and the closer fourteenth magnitude companion as δ Ori B. Confusingly, some catalogues list δ Ori Ab as component B or component D and this type of unreconcilable discrepancy is not unique to Mintaka and the reasons for it have yet to be clarified. Mintaka was also seen by astrologers as a portent of good fortune, the three belt stars were collectively known by many names in many cultures. Arabic terms include Al Nijād the Belt, Al Nasak the Line, Al Alkāt the Golden Grains or Nuts and, in modern Arabic, in Chinese mythology they were also known as The Weighing Beam. The belt was also the Three Stars mansion, one of the Twenty-eight mansions of the Chinese constellations and it is one of the western mansions of the White Tiger. In pre-Christian Scandinavia, the belt was known as Friggs Distaff or Freyjas distaff, similarly Jacobs Staff and Peters Staff were European biblical derived terms, as were the Three Magi, or the Three Kings
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Canopus
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Canopus, also designated Alpha Carinae, is the brightest star in the southern constellation of Carina, and the second brightest star in the night-time sky, after Sirius. Canopuss visual magnitude is −0.74, and it has a magnitude of −5.71. Canopus is a giant of spectral type A9, so it is essentially white when seen with the naked eye. It is located in the far southern sky, at a year 2000 declination of −52° 42′ and its name is generally considered to originate from the mythological Canopus, who was a navigator for Menelaus, king of Sparta. In Indian Vedic literature, the star Canopus is associated with the sage Agastya, Agastya, the star, is said to be the cleanser of waters and its rising coincides with the calming of the waters of the Indian Ocean. It is considered the son of Pulasthya, son of Brahma, Canopus was not visible to the mainland ancient Greeks and Romans, it was, however, visible to the ancient Egyptians. Hence Aratus did not write of the star as it remained below the horizon, while Eratosthenes and Ptolemy—observing from Alexandria—did, the Navajo named it Ma’ii Bizò‘. The Bedouin people of the Negev and Sinai also knew it as Suhayl, due to the fact that it disappears below the horizon in those regions, it became associated with a changeable nature, as opposed to always-visible Polaris, which was circumpolar and hence steadfast. It is also referred to by its Arabic name, سهيل, called the Old Man of the South Pole in Chinese, Canopus appears on the medieval Chinese manuscript the Dunhuang star chart, despite not being visible from the Chinese capital of Changan. The Chinese astronomer Yi Xing had journeyed south to chart Canopus, however, it was already mentioned by Sima Qian in the second century BC, drawing on sources from the Warring States period, as the southern counterpart of Sirius. Bright stars were important to the ancient Polynesians for navigation between the islands and atolls of the Pacific Ocean. Low on the horizon, they acted as stellar compasses to assist mariners in charting courses to particular destinations. The Hawaiian people called Canopus Ke Alii-o-kona-i-ka-lewa, The chief of the expanse, it was one of the stars used by Hawaii-loa. The Māori people of New Zealand/Aotearoa had several different names for Canopus, ariki, was known as a solitary star that appeared in the east, prompting people to weep and chant. They also named it Atutahi, Aotahi or Atuatahi, Stand Alone and its solitary nature indicates it is a tapu star, as tapu people are often solitary. Its appearance at the beginning of the Maruaroa season foretells the coming winter, light rays to the south indicate a cold wet winter, food was offered to the star on its appearance. This name has several different mythologies attached to it as well, one story tells of how Atutahi was left outside of the basket representing the Milky Way when Tāne wove it. Another related myth surrounding the star says that Atutahi was the child of Rangi
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Red clump
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The red clump is a clustering of red giants in the Hertzsprung–Russell diagram at around 5,000 K and absolute magnitude +0.5, slightly hotter than most red-giant-branch stars of the same luminosity. It is visible as a dense region of the red giant branch or a bulge towards hotter temperatures. It is most distinct in many, but not all, galactic open clusters, the red clump giants are cool horizontal branch stars, stars originally similar to the sun which have undergone a helium flash and are now fusing helium in their cores. Red clump stellar properties vary depending on their origin, most notably on the metallicity of the stars, the absolute visual magnitude of red clump giants near the sun has been measured at an average of +0.81 with metallicities between −0.6 and +0.4 dex. There is a spread in the properties of red clump stars even within a single population of similar stars such as an open cluster. Although red clump stars are hotter than red-giant-branch stars, the two regions overlap and the status of individual stars can only be assigned with a detailed chemical abundance study. Modelling of the branch has shown that stars have a strong tendency to cluster at the cool end of the zero age horizontal branch. This tendency is weaker in low metallicity stars, so the red clump is usually more prominent metal-rich clusters, however, there are other effects, and there are well-populated red clumps in some metal-poor globular clusters. Stars with a mass to the sun evolve towards the tip of the red giant branch with a degenerate helium core. More massive stars leave the red giant branch early and perform a loop, but all stars with a degenerate core reach the tip with very similar core masses, temperatures. After the helium flash they lie along the ZAHB, all with helium cores just under 0.5 M☉, lower envelope masses result in weaker hydrogen shell fusion and give hotter and slightly less luminous stars strung along the horizontal branch. Different initial masses and natural variations in mass loss rates on the red giant branch cause the variations in the masses even though the helium cores are all the same size. Low metallicity stars are more sensitive to the size of the envelope, so with the same envelope masses they are spread further along the horizontal branch. Although red clump stars lie consistently to the hot side of the red giant branch that they evolved from, red clump and this occurs in ω Centauri where metal-poor red-giant-branch stars have the same or hotter temperatures as more metal-rich red clump giants. Other stars, not strictly horizontal branch stars, can lie in the region of the H-R diagram. Stars too massive to develop a degenerate helium core on the red giant branch will ignite helium before the tip of the red giant branch and perform a blue loop. For stars only a more massive than the sun, around 2 M☉. This means that these imposters are much less common in the H-R diagram, stars with 2 -3 M☉ will also pass through the red clump as they evolve along the subgiant branch
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Arcturus
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Arcturus, also designated Alpha Boötis, is the brightest star in the constellation of Boötes, the fourth-brightest in the night sky, and the brightest in the northern celestial hemisphere. Together with Spica and Denebola, Arcturus is part of the Spring Triangle asterism and, by extension, also of the Great Diamond along with the star Cor Caroli. It is 1.08 ±0.06 times as massive as the Sun, α Boötis is the stars Bayer designation. The traditional name Arcturus derives from Ancient Greek Ἀρκτοῦρος and means Guardian of the Bear, ultimately from ἄρκτος, bear and οὖρος, watcher and it has been known by this name since at least the time of Hesiod. This is a reference to its being the brightest star in the constellation of Boötes, which is next to the constellations of Ursa Major and Ursa Minor, in 2016, the International Astronomical Union organized a Working Group on Star Names to catalog and standardize proper names for stars. The WGSNs first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN, which included Arcturus for this star and it is now so entered in the IAU Catalog of Star Names. However, Alpha Centauri is a star, whose unresolved components to the naked eye are both fainter than Arcturus. This makes Arcturus the third-brightest individual star, just ahead of Alpha Centauri A, the French mathematician and astronomer Jean-Baptiste Morin observed Arcturus in the daytime with a telescope in 1635, and Arcturus has been seen at or just before sunset with the naked eye. Arcturus is visible from both Earths hemispheres as it is located 19° north of the celestial equator, the star culminates at midnight on 27 April, and at 9PM on June 10 being visible during the late northern spring or the southern autumn. From the northern hemisphere, a way to find Arcturus is to follow the arc of the handle of the Big Dipper. By continuing in this path, one can find Spica, Arc to Arcturus, ptolemy described Arcturus as subrufa, it has a B-V color index of +1.23, roughly midway between Pollux and Aldebaran. Based upon an annual parallax shift of 88.83 milliarcseconds as measured by the Hipparcos satellite, Arcturus is a type K0 III red giant star. With an absolute magnitude of −0.30 it is, together with Vega and Sirius, the lower output in visible light is due to a lower efficacy as the star has a lower surface temperature than the Sun. With a near-infrared J band magnitude of −2.2, only Betelgeuse, as a single star, the mass of Arcturus cannot be measured directly, but models suggest it is slightly larger than that of the Sun. It has likely exhausted the hydrogen from its core and is now in its active hydrogen shell burning phase and it will continue to expand before entering horizontal branch stage of its life cycle. They suggested that the most plausible explanation for the variability of Arcturus is stellar oscillations, asteroseismological measurements allow direct calculation of the mass and radius, giving values of 0.8 ±0.2 M☉ and 27.9 ±3.4 R☉. This form of modelling is still relatively inaccurate, but a check on other models. Astronomers term metals those elements with atomic numbers than helium
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Mira
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Mira /ˈmaɪrə/, alternatively designated Omicron Ceti is a red giant star estimated to be 200–400 light years from the Sun in the constellation of Cetus. ο Ceti is a stellar system, consisting of a variable red giant along with a white dwarf companion. Mira A is a variable star and was the first non-supernova variable star discovered. ο Ceti is the stars Bayer designation and it was named Mira by Johannes Hevelius in his Historiola Mirae Stellae. 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 Mira for this star. Evidence that the variability of Mira was known in ancient China, what is certain is that the variability of Mira was recorded by the astronomer David Fabricius beginning on August 3,1596. Observing what he thought was the planet Mercury, he needed a star for comparing positions. By August 21, however, it had increased in brightness by one magnitude, Fabricius assumed it was a nova, but then saw it again on February 16,1609. In 1638 Johannes Holwarda determined a period of the stars reappearances, eleven months, Johannes Hevelius was observing it at the same time and named it Mira in 1662, for it acted like no other known star. Ismail Bouillaud then estimated its period at 333 days, less one day off the modern value of 332 days. Bouillauds measurement may not have been erroneous, Mira is known to vary slightly in period, the star is estimated to be a 6-billion-year-old red giant. There is considerable speculation as to whether Mira had been observed prior to Fabricius, certainly Algols history suggests that Mira might have been known too. Karl Manitius, a translator of Hipparchus Commentary on Aratus, has suggested that certain lines from that second-century text may be about Mira. The other pre-telescopic Western catalogs of Ptolemy, al-Sufi, Ulugh Beg and this binary star system consists of a red giant undergoing mass loss and a high temperature white dwarf companion that is accreting mass from the primary. Such an arrangement of stars is known as a symbiotic system, examination of this system by the Chandra X-ray Observatory shows a direct mass exchange along a bridge of matter from the primary to the white dwarf. The two stars are separated by about 70 astronomical units. Mira A is currently an asymptotic giant branch star, in the thermally pulsing AGB phase, each pulse lasts a decade or more, and an amount of time on the order of 10,000 years passes between each pulse. With every pulse cycle Mira increases in luminosity and the pulses grow stronger and this is also causing dynamic instability in Mira, resulting in dramatic changes in luminosity and size over shorter, irregular time periods
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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
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Instability strip
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RV Tauri variables are also often considered to lie on the instability strip, occupying the area to the right of the brighter Cepheids, since their pulsations are attributed to the same mechanism. The instability strip intersects the main sequence in the region of A and F stars and extends to G, the lower part of instability strip appears as the Hertzsprung gap on the Hertzsprung–Russell diagram. Above the main sequence, the vast majority of stars in the instability strip are variable, where the instability strip intersects the main sequence, the vast majority of stars are stable, but there are some variables, including the roAp stars. Stars in the instability strip pulsate due to He III, in normal A-F-G stars He is neutral in the stellar photosphere. Deeper below the photosphere, at about 25, 000–30, 000K, second ionization starts at about 35, 000–50, 000K. When the star contracts, the density and temperature of the He II layer increases and he II starts to transform into He III. This causes the opacity of the star to increase and the flux from the interior of the star is effectively absorbed. The temperature of the rises and it begins to expand. After expansion, He III begins to recombine into He II and this lowers the surface temperature of the star. The outer layers contract and the cycle starts from the beginning, the phase shift between a stars radial pulsations and brightness variations depends on the distance of He II zone from the stellar surface in the stellar atmosphere. For most Cepheids, this creates a distinctly asymmetrical observed light curve, rising rapidly to maximum, there are several types of pulsating star not found on the instability strip and with pulsations driven by different mechanisms. At cooler temperatures are the long period variable AGB stars, at hotter temperatures are the Beta Cephei and PV Telescopii, variables. Right at the edge of the instability strip near the main sequence are Gamma Doradus variables, the band of White dwarfs has three separate regions types of variable, DOV, DBV, and DAV white dwarfs. Each of these types of pulsating variable has an associated instability strip created by variable opacity partial ionisation regions other than helium, most high luminosity supergiants are somewhat variable, including the Alpha Cygni variables. In the specific region of more luminous stars above the instability strip are found the yellow hypergiants which have irregular pulsations and eruptions, the hotter luminous blue variables may be related and show similar short- and long-term spectral and brightness variations with irregular eruptions
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Cepheid variable
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A Cepheid variable is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. This robust characteristic of classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt after studying thousands of stars in the Magellanic Clouds. This discovery allows one to know the true luminosity of a Cepheid by simply observing its pulsation period and this in turn allows one to determine the distance to the star, by comparing its known luminosity to its observed brightness. The term Cepheid originates from Delta Cephei in the constellation Cepheus, identified by John Goodricke in 1784, Cepheid variables are divided into two subclasses which exhibit markedly different masses, ages, and evolutionary histories, classical Cepheids and type II Cepheids. Delta Scuti variables are A class stars on or near the main sequence at the end of the instability strip and were originally referred to as dwarf Cepheids. RR Lyrae variables have short periods and lie on the instability strip where it crosses the horizontal branch, Delta Scuti variables and RR Lyrae variables are not generally treated with Cepheid variables although their pulsations originate with the same helium ionisation kappa mechanism. Classical Cepheids undergo pulsations with very regular periods on the order of days to months, classical Cepheids are Population I variable stars which are 4–20 times more massive than the Sun, and up to 100,000 times more luminous. These Cepheids are yellow bright giants and supergiants of spectral class F6 – K2, classical Cepheids are used to determine distances to galaxies within the Local Group and beyond, and are a means by which the Hubble constant can be established. Classical Cepheids have also used to clarify many characteristics of our galaxy, such as the Suns height above the galactic plane. A group of classical Cepheids with small amplitudes and sinusoidal light curves are often separated out as Small Amplitude Cepheids or s-Cepheids, type II Cepheids are population II variable stars which pulsate with periods typically between 1 and 50 days. Type II Cepheids are typically metal-poor, old, low mass objects, type II Cepheids are divided into several subgroups by period. Stars with periods between 1 and 4 days are of the BL Her subclass, 10–20 days belong to the W Virginis subclass, type II Cepheids are used to establish the distance to the Galactic Center, globular clusters, and galaxies. A group of pulsating stars on the instability strip have periods of less than 2 days, similar to RR Lyrae variables, anomalous Cepheid variables have masses higher than type II Cepheids, RR Lyrae variables, and our sun. It is unclear whether they are stars on a turned-back horizontal branch, blue stragglers formed through mass transfer in binary systems. A small proportion of Cepheid variables have been observed to pulsate in two modes at the time, usually the fundamental and first overtone, occasionally the second overtone. A very small number pulsate in three modes, or a combination of modes including higher overtones. On September 10,1784, Edward Pigott detected the variability of Eta Aquilae, however, the eponymous star for classical Cepheids is Delta Cephei, discovered to be variable by John Goodricke a few months later. A relationship between the period and luminosity for classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt in an investigation of thousands of stars in the Magellanic Clouds
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. Bibcode:2004MmSAI..75..694E.
