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Der in die Jahre gekommene Rockstar Jackson Maine entdeckt die talentierte Sängerin Ally und verliebt sich in sie. Ally hätte ihren großen Traum von einer Musikkarriere fast aufgegeben. Jackson fördert sie und nimmt sie schließlich sogar mit auf. Der A*-Algorithmus („A Stern“ oder englisch „a star“, auch A*-Suche) gehört zur Klasse der informierten Suchalgorithmen. Er dient in der Informatik der. A Star Is Born ist ein Musikfilm von Bradley Cooper, der am August im Rahmen der Filmfestspiele von Venedig seine Weltpremiere feierte und am 3. A Star Is Born [dt./OV]. (4,)2h 16minX-Ray Der erfahrene Musiker Jackson Maine entdeckt - und verliebt sich in - die angeschlagene Künstlerin Ally​. von mehr als Ergebnissen oder Vorschlägen für "a star is born".

a star

A Star Is Born [dt./OV]. (4,)2h 16minX-Ray Der erfahrene Musiker Jackson Maine entdeckt - und verliebt sich in - die angeschlagene Künstlerin Ally​. siriussportsclub.se: Seit dem 7. Juni gibt es „A Star Is Born“ bei Netflix. Als Stream gibt es dort allerdings auch nur die reguläre Kinofassung. Seasoned musician Jackson Maine discovers—and falls in love with—struggling artist Ally. She has just about given up on her dream as a singer until Jack. Selbst wenn wir „Be a Star“ nicht gewinnen sollten, hofften wir, dass irgendwelche Talentsucher „Be a Star“ sehen und uns einen Plattenvertrag anbieten. Star. is. born. Bob Marley the Paradise Hound Am erblickte ich morgens um Uhr das Licht der Welt. Was für ein Ereignis für meine liebevollen. Seasoned musician Jackson Maine discovers—and falls in love with—struggling artist Ally. She has just about given up on her dream as a singer until Jack. 20 Userkritiken zum Film A Star Is Born von Bradley Cooper mit Lady Gaga, Bradley Cooper, Sam Elliott - siriussportsclub.se siriussportsclub.se: Seit dem 7. Juni gibt es „A Star Is Born“ bei Netflix. Als Stream gibt es dort allerdings auch nur die reguläre Kinofassung.

Gravitational microlensing has been used to measure the mass of a single star. The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time.

It has units of power. The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface.

The rapidly rotating star Vega , for example, has a higher energy flux power per unit area at its poles than along its equator. Patches of the star's surface with a lower temperature and luminosity than average are known as starspots.

Small, dwarf stars such as our Sun generally have essentially featureless disks with only small starspots.

Giant stars have much larger, more obvious starspots, [] and they also exhibit strong stellar limb darkening.

That is, the brightness decreases towards the edge of the stellar disk. The apparent brightness of a star is expressed in terms of its apparent magnitude.

It is a function of the star's luminosity, its distance from Earth, the extinction effect of interstellar dust and gas, and the altering of the star's light as it passes through Earth's atmosphere.

Intrinsic or absolute magnitude is directly related to a star's luminosity, and is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs Both the apparent and absolute magnitude scales are logarithmic units : one whole number difference in magnitude is equal to a brightness variation of about 2.

On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter the star.

The brightest stars, on either scale, have negative magnitude numbers. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus.

This is because Sirius is merely 8. This star is at least 5,, times more luminous than the Sun. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered.

These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth. The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line.

The classifications were since reordered by temperature, on which the modern scheme is based. Stars are given a single-letter classification according to their spectra, ranging from type O , which are very hot, to M , which are so cool that molecules may form in their atmospheres.

A variety of rare spectral types are given special classifications. The most common of these are types L and T , which classify the coldest low-mass stars and brown dwarfs.

Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures as classes O0 and O1 may not exist.

In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by their surface gravity.

Main sequence stars fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.

Additional nomenclature, in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum.

For example, an " e " can indicate the presence of emission lines; " m " represents unusually strong levels of metals, and " var " can mean variations in the spectral type.

White dwarf stars have their own class that begins with the letter D. This is followed by a numerical value that indicates the temperature.

Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.

During their stellar evolution, some stars pass through phases where they can become pulsating variables.

Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star.

This category includes Cepheid and Cepheid-like stars , and long-period variables such as Mira.

Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events. Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties.

This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.

Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.

The interior of a stable star is in a state of hydrostatic equilibrium : the forces on any small volume almost exactly counterbalance each other.

The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star.

The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core.

The temperature at the core of a main sequence or giant star is at least on the order of 10 7 K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.

As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core.

Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core.

Eventually the helium content becomes predominant, and energy production ceases at the core. Instead, for stars of more than 0.

In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium.

There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior.

The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below. The radiation zone is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, since convective heat transfer is inefficient in that zone.

In this region the plasma will not be perturbed, and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone.

This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity making radiatative heat transfer inefficient as in the outer envelope.

The occurrence of convection in the outer envelope of a main sequence star depends on the star's mass.

Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers.

Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers. The photosphere is that portion of a star that is visible to an observer.

This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate into space.

It is within the photosphere that sun spots , regions of lower than average temperature, appear. Above the level of the photosphere is the stellar atmosphere.

In a main sequence star such as the Sun, the lowest level of the atmosphere, just above the photosphere, is the thin chromosphere region, where spicules appear and stellar flares begin.

Beyond this is the corona , a volume of super-heated plasma that can extend outward to several million kilometres.

The corona region of the Sun is normally only visible during a solar eclipse. From the corona, a stellar wind of plasma particles expands outward from the star, until it interacts with the interstellar medium.

For the Sun, the influence of its solar wind extends throughout a bubble-shaped region called the heliosphere.

A variety of nuclear fusion reactions take place in the cores of stars, that depend upon their mass and composition.

When nuclei fuse, the mass of the fused product is less than the mass of the original parts. The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate.

In the Sun, with a million-kelvin core, hydrogen fuses to form helium in the proton—proton chain reaction : []. The energy released by this reaction is in millions of electron volts, which is actually only a tiny amount of energy.

However enormous numbers of these reactions occur constantly, producing all the energy necessary to sustain the star's radiation output.

In comparison, the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5. In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon called the carbon-nitrogen-oxygen cycle.

In massive stars, heavier elements can also be burned in a contracting core through the neon-burning process and oxygen-burning process.

The final stage in the stellar nucleosynthesis process is the silicon-burning process that results in the production of the stable isotope iron As an O-class main sequence star, it would be 8 times the solar radius and 62, times the Sun's luminosity.

From Wikipedia, the free encyclopedia. Astronomical spheroid of plasma. This article is about the astronomical object. For other uses, see Star disambiguation.

Main articles: Stellar designation , Astronomical naming conventions , and Star catalogue. Main article: Stellar evolution.

Main article: Star formation. Main article: Main sequence. Main articles: Supergiant star , Hypergiant , and Wolf—Rayet star. Main article: Stellar age estimation.

See also: Metallicity and Molecules in stars. Main articles: List of largest stars , List of least voluminous stars , and Solar radius.

Main article: Stellar kinematics. Main article: Stellar magnetic field. Main article: Stellar mass.

Main article: Stellar rotation. Main articles: Apparent magnitude and Absolute magnitude. Main article: Stellar classification.

Main article: Variable star. Main article: Stellar structure. Main article: Stellar nucleosynthesis.

Stars portal Astronomy portal. Fusor astronomy Exoplanet host stars Lists of stars List of largest known stars Outline of astronomy Sidereal time Star clocks Star count Stars and planetary systems in fiction Stellar astronomy Stellar dynamics.

European Space Agency. Retrieved 21 September The Fundamentals of Modern Astrophysics. June 29, Nobel Foundation. Retrieved Rochester Institute of Technology.

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Bibcode : PASP Mayall, N. Part II. The Astronomical Aspects". The Observatory. Bibcode : Obs Messier's nebulae and star clusters.

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Ashgate Publishing, Ltd. Retrieved 22 May Scientia Astrophysical Organization. This is essential to guarantee that the path returned is optimal if the heuristic function is admissible but not consistent.

In this example, edges are railroads and h x is the great-circle distance the shortest possible distance on a sphere to the target.

The algorithm is searching for a path between Washington, D. The first detail to note is that the way the priority queue handles ties can have a significant effect on performance in some situations.

When a path is required at the end of the search, it is common to keep with each node a reference to that node's parent.

At the end of the search these references can be used to recover the optimal path. If these references are being kept then it can be important that the same node doesn't appear in the priority queue more than once each entry corresponding to a different path to the node, and each with a different cost.

A standard approach here is to check if a node about to be added already appears in the priority queue. If it does, then the priority and parent pointers are changed to correspond to the lower cost path.

A standard binary heap based priority queue does not directly support the operation of searching for one of its elements, but it can be augmented with a hash table that maps elements to their position in the heap, allowing this decrease-priority operation to be performed in logarithmic time.

Alternatively, a Fibonacci heap can perform the same decrease-priority operations in constant amortized time. Every time we process a node we assign C to all of its newly discovered neighbors.

After each single assignment, we decrease the counter C by one. A search algorithm is said to be admissible if it is guaranteed to return an optimal solution.

To compute approximate shortest paths, it is possible to speed up the search at the expense of optimality by relaxing the admissibility criterion.

In the worst case of an unbounded search space, the number of nodes expanded is exponential in the depth of the solution the shortest path d : O b d , where b is the branching factor the average number of successors per state.

The time complexity is polynomial when the search space is a tree, there is a single goal state, and the heuristic function h meets the following condition:.

Special care needs to be taken for the stopping criterion. From Wikipedia, the free encyclopedia. Algorithm used for pathfinding and graph traversal.

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Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass , age, metallicity chemical composition , and many other properties of a star by observing its motion through space, its luminosity , and spectrum respectively.

The total mass of a star is the main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement.

A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung—Russell diagram H—R diagram.

Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined. A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements.

When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process.

The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.

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.

Binary and multi-star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits.

When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.

Historically, stars have been important to civilizations throughout the world. They have been part of religious practices and used for celestial navigation and orientation.

Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.

The oldest accurately dated star chart was the result of ancient Egyptian astronomy in BC. The first star catalogue in Greek astronomy was created by Aristillus in approximately BC, with the help of Timocharis.

In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear. Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars.

They built the first large observatory research institutes, mainly for the purpose of producing Zij star catalogues.

Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky Way galaxy as a multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in In , Giordano Bruno suggested that the stars were like the Sun, and may have other planets , possibly even Earth-like, in orbit around them, [25] an idea that had been suggested earlier by the ancient Greek philosophers , Democritus and Epicurus , [26] and by medieval Islamic cosmologists [27] such as Fakhr al-Din al-Razi.

To explain why these stars exerted no net gravitational pull on the Solar System, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.

The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancient Greek astronomers Ptolemy and Hipparchus.

William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the s, he established a series of gauges in directions and counted the stars observed along each line of sight.

From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core.

His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction. The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi.

By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines —the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies.

In , Secchi began classifying stars into spectral types. Cannon during the s. The first direct measurement of the distance to a star 61 Cygni at Parallax measurements demonstrated the vast separation of the stars in the heavens.

In , Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion. Edward Pickering discovered the first spectroscopic binary in when he observed the periodic splitting of the spectral lines of the star Mizar in a day period.

Detailed observations of many binary star systems were collected by astronomers such as Friedrich Georg Wilhelm von Struve and S.

Burnham , allowing the masses of stars to be determined from computation of orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in The photograph became a valuable astronomical tool.

Karl Schwarzschild discovered that the color of a star and, hence, its temperature, could be determined by comparing the visual magnitude against the photographic magnitude.

The development of the photoelectric photometer allowed precise measurements of magnitude at multiple wavelength intervals. In Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory.

Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century.

In , the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars.

Successful models were developed to explain the interiors of stars and stellar evolution. Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her PhD thesis.

This allowed the chemical composition of the stellar atmosphere to be determined. With the exception of supernovae, individual stars have primarily been observed in the Local Group , [36] and especially in the visible part of the Milky Way as demonstrated by the detailed star catalogues available for our galaxy.

However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed.

The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located at a distance of one billion light years [40] —ten times further than the most distant star cluster previously observed.

In April, , astronomers reported the detection of the most distant "ordinary" i. In May , astronomers reported the detection of the most distant oxygen ever detected in the Universe—and the most distant galaxy ever observed by Atacama Large Millimeter Array or the Very Large Telescope —with the team inferring that the signal was emitted The concept of a constellation was known to exist during the Babylonian period.

Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths.

Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology. As well as certain constellations and the Sun itself, individual stars have their own myths.

Their names were assigned by later astronomers. Circa , the names of the constellations were used to name the stars in the corresponding regions of the sky.

The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation.

Later a numbering system based on the star's right ascension was invented and added to John Flamsteed 's star catalogue in his book "Historia coelestis Britannica" the edition , whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.

The only internationally recognized authority for naming celestial bodies is the International Astronomical Union IAU.

A number of private companies sell names of stars, which the British Library calls an unregulated commercial enterprise.

This now-discontinued ISR practice was informally labeled a scam and a fraud, [54] [55] [56] [57] and the New York City Department of Consumer and Worker Protection issued a violation against ISR for engaging in a deceptive trade practice.

Although stellar parameters can be expressed in SI units or CGS units , it is often most convenient to express mass , luminosity , and radii in solar units, based on the characteristics of the Sun.

In , the IAU defined a set of nominal solar values defined as SI constants, without uncertainties which can be used for quoting stellar parameters:.

However, one can combine the nominal solar mass parameter with the most recent CODATA estimate of the Newtonian gravitational constant G to derive the solar mass to be approximately 1.

Although the exact values for the luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties, the IAU nominal constants will remain the same SI values as they remain useful measures for quoting stellar parameters.

In , the IAU defined the astronomical constant to be an exact length in meters: ,,, m. Stars condense from regions of space of higher matter density, yet those regions are less dense than within a vacuum chamber.

These regions—known as molecular clouds —consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements.

One example of such a star-forming region is the Orion Nebula. Such feedback effects, from star formation, may ultimately disrupt the cloud and prevent further star formation.

All stars spend the majority of their existence as main sequence stars , fueled primarily by the nuclear fusion of hydrogen into helium within their cores.

However, stars of different masses have markedly different properties at various stages of their development.

The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosities and the impact they have on their environment.

Accordingly, astronomers often group stars by their mass: [63]. The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density—often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the collision of galaxies as in a starburst galaxy.

As the cloud collapses, individual conglomerations of dense dust and gas form " Bok globules ". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises.

When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium , a protostar forms at the core.

These newly formed stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig—Haro objects.

Early in their development, T Tauri stars follow the Hayashi track —they contract and decrease in luminosity while remaining at roughly the same temperature.

Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track.

Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed.

The fragmentation of the cloud into multiple stars distributes some of that angular momentum. The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters.

These interactions tend to split apart more widely separated soft binaries while causing hard binaries to become more tightly bound.

This produces the separation of binaries into their two observed populations distributions. Such stars are said to be on the main sequence , and are called dwarf stars.

Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.

Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible.

The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it. Massive stars consume their fuel very rapidly and are short-lived.

Low mass stars consume their fuel very slowly. Stars less massive than 0. The combination of their slow fuel-consumption and relatively large usable fuel supply allows low mass stars to last about one trillion 10 12 years; the most extreme of 0.

Red dwarfs become hotter and more luminous as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and decline in temperature.

Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call the chemical concentration of these elements in a star, its metallicity.

A star's metallicity can influence the time the star takes to burn its fuel, and controls the formation of its magnetic fields, [79] which affects the strength of its stellar wind.

Over time, such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres. As stars of at least 0.

Their outer layers expand and cool greatly as they form a red giant. As the hydrogen shell burning produces more helium, the core increases in mass and temperature.

In a red giant of up to 2. Finally, when the temperature increases sufficiently, helium fusion begins explosively in what is called a helium flash , and the star rapidly shrinks in radius, increases its surface temperature, and moves to the horizontal branch of the HR diagram.

For more massive stars, helium core fusion starts before the core becomes degenerate, and the star spends some time in the red clump , slowly burning helium, before the outer convective envelope collapses and the star then moves to the horizontal branch.

After the star has fused the helium of its core, the carbon product fuses producing a hot core with an outer shell of fusing helium. The star then follows an evolutionary path called the asymptotic giant branch AGB that parallels the other described red giant phase, but with a higher luminosity.

The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate. During their helium-burning phase, a star of more than 9 solar masses expands to form first a blue and then a red supergiant.

Particularly massive stars may evolve to a Wolf-Rayet star , characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss.

When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rises enough to fuse carbon see Carbon-burning process.

This process continues, with the successive stages being fueled by neon see neon-burning process , oxygen see oxygen-burning process , and silicon see silicon-burning process.

Near the end of the star's life, fusion continues along a series of onion-layer shells within a massive star.

Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.

The final stage occurs when a massive star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy.

As a star's core shrinks, the intensity of radiation from that surface increases, creating such radiation pressure on the outer shell of gas that it will push those layers away, forming a planetary nebula.

If what remains after the outer atmosphere has been shed is less than roughly 1. White dwarfs lack the mass for further gravitational compression to take place.

Eventually, white dwarfs fade into black dwarfs over a very long period of time. In massive stars, fusion continues until the iron core has grown so large more than 1.

This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos, and gamma rays in a burst of electron capture and inverse beta decay.

The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become so bright that they may briefly outshine the star's entire home galaxy.

When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before.

A supernova explosion blows away the star's outer layers, leaving a remnant such as the Crab Nebula. Within a black hole, the matter is in a state that is not currently understood.

The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars.

These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.

The post—main-sequence evolution of binary stars may be significantly different from the evolution of single stars of the same mass.

If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its Roche lobe , the region around a star where material is gravitationally bound to that star, leading to transfer of material to the other.

When the Roche lobe is violated, a variety of phenomena can result, including contact binaries , common-envelope binaries, cataclysmic variables , and type Ia supernovae.

Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust.

A multi-star system consists of two or more gravitationally bound stars that orbit each other. The simplest and most common multi-star system is a binary star, but systems of three or more stars are also found.

For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of binary stars.

These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars.

Such systems orbit their host galaxy. It has been a long-held assumption that the majority of stars occur in gravitationally bound, multiple-star systems.

The nearest star to the Earth, apart from the Sun, is Proxima Centauri , which is Travelling at the orbital speed of the Space Shuttle 8 kilometres per second—almost 30, kilometres per hour , it would take about , years to arrive.

Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common.

These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity of the cluster to which it belongs.

Almost everything about a star is determined by its initial mass, including such characteristics as luminosity, size, evolution, lifespan, and its eventual fate.

Some stars may even be close to The oldest star yet discovered, HD , nicknamed Methuselah star, is an estimated The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly.

The most massive stars last an average of a few million years, while stars of minimum mass red dwarfs burn their fuel very slowly and can last tens to hundreds of billions of years.

Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure.

The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system. Due to their great distance from the Earth, all stars except the Sun appear to the unaided eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere.

The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight.

Other than the Sun, the star with the largest apparent size is R Doradus , with an angular diameter of only 0. The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required to produce images of these objects.

Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon or the rise in brightness when it reappears , the star's angular diameter can be computed.

The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy.

The components of motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its proper motion.

The proper motion of a star, its parallax , is determined by precise astrometric measurements in units of milli- arc seconds mas per year.

With knowledge of the star's parallax and its distance, the proper motion velocity can be calculated. Together with the radial velocity, the total velocity can be calculated.

Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.

When both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed.

Among nearby stars, it has been found that younger population I stars have generally lower velocities than older, population II stars.

The latter have elliptical orbits that are inclined to the plane of the galaxy. The magnetic field of a star is generated within regions of the interior where convective circulation occurs.

This movement of conductive plasma functions like a dynamo , wherein the movement of electrical charges induce magnetic fields, as does a mechanical dynamo.

Those magnetic fields have a great range that extend throughout and beyond the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation.

This surface activity produces starspots , which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic field flux lines that rise from a star's surface into the star's outer atmosphere, its corona.

The coronal loops can be seen due to the plasma they conduct along their length. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.

Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation with time.

Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity.

The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time. This generation of supermassive population III stars is likely to have existed in the very early universe i.

The combination of the radius and the mass of a star determines its surface gravity. Giant stars have a much lower surface gravity than do main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs.

The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.

The rotation rate of stars can be determined through spectroscopic measurement , or more exactly determined by tracking their starspots.

Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum —the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin.

A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind. The pulsar at the heart of the Crab nebula , for example, rotates 30 times per second.

The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's color index.

Note that the effective temperature is only a representative of the surface, as the temperature increases toward the core.

The stellar temperature will determine the rate of ionization of various elements, resulting in characteristic absorption lines in the spectrum.

The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star see classification below.

Smaller stars such as the Sun have surface temperatures of a few thousand K. The energy produced by stars, a product of nuclear fusion, radiates to space as both electromagnetic radiation and particle radiation.

The particle radiation emitted by a star is manifested as the stellar wind, [] which streams from the outer layers as electrically charged protons and alpha and beta particles.

Although almost massless, there also exists a steady stream of neutrinos emanating from the star's core.

The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion product.

This energy is converted to other forms of electromagnetic energy of lower frequency, such as visible light, by the time it reaches the star's outer layers.

The color of a star, as determined by the most intense frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere.

In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum , from the longest wavelengths of radio waves through infrared , visible light, ultraviolet , to the shortest of X-rays , and gamma rays.

From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics.

Using the stellar spectrum , astronomers can also determine the surface temperature, surface gravity , metallicity and rotational velocity of a star.

If the distance of the star is found, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models.

Mass can be calculated for stars in binary systems by measuring their orbital velocities and distances. Gravitational microlensing has been used to measure the mass of a single star.

The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time. It has units of power.

The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface.

The rapidly rotating star Vega , for example, has a higher energy flux power per unit area at its poles than along its equator.

Patches of the star's surface with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as our Sun generally have essentially featureless disks with only small starspots.

Giant stars have much larger, more obvious starspots, [] and they also exhibit strong stellar limb darkening.

That is, the brightness decreases towards the edge of the stellar disk. The apparent brightness of a star is expressed in terms of its apparent magnitude.

It is a function of the star's luminosity, its distance from Earth, the extinction effect of interstellar dust and gas, and the altering of the star's light as it passes through Earth's atmosphere.

Intrinsic or absolute magnitude is directly related to a star's luminosity, and is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs Both the apparent and absolute magnitude scales are logarithmic units : one whole number difference in magnitude is equal to a brightness variation of about 2.

On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter the star.

The brightest stars, on either scale, have negative magnitude numbers. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus.

This is because Sirius is merely 8. This star is at least 5,, times more luminous than the Sun. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered.

These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth. The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line.

The classifications were since reordered by temperature, on which the modern scheme is based. Stars are given a single-letter classification according to their spectra, ranging from type O , which are very hot, to M , which are so cool that molecules may form in their atmospheres.

A variety of rare spectral types are given special classifications. The most common of these are types L and T , which classify the coldest low-mass stars and brown dwarfs.

Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures as classes O0 and O1 may not exist.

In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by their surface gravity.

Main sequence stars fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.

Additional nomenclature, in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum.

For example, an " e " can indicate the presence of emission lines; " m " represents unusually strong levels of metals, and " var " can mean variations in the spectral type.

White dwarf stars have their own class that begins with the letter D. This is followed by a numerical value that indicates the temperature.

Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.

During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star.

This category includes Cepheid and Cepheid-like stars , and long-period variables such as Mira. Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.

Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties.

This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.

Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.

The interior of a stable star is in a state of hydrostatic equilibrium : the forces on any small volume almost exactly counterbalance each other.

The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star.

The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core.

The temperature at the core of a main sequence or giant star is at least on the order of 10 7 K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.

As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core.

Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core.

Eventually the helium content becomes predominant, and energy production ceases at the core. Instead, for stars of more than 0.

In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium.

There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior.

The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below. The radiation zone is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, since convective heat transfer is inefficient in that zone.

In this region the plasma will not be perturbed, and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone.

This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity making radiatative heat transfer inefficient as in the outer envelope.

After each single assignment, we decrease the counter C by one. A search algorithm is said to be admissible if it is guaranteed to return an optimal solution.

To compute approximate shortest paths, it is possible to speed up the search at the expense of optimality by relaxing the admissibility criterion.

In the worst case of an unbounded search space, the number of nodes expanded is exponential in the depth of the solution the shortest path d : O b d , where b is the branching factor the average number of successors per state.

The time complexity is polynomial when the search space is a tree, there is a single goal state, and the heuristic function h meets the following condition:.

Special care needs to be taken for the stopping criterion. From Wikipedia, the free encyclopedia. Algorithm used for pathfinding and graph traversal.

Record it! See also: Dijkstra's algorithm. Artificial intelligence a modern approach. Norvig, Peter 4th ed.

Boston: Pearson. Lecture Notes in Computer Science. International Journal of Geographical Information Science. Cambridge: Cambridge University Press.

This excludes, for example, algorithms that search backward from the goal or in both directions simultaneously. Journal of the ACM.

Artificial Intelligence. Machine Intelligence. California, USA. Vienna, Austria. Kim Karlsruhe, Germany.

Archived from the original PDF on Archived from the original on Retrieved Artificial Intelligence: A Modern Approach 2nd ed.

Prentice Hall. Artificial Intelligence: A Modern Approach 3rd ed. IIE Transactions, , JAIR 28 : Thrun, L. Saul, and B.

MIT Press.

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