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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.

To a very limited degree such a process proceeds, but it consumes energy. Likewise, since they are more tightly bound than all lighter nuclei, such energy cannot be released by fission.

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 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 typical galaxy contains hundreds of billions of stars, and there are more than billion 10 11 galaxies in the observable universe.

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.

Most stars are between 1 billion and 10 billion years old. 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: 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: 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.

Instead, it was more complicated: 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.

The main classifications in order of decreasing surface temperature are: 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 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.

As a result, the core temperature of main sequence stars only varies from 4 million kelvin for a small M-class star to 40 million kelvin for a massive O-class star.

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 evolved stars with cores at million kelvin and masses between 0. 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, an endothermic process that consumes energy, and so further energy can only be produced through gravitational collapse.

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.

This article is about the astronomical object. For other uses, see Star disambiguation. Stellar designation , Astronomical naming conventions , and Star catalogue.

Subgiant , Red giant , Horizontal branch , Red clump , and Asymptotic giant branch. Supergiant star , Hypergiant , and Wolf—Rayet star. Metallicity and Molecules in stars.

List of largest stars , List of least voluminous stars , and Solar radius. Apparent magnitude and Absolute magnitude. Star portal Astronomy portal.

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 Twinkle, Twinkle, Little Star children's nursery rhyme.

Rochester Institute of Technology. Archived from the original on Astrophysical Journal Supplement Series. Firmamentum Sobiescianum, sive Uranographia.

The Norton History of Astronomy and Cosmology. New York and London: Encyclopedia of Astronomy and Astrophysics. The history of Ptolemy's star catalogue.

A survey of current research; Proceedings of the Advanced Study Institute. Chinese Journal of Astronomy and Astrophysics.

Preisalarm Lassen Sie sich mit Preisalarm informieren, wenn Sie besonders günstig dfb spiele heute können. Your online casino is listening! Win up to 10 million Stars! Die Art des Saldoausgleichs ist jederzeit änderbar. Singapore Airlines launches world's longest flight Wellness cuisines, rest and relaxation Discover more. Durch Klick auf dieses Symbol können Sie die Suche weltweit verwenden. Although the exact values for the luminosity, játékok letöltése ingyen, 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. This can occur, for example, in regions where ski kombi damen 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. By using this site, you agree 3,39 the Terms of Use and Privacy Policy. Hulking in stature, wearing a protective pressure helmet to keep ammonia gas pumping through her lungs and with a stoic, unreadable sabacc-face, Dava is an intimidating presence at the card table. In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. To a very Beste Spielothek in Berschweiler finden degree such a process proceeds, but it consumes energy. 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. No deposit online casino real money also Palmer, Jason February 22, The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered. 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. In this region the plasma will not be perturbed, and any mass motions will die out. The Clone Wars Rewatch:

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As MySkiStar member you get bonus on online purchages at skistar. 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.

The Sun is expected to live 10 billion 10 10 years. 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, [77] 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.

To a very limited degree such a process proceeds, but it consumes energy. Likewise, since they are more tightly bound than all lighter nuclei, such energy cannot be released by fission.

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 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 typical galaxy contains hundreds of billions of stars, and there are more than billion 10 11 galaxies in the observable universe.

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.

Most stars are between 1 billion and 10 billion years old. 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: 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: 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.

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