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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.

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.

NASA Observatorium. Archived from the original on Astrophysical Journal Supplement Series. Bibcode : ApJS History of Astronomy. Firmamentum Sobiescianum, sive Uranographia.

Bibcode : Cent The Norton History of Astronomy and Cosmology. New York and London: W. Encyclopedia of Astronomy and Astrophysics. Bibcode : eaa..

The history of Ptolemy's star catalogue. Cambridge, UK: Dordrecht, D. Reidel Publishing Co. Bibcode : ASIC Chinese Journal of Astronomy and Astrophysics.

Bibcode : ChJAA NAOA News. March 5, University of Arizona. April Part I. The Ancient Oriental Chronicles". Publications of the Astronomical Society of the Pacific.

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

Cambridge University Press. Hasanuddin University. Stanford Encyclopedia of Philosophy. August 17, Vistas in Astronomy. Bibcode : VA Bibcode : ASPC..

Bibcode : Natur Fairfield University. The Binary Stars. New York: Dover Publications Inc. Astrophysical Journal. Bibcode : ApJ University of California.

The New Cosmos 5th ed. New York: Springer. The Outer Disk of M31". The Astronomical Journal. Bibcode : AJ December 8, October 26, Hubble Site.

May 25, UBC Public Affairs. January 8, Retrieved 28 February Bibcode : NatAs Retrieved 2 April Bibcode : Natur. Mesopotamian astrology: an introduction to Babylonian and Assyrian celestial divination.

Carsten Niebuhr Institute Publications. Museum Tusculanum Press. Frosty Drew Observatory. Space Law: A Treatise. Ashgate Publishing, Ltd. Retrieved 22 May Scientia Astrophysical Organization.

British Library. The British Library Board. International Astronomical Union. Skeptical Inquirer. The Straight Dope. Times Magazine. Time Inc.

Bad astronomy: misconceptions and misuses revealed, from astrology to the moon landing "hoax". John Wiley and Sons. Archived from the original on January 11, Astronomical Journal.

Annual Review of Astronomy and Astrophysics. The origin and evolution of planetary nebulae. Cambridge astrophysics series.

Bibcode : RMxAC.. Astrophysical Journal, Part 1. Monthly Notices of the Royal Astronomical Society. The Origin of Stars.

Imperial College Press. In Benvenuti, Piero; Macchetto, F. Proceedings of a workshop held in Paris, France, December 4—8, Space Telescope Science Institute.

Bibcode : swhs. The origin of stars. II — Distribution of the orbital elements in an unbiased sample". Present and Future".

The Astrophysical Journal. Astronomy and Astrophysics. Royal Greenwich Observatory. Swinburne Astronomy Online Encyclopedia of Astronomy.

Swinburne University of Technology. UCL Astrophysics Group. June 18, See also Palmer, Jason February 22, Penn Stats College of Science. Goddard Space Flight Center.

April 6, Classical and Quantum Gravity. Bibcode : CQGra.. The New York Times. Retrieved 17 October Centre for Astrophysics and Supercomputing.

Retrieved 28 January CBS News. Hubble News Desk. January 14, Harvard-Smithsonian Center for Astrophysics. January 30, CNN News. June 2, Bond; E.

Nelan; D. VandenBerg; G. Schaefer; D. Harmer The Astrophysical Journal Letters. Cosmological parameters See Table 4 on page 31 of pfd ".

July 13, Is the technique really accurate enough to use it to verify the age of the universe? Scientific American.

July See Table 1. Astrophysics: Decoding the Cosmos. Bibcode : adc.. April 17, The Observation and Analysis of Stellar Photospheres.

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Bibcode : JApAS.. Harper; et al. September Astronomy Reports. Bibcode : ARep September 10, American Scientist. Bibcode : AmSci..

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Bibcode : LRSP Mercury Magazine. Bibcode : Mercu.. February 11, September 22, Retrieved 17 June Retrieved 14 December See also.

What's in the sky? Planetarium Sky rotation diagram Star atlas Orrery. The In-The-Sky. Begin typing the name of a town near to you, and then select the town from the list of options which appear below.

Time slider. Date slider. Limiting magnitude Bright Intermediate Faint Showing stars down to mag —.

The sky above Melun. Age of Moon — Sidereal Time —. Rey's designs. Terms of use. Search for object Search for object. Melun Latitude: Longitude: Timezone:

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Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. For most of its active life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space.

Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, and for some stars by supernova nucleosynthesis when it explodes.

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.

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Location: Melun Charts of the Night Sky. See also. What's in the sky? It will be descending sunward, and will disappear into the sun's glow well before it reaches superior conjunction on Aug.

Mercury will re-appear low in the western sky after sunset for the final week of the month. This time, the planet will show a waning, nearly fully-illuminated phase, and a disk size of approximately 5 arc-seconds.

During August, Venus will shine very brightly in the eastern pre-dawn sky. For the first half of the month, it will move prograde east through the stars of eastern Taurus and then through northern Orion.

On Aug. The planet will diminish slightly in visual brightness during August. Viewed in a telescope, Venus will exhibit a half-illuminated phase, and its apparent disk diameter will shrink from 27 to 20 arc-seconds.

During August, Mars will shine prominently among the modest stars of Pisces in the late evening and overnight sky as the Earth continues to overtake the reddish planet.

Visually, Mars will nearly double in brightness during August — from magnitude Meanwhile, its apparent disk size will grow from On Saturday night, Aug.

During August, Jupiter will already be shining low in the southeast when the evening sky begins to darken. Recently past opposition, the planet will be a fine observing target all night long as it moves retrograde westward through the stars of northeastern Sagittarius — and only 8 degrees to the west of dimmer Saturn.

During August, Jupiter will decrease slightly in brightness from magnitude The tableau will repeat on Aug. Commencing at a.

After its recent opposition, Saturn will be well-positioned for observing all night during August while it moves retrograde westward through the stars of northeastern Sagittarius.

The planet will also remain just 8 degrees to the east of Jupiter, which will outshine Saturn by a factor of 10 — delaying the dimmer planet's appearance, low the southeastern sky, until well after sunset.

The rings, and many of Saturn's moons, are easily visible in backyard telescopes. During August, Saturn will diminish slightly in brightness and apparent size.

As August begins, blue-green Uranus magnitude 5. The slow-moving planet can be found by looking 10 degrees south of Aries' brightest star Hamal.

On the night of Aug. During August, Blue-tinted Neptune magnitude 7. Asterism: A noteworthy or striking pattern of stars within a larger constellation.

Degrees measuring the sky : The sky is degrees all the way around, which means roughly degrees from horizon to horizon. It's easy to measure distances between objects: Your fist on an outstretched arm covers about 10 degrees of sky, while a finger covers about one degree.

Visual Magnitude: This is the astronomer's scale for measuring the brightness of objects in the sky. The dimmest object visible in the night sky under perfectly dark conditions is about magnitude 6.

Brighter stars are magnitude 2 or 1. The brightest objects get negative numbers. Venus can be as bright as magnitude minus 4.

The full moon is minus Adjust to the dark: If you wish to observe faint objects, such as meteors or dim stars, give your eyes at least 15 minutes to adjust to the darkness.

Light Pollution: Even from a big city, one can see the moon, a handful of bright stars and sometimes the brightest planets.

But to fully enjoy the heavens — especially a meteor shower, the constellations, or to see the amazing swath across the sky that represents our view toward the center of the Milky Way Galaxy — rural areas are best for night sky viewing.

If you're stuck in a city or suburban area, a building can be used to block ambient light or moonlight to help reveal fainter objects.

If you're in the suburbs, simply turning off outdoor lights can help. Prepare for skywatching: If you plan to be out for more than a few minutes, and it's not a warm summer evening, dress warmer than you think necessary.

An hour of observing a winter meteor shower can chill you to the bone. A blanket or lounge chair will prove much more comfortable than standing or sitting in a chair and craning your neck to see overhead.

Daytime skywatching: When Venus is visible that is, not in front of or behind the sun it can often be spotted during the day. But you'll need to know where to look.

A sky map is helpful. When the sun has large sunspots, they can be seen without a telescope. However, it's unsafe to look at the sun without protective eyewear.

See our video on how to safely observe the sun, or our safe sunwatching infographic. Join our Space Forums to keep talking space on the latest missions, night sky and more!

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The night sky is more than just the moon and stars, if you know when and where to look.

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