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22 septembre 2013 7 22 /09 /septembre /2013 08:36

The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun.[93] By contrast, the super-metal-rich star μ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron.[94] Lenovo 04W0872 laptop keyboard

There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements.

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. ACER Aspire 7745G Laptop Keyboard

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 0.057 arcseconds.[96]

The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, DELL NSK-DD101 Laptop Keyboard

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.[97] SONY VAIO VGN-C2S Series Laptop Keyboard

Stars range in size from neutron stars, which vary anywhere from 20 to 40 km (25 mi) in diameter, to supergiants likeBetelgeuse in the Orion constellation, which has a diameter approximately 650 times that of the Sun—about 900,000,000 km (560,000,000 mi). Betelgeuse, however, has a much lower density than the Sun. HP Pavilion dv6-2112sa Laptop Keyboard

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.  Lenovo 0A62075 Laptop Keyboard

Radial velocity is measured by the doppler shift of the star's spectral lines, and is given in units of km/s. The proper motion of a star is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. By determining the parallax of a star, the proper motion can then be converted into units of velocity. HP Pavilion G7-1081NR Laptop Keyboard

Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.[100]

Once both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. SONY VAIO VGN-FS742/W Laptop Keyboard

Among nearby stars, it has been found that 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.[101]Comparison of the kinematics of nearby stars has also led to the identification of stellar associations. ASUS X53S Laptop Keyboard

These are most likely groups of stars that share a common point of origin in giant molecular clouds.

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, generating magnetic fields that extend throughout the star. HP 605344-001 Laptop Keyboard

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. FUJITSU CP270342-02 Laptop Keyboard

 Coronal loops are arching magnetic fields that reach out into the corona from active regions.Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.[103]

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 as the star grows older. GATEWAY NV-54 Laptop Keyboard

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.[104]During the Maunder minimum, for example, the Sun underwent a 70-year period with almost no sunspot activity. HP G42-362LA Laptop Keyboard

One of the most massive stars known is Eta Carinae,[105] which, with 100–150 times as much mass as the Sun, will have a lifespan of only several million years. A study of the Arches cluster suggests that 150 solar masses is the upper limit for stars in the current era of the universe.[106] The reason for this limit is not precisely known,  SONY KFRSBA019A Laptop Keyboard

but it is partially due to the Eddington luminosity which defines the maximum amount of luminosity that can pass through the atmosphere of a star without ejecting the gases into space. However, a star named R136a1 in the RMC 136a star cluster has been measured at 265 solar masses, which puts this limit into question.[107] Lenovo 63Y0047 Laptop Keyboard

A study determined that stars larger than 150 solar masses in R136 were created through the collision and merger of massive stars in close binary systems, providing a way to sidestep the 150 solar mass limit.

The first stars to form after the Big Bang may have been larger, up to 300 solar masses or more,[109] SONY KFRMBA152B Laptop Keyboard

due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive, population III stars is long extinct, however, and currently only theoretical.

With a mass only 93 times that of Jupiter, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core.[110] Compaq Presario CQ42-228LA Laptop Keyboard

For stars with similar metallicity to the Sun, the theoretical minimum mass the star can have, and still undergo fusion at the core, is estimated to be about 75 times the mass of Jupiter.[111][112]When the metallicity is very low, however, a recent study of the faintest stars found that the minimum star size seems to be about 8.3% of the solar mass, or about 87 times the mass of Jupiter.[112][113] ACER Aspire 7745G Laptop Keyboard

Smaller bodies are called brown dwarfs, which occupy a poorly defined grey area between stars and gas giants.

The combination of the radius and the mass of a star determines the surface gravity. Giant stars have a much lower surface gravity than main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs.  HP Probook 4515S Laptop Keyboard

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 the rotation rate of starspots. SONY VAIO VGN-N31S/W Laptop Keyboard

Young stars can have a rapid rate of rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial rotation velocity of about 225 km/s or greater, giving it an equatorial diameter that is more than 50% larger than the distance between the poles. This rate of rotation is just below the critical velocity of 300 km/s where the star would break apart.[114] SONY VAIO VGN-CS14G/B Laptop Keyboard

By contrast, the Sun only rotates once every 25 – 35 days, with an equatorial velocity of 1.994 km/s. The star's magnetic field and the stellar wind serve to slow down a main sequence star's rate of rotation by a significant amount as it evolves on the main sequence.[115]

Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. ASUS F3Sv laptop keyboard

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.[116]

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 In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second.[117] The rotation rate of the pulsar will gradually slow due to the emission of radiation.

The surface temperature of a main sequence star is determined by the rate of energy production at the core and its radius, and is often estimated from the star's color index.[118]  HP Mini 210-1124TU laptop keyboard

 It is normally given as the effective temperature, which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. Note that the effective temperature is only a representative value, as the temperature increases toward the core.[119] The temperature in the core region of a star is several million kelvins.[120] SONY VAIO VGN-NW23GF laptop keyboard

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).[33] SONY VAIO VGN-NW320F/T laptop keyboard

Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3,600 K; but they also have a high luminosity due to their large exterior surface area. SONY VAIO VPC-F12 laptop keyboard

The energy produced by stars, as a product of nuclear fusion, radiates into space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind,[122] which streams from the outer layers as free protons, and electrically charged alpha, and beta particles. SONY VAIO PCG-FR55E/B laptop keyboard

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 nucleusof a new heavier element, gamma ray photons are released from the nuclear fusion product.  HP G62-b21SL laptop keyboard

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 itsphotosphere.[123]  COMPAQ Presario V6405CA laptop keyboard

Besides visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. 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. HP Pavilion G7-1077NR laptop keyboard

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.  FUJITSU Lifebook S7111 laptop keyboard

If the distance of the star is known, 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. SONY VAIO VGN-CR410E laptop keyboard

Gravitational microlensing has been used to measure the mass of a single star.[124]) With these parameters, astronomers can also estimate the age of the 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. HP Mini 110-3538tu laptop keyboard

The luminosity of a star is determined by the radius and the surface temperature. However, many stars do not radiate a uniform flux (the amount of energy radiated per unit area) across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux at its poles than along its equator.[126] ASUS X85S laptop keyboard

Surface patches 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. Larger, giant stars have much larger, more obvious starspots,[127] and they also exhibit strong stellar limb darkening. DELL PK1303Q0100 Laptop Keyboard

That is, the brightness decreases towards the edge of the stellar disk.[128] Red dwarf flare stars such as UV Ceti may also possess prominent starspot features.

The apparent brightness of a star is expressed in terms of its apparent magnitude, which is the brightness of a star and is a function of the star's luminosity, distance from Earth, and the altering of the star's light as it passes through Earth's atmosphere.  ASUS X53S Laptop Keyboard

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 (32.6 light-years).

Both the apparent and absolute magnitude scales are logarithmic units: SAMSUNG R522 Laptop Keyboard

one whole number difference in magnitude is equal to a brightness variation of about 2.5 times[131] (the 5th root of 100 or approximately 2.512). This means that a first magnitude (+1.00) star is about 2.5 times brighter than a second magnitude (+2.00) star, and approximately 100 times brighter than a sixth magnitude (+6.00) star. SONY VAIO PCG-FR55E Laptop Keyboard

The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.

On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter. The brightest stars, on either scale, have negative magnitude numbers. DELL M711P Laptop Keyboard

The variation in brightness (ΔL) between two stars is calculated by subtracting the magnitude number of the brighter star (mb) from the magnitude number of the fainter star (mf), then using the difference as an exponent for the base number 2.512.

Relative to both luminosity and distance from Earth, a star's absolute magnitude (M) and apparent magnitude (m) are not equivalent;[131]  HP AEAT5U00010 Laptop Keyboard

for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41.

The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun,  HP 540 Laptop Keyboard

while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. DELL PVDG3 Laptop Keyboard

This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.

As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of −14.2. This star is at least 5,000,000 times more luminous than the Sun.[132]  HP G62-361TX Laptop Keyboard

 The least luminous stars that are currently known are located in the NGC 6397 cluster. 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.  DELL Vostro 1014 Laptop Keyboard

The current stellar classification system originated in the early 20th century, when stars were classified from A to Qbased on the strength of the hydrogen line.[135] It was not known at the time that the major influence on the line strength was temperature; the hydrogen line strength reaches a peak at over 9000 K, and is weaker at both hotter and cooler temperatures.  DELL Vostro 3550 Laptop Keyboard

When the classifications were reordered by temperature, it more closely resembled the modern scheme.[136]

Stars are given a single-letter classification according to their spectra, ranging from type O, which are very hot, toM, which are so cool that molecules may form in their atmospheres. HP Pavilion dv5-1104tu Laptop Keyboard

The main classifications in order of decreasing surface temperature are: O, B, A, F, G, K, and M. A variety of rare spectral types have 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. GATEWAY NV-59 Laptop Keyboard

However, this system breaks down at extreme high temperatures: class O0 and O1 stars may not exist.[137]

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 the surface gravity. ACER Aspire 5930 Laptop Keyboard

These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs); some authors addVII (white dwarfs). Most stars belong to the main sequence, which consists of ordinary hydrogen-burning stars. These fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.[137] DELL Vostro 3450 Laptop Keyboard

The Sun is a main sequence G2V yellow dwarf of intermediate temperature and ordinary size.

Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; ACER Aspire 5610Z Laptop Keyboard

"m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.[137]

White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DADBDCDODZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature index. TOSHIBA NSK-TAJ01 Laptop Keyboard

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.  Lenovo Thinkpad T420 Laptop Keyboard

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.[139]  HP Mini 110-1032NR Laptop Keyboard

Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.[139] This group includes protostars, Wolf-Rayet stars, and Flare stars, as well as giant and supergiant stars.

Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties. This group includes novaeand supernovae.HP Pavilion zv6233ea Laptop Keyboard

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.[4] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.[140] SONY VAIO VGN-FS940 laptop keyboard

 Some novae are also recurrent, having periodic outbursts of moderate amplitude.[139]

Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.[139] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.3 to 3.5 over a period of 2.87 days. SONY VAIO VGN-FS730F laptop keyboard

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. HP Pavilion dv3-2157cl laptop keyboard

The temperature at the core of a main sequence or giant star is at least on the order of 107 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.[141][142] SONY Vaio PCG-K315S laptop keyboard

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. HP Pavilion dv3-2157cl laptop keyboard

Eventually the helium content becomes predominant and energy production ceases at the core. Instead, for stars of more than 0.4 solar masses, fusion occurs in a slowly expanding shell around the degeneratehelium core.[143]

In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. HP Pavilion dv6-2030ev laptop keyboard

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 within the stellar interior where radiative transfer is sufficiently efficient to maintain the flux of energy. HP G61-465SL laptop keyboard

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 highopacity as in the outer envelope.[142]  LENOVO IdeaPad S10 20015 laptop keyboard

The occurrence of convection in the outer envelope of a main sequence star depends on the 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.[144] SONY VAIO VGN-FS315M laptop keyboard

Red dwarf stars with less than 0.4 solar masses are convective throughout, which prevents the accumulation of a helium core.[2] For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.

The portion of a star that is visible to an observer is called the photosphere. HP Mini 110-1150EV laptop keyboard

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 out into space. It is within the photosphere that sun spots, or regions of lower than average temperature, appear.

Above the level of the photosphere is the stellar atmosphere. HP Envy 15-1090eg laptop keyboard

In a main sequence star such as the Sun, the lowest level of the atmosphere is the thin chromosphere region, where spicules appear and stellar flares begin. This is surrounded by a transition region, where the temperature rapidly increases within a distance of only 100 km (62 mi). SONY Vaio VGN-FW390 laptop keyboard

Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.[145] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.[144] Despite its high temperature, the corona emits very little light. The corona region of the Sun is normally only visible during a solar eclipse. TOSHIBA Satellite M105-S3004 laptop keyboard

From the corona, a stellar wind of plasma particles expands outward from the star, propagating until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout the bubble-shaped region of the heliosphere.

A variety of different nuclear fusion reactions take place inside the cores of stars, depending upon their mass and composition, as part of stellar nucleosynthesis. GATEWAY MX8715 laptop keyboard

The net mass of the fused atomic nuclei is smaller than the sum of the constituents. This lost mass is released as electromagnetic energy, according to the mass-energy equivalence relationshipE = mc2.[1]

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. SONY Vaio VGN-AW330J laptop keyboard

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 massive stars, heavier elements can also be burned in a contracting core through the neon burning process and oxygen burning process. SONY Vaio PCG-K315S Laptop Keyboard

The final stage in the stellar nucleosynthesis process is the silicon burning process that results in the production of the stable isotope iron-56. Fusion can not proceed any further except through an endothermic process, and so further energy can only be produced through gravitational collapse.[147] HP Business Notebook nx9000 Laptop Keyboard

The example below shows the amount of time required for a star of 20 solar masses to consume all of its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity.

Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties"[1]  HP pavilion DV7-1000 Laptop Keyboard

In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitative study of the subject.[2] Astronomy focuses on celestial objects, space, and the physical universe as a whole. Observations of the Sun, Moon, stars, and planets have formed the basis of timekeeping and navigation.  IBM Lenovo ThinkPad X300 Laptop Keyboard

Astronomy is a branch of science, but unlike other sciences, which have Earth-based laboratories in which controlled experiments are performed, astronomy has its labs located in the heavens far beyond the reach, let alone control, of the terrestrial observer.[3] "So how can one be sure that what one sees out there is subject to the same rules and disciplines of science that govern the local laboratory experiments of physics and chemistry?"[3] HP 519004-001 Laptop Keyboard

"The most incomprehensible thing about the universe is that it is comprehensible." – Albert Einstein.[3]

Ptolemy's Almagest, although a brilliant treatise on theoretical astronomy combined with a practical handbook for computation, nevertheless includes many compromises to reconcile discordant observations.  ACER Aspire One D150-1577 Laptop Keyboard

Theoretical astronomy is usually assumed to have begun with Johannes Kepler (1571–1630), and Kepler's laws. It is co-equal with observation. The general history of astronomy deals with the history of the descriptive and theoretical astronomy of the solar system, from the late sixteenth century to the end of the nineteenth century.  FUJITSU Amilo Mini Ui 3520 Laptop Keyboard

The major categories of works on the history of modern astronomy include general histories, national and institutional histories, instrumentation, descriptive astronomy, theoretical astronomy, positional astronomy, and astrophysics. Astronomy was early to adopt computational techniques to model stellar and galactic formation and celestial mechanics.  HP G61-465SL Laptop Keyboard

From the point of view of theoretical astronomy, not only must the mathematical expression be reasonably accurate but it should preferably exist in a form which is amenable to further mathematical analysis when used in specific problems. Most of theoretical astronomy uses Newtonian theory of gravitation, considering that the effects of general relativity are weak for most celestial objects. HP Pavilion ze5715ca Laptop Keyboard

The obvious fact is that theoretical astronomy cannot (and does not try) to predict the position, size and temperature of every star in the heavens. Theoretical astronomy by and large has concentrated upon analyzing the apparently complex but periodic motions of celestial objects.

Astrochemistry, the overlap of the disciplines of astronomy and chemistry,  GATEWAY MX8715 Laptop Keyboard

is the study of the abundance and reactions of chemical elements and molecules in space, and their interaction with radiation. The formation, atomic and chemical composition, evolution and fate of molecular gas clouds, is of special interest because it is from these clouds that solar systems form. DELL Vostro 1015 Laptop Keyboard

Infrared astronomy, for example, has revealed that the interstellar medium contains a suite of complex gas-phase carbon compounds called aromatic hydrocarbons, often abbreviated (PAHs or PACs). These molecules composed primarily of fused rings of carbon (either neutral or in an ionized state) are said to be the most common class of carbon compound in the galaxy. TOSHIBA Satellite L455 Laptop Keyboard

They are also the most common class of carbon molecule in meteorites and in cometary and asteroidal dust (cosmic dust). These compounds, as well as the amino acids, nucleobases, and many other compounds in meteorites, carry deuterium and isotopes of carbon, nitrogen, and oxygen that are very rare on earth, attesting to their extraterrestrial origin. Lenovo 39T7417 laptop keyboard

The PAHs are thought to form in hot circumstellar environments (around dying carbon rich red giant stars).

The sparseness of interstellar and interplanetary space results in some unusual chemistry, since symmetry-forbidden reactions cannot occur except on the longest of timescales. ACER TravelMate 8204WLMi laptop keyboard

For this reason, molecules and molecular ions which are unstable on Earth can be highly abundant in space, for example the H3+ ion. Astrochemistry overlaps with astrophysics and nuclear physics in characterizing the nuclear reactions which occur in stars, the consequences for stellar evolution, as well as stellar 'generations'.  HP Mini 110-3102sl laptop keyboard

Indeed, the nuclear reactions in stars produce every naturally occurring chemical element. As the stellar 'generations' advance, the mass of the newly formed elements increases. A first-generation star uses elemental hydrogen (H) as a fuel source and produces helium(He). Hydrogen is the most abundant element, TOSHIBA Satellite P300-1EI laptop keyboard

and it is the basic building block for all other elements as its nucleus has only one proton. Gravitational pull toward the center of a star creates massive amounts of heat and pressure, which cause nuclear fusion. Through this process of merging nuclear mass, heavier elements are formed.  HP Pavilion dv6-2114sa laptop keyboard

Lithium, carbon, nitrogen and oxygen are examples of elements that form in stellar fusion. After many stellar generations, very heavy elements are formed (e.g. iron and lead).

Theoretical astronomers use a wide variety of tools which include analytical models (for example, polytropes to approximate the behaviors of a star) HP Mini 110-1116NR laptop keyboard

andcomputational numerical simulations. Each has some advantages. Analytical models of a process are generally better for giving insight into the heart of what is going on. Numerical models can reveal the existence of phenomena and effects that would otherwise not be seen.[4][5] HP Pavilion dv6-3106tu laptop keyboard

Astronomy theorists endeavor to create theoretical models and figure out the observational consequences of those models. This helps observers look for data that can refute a model or help in choosing between several alternate or conflicting models. SONY VAIO VGN-SR290 laptop keyboard

Theorists also try to generate or modify models to take into account new data. Consistent with the general scientific approach, in the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model. SONY VAIO VGN-CS31Z/Q laptop keyboard

Of the topics approached with the tools of theoretical physics, particular consideration is often given to stellar photospheres, stellar atmospheres, the solar atmosphere, planetary atmospheres, gaseous nebulae, nonstationary stars, and the interstellar medium. Special attention is given to the internal structure of stars. SONY VAIO VGN-CS31S/R laptop keyboard

and the New York City Department of Consumer Affairs issued a violation against ISR for engaging in a deceptive trade practice.

Reaction equations and large reaction networks are an important tool in theoretical astrochemistry, especially as applied to the gas-grain chemistry of the interstellar medium. IBM 42T3297 laptop keyboard

Theoretical astrochemistry offers the prospect of being able to place constraints on the inventory of organics for exogenous delivery to the early Earth.

"An important goal for theoretical astrochemistry is to elucidate which organics are of true interstellar origin, and to identify possible interstellar precursors and reaction pathways for those molecules which are the result of aqueous alterations."[11]  HP Pavilion dv6-2135er laptop keyboard

One of the ways this goal can be achieved is through the study of carbonaceous material as found in some meteorites. Carbonaceous chondrites (such as C1 and C2) include organic compounds such as amines and amides; alcohols, aldehydes, and ketones; aliphatic and aromatic hydrocarbons; sulfonic and phosphonic acids; HP Pavilion G7T-1000 Series laptop keyboard

amino, hydroxycarboxylic, and carboxylic acids; purines and pyrimidines; and kerogen-type material.[11] The organic inventories of primitive meteorites display large and variable enrichments in deuterium, 13C and 15N which is indicative of their retention of an interstellar heritage.

The chemical composition of comets should reflect both the conditions in the outer solar ASUS X53S laptop keyboard

nebula some 4.5 x 109 ayr, and the nature of the natal interstellar cloud from which the Solar system was formed.[12] While comets retain a strong signature of their ultimate interstellar origins, significant processing must have occurred in the protosolar nebula.[12] Early models of coma chemistry showed that reactions can occur rapidly in the inner coma, SONY Vaio PCG-K23 laptop keyboard

where the most important reactions are proton transfer reactions.[12] Such reactions can potentially cycle deuterium between the different coma molecules, altering the initial D/H ratios released from the nuclear ice, and necessitating the construction of accurate models of cometary deuterium chemistry, so that gas-phase coma observations can be safely extrapolated to give nuclear D/H ratios. SONY VGN-FE790G/N laptop keyboard

While the lines of conceptual understanding between theoretical astrochemistry and theoretical chemical astronomy often become blurred so that the goals and tools are the same, there are subtle differences between the two sciences. Theoretical chemistry as applied to astronomy seeks to find new ways to observe chemicals in celestial objects, for example. ACER PK1306F0200 laptop keyboard

This often leads to theoretical astrochemistry having to seek new ways to describe or explain those same observations.

Like theoretical chemical astronomy, the lines of conceptual understanding between theoretical astrophysics and theoretical physical astronomy are often blurred, but, again, there are subtle differences between these two sciences.HP G42-164LA Laptop Keyboard

Theoretical physics as applied to astronomy seeks to find new ways to observe physical phenomena in celestial objects and what to look for, for example. This often leads to theoretical astrophysics having to seek new ways to describe or explain those same observations, with hopefully a convergence to improve our understanding of the local environment of Earth and the physical Universe.  HP Pavilion dv6-2157sb Laptop Keyboard

From the Systeme Internationale (SI) comes the second as defined by the duration of 9 192 631 770 cycles of a particular hyperfine structure transition in the ground state of 133Cesium.[17] For practical usability a device is required that attempts to produce the SI second (s) such as an atomic clock. But not all such clocks agree. HP Mini 1116NR Laptop Keyboard

The weighted mean of many clocks distributed over the whole Earth defines the Temps Atomique International; i.e., the Atomic Time TAI.[17] From theGeneral theory of relativity the time measured depends on the altitude on Earth and the spatial velocity of the clock so that TAI refers to a location on sea level that rotates with the Earth. HP Pavilion DV7-3065dx Laptop Keyboard

Since the Earth's rotation is irregular, any time scale derived from it such as Greenwich Mean Time led to recurring problems in predicting the Ephemerides for the positions of the Moon, Sun, planets and their natural satellites.[17] In 1976 the International Astronomical Union (IAU) resolved that the theoretical basis for ephemeris time (ET) was wholly non-relativistic, HP Pavilion dv5-1104tu Laptop Keyboard

and therefore, beginning in 1984 ephemeris time would be replaced by two further time scales with allowance for relativistic corrections. Their names, assigned in 1979,[18]emphasized their dynamical nature or origin, Barycentric Dynamical Time (TDB) and Terrestrial Dynamical Time (TDT). Both were defined for continuity with ET and were based on what had become the standard SI second, which in turn had been derived from the measured second of ET. ACER Aspire 7520 Laptop Keyboard

During the period 1991–2006, the TDB and TDT time scales were both redefined and replaced, owing to difficulties or inconsistencies in their original definitions. The current fundamental relativistic time scales are Geocentric Coordinate Time (TCG) and Barycentric Coordinate Time (TCB). DELL Latitude D620 Laptop Keyboard

Both of these have rates that are based on the SI second in respective reference frames (and hypothetically outside the relevant gravity well), but due to relativistic effects, their rates would appear slightly faster when observed at the Earth's surface, and therefore diverge from local Earth-based time scales using the SI second at the Earth's surface.[19] HP G62-a30SP Laptop Keyboard

The currently defined IAU time scales also include Terrestrial Time (TT) (replacing TDT, and now defined as a re-scaling of TCG, chosen to give TT a rate that matches the SI second when observed at the Earth's surface),[20] and a redefined Barycentric Dynamical Time (TDB), a re-scaling of TCB to give TDB a rate that matches the SI second at the Earth's surface. DELL FM8 Laptop Keyboard,Lenovo 45N2106 Laptop Keyboard,DELL PVDG3 Laptop Keyboard

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