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کیهان شناسی

کیهان شناسی

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compositon & structuer

The galaxy consists of a bar-shaped core region surrounded by a disk of gas, dust and stars forming four distinct arm structures spiraling outward in a logarithmic spiral shape (see Spiral arms). The mass distribution within the galaxy closely resembles the Sbc Hubble classification, which is a spiral galaxy with relatively loosely wound arms.[30] Astronomers first began to suspect that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy, in the 1990s.[31] Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005[32] that showed the galaxy's central bar to be larger than previously suspected.

Estimates for the mass of the Milky Way vary, depending upon the method and data used. Recent estimates at the low end have placed the mass of the Milky Way at 5.8×1011 solar masses (M), somewhat smaller than the Andromeda Galaxy.[33][34][35] Other measurements by the Very Long Baseline Array (VLBA) have found velocities as large as 254 km/s for stars at the edge of the Milky Way, higher than the previously accepted value of 220 km/s.[36] As the orbital velocity depends on the mass enclosed, this implies that the Milky Way is more massive, roughly equaling the mass of Andromeda Galaxy at 7×1011 M within 50 kiloparsecs (160,000 ly) of its center.[37] A recent measurement of the radial velocity of halo stars finds the mass enclosed within 80 kiloparsecs is 7×1011 M.[38] Most of the mass of the galaxy is thought to be dark matter, which forms a dark matter halo that is spread out relatively uniformly to a distance beyond one hundred kiloparsecs from the Galactic Center. Modelling of the Milky Way suggests that the overall mass of the entire galaxy lies in the range 1-1.5×1012 M[5]

This mass in baryonic matter is estimated to include 200 to 400 billion stars.[39] Its integrated absolute visual magnitude has been estimated to be −20.9.[40]
+ نوشته شده در  Tue 31 Jan 2012ساعت 4:34 PM  توسط parsa razavi  | 

appearance

All the stars that can be seen in the night sky are part of the Milky Way galaxy, but for the purposes of the astronomical activity of observing the celestial sphere the term "Milky Way" is limited to the hazy band of white light arching around the entire night sky.[14] The light originates from un-resolved stars and other material that lie within the galactic plane. Dark regions within the band, such as the Great Rift and the Coalsack, correspond to areas where light from distant stars is blocked by dark nebulae.

The Milky Way has a relatively low surface brightness, with its visibility being greatly affected by the brightness of the night sky due to light pollution, stray light from the moon, and so forth. It becomes readily visible at limiting magnitudes of +5.1 or better, while showing a great deal of detail at +6.1.[15] This makes the Milky Way difficult to see from any brightly-lit urban or suburban location, but very prominent when viewed from a rural area.[16]

The center of the galaxy lies in the direction of the constellation Sagittarius, and it is here that the Milky Way looks brightest. From Sagittarius, the hazy band of white light appears to pass westward through the constellations of Scorpius, Ara, Norma, Triangulum Australe, Circinus, Centaurus, Musca, Crux, Carina, Vela, Puppis, Canis Major, Monoceros, Orion and Gemini, Taurus, to the galactic anticenter in Auriga. From there, it passes through Perseus, Andromeda, Cassiopeia, Cepheus and Lacerta, Cygnus, Vulpecula, Sagitta, Aquila, Ophiuchus, Scutum, and back to Sagittarius. The fact that the band divides the night sky into two roughly equal hemispheres indicates that the Solar System lies close to the galactic plane.

The galactic plane is inclined by about 60 degrees to the ecliptic (the plane of the Earth's orbit). Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth's equatorial plane and the plane of the ecliptic relative to the galactic plane. The north galactic pole is situated at right ascension 12h 49m, declination +27.4° (B1950) near beta Comae Berenices, and the south galactic pole is near alpha Sculptoris.

+ نوشته شده در  Tue 31 Jan 2012ساعت 4:31 PM  توسط parsa razavi  | 

size

The stellar disk of the Milky Way galaxy is approximately 100,000 light-years (30 kiloparsecs, 9×1017 km) in diameter, and is considered to be, on average, about 1,000 ly (0.3 kpc) thick.[1] It is estimated to contain at least 100 billion stars[17] and possibly up to 400 billion stars.[18] The exact figure depends on the number of very low-mass, or dwarf stars, which are hard to detect, especially at distance of more than 300 ly (90 pc) from the Sun. Hence, current estimates of the total number remain highly uncertain, though it is often speculated to be around 250 billion.[citation needed] This can be compared to the one trillion (1012) stars of the neighboring Andromeda Galaxy.[19]

The disk of stars in the Milky Way does not have a sharp edge; a radius beyond which there are no stars. Rather, the concentration of stars drops smoothly with distance from the center of the galaxy. Beyond a radius of roughly 40,000 ly (12 kpc), the number of stars per cubic parsec drops much faster with radius,[20] for reasons that are not understood. Recent[when?] estimates give the galaxy a population of at least 50 billion planets, 10 billion of which could be located in the habitable zone of their parent star.[21][22] New data[when?] suggests there may be up to twice as many free-floating planets in the Milky Way as there are stars.[23] In 2011, new evidence obtained via gravitational microlensing indicated an average of at least one bound planet per star in the Milky Way, with Earth-sized planets being more numerous than gas giants.[13

Filling the place between the stars within and around the stellar disk is a disk of gas called the interstellar medium. The disk of gas has at least a comparable extent in radius to the stars,[24] while the thickness of the gas layer ranges from hundreds of light years for the colder gas to thousands of light years for warmer gas.[25][26]

As a guide to the relative physical scale of the Milky Way, if the Solar System out to the orbit of Pluto were reduced to the size of a US quarter (approximately 2.5 cm in diameter) the Milky Way would span some 2,045 km across covering an area of 3.286 million square kilometers, an area equivalent to about a third of the United States (3.276 million square km), the total area of India (3.287 million square km), or the combined areas of Great Britain, France, Spain, Germany, Italy, Poland, Sweden, Finland and Norway (3.378 million square km).[27]

The Galactic Halo extends outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose perigalacticon is at about 180,000 ly (55 kpc).[28] At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds, and the objects would likely be ejected from the vicinity of the Milky Way

]

+ نوشته شده در  Tue 31 Jan 2012ساعت 4:31 PM  توسط parsa razavi  | 

milky way galexy

The Milky Way is the galaxy that contains the Solar System.[10][a] This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky. The term "Milky Way" is a translation of the Latin for "milky road", Via Lactea, in turn derived from the Greekkyklos galaktikos or "milky circle", "milk" ("γάλα") also being the root for the Greek word for galaxy, γαλαξίας (galaxias).[11][12]

The galaxy has this appearance because it is a disk-shaped structure that is being viewed edge-on. Earth is located within the galactic plane of this disk, around two thirds of the way out from the center, on the inner edge of a spiral-shaped concentration of gas and dust called the Orion–Cygnus Arm. The concept of this faint band of light being made up of stars was proven in 1610 when Galileo Galilei used his telescope to resolve it into individual stars. In the 1920s observations by astronomer Edwin Hubble showed that the Milky Way was just one of around 200 billion galaxies in the observable universe.

The Milky Way is a barred spiral galaxy 100,000-120,000 light-years in diameter containing 200–400 billion stars. The galaxy is estimated to contain at least as many planets, 10 billion of which could be located in the habitable zone of their parent star.[13] Depending on its structure the entire galaxy has a rotational rate of once every 15 to 50 million years. The galaxy is also moving at a velocity of 552 to 630 km per second, depending on the relative frame of reference. It is estimated to be about 13.2 billion years old, nearly as old as the Universe. The Milky Way is part of the Local Group of galaxies.

+ نوشته شده در  Tue 31 Jan 2012ساعت 4:29 PM  توسط parsa razavi  | 

origin

White holes appear as part of a solution to the Einstein field equations known as the maximally extended version of the Schwarzschild metric describing an eternal black hole with no charge and no rotation. Here, "maximally extended" refers to the idea that the spacetime should not have any "edges": for any possible trajectory of a free-falling particle (following a geodesic) in the spacetime, it should be possible to continue this path arbitrarily far into the particle's future, unless the trajectory hits a gravitational singularity like the one at the center of the black hole's interior. In order to satisfy this requirement, it turns out that in addition to the black hole interior region which particles enter when they fall through the event horizon from the outside, there must be a separate white hole interior region which allows us to extrapolate the trajectories of particles which an outside observer sees rising up away from the event horizon. For an observer outside using Schwarzschild coordinates, infalling particles take an infinite time to reach the black hole horizon infinitely far in the future, while outgoing particles which pass the observer have been traveling outward for an infinite time since crossing the white hole horizon infinitely far in the past (however, the particles or other objects experience only a finite proper time between crossing the horizon and passing the outside observer). The black hole/white hole appears "eternal" from the perspective of an outside observer, in the sense that particles traveling outward from the white hole interior region can pass the observer at any time, and particles traveling inward which will eventually reach the black hole interior region can also pass the observer at any time.

Just as there are two separate interior regions of the maximally extended spacetime, there are also two separate exterior regions, sometimes called two different "universes", with the second universe allowing us to extrapolate some possible particle trajectories in the two interior regions. This means that the interior black-hole region can contain a mix of particles that fell in from either universe (and thus an observer who fell in from one universe might be able to see light that fell in from the other one), and likewise particles from the interior white-hole region can escape into either universe. All four regions can be seen in a spacetime diagram which uses Kruskal–Szekeres coordinates, see figure.[4]

In this spacetime, it is possible to come up with coordinate systems such that if you pick a hypersurface of constant time (a set of points that all have the same time coordinate, such that every point on the surface has a space-like separation, giving what is called a 'space-like surface') and draw an "embedding diagram" depicting the curvature of space at that time, the embedding diagram will look like a tube connecting the two exterior regions, known as an "Einstein-Rosen bridge" or Schwarzschild wormhole.[4] Depending on where the space-like hypersurface is chosen, the Einstein-Rosen bridge can either connect two black hole event horizons in each universe (with points in the interior of the bridge being part of the black hole region of the spacetime), or two white hole event horizons in each universe (with points in the interior of the bridge being part of the white hole region). It is impossible to use the bridge to cross from one universe to the other, however, because it is impossible to enter a white hole event horizon from the outside, and anyone entering a black hole horizon from either universe will inevitably hit the black hole singularity.

Note that the maximally extended Schwarzschild metric describes an idealized black hole/white hole that exists eternally from the perspective of external observers; a more realistic black hole that forms at some particular time from a collapsing star would require a different metric. When the infalling stellar matter is added to a diagram of a black hole's history, it removes the part of the diagram corresponding to the white hole interior region.[5] But because the equations of general relativity are time-reversible (they exhibit T-symmetry), general relativity must also allow the time-reverse of this type of "realistic" black hole that forms from collapsing matter. The time-reversed case would be a white hole that has existed since the beginning of the universe, and which emits matter until it finally "explodes" and disappears.[6] Despite the fact that such objects are permitted theoretically, they are not taken as seriously as black holes by physicists, since there would be no processes that would naturally lead to their formation, they could only exist if they were built into the initial conditions of the Big Bang.[6] Additionally, it is predicted that such a white hole would be highly "unstable" in the sense that if any small amount of matter fell towards the horizon from the outside, this would prevent the white hole's explosion as seen by distant observers, with the matter emitted from the singularity never able to escape the white hole's gravitational radius.[7]
+ نوشته شده در  Tue 24 Jan 2012ساعت 11:44 AM  توسط parsa razavi  | 

white hole


aed below, the white hole event horizon in the past becomes a black hole event horizon in the future, so any object falling towards it will eventually reach the black hole horizon).

In quantum mechanics, the black hole emits Hawking radiation, and so can come to thermal equilibrium with a gas of radiation. Since a thermal equilibrium state is time reversal invariant, Stephen Hawking argued that the time reverse of a black hole in thermal equilibrium is again a black hole in thermal equilibrium.[2] This implies that black holes and white holes are the same object. The Hawking radiation from an ordinary black hole is then identified with the white hole emission. Hawking's semi-classical argument is reproduced in a quantum mechanical AdS/CFT treatment,[3] where a black hole in anti-de Sitter space is described by a thermal gas in a gauge theory, whose time reversal is the same as itself.

+ نوشته شده در  Tue 24 Jan 2012ساعت 11:41 AM  توسط parsa razavi  | 

A white hole, in general relativity, is a hypothetical region of spacetime which cannot be entered from the outside, but from which matter and light may escape. In this sense it is the reverse of a black hole, which can be entered from the outside, but from which nothing, including light, may escape. White holes appear in the theory of eternal black holes. In addition to a black hole region in the future, such a solution of the Einstein equations has a white hole region in its past.[1] However, this region does not exist for black holes that have formed through gravitational collapse, nor are there any known physical processes through which a white hole could be formed.

Like black holes, white holes have properties like mass, charge, and angular momentum. They attract matter like any other mass, but objects falling towards a white hole would never actually reach the white hole's event horizon (though in the case of the maximally extended Schwarzschild solution, discussed below, the white hole event horizon in the past becomes a black hole event horizon in the future, so any object falling towards it will eventually reach the black hole horizon).

In quantum mechanics, the black hole emits Hawking radiation, and so can come to thermal equilibrium with a gas of radiation. Since a thermal equilibrium state is time reversal invariant, Stephen Hawking argued that the time reverse of a black hole in thermal equilibrium is again a black hole in thermal equilibrium.[2] This implies that black holes and white holes are the same object. The Hawking radiation from an ordinary black hole is then identified with the white hole emission. Hawking's semi-classical argument is reproduced in a quantum mechanical AdS/CFT treatment,[3] where a black hole in anti-de Sitter space is described by a thermal gas in a gauge theory, whose time reversal is the same as itself.

+ نوشته شده در  Tue 24 Jan 2012ساعت 11:40 AM  توسط parsa razavi  | 

معذرت می خواهم از معلم خوبم چون اطلاعات من در برابر او هیچ است از ویکی پدیا کمک گرفتم

+ نوشته شده در  Sun 22 Jan 2012ساعت 11:41 AM  توسط parsa razavi  | 

خواص فیزیکی

ساده ترین نوع سیاهچاله‌ها آنهایی هستند که تنها جرم دارند و بار الکتریکی و تکانه زاویه‌ای ندارند. این سیاهچاله‌ها را اغلب با نام سیاهچاله‌های شوارتزشیلد می‌نامند که بر گرفته از نام کارل شوارتزشیلد است که جوابی برای معادلات میدانی انیشتین در سال ۱۹۱۶ ارائه نمود.[۱۳] بنا بر قضیه بیرخوف در نسبیت عام، تنها جواب خلا است که متقارن کروی است. این بدان معنی است که تفاوتی میان میدان گرانشی یک سیاهچاله و یک جسم کروی با همان جرم وجود ندارد. بنابراین سیاهچاله تنها در محدوده نزدیک به افق آن است که همه چیز حتی نور را به درون می‌کشد و در فواصل دورتر کاملا مانند هر جسم دیگری با همان میزان جرم رفتار می‌کند.[۳۶]

راه حل‌هایی برای معادلات انیشتین که سیاهچاله‌های کلی تری را توصیف می‌کنند نیز وجود دارند. مثلا متریک رایسنر-نوردستروم سیاهچاله‌های باردار و متریک کر سیاهچاله‌های چرخان را توصیف می‌کنند. کلی ترین جواب موجود برای سیاهچاله‌های ثابت متریک کر-نیومن است که سیاهچاله‌هایی را توصیف می‌کند که هم بار الکتریکی وهم تکانه زاویه‌ای دارند.[۳۷]

در حالیکه جرم سیاهچاله می‌تواند هر مقداری داشته باشد، بار و تکانه زاویه‌ای آن توسط جرم محدود می‌شوند. چنانچه واحدهای پلانک را بکار بریم، کل بار الکتریکی Q و مجموع تکانه زاویه‌ای J در این رابطه صدق می‌کنند(M جرم سیاهچاله‌است): Q^2+\left (\tfrac{J}{M} \right)^2\le M^2\, . سیاهچاله‌هایی که نابرابری فوق را اشباع می‌کنند، سیاهچاله‌های اکسترمال نامیده می‌شوند. جواب‌هایی نیز برای معادلات انیشتین موجودند که این نابرابری را نقض می‌کنند اما این جواب‌ها افق رویداد ندارند. این جوابها را تکینگی‌های برهنه می‌نامند که از بیرون قابل مشاهده‌اند و در نتیجه نمی‌توانند فیزیکی باشند. فرضیه سانسور کیهانی شکل گیری چنین تکینگی‌هایی را در جریان رمبش نامحتمل می‌شمرد. [۳۸]

به دلیل قدرت نسبی الکترومغناطیس سیاهچاله‌هایی که از رمبش ستارگان تشکیل می‌شوند تمایل دارند که بار تقریبا خنثی ستاره را حفظ کنند. اما انتظار می‌رود که چرخش یک ویژگی مشترک در اجسام فشرده باشد. نامزد سیاهچاله قرار گرفته در دوتایی پرتو ایکس جی‌آراس ۱۹۱۵+۱۰۵ [۳۹] به نظر می‌رسد که تکانه زاویه‌ای نزدیک به حداکثر مقدار مجاز داشته باشد. 

+ نوشته شده در  Sun 22 Jan 2012ساعت 11:39 AM  توسط parsa razavi  | 

سیاه چاله (برای کسانی که انگلیسی بلدند)

A black hole is a region of spacetime from which nothing, not even light, can escape.[1] The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that marks the point of no return. It is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics.[2] Quantum mechanics predicts that black holes emit radiation like a black body with a finite temperature. This temperature is inversely proportional to the mass of the black hole, making it difficult to observe this radiation for black holes of stellar mass or greater.

Objects whose gravity field is too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was not fully appreciated for another four decades. Long considered a mathematical curiosity, it was during the 1960s that theoretical work showed black holes were a generic prediction of general relativity. The discovery of neutron stars sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality.

Black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle. After a black hole has formed it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form. There is general consensus that supermassive black holes exist in the centers of most galaxies. In particular, there is strong evidence of a black hole of more than 4 million solar masses at the center of our galaxy, the Milky Way.

Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter and with light and other electromagnetic radiation. From stellar movement, the mass and location of an invisible companion object can be calculated; in a number of cases the only known object capable of meeting these criteria is a black hole. Astronomers have identified numerous stellar black hole candidates in binary systems by studying the movement of their companion stars in this way.

+ نوشته شده در  Sun 22 Jan 2012ساعت 11:15 AM  توسط parsa razavi  |