کهکشان مارپیچی
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![]() یک مثال از کهکشان مارپیچی کهکشان فرفره یک کهکشان مارپیچی (به انگلیسی: Spiral Galaxy) یک نوع از سه نوع اصلی کهکشانها است که نخستینبار توسط ادوین هابل و در سال ۱۹۳۶ میلادی ردهبندی شد.[۱] ساختار[ویرایش]کهکشانهای مارپیچی چند بخش اصلی دارند:
بازوهای مارپیچی[ویرایش]بازوهای مارپیچی نقاطی هستند که از مرکز مارپیچی و کهکشانهای مارپیچی بسته کشیده شدهاند. این نواحی باریک به شکل مارپیچ میباشند و از این رو به آنها کهکشانهای مارپیچی میگویند. طبقهبندی متفاوت کهکشانهای مارپیچی بستگی به ساختار بازوهای مشخص در آنهاست. کهکشانهای Sc و SBc به علت بعد مسافت بازوهای شل و آویزانی دارند، در حالی که که کهشانهای Sa و SBa کهکشانهایی با بازوهای تنگ و به هم پیچیده شدهای هستند (با مراجعه به طبقهبندی هابل). در هر دو حالت بازوهای مارپیچی از تعداد بسیار زیادی ستارهٔ آبی و بزرگ و جوان تشکیل شدهاند (ناشی از چگالی جرمی و نرخ بالای شکلگیری ستارگان) بازوها را قابل توجه میسازد. برآمدگیهای کهکشانی[ویرایش]برآمدگیها گروه عظیمالجثه و بسیار فشرده ستارگان هستند. این اصطلاح به گروه مرکزی ستارگان در اکثر کهکشانهای مارپیچی اشاره میکند. برآمدگی (bulg) کهکشان Sa معمولاً از ستارههای II تشکیل شدهاست، که ستارههای سرخرنگ و پیر همراه با حجم فلزی کم میباشند. افزونبر این کهکشانهای sa و sba تمایل به بزرگ بودن دارند در مقابل ان برآمدگیهای (bulges) کهکشانهای Sc و SBc بسیار کوچکتر هستند و غالباً از تراکم ستارههای جوان و آبی I تشکیل شدهاند بعضی از برآمدگیها ویژگیهای مشابهی با کهکشانهای بیضوی دارند (تمایل به سمت جرم و درخشندگی کم) و بقیه دارای چگالی دیسک مرکزی بسیار بالا همراه با ویژگیهای مشابه صفحهٔ کهکشان میباشند اینگونه تصور میشود که بسیاری از برآمدگیها در مرکزشان میزبان سیاهچالهٔ ابر پر جرم میباشند، اگرچه سیاهچاله تاکنون به صورت مستقیم مشاهد نشدهاست اما شواهد غیرمستقیم آن وجود دارد. به عنوان مثال در کهکشان خودمان شیئی که Sagittarius A* نامیده میشود احتمالاً یک سیاهچالهٔ ابر پر جرم میباشد یک ارتباط قوی بین جرم سیاهچاله و سرعت انشار ستارهها در برآمدگی وجود دارد رابطهٔ M-sigma. شبه کرهٔ بسیار عظیم[ویرایش]جثهٔ ستارهها در کهکشانهای مارپیچی اگرچه در صفحهٔ کهکشان قرار گرفتهاند اما ستارگان کمی وجود دارند که در یک مدار دایرهای به دور مرکز کهکشان قرار گرفته باشند بلکه آنها در یک هالهٔ کرهای به دور هستهٔ کهکشانی قرار گرفتهاند، به هر حال بعضی از ستارگان در یک هالهٔ کروی یا کره کهکشانی ساکن شدهاند. رفتار مداری این ستارگان مورد بحث است، آنها ممکن است برگشت یا کج شدن مدارها را توصیف نمایند، امّا حرکت ستارگان در یک مدار منظم را هرگز. هالههای ستارهای ممکن است از کهکشانهای کوچک که ادغام میشوند با کهکشانهای مارپیچی حاصل شوند برای مثال کهکشان بیضوی Sagittarius Dwarf در فرایند ادغام با راه شیری است و مشاهدات نشان میدهد که بعضی از ستارگان در هاله که در راه شیری وجود دارند حاصل ان است بر خلاف دیسک کهکشانی به نظر میرسد که هاله عاری از غبار است و علاوه بر این ستارگان در هالهٔ کهکشانی از تراکم II پیرتر و با نسبت فلزی کمتر نسبت به تراکم I در دیسک کهکشانی (اما بسیار شبیه نسبت به برآمدگی کهکشانی) هستند هالهٔ کهکشانی همچنین از خوشههای کروی تشکیل شدهاست. حرکت هالههای ستاره آنها را در فرصت مناسب میان دیسک میاورد و تصور میشود که تعدادی از کوتولههای قرمزز نزدیک به خورشید متعلق به هالههای کهکشانی میباشند برای مثال Kapteyn's Star و Groombridge ۱۸۳۰ بر طبق حرکت نامنظم آنها به دور مرکز کهکشان اگر آنها این حرکت را همواره انجام دهند این ستارهها اغلب آشکار میوند به صورت غیرطبیعی در حرکت مناسب بالا منشأ ناحیهٔ ساختار مارپیچ[ویرایش]برتیل لیندبلد در سال ۱۹۲۵ پیشگام مطالعه دربارهٔ دوران کهکشان و شکلگیری بازوهای مارپیچ بود. او پی برد که ایدهٔ مرتب شدن ستارگان در قالب مارپیچ به علت مسئلهٔ غیرقابل حل مارپیچی غیرقابل دفاع است. نظر به اینکه سرعت زاویهای دوران دیسک کهکشانی با فاصله گرفتن از مرکز کهکشان تغییر میکند (از طریق یک مدل گرانشی منظومه شمسی) یک بازوی محوری (شیبه دود) سریعاً به هنگام چرخش کهکشان خم خواهد شد بازو در چرخش کم کهکشان خم شدن خود را افزایش میدهد تاجایی به دور کهکشان میپیچد. این مسئله، مسئلهٔ مارپیچ نامیده میشود. اندازهگیری تا قبل از سال ۱۹۶۰ نشان میداد که سرعت مداری ستارهها در کهکشانهای مارپیچی با در نظر گرفتن فاصلهشان از مرکز کهکشان بسیار بالاتر انتظار ما در دینامیک نیوتن است اما هنوز نمیتواند استحکام ساختار مارپیچی را توضیح دهد دو تئوری یا مدل برای ساختار کهکشانهای مارپیچ وجود دارد.
مدل موج چگالی[ویرایش]لیند بلد پیشنهاد کرد که بازوها نمایش دهندهٔ مکانهای تقویتکنندهٔ چگالی میباشند (density waves) که باعث میشوند ستارهها و گاز در آنها دوران بسیار اهستهای داشته باشد همانگونه که گاز داخل یک موج چگالی میشود فشار وارد میکند و یک ستارهٔ جدید را درست میکند بعضی از ستارههای ابی جوان که بازوها را روشن میکنند این ایده تحت عنوان density wave theory توسط c.c Lin و frnk Shu، در سال ۱۹۶۴ بسط داده شد آنها پیشنهاد دادند که بازوهای مارپیچی آشکارسازی موج چگالی میباشند تلاش برای توضیح ساختار کشیدگی بزرگ مارپیچها در شرایط تکثیر موجهای کوتاه دامنه با سرعت زاویهای مناسب که با سرعت متفاوت از ستارهها و گازهای کهکشان اطراف کهکشان میچرخند. نظریهٔ تاریخی لین و شو[ویرایش]نخستین نظریهٔ مورد قبول برای ساختار مارپیچی توسط لین و شو در سال ۱۹۶۴ ابداع شد.
مثالهای مهم[ویرایش]جستارهای وابسته[ویرایش]اجزا[ویرایش]ردهبندی[ویرایش]دیگر[ویرایش]منابع[ویرایش]پیوند به بیرون[ویرایش]
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![]() An example of a spiral galaxy, the Pinwheel Galaxy (also known as Messier 101 or NGC 5457) Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae[1] and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters. Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them. Roughly two-thirds of all spirals are observed to have an additional component in the form of a bar-like structure,[2] extending from the central bulge, at the ends of which the spiral arms begin. The proportion of barred spirals relative to barless spirals has likely changed over the history of the universe, with only about 10% containing bars about 8 billion years ago, to roughly a quarter 2.5 billion years ago, until present, where over two-thirds of the galaxies in the visible universe (Hubble volume) have bars.[3] The Milky Way is a barred spiral, although the bar itself is difficult to observe from Earth's current position within the galactic disc.[4] The most convincing evidence for the stars forming a bar in the galactic center comes from several recent surveys, including the Spitzer Space Telescope.[5] Together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in today's universe.[6] They are mostly found in low-density regions and are rare in the centers of galaxy clusters.[7] StructureSpiral galaxies may consist of several distinct components:
The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy. Spiral armsSpiral arms are regions of stars that extend from the center of barred and unbarred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name. Naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very "loose" arms, whereas Sa and SBa galaxies have tightly wrapped arms (with reference to the Hubble sequence). Either way, spiral arms contain many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so bright. Bulge![]() Spiral galaxy NGC 1589[8] A bulge is a large, tightly packed group of stars. The term refers to the central group of stars found in most spiral galaxies, often defined as the excess of stellar light above the inward extrapolation of the outer (exponential) disk light. Using the Hubble classification, the bulge of Sa galaxies is usually composed of Population II stars, which are old, red stars with low metal content. Further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller[9] and are composed of young, blue Population I stars. Some bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity); others simply appear as higher density centers of disks, with properties similar to disk galaxies. Many bulges are thought to host a supermassive black hole at their centers. In our own galaxy, for instance, the object called Sagittarius A* is believed to be a supermassive black hole. There are many lines of evidence for the existence of black holes in spiral galaxy centers, including the presence of active nuclei in some spiral galaxies, and dynamical measurements that find large compact central masses in galaxies such as Messier 106. BarBar-shaped elongations of stars are observed in roughly two-thirds of all spiral galaxies.[10][11] Their presence may be either strong or weak. In edge-on spiral (and lenticular) galaxies, the presence of the bar can sometimes be discerned by the out-of-plane X-shaped or (peanut shell)-shaped structures[12][13] which typically have a maximum visibility at half the length of the in-plane bar. Spheroid![]() Spiral galaxy NGC 1345 The bulk of the stars in a spiral galaxy are located either close to a single plane (the galactic plane) in more or less conventional circular orbits around the center of the galaxy (the Galactic Center), or in a spheroidal galactic bulge around the galactic core. However, some stars inhabit a spheroidal halo or galactic spheroid, a type of galactic halo. The orbital behaviour of these stars is disputed, but they may exhibit retrograde and/or highly inclined orbits, or not move in regular orbits at all. Halo stars may be acquired from small galaxies which fall into and merge with the spiral galaxy—for example, the Sagittarius Dwarf Spheroidal Galaxy is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it. Unlike the galactic disc, the halo seems to be free of dust, and in further contrast, stars in the galactic halo are of Population II, much older and with much lower metallicity than their Population I cousins in the galactic disc (but similar to those in the galactic bulge). The galactic halo also contains many globular clusters. The motion of halo stars does bring them through the disc on occasion, and a number of small red dwarfs close to the Sun are thought to belong to the galactic halo, for example Kapteyn's Star and Groombridge 1830. Due to their irregular movement around the center of the galaxy, these stars often display unusually high proper motion. Oldest spiral galaxyThe oldest spiral galaxy on file is BX442. At eleven billion years old, it is more than two billion years older than any previous discovery. Researchers think the galaxy's shape is caused by the gravitational influence of a companion dwarf galaxy. Computer models based on that assumption indicate that BX442's spiral structure will last about 100 million years.[15][16] RelatedIn June 2019, citizen scientists through Galaxy Zoo reported that the usual Hubble classification, particularly concerning spiral galaxies, may not be supported, and may need updating.[17][18] Origin of the spiral structureThe pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925. He realized that the idea of stars arranged permanently in a spiral shape was untenable. Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy (via a standard solar system type of gravitational model), a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is called the winding problem. Measurements in the late 1960s showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from Newtonian dynamics but still cannot explain the stability of the spiral structure. Since the 1970s, there have been two leading hypotheses or models for the spiral structures of galaxies:
These different hypotheses are not mutually exclusive, as they may explain different types of spiral arms. Density wave modelBertil Lindblad proposed that the arms represent regions of enhanced density (density waves) that rotate more slowly than the galaxy's stars and gas. As gas enters a density wave, it gets squeezed and makes new stars, some of which are short-lived blue stars that light the arms.[20] Historical theory of Lin and ShuThe first acceptable theory for the spiral structure was devised by C. C. Lin and Frank Shu in 1964,[21] attempting to explain the large-scale structure of spirals in terms of a small-amplitude wave propagating with fixed angular velocity, that revolves around the galaxy at a speed different from that of the galaxy's gas and stars. They suggested that the spiral arms were manifestations of spiral density waves – they assumed that the stars travel in slightly elliptical orbits, and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic center. This is illustrated in the diagram to the right. It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits.[22] Star formation caused by density wavesThe following hypotheses exist for star formation caused by density waves:
![]() The bright galaxy NGC 3810 demonstrates classical spiral structure in this very detailed image from Hubble. Credit: ESA/Hubble and NASA. More young stars in spiral armsSpiral arms appear visually brighter because they contain both young stars and more massive and luminous stars than the rest of the galaxy. As massive stars evolve far more quickly,[23] their demise tends to leave a darker background of fainter stars immediately behind the density waves. This make the density waves much more prominent.[20] Spiral arms simply appear to pass through the older established stars as they travel in their galactic orbits, so they also do not necessarily follow the arms.[20] As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the local higher density. Also the newly created stars do not remain forever fixed in the position within the spiral arms, where the average space velocity returns to normal after the stars depart on the other side of the arm.[22] Gravitationally aligned orbitsCharles Francis and Erik Anderson showed from observations of motions of over 20,000 local stars (within 300 parsecs) that stars do move along spiral arms, and described how mutual gravity between stars causes orbits to align on logarithmic spirals. When the theory is applied to gas, collisions between gas clouds generate the molecular clouds in which new stars form, and evolution towards grand-design bisymmetric spirals is explained.[24] Distribution of stars in spiralsThe stars in spirals are distributed in thin disks radial with intensity profiles such that[25][26][27] with being the disk scale-length; is the central value; it is useful to define: as the size of the stellar disk, whose luminosity is . The spiral galaxies light profiles, in terms of the coordinate , do not depend on galaxy luminosity. Spiral nebulaBefore it was understood that spiral galaxies existed outside of our Milky Way galaxy, they were often referred to as spiral nebulae. The question of whether such objects were separate galaxies independent of the Milky Way, or a type of nebula existing within our own galaxy, was the subject of the Great Debate of 1920, between Heber Curtis of Lick Observatory and Harlow Shapley of Mt. Wilson Observatory. Beginning in 1923, Edwin Hubble[28][29] observed Cepheid variables in several spiral nebulae, including the so-called "Andromeda Nebula", proving that they are, in fact, entire galaxies outside our own. The term spiral nebula has since fallen out of use. Milky WayThe Milky Way was once considered an ordinary spiral galaxy. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1960s.[30][31] Their suspicions were confirmed by Spitzer Space Telescope observations in 2005,[32] which showed that the Milky Way's central bar is larger than what was previously suspected. Famous examples
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