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Introduction TheSun, aG-type main-sequence star, the closest to Earth Astar is a luminousspheroid ofplasma held together byself-gravity. Thenearest star to Earth is theSun. Many other stars are visible to the naked eye atnight; their immense distances from Earth make them appear asfixed points of light. The most prominent stars have been categorised intoconstellations andasterisms, and many of the brightest stars haveproper names.Astronomers have assembledstar catalogues that identify the known stars and provide standardizedstellar designations. Theobservable universe contains an estimated1022 to1024 stars. Only about 4,000 of these stars are visible to the naked eye—all within theMilky Waygalaxy. A star's lifebegins with thegravitational collapse of a gaseousnebula of material largely comprisinghydrogen, helium, and traces of heavier elements. Itstotal mass mainly determines itsevolution and eventual fate. A star shines formost of its active life due to thethermonuclear fusion of hydrogen intohelium in its core. This process releases energy that traverses the star's interior andradiates intoouter space. At the end of a star's lifetime, fusion ceases and its core becomes astellar remnant: awhite dwarf, aneutron star, or—if it is sufficiently massive—ablack hole. Stellar nucleosynthesis in stars or their remnants creates almost all naturally occurringchemical elements heavier thanlithium.Stellar mass loss orsupernova explosions return chemically enriched material to theinterstellar medium. These elements are then recycled into new stars. Astronomers can determine stellar properties—including mass, age,metallicity (chemical composition),variability,distance, and motion throughspace—by carrying out observations of a star'sapparent brightness,spectrum, andchanges in its position in the sky over time. Stars can form orbital systems with otherastronomical objects, as inplanetary systems andstar systems withtwo ormore stars. When two such stars orbit closely, their gravitational interaction can significantly impact their evolution. Stars often form part of much larger gravitationally bound structures, such asstar clusters and galaxies. (Full article...) Selected star -show another Photo credit:NASA Canopus (/kəˈnoʊpəs/; α Car, α Carinae,Alpha Carinae) is the brighteststar in the southernconstellation ofCarina, and thesecond brightest star in the night-time sky, afterSirius. Canopus'svisual magnitude is −0.72, and it has anabsolute magnitude of −5.65. Canopus is a supergiant ofspectral type F. Canopus is essentially white when seen with the naked eye (although F-type stars are sometimes listed as "yellowish-white"). It is located in the far southern sky, at adeclination of −52° 42' (2000) and aright ascension of 06h24.0m. Its name comes from the mythologicalCanopus, who was a navigator forMenelaus, king of Sparta. Canopus is the most intrinsically bright star within approximately 700 light years, and it has been the brightest star in Earth's sky during three different epochs over the past four million years. Other stars appear brighter only during relatively temporary periods, during which they are passing the Solar System at a much closer distance than Canopus. About 90,000 years ago,Sirius moved close enough that it became brighter than Canopus, and that will remain the case for another 210,000 years. But in 480,000 years, Canopus will once again be the brightest, and will remain so for a period of about 510,000 years. Selected article -show another Photo credit:NASA Stars of different mass and age have varyinginternal structures.Stellar structure models describe the internal structure of a star in detail and make detailed predictions about theluminosity, thecolor and thefuture evolution of the star. Different layers of the stars transport heat up and outwards in different ways, primarilyconvection andradiative transfer, butthermal conduction is important inwhite dwarfs. The internal structure of amain sequence star depends upon the mass of the star. In solar mass stars (0.3–1.5solar masses), including theSun, hydrogen-to-helium fusion occurs primarily viaproton-proton chains, which do not establish a steep temperature gradient. Thus, radiation dominates in the inner portion of solar mass stars. The outer portion of solar mass stars is cool enough that hydrogen is neutral and thus opaque to ultraviolet photons, so convection dominates. Therefore,solar mass stars have radiative cores with convective envelopes in the outer portion of the star. In massive stars (greater than about 1.5solar masses), the core temperature is above about 1.8×107K, sohydrogen-to-heliumfusion occurs primarily via theCNO cycle. In the CNO cycle, the energy generation rate scales as the temperature to the 17th power, whereas the rate scales as the temperature to the 4th power in the proton-proton chains. Due to the strong temperature sensitivity of the CNO cycle, the temperature gradient in the inner portion of the star is steep enough to make the coreconvective. The simplest commonly used model of stellar structure is the spherically symmetric quasi-static model, which assumes that astar is in asteady state and that it isspherically symmetric. It contains four basic first-orderdifferential equations: two represent howmatter andpressure vary with radius; two represent howtemperature andluminosity vary with radius. Selected image -show another Photo credit:NASA Messier 82 (also known as NGC 3034, Cigar Galaxy or M82) is the prototypenearbystarburst galaxy about 12 millionlight-years away in theconstellationUrsa Major. The starburst galaxy is five times as bright as the wholeMilky Way and one hundred times as bright as our galaxy's center. M82 was previously believed to be anirregular galaxy. However, in2005, two symmetricspiral arms were discovered in thenear-infrared (NIR) images of M82, and is now considered aspiral galaxy. Did you know?
SubcategoriesTo display all subcategories click on the ► Selected biography -show another Photo credit:State Post Bureau of the People's Republic of China Zhang Heng (simplified Chinese:张衡;traditional Chinese:張衡;pinyin:Zhāng Héng;Wade–Giles:Chang Heng) (CE 78–139) was aChineseastronomer,mathematician,inventor,geographer,cartographer,artist,poet,statesman andliterary scholar fromNanyang,Henan. He lived during theEastern Han Dynasty (CE 25–220) ofChina. He was educated in the capital cities ofLuoyang andChang'an, and began his career as a minor civil servant in Nanyang. Eventually, he became Chief Astronomer, Prefect of the Majors for Official Carriages, and then Palace Attendant at the imperial court. His uncompromising stances on certain historical and calendrical issues led to Zhang being considered a controversial figure, which prevented him from becoming an official court historian. His political rivalry with the palaceeunuchs during the reign ofEmperor Shun (r. 125–144) led to his decision to retire from the central court to serve as an administrator of Hejian, inHebei. He returned home to Nanyang for a short time, before being recalled to serve in the capital once more in 138. He died there a year later, in 139. Zhang applied his extensive knowledge of mechanics and gears in several of his inventions. He invented the world's firstwater-poweredarmillary sphere, to represent astronomical observation; improved the inflowwater clock by adding another tank; and invented the world's firstseismometer, which discerned thecardinal direction of anearthquake 500 km (310 mi) away. Furthermore, he improved previous Chinese calculations of the formula forpi. In addition to documenting about 2,500 stars in his extensivestar catalogue, Zhang also posited theories about theMoon and its relationship to theSun; specifically, he discussed the Moon's sphericity, its illumination by reflecting sunlight on one side and remaining dark on the other, and the nature ofsolar andlunareclipses. Hisfu (rhapsody) andshi poetry were renowned and commented on by later Chinese writers. Zhang received many posthumous honors for his scholarship and ingenuity, and is considered apolymath by some scholars. Some modern scholars have also compared his work in astronomy to that ofPtolemy (CE 86–161). TopicsWikiProjectsThings to doAssociated WikimediaThe followingWikimedia Foundation sister projects provide more on this subject:
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