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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 can form part of a much larger gravitationally bound structure, such as astar cluster or a galaxy. (Full article...) Selected star -show another Photo credit:commons:user:Riffsyphon1024 andcommons:user:Mysid Aldebaran (α Tau, α Tauri,Alpha Tauri) is ared giantstar located about 65light years away in thezodiacconstellation ofTaurus. With an averageapparent magnitude of 0.87 it is the brightest star in the constellation and isone of the brightest stars in the nighttime sky. The nameAldebaran isArabic (الدبرانal-dabarān) and translates literally as "the follower", presumably because this bright star appears to follow thePleiades, or "Seven Sisters"star cluster in the night sky. In 1997 asubstellar companion was reported but subsequent observations have not confirmed this claim. Aldebaran is classified as a type K5III star. It is anorange giant star that has moved off themain sequence line of theHertzsprung–Russell diagram. It has exhausted thehydrogen fuel in its core andhydrogen fusion has ceased there. Although not yet hot enough forfusing helium, the core temperature of the star has greatly increased due to gravitational pressure and the star has expanded to a diameter of 44.2 times the diameter of theSun, Richichi & Roccatagliata (2005) derived an angular diameter of 20.58±0.03 milliarcsec, which given a distance of 65 light years yields a diameter of 61 million km.</ref> approximately 61 million kilometres (see10 gigametres for similar sizes). TheHipparcos satellite has measured it as 65.1light-years (20.0 pc) away, and it shines with 150 times the Sun's luminosity. Aldebaran is a slightlyvariable star, of theslow irregular variable typeLB. It varies by about 0.2 in apparent magnitude. Selected article -show another Photo credit:user:Pascalou petit Astellar magnetic field is amagnetic field generated by the motion of conductiveplasma inside astar. This motion is created throughconvection, which is a form of energy transport involving the physical movement of material. A localizedmagnetic field exerts a force on the plasma, effectively increasing the pressure without a comparable gain in density. As a result the magnetized region rises relative to the remainder of the plasma, until it reaches the star'sphotosphere. This createsstarspots on the surface, and the related phenomenon ofcoronal loops. The magnetic field of a star can be measured by means of theZeeman effect. Normally the atoms in a star's atmosphere will absorb certain frequencies of energy in theelectromagnetic spectrum, producing characteristic darkabsorption lines in the spectrum. When the atoms are within a magnetic field, however, these lines become split into multiple, closely spaced lines. The energy also becomespolarized with an orientation that depends on orientation of the magnetic field. Thus the strength and direction of the star's magnetic field can be determined by examination of the Zeeman effect lines. A star with a magnetic field will generate amagnetosphere that extends outward into the surrounding space. Field lines from this field originate at one magnetic pole on the star then end at the other pole, forming a closed loop. The magnetosphere contains charged particles that are trapped from thestellar wind, which then move along these field lines. As the star rotates, the magnetosphere rotates with it, dragging along the charged particles. Selected image -show another Photo credit:NASA ThePinwheel Galaxy (also known asMessier 101 orNGC 5457) is a face-onspiral galaxy about 27 millionlight-years away in theconstellationUrsa Major, discovered byPierre Méchain. On February 28, 2006,NASA and theESA released a very detailed image of Pinwheel Galaxy, which was the largest and most detailed image of a galaxy byHubble Space Telescope at the time. The image was composed from 51 individual exposures, plus some extra ground-based photos. M101 is a relatively large galaxy compared to theMilky Way. With a diameter of 170,000 light-years it is nearly twice the size of the Milky Way. It has a disk mass on the order of 100 billion solar masses, along with a small bulge of about 3 billion solar masses. Did you know?
SubcategoriesTo display all subcategories click on the ► Selected biography -show another Photo credit:Portrait from Toruń Nicolaus Copernicus (19 February 1473 – 24 May 1543) was the firstastronomer to formulate a comprehensiveheliocentriccosmology, which displaced theEarth from the center of theuniverse. Nicolaus Copernicus was born on 19 February 1473 in the city ofToruń (Thorn) inRoyal Prussia, part of theKingdom of Poland. Copernicus' epochal book,De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres), published just before his death in 1543, is often regarded as the starting point of modernastronomy and the definingepiphany that began theScientific Revolution. Hisheliocentric model, with the Sun at the center of the universe, demonstrated that the observed motions of celestial objects can be explained without putting Earth at rest in the center of the universe. His work stimulated further scientific investigations, becoming alandmark in thehistory of science that is often referred to as theCopernican Revolution. Among the greatpolymaths of theRenaissance, Copernicus was amathematician,astronomer,physician,quadrilingualpolyglot,classical scholar,translator,artist,Catholic cleric,jurist,governor,military leader,diplomat andeconomist. Among his many responsibilities, astronomy figured as little more than anavocation – yet it was in that field that he made his mark upon the world.  TopicsWikiProjectsThings to doAssociated WikimediaThe followingWikimedia Foundation sister projects provide more on this subject:
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