Portal:Stars
IntroductionA star is a luminous spheroid of plasma held together by self-gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night; their immense distances from Earth make them appear as fixed points of light. The most prominent stars have been categorised into constellations and asterisms, and many of the brightest stars have proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable universe contains an estimated 1022 to 1024 stars. Only about 4,000 of these stars are visible to the naked eye—all within the Milky Way galaxy. A star's life begins with the gravitational collapse of a gaseous nebula of material largely comprising hydrogen, helium, and trace heavier elements. Its total mass mainly determines its evolution and eventual fate. A star shines for most of its active life due to the thermonuclear fusion of hydrogen into helium in its core. This process releases energy that traverses the star's interior and radiates into outer space. At the end of a star's lifetime as a fusor, its core becomes a stellar remnant: a white dwarf, a neutron star, or—if it is sufficiently massive—a black hole. Stellar nucleosynthesis in stars or their remnants creates almost all naturally occurring chemical elements heavier than lithium. Stellar mass loss or supernova explosions return chemically enriched material to the interstellar medium. These elements are then recycled into new stars. Astronomers can determine stellar properties—including mass, age, metallicity (chemical composition), variability, distance, and motion through space—by carrying out observations of a star's apparent brightness, spectrum, and changes in its position in the sky over time. Stars can form orbital systems with other astronomical objects, as in planetary systems and star systems with two or more 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 a star cluster or a galaxy. (Full article...) Selected star - Photo credit: Rutherfurd Observatory
Rigel, also known by its Bayer designation Beta Orionis (β Ori, β Orionis), is the brightest star in the constellation Orion and the seventh brightest star in the night sky, with visual magnitude 0.13. The star as seen from Earth is actually a triple star system, with the primary star (Rigel A) a blue-white supergiant of absolute magnitude −7.84 and around 120,000 times as luminous as the Sun. An Alpha Cygni variable, it pulsates periodically. Visible in small telescopes, Rigel B is itself a spectroscopic binary system, consisting of two main sequence blue-white stars of spectral type B9. If viewed from a distance of 1 astronomical unit, it would span an angular diameter of 35° and shine at magnitude −38. Like other blue supergiants, Rigel has exhausted burning its core hydrogen fuel and left the main sequence, expanding and brightening as it progresses across the Hertzsprung–Russell diagram. It will end its stellar life as a type II supernova, exploding and in the process flinging out material that will serve to seed future generations of stars. As it is both bright and moving through a region of nebulosity, Rigel lights up several dust clouds in its vicinity, most notably the IC 2118 (the Witch Head Nebula). Rigel is also associated with the Orion Nebula, which—while more or less along the same line of sight as the star—is almost twice as far away from Earth. Despite the difference in distance, projecting Rigel's path through space for its expected age brings it close to the nebula. As a result, Rigel is sometimes classified as an outlying member of the Orion OB1 Association, along with many of the other bright stars in that region of the sky. Selected article -  Photo credit: commons:user:Borb
Stellar nucleosynthesis is the collective term for the nucleosynthesis, or nuclear reactions, taking place in stars to build the nuclei of the elements heavier than hydrogen. Some small quantity of these reactions also occur on the stellar surface under various circumstances. For the creation of elements during the explosion of a star, the term supernova nucleosynthesis is used. The processes involved began to be understood early in the 20th century, when it was first realized that the energy released from nuclear reactions accounted for the longevity of the Sun as a source of heat and light. The prime energy producer in the sun is the fusion of hydrogen to helium, which occurs at a minimum temperature of 3 million kelvin. Hydrogen burning is an expression that astronomers sometimes use for the stellar process that results in the nuclear fusion of four protons to form a nucleus of helium-4. (This should not be confused with the combustion of hydrogen in an oxidizing atmosphere.) There are two predominant processes by which stellar hydrogen burning occurs. Selected image - Photo credit: ESA/Hubble
The Large Magellanic Cloud (LMC) is a nearby irregular galaxy, once thought to be a satellite of our own. At a distance of slightly less than 50 kiloparsecs (≈ 160,000 light-years), the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy, lying closer to the center of the Milky Way. It has a mass equivalent to approximately 10 billion times the mass of our Sun (1010 solar masses), making it roughly 1/10 as massive as the Milky Way, and a diameter of about 14,000 light-years, though the LMC is the fourth largest galaxy in the Local Group. Did you know?
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Selected biography - Photo credit: Unknown artist, uploaded by User:ArtMechanic
Johannes Kepler (/ˈkɛplər/; German: [joˈhanəs ˈkɛplɐ, -nɛs -] ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws of planetary motion, and his books Astronomia nova, Harmonice Mundi, and Epitome Astronomiae Copernicanae, influencing among others Isaac Newton, providing one of the foundations for his theory of universal gravitation. The variety and impact of his work made Kepler one of the founders and fathers of modern astronomy, the scientific method, natural and modern science. He has been described as the "father of science fiction" for his novel Somnium. Kepler was a mathematics teacher at a seminary school in Graz, where he became an associate of Prince Hans Ulrich von Eggenberg. Later he became an assistant to the astronomer Tycho Brahe in Prague, and eventually the imperial mathematician to Emperor Rudolf II and his two successors Matthias and Ferdinand II. He also taught mathematics in Linz, and was an adviser to General Wallenstein. Additionally, he did fundamental work in the field of optics, being named the father of modern optics, in particular for his Astronomiae pars optica. He also invented an improved version of the refracting telescope, the Keplerian telescope, which became the foundation of the modern refracting telescope, while also improving on the telescope design by Galileo Galilei, who mentioned Kepler's discoveries in his work. Kepler lived in an era when there was no clear distinction between astronomy and astrology, but there was a strong division between astronomy (a branch of mathematics within the liberal arts) and physics (a branch of natural philosophy). Kepler also incorporated religious arguments and reasoning into his work, motivated by the religious conviction and belief that God had created the world according to an intelligible plan that is accessible through the natural light of reason. Kepler described his new astronomy as "celestial physics", as "an excursion into Aristotle's Metaphysics", and as "a supplement to Aristotle's On the Heavens", transforming the ancient tradition of physical cosmology by treating astronomy as part of a universal mathematical physics. (Full article...) TopicsThings to do
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