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: User:Dbenbenn and User:Qef
Alpha Centauri (α Centauri / α Cen); (also known as Rigil Kentaurus, Rigil Kent, or Toliman) is the binary star system Alpha Centauri AB (α Cen AB), of which Alpha Centauri A (α Cen A) is the brightest star in the southern constellation of Centaurus. To the unaided eye it appears as a single star, whose total visual magnitude would identify it as the third brightest star in the night sky. Alpha Centauri AB is 1.34 parsec or 4.37 light years away from our Sun. The two stars are the closest stars to the Sun after their companion Proxima Centauri, at 0.21 light-year away from the two, and at 4.243 light-years away from the Sun. At −0.27v visual magnitude, Alpha Centauri appears to the naked-eye as a single star and is fainter than Sirius and Canopus. The next brightest star in the night sky is Arcturus. When considered among the individual brightest stars in the sky (excluding the Sun), Alpha Centauri A is the fourth brightest at −0.01 magnitude being only fractionally fainter than Arcturus at −0.04v magnitude. Alpha Centauri B at 1.33v magnitude is twenty-first in brightness. Selected article - Photo credit: User:Oliverbeatson
All stars are born from collapsing clouds of gas and dust, often called nebulae or molecular clouds. Nuclear fusion powers a star for most of its life. Stars similar to our Sun gradually grow in size until they reach a red giant phase, after which the core collapses into a dense white dwarf and the outer layers are expelled as a planetary nebula. Larger stars can explode in a supernova as their cores collapse into an extremely dense neutron star or black hole. It is not clear how red dwarfs die because of their extremely long life spans, but they probably experience a gradual death in which their outer layers are expelled over time. Stellar evolution is not studied by observing the life of a single star, as most stellar changes occur too slowly to be detected, even over many centuries. Instead, astrophysicists come to understand how stars evolve by observing numerous stars at various points in their lifetime, and by simulating stellar structure using computer models. A stellar evolutionary model is a mathematical model that can be used to compute the evolutionary phases of a star from its formation until it becomes a remnant. The mass and chemical composition of the star are used as the inputs, and the luminosity and surface temperature are the only constraints. The model formulae are based upon the physical understanding of the star, usually under the assumption of hydrostatic equilibrium. Selected image - Photo credit: commons:User:Pbroks13
An illustration of the structure of the Sun:
1. Core 2. Radiative zone 3. Convective zone 4. Photosphere 5. Chromosphere 6. Corona 7. Sunspot 8. Granules 9. Prominence Did you know?
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Selected biography - Photo credit: Unknown artist, uploaded by User:Salvatore Ingala
Claudius Ptolemaeus (Greek: Κλαύδιος Πτολεμαῖος Klaúdios Ptolemaîos; c. AD 90 – c. 168), known in English as Ptolemy /ˈtɒləmɪ/, was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer and a poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the Thebaid. He died in Alexandria around AD 168. Ptolemy was the author of several scientific treatises, at least three of which were of continuing importance to later Islamic and European science. The first is the astronomical treatise now known as the Almagest (in Greek, Ἡ Μεγάλη Σύνταξις, "The Great Treatise", originally Μαθηματικὴ Σύνταξις, "Mathematical Treatise"). The second is the Geography, which is a thorough discussion of the geographic knowledge of the Greco-Roman world. The third is the astrological treatise known sometimes in Greek as the Apotelesmatika (Ἀποτελεσματικά), more commonly in Greek as the Tetrabiblos (Τετράβιβλος, "Four Books"), and in Latin as the Quadripartitum (or "Four Books") in which he attempted to adapt horoscopic astrology to the Aristotelian natural philosophy of his day. In Almagest, considered to be one of the most influential scientific texts of all time, Ptolemy presented his astronomical models in convenient tables, which could be used to compute the future or past position of the planets. The Almagest also contains a star catalogue, which is an appropriated version of a catalogue created by Hipparchus. His Planetary Hypotheses went beyond the mathematical model of the Almagest to present a physical realization of the universe as a set of nested spheres. TopicsThings to do
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