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: ESO/P. Kervella
Betelgeuse is a semiregular variable star located approximately 640 light-years from the Earth. With an apparent magnitude ranging between 0.3 and 1.2, it is the ninth brightest star in the night sky. Although Betelgeuse has the Bayer designation Alpha Orionis (α Orionis / α Ori), it is most often the second brightest star in the constellation Orion behind α; Rigel (Beta Orionis) is usually brighter (Betelgeuse is a variable star and is on occasion brighter than Rigel). The star marks the upper right vertex of the Winter Triangle and center of the Winter Hexagon. Betelgeuse is a red supergiant, and one of the largest and most luminous stars known. For comparison, if the star were at the center of the Solar System its surface might extend out to between the orbits of Mars and Jupiter, wholly engulfing Mercury, Venus, the Earth and Mars. The angular diameter of Betelgeuse was first measured in 1920–1921 by Albert Abraham Michelson and Francis G. Pease using the 100 inch (2.5 m) John D. Hooker astronomical interferometer telescope atop Mount Wilson Observatory. Astronomers believe Betelgeuse is only a few million years old, but has evolved rapidly because of its high mass. Due to its age, Betelgeuse may go supernova within the next millennium (because it is hundreds of light years away, it possibly may have done so already). Selected article - Photo credit: user:Lviatour
A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph. The Latin root of the word corona means crown. The high temperature of the corona gives it unusual spectral features, which led some to suggest, in the 19th century, that it contained a previously unknown element, "coronium". These spectral features have since been traced to highly ionized iron (Fe-XIV) which indicates a plasma temperature in excess of 106 kelvin. The fact that the Sun has a million degree corona was first discovered by Gotrian in 1939 and Bengt Edlén in 1941 by identifying the coronal lines (observed since 1869) as transitions from low lying metastable levels of the ground configuration of highly ionized metals (the green FeXIV line at 5303 Å, but also the red line FeX at 6374 Å). Light from the corona comes from three primary sources, which are called by different names although all of them share the same volume of space. The K-corona (K for kontinuierlich, "continuous" in German) is created by sunlight scattering off free electrons; Doppler broadening of the reflected photospheric absorption lines completely obscures them, giving the spectral appearance of a continuum with no absorption lines. The F-corona (F for Fraunhofer) is created by sunlight bouncing off dust particles, and is observable because its light contains the Fraunhofer absorption lines that are seen in raw sunlight; the F-corona extends to very high elongation angles from the Sun, where it is called the Zodiacal light. The E-corona (E for emission) is due to spectral emission lines produced by ions that are present in the coronal plasma; it may be observed in broad or forbidden or hot spectral emission lines and is the main source of information about the corona's composition. The sun's corona is much hotter (by a factor of nearly 200) than the visible surface of the Sun: the photosphere's average temperature is 5800 kelvin compared to the corona's one to three million kelvin. Selected image - Photo credit: Urania's Mirror (Sidney Hall/Adam Cuerden)
Andromeda as depicted in Urania's Mirror, set of constellation cards published in London c.1825. Did you know?
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Selected biography - 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 a Chinese astronomer, mathematician, inventor, geographer, cartographer, artist, poet, statesman and literary scholar from Nanyang, Henan. He lived during the Eastern Han Dynasty (CE 25–220) of China. He was educated in the capital cities of Luoyang and Chang'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 palace eunuchs during the reign of Emperor Shun (r. 125–144) led to his decision to retire from the central court to serve as an administrator of Hejian, in Hebei. 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 first water-powered armillary sphere, to represent astronomical observation; improved the inflow water clock by adding another tank; and invented the world's first seismometer, which discerned the cardinal direction of an earthquake 500 km (310 mi) away. Furthermore, he improved previous Chinese calculations of the formula for pi. In addition to documenting about 2,500 stars in his extensive star catalogue, Zhang also posited theories about the Moon and its relationship to the Sun; specifically, he discussed the Moon's sphericity, its illumination by reflecting sunlight on one side and remaining dark on the other, and the nature of solar and lunar eclipses. His fu (rhapsody) and shi poetry were renowned and commented on by later Chinese writers. Zhang received many posthumous honors for his scholarship and ingenuity, and is considered a polymath by some scholars. Some modern scholars have also compared his work in astronomy to that of Ptolemy (CE 86–161).
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