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: Harvard-Smithsonian/NASA
Mira, /ˈmaɪrə/, also known as Omicron Ceti (or ο Ceti / ο Cet), is a red giant star estimated 200-400 light years away in the constellation Cetus. Mira is a binary star, consisting of the red giant Mira A along with Mira B. Mira A is also an oscillating variable star and was the first non-supernova variable star discovered, with the possible exception of Algol. Apart from the unusual Eta Carinae, Mira is the brightest periodic variable in the sky that is not visible to the naked eye for part of its cycle. Its distance is uncertain; pre-Hipparcos estimates centered around 220 light-years, while Hipparcos data suggests a distance of 418 light-years, albeit with a margin of error of ~14%. Evidence that the variability of Mira was known in ancient China, Babylon or Greece is at best only circumstantial. In 1638 Johannes Holwarda determined a period of the star's reappearances, eleven months; he is often credited with the discovery of Mira's variability. Johannes Hevelius was observing it at the same time and named it "Mira" (meaning "wonderful" or "astonishing," in Latin) in 1662's Historiola Mirae Stellae, for it acted like no other known star. Ismail Bouillaud then estimated its period at 333 days, less than one day off the modern value of 332 days (and perfectly forgivable, as Mira is known to vary slightly in period, and may even be slowly changing over time). The star is estimated to be a 6 billion year old red giant.
Selected article - Photo credit: NASA/ESA
The Crab Pulsar (PSR B0531+21) is a relatively young neutron star. The star is the central star in the Crab Nebula, a remnant of the supernova SN 1054, which was widely observed on Earth in the year 1054. Discovered in 1968, the pulsar was the first to be connected with a supernova remnant. The optical pulsar is roughly 25 km in diameter and the pulsar "beams" rotate once every 33 milliseconds, or 30 times each second. The outflowing relativistic wind from the neutron star generates synchrotron emission, which produces the bulk of the emission from the nebula, seen from radio waves through to gamma rays. The most dynamic feature in the inner part of the nebula is the point where the pulsar's equatorial wind slams into the surrounding nebula, forming a termination shock. The shape and position of this feature shifts rapidly, with the equatorial wind appearing as a series of wisp-like features that steepen, brighten, then fade as they move away from the pulsar into the main body of the nebula. The period of the pulsar's rotation is slowing by 38 nanoseconds per day due to the large amounts of energy carried away in the pulsar wind. The Crab Nebula is often used as a calibration source in X-ray astronomy. It is very bright in X-rays and the flux density and spectrum are known to be constant, with the exception of the pulsar itself. The pulsar provides a strong periodic signal that is used to check the timing of the X-ray detectors. In X-ray astronomy, 'crab' and 'millicrab' are sometimes used as units of flux density. A millicrab corresponds to a flux density of about 2.4x10−11 erg s−1 cm−2 (2.4x10−14 W m−2) in the 2–10 keV X-ray band, for a "crab-like" X-ray spectrum, which is roughly a powerlaw in photon energy, I(E)=9.5 E−1.1. Very few X-ray sources ever exceed one crab in brightness. Selected image - Photo credit: Digitized Sky Survey, ESA/ESO/NASA
Orion's Belt or the Belt of Orion is an asterism in the constellation Orion, consisting of the three bright stars Alnitak, Alnilam and Mintaka. The stars are more or less evenly spaced in a straight line, and so can be visualized as the belt of the hunter's clothing. 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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