A* search algorithm

In computer science, A* (pronounced as "A star" ( listen)) is a computer algorithm that is widely used in pathfinding and graph traversal, the process of plotting an efficiently traversable path between multiple points, called nodes. Noted for its performance and accuracy, it enjoys widespread use. However, in practical travel-routing systems, it is generally outperformed by algorithms which can pre-process the graph to attain better performance, although other work has found A* to be superior to other approaches.

Peter Hart, Nils Nilsson and Bertram Raphael of Stanford Research Institute (now SRI International) first described the algorithm in 1968. It is an extension of Edsger Dijkstra's 1959 algorithm. A* achieves better performance by using heuristics to guide its search.

History

In 1968, AI researcher Nils Nilsson was trying to improve the path planning done by Shakey the Robot, a prototype robot that could navigate through a room containing obstacles. This path-finding algorithm, that Nilsson called A1, was a faster version of the then best known method, Dijkstra's algorithm, for finding shortest paths in graphs. Bertram Raphael suggested some significant improvements upon this algorithm, calling the revised version A2. Then Peter E. Hart introduced an argument that established A2, with only minor changes, to be the best possible algorithm for finding shortest paths. Hart, Nilsson and Raphael then jointly developed a proof that the revised A2 algorithm was optimal for finding shortest paths under certain well-defined conditions.

John

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Star

A star is a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun. Other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, and the brightest stars gained proper names. Extensive catalogues of stars have been assembled by astronomers, which provide standardized star designations.

For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Once the hydrogen in the core of a star is nearly exhausted, almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime and, for some stars, by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, luminosity, and spectrum respectively. The total mass of a star is the principal determinant of its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined.

Chatham County Line

Chatham County Line is an American bluegrass musical group. Formed in Raleigh, North Carolina, in 1999 from members of the band Stillhouse, the band has released seven albums including six on the Yep Roc label (whom they were linked with by the producer Chris Stamey), and have become popular in Europe as well as their native United States. Their most recent tour took in numerous European destinations, including the Lowlands rock festival in the Netherlands. Chatham County Line made their Canadian debut in July 2009 at the Winnipeg Folk Festival, where they performed beside The Del McCoury Band and The Punch Brothers.

The members met in 1996 when lead singer-songwriter Dave Wilson was playing for Stillhouse. Wilson is the son of Charlotte poet Dede Wilson and was living in the Blue House, a Raleigh crash pad and romper room for the area's hottest young musicians. The other members are banjo picker Chandler Holt, upright bassist Greg Readling, and multi-instrumentalist John Teer. Wilson and Readling were playing in the Blue House as "Stillhouse" when Teer and Holt became intrigued "to hear these guys playing original country music that didn't suck" as Holt recalls. Holt and Teer befriended Wilson at the Blue House and began sitting in with the band. Wilson, over a beer one night, asked the others if they were interested in starting a bluegrass band.