Sunday, September 22, 2013

A star is a massive

star is a massive, luminous sphere of plasma held together by its own gravity. Our nearest star is the Sun, which is the source of most of the energy on Earth. Some other stars are visible from Earth during the night, appearing as a multitude of fixed luminous points due to their immense distance.  HP Envy 15 Laptop Keyboard
Historically, the most prominent stars were grouped into constellations and asterisms, and the brightest stars gained proper names. Extensivecatalogues 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. COMPAQ Presario CQ50-106CA Laptop Keyboard
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 nucleosynthesiswhen it explodes. Near the end of its life, a star can also contain degenerate matter.  ACER Aspire 5810T-8929 Laptop Keyboard
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, DELL Vostro 1510 Laptop Keyboard
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.  ACER Aspire 5742 Laptop Keyboard
A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process.[1] COMPAQ NSK-H5401 Laptop Keyboard
The remainder of the star's interior carries energy away from the core through a combination of radiative andconvective processes. The star's internal pressure prevents it from collapsing further under its own gravity. Once the hydrogen fuel at the core is exhausted, a star with at least 0.4 times the mass of the Sun[2] expands to become a red giant, DELL Vostro 1014 Laptop Keyboard
in some cases fusing heavier elements at the core or in shells around the core. The star then evolves into a degenerate form, recycling a portion of its matter into the interstellar environment, where it will contribute to the formation of a new generation of stars with a higher proportion of heavy elements.[3] SONY VAIO VGN-CR23G laptop keyboard
Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or (if it is sufficiently massive) a black hole.
Binary and multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.[4DELL 0PVDG3 laptop keyboard
] Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.
Historically, stars have been important to civilizations throughout the world. They have been part of religious practices and used for celestial navigation and orientation. HP Pavilion ze5170 laptop keyboard
Many ancient astronomers believed that stars were permanently affixed to aheavenly sphere, and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.[5] The motion of the Sun against the background stars (and the horizon) was used to create calendars,
which could be used to regulate agricultural practices.[7] The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun.
The oldest accurately dated star chart appeared in ancient Egyptian astronomy in 1534 BC.[8] HP Pavilion dv6-3030sp laptop keyboard
The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during theKassite Period (ca. 1531–1155 BC).[9]
The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC, with the help ofTimocharis.[10]  HP Pavilion DV8310CA laptop keyboard
 The star catalog of Hipparchus (2nd century BC) included 1020 stars and was used to assemble Ptolemy's star catalogue.[11] Hipparchus is known for the discovery of the first recorded nova (new star).[12] Many of the constellations and star names in use today derive from Greek astronomy. SONY VAIO VGN-CR21Z laptop keyboard
In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear.[13] In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185.[14] The brightest stellar event in recorded history was the SN 1006 supernova, which was observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers.[15] HP Pavilion dv5-1114ez laptop keyboard
 The SN 1054 supernova, which gave birth to the Crab Nebula, was also observed by Chinese and Islamic astronomers.[16][17][18]
Medieval Islamic astronomers gave Arabic names to many stars that are still used today, and they invented numerousastronomical instruments that could compute the positions of the stars. HP Pavilion dv5-1033ez laptop keyboard
They built the first large observatory research institutes, mainly for the purpose of producing Zij star catalogues.[19] Among these, the Book of Fixed Stars (964) was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters (including the Omicron Velorum and Brocchi's Clusters) and galaxies (including the Andromeda Galaxy).[20] SONY VGN-FE48E laptop keyboard
According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky Way galaxy as a multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in 1019.[21]  Lenovo 3000 C200 laptop keyboard
According to Josep Puig, the Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars which almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence.[22] TOSHIBA Satellite m502 laptop keyboard
Early Europeanastronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable. In 1584 Giordano Bruno suggested that the stars were like the Sun, and may have other planets, possibly even Earth-like, in orbit around them,[23] TOSHIBA Satellite P300 Laptop Keyboard
an idea that had been suggested earlier by the ancient Greek philosophers, Democritusand Epicurus,[24] and by medieval Islamic cosmologists[25] such as Fakhr al-Din al-Razi.[26] By the following century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers. HP Pavilion DV8310CA Laptop Keyboard
To explain why these stars exerted no net gravitational pull on the Solar System, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.[27]
The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. ACER TravelMate 4050 Laptop Keyboard
Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions from the time of the ancient Greek astronomers Ptolemy andHipparchus.[23]
William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. TOSHIBA Satellite L300 Laptop Keyboard
During the 1780s, he performed a series of gauges in 600 directions, and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction.[2HP Mini 1001TU Laptop Keyboard
8] In addition to his other accomplishments, William Herschel is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems.
The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, SONY VAIO VGN-NR21E/S Laptop Keyboard
they found differences in the strength and number of their absorption lines—the dark lines in a stellar spectra due to the absorption of specific frequencies by the atmosphere. In 1865 Secchi began classifying stars into spectral types.[29] However, the modern version of the stellar classification scheme was developed byAnnie J. Cannon during the 1900s. Dell Inspiron 2500 Laptop Keyboard
The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 byFriedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.[23] Observation of double stars gained increasing importance during the 19th century. HP Pavilion G7-1167DX Laptop Keyboard
In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius, and inferred a hidden companion.Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104-day period. SAMSUNG N140 Laptop Keyboard
Detailed observations of many binary star systems were collected by astronomers such as William Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of the orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.[30] TOSHIBA NSK-TBD01 Laptop Keyboard
The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschilddiscovered that the color of a star, and hence its temperature, could be determined by comparing the visual magnitude against the photographic magnitude.  ACER TravelMate 6292 Laptop Keyboard
The development of the photoelectric photometer allowed very precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope.[31]
Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. TOSHIBA Satellite L300 Laptop Keyboard
In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution.Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis.[32] TOSHIBA Satellite A215-S4807 Laptop Keyboard
The spectra of stars were further understood through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.[33]
With the exception of supernovae, individual stars have primarily been observed in our Local Group of galaxies,[34] and especially in the visible part of theMilky Way (as demonstrated by the detailed star catalogues available for our galaxy).[35] HP G72-b67CA Laptop Keyboard
But some stars have been observed in the M100 galaxy of the Virgo Cluster, about 100 million light years from the Earth.[36] In the Local Supercluster it is possible to see star clusters, and current telescopes could in principle observe faint individual stars in the Local Cluster[37] (see Cepheids). DELL Latitude D620 Laptop Keyboard
However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located at a distance of one billion light years[38]—ten times further than the most distant star cluster previously observed. HP 597635-001 Laptop Keyboard
The concept of the constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology.[39] ACER Aspire 5742 Laptop Keyboard
Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.
As well as certain constellations and the Sun itself, individual stars have their own myths.[40] To the Ancient Greeks, some "stars", ACER Aspire 5336 Laptop Keyboard
known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter andSaturn were taken.[40] (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)  HP 605344-001 Laptop Keyboard
Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star's right ascensionwas DELL PVDG3 Laptop Keyboard
invented and added to John Flamsteed's star catalogue in his book "Historia coelestis Britannica" (the 1712 edition), whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.[41][42]
The only internationally recognized authority for naming celestial bodies is the International Astronomical Union (IAU)[43]  SAMSUNG N140 Laptop Keyboard
A number of private companies sell names of stars, which the British Library calls an unregulated commercial enterprise.[44][45] However, the IAU has disassociated itself from this commercial practice, and these names are neither recognized by the IAU nor used by them.[46] One such star naming company is the International Star Registry, which, TOSHIBA Mini NB 505-SP0160 Laptop Keyboard
during the 1980s, was accused of deceptive practice for making it appear that the assigned name was official. This now-discontinued ISR practice was informally labeled a scam and a fraud,
Stars are formed within extended regions of higher density in the interstellar medium, although the density is still lower than the inside of a vacuum chamber. SONY VAIO VGN-N31S/W Laptop Keyboard
These regions are called molecular clouds and consist mostly of hydrogen, with about 23–28% helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.[55] As massive stars are formed from molecular clouds, they powerfully illuminate those clouds. They also ionize the hydrogen, creating an H II region. HP Pavilion dv6-3217cl laptop keyboard
All stars spend the majority of their lives as main sequence stars, fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their lives. The ultimate fate of more massive stars is different from that of less massive stars, as is their luminosity and the impact they have on their environment. SONY VAIO VGN-NW15G/S laptop keyboard
Therefore, stars are often grouped by mass. Very low mass stars with masses below 0.5 solar masses do not enter the asymptotic giant branch (AGB) but evolve directly into white dwarfs. Low mass stars (including the Sun) with a mass above about 0.5 and below about 1.8–2.2 solar masses (depending on composition) DELL PK130CW1A08 laptop keyboard
do enter the AGB, where they develop a degenerate helium core.Intermediate-mass stars undergo helium fusion and develop a degenerate carbon-oxygen core. Massive stars have a minimum mass of 7–10 solar masses, but this may be as low as 5–6 solar masses. These stars undergo carbon fusion, with their lives ending in a core-collapse supernova explosion. ASUS A6J laptop keyboard
The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density often triggered by shock waves from nearby supernovae (massive stellar explosions), the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy). SONY VAIO VGN-CR21SR laptop keyboard
Once a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability, it begins to collapse under its own gravitational force.
As the cloud collapses, individual conglomerations of dense dust and gas form what are known as Bok globules. ACER TravelMate 8210 laptop keyboard
As a globule collapses and the density increases, the gravitational energy is converted into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core.[58] These pre–main sequence stars are often surrounded by aprotoplanetary disk and powered mainly by the release of gravitational energy. TOSHIBA Satellite L775D-S7340 laptop keyboard
The period of gravitational contraction lasts about 10–15 million years.
Early stars of less than 2 solar masses are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly born stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig–Haro objects.[59][60] SONY VAIO PCG-FR130 laptop keyboard
These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed.[61]
Early in their life, T Tauri stars follow the Hayashi track--they contract and decrease in luminosity while remaining at roughly the same temperature. TOSHIBA Satellite C655-S9520D laptop keyboard
Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track.
Stars spend about 90% of their lifetime fusing hydrogen into helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence and are called dwarf stars.  HP Pavilion DV7-3060us laptop keyboard
Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.[62] The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion (4.6 × 109) years ago.[63]  HP G62-b12SL laptop keyboard
Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses 10−14 solar masses every year,[64] or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose 10−7 to 10−5 solar masses each year, significantly affecting their evolution.[65] TOSHIBA NB100 laptop keyboard
 Stars that begin with more than 50 solar masses can lose over half their total mass while on the main sequence.
The duration that a star spends on the main sequence depends primarily on the amount of fuel it has to fuse and the rate at which it fuses that fuel, i.e. its initial mass and its luminosity. TOSHIBA Satellite L200 laptop keyboard
For the Sun, its life is estimated to be about 10 billion (1010) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than 0.25 solar masses, called red dwarfs, are able to fuse nearly all of their mass as fuel while stars of about 1 solar mass can only use about 10% of their mass as fuel. Lenovo 0A62075 laptop keyboard
The combination of their slow fuel consumption and relatively large usable fuel supply allows stars about 0.25 times the mass of the Sun to last for about one trillion (1012) years according to stellar evolution calculations, while the least-massive hydrogen-fusing stars (0.08 solar masses) will last for about 12 trillion years.[67] TOSHIBA Satellite L750-ST4N02 Laptop Keyboard
At the end of their lives, red dwarfs simply become dimmer and dimmer.[2] However, since the lifespan of such stars is greater than the current age of the universe (13.8 billion years), no stars under about 0.85 solar masses[68] are expected to have moved off of the main sequence. ACER Aspire 5742 Laptop Keyboard
Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. In astronomy all elements heavier than helium are considered a "metal", and the chemical concentration of these elements is called the metallicity. The metallicity can influence the duration that a star will burn its fuel, control the formation of magnetic fields[69] and modify the strength of the stellar wind.[70] HP 605344-001 Laptop Keyboard
Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time these clouds become increasingly enriched in heavier elements as older stars die and shed portions of theiratmospheres. SAMSUNG N150 Laptop Keyboard
As stars of at least 0.4 solar masses[2] exhaust their supply of hydrogen at their core, their outer layers expand greatly and cool to form a red giant. In about 5 billion years, when the Sun enters this phase, it will expand to a maximum radius of roughly 1 astronomical unit (150 million kilometres), 250 times its present size. As a giant, the Sun will lose roughly 30% of its current mass.[63][71 HP G72-a10SV Laptop Keyboard
In a red giant of up to 2.25 solar masses, hydrogen fusion proceeds in a shell surrounding the core.[72] Eventually the core is compressed enough to start helium fusion, and the star now gradually shrinks in radius and its surface temperature increases. For larger stars, the core region transitions directly from fusing hydrogen to fusing helium.[4] DELL Vostro 1015 Laptop Keyboard
After the star has consumed the helium at the core, fusion continues in a shell around a hot core of carbon and oxygen. The star then follows an evolutionary path that parallels the original red giant phase, but at a higher surface temperature.
During their helium-burning phase, very high mass stars with more than nine solar masses expand to form red supergiants. SONY VAIO VGN-N31S/W Laptop Keyboard
Once this fuel is exhausted at the core, they continue to fuse elements heavier than helium.
The core contracts until the temperature and pressure are sufficient to fuse carbon (see carbon burning process). This process continues, with the successive stages being fueled by neon (see neon burning process), SONY VAIO VGN-CR150F Laptop Keyboard
oxygen (see oxygen burning process), and silicon (see silicon burning process). Near the end of the star's life, fusion continues along a series of onion-layer shells within the star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.[73] DELL Inspiron 9300 Laptop Keyboard
The final stage is reached when a massive star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy—the process would, on the contrary, consume energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by fission.[72] DELL Latitude D505 Laptop Keyboard
In relatively old, very massive stars, a large core of inert iron will accumulate in the center of the star. The heavier elements in these stars can work their way to the surface, forming evolved objects known as Wolf-Rayet stars that have a dense stellar wind which sheds the outer atmosphere. HP Pavilion DV3000 Laptop Keyboard

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