A 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. 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 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. 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. 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. 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. 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).
The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC, with the help ofTimocharis. 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. Hipparchus is known for the discovery of the first recorded nova (new star). 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. In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185. 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. 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.
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. 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). 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. 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. 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, TOSHIBA Satellite P300 Laptop Keyboard
an idea that had been suggested earlier by the ancient Greek philosophers, Democritusand Epicurus, and by medieval Islamic cosmologists such as Fakhr al-Din al-Razi. 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.
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.
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. 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. 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. 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