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Gamma Arae
GAMMA ARAE (γ Ara, γ Arae) is a star in the southern constellation of Ara . With an apparent visual magnitude of 3.3, it is the fourth-brightest star in the constellation and is readily visible to the naked eye. From parallax measurements made during the Hipparcos mission, the distance to this star can be estimated as 1,110 light-years (340 parsecs ) from Earth
Earth
. This is an enormous star with 23 times the radius of the Sun
Sun
. It is radiating 120,000 as much energy as the Sun
Sun
from its outer envelope at an effective temperature of 21,500 K . This heat gives the star the blue-white glow of a B-type star . The spectrum shows it to match a stellar classification of B1 Ib, with the luminosity class of 'Ib' indicating this is a lower luminosity supergiant star. It is a relatively young body, with an estimated age of around 15.7 million years
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Star Catalogue
A STAR CATALOGUE (Commonwealth English ) or STAR CATALOG (American English ), is an astronomical catalogue that lists stars . In astronomy , many stars are referred to simply by catalogue numbers. There are a great many different star catalogues which have been produced for different purposes over the years, and this article covers only some of the more frequently quoted ones. Star
Star
catalogues were compiled by many different ancient peoples, including the Babylonians , Greeks , Chinese , Persians , and Arabs . Most modern catalogues are available in electronic format and can be freely downloaded from space agencies data center. Completeness and accuracy is described by the weakest apparent magnitude V (largest number) and the accuracy of the positions
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Megayear
A YEAR is the orbital period of the Earth
Earth
moving in its orbit around the Sun
Sun
. Due to the Earth's axial tilt , the course of a year sees the passing of the seasons , marked by changes in weather , the hours of daylight , and, consequently, vegetation and soil fertility . In temperate and subpolar regions around the globe, four seasons are generally recognized: spring , summer , autumn and winter . In tropical and subtropical regions several geographical sectors do not present defined seasons; but in the seasonal tropics , the annual wet and dry seasons are recognized and tracked. A calendar year is an approximation of the number of days of the Earth's orbital period as counted in a given calendar . The Gregorian, or modern, calendar , presents its calendar year to be either a common year of 365 days or a leap year of 366 days, as do the Julian calendars ; see below
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Cape Photographic Durchmusterung
In astronomy , DURCHMUSTERUNG or BONNER DURCHMUSTERUNG (BD), is the comprehensive astrometric star catalogue of the whole sky, compiled by the Bonn
Bonn
Observatory ( Germany
Germany
) from 1859 to 1903. The name comes from Durchmusterung ("run-through examination"), a German word used for a systematic survey of objects or data. The term has sometimes been used for other astronomical surveys, including not only stars but also the search for other celestial objects. Special tasks are the celestial scanning in electromagnetic wavelengths which are shorter or longer than visible light waves
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Henry Draper Catalogue
The HENRY DRAPER CATALOGUE (HD) is an astronomical star catalogue published between 1918 and 1924, giving spectroscopic classifications for 225,300 stars; it was later expanded by the HENRY DRAPER EXTENSION (HDE), published between 1925 and 1936, which gave classifications for 46,850 more stars, and by the HENRY DRAPER EXTENSION CHARTS (HDEC), published from 1937 to 1949 in the form of charts, which gave classifications for 86,933 more stars. In all, 359,083 stars were classified as of August 2017. The original HD catalogue covers the entire sky almost completely down to an apparent photographic magnitude of about 9; the extensions added fainter stars in certain areas of the sky. The construction of the Henry Draper
Henry Draper
Catalogue was part of a pioneering effort to classify stellar spectra, and its catalogue numbers are commonly used as a way of identifying stars
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Hipparcos Catalogue
HIPPARCOS was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry , the accurate measurement of the positions of celestial objects on the sky. This permitted the accurate determination of proper motions and parallaxes of stars, allowing a determination of their distance and tangential velocity . When combined with radial velocity measurements from spectroscopy , this pinpointed all six quantities needed to determine the motion of stars. The resulting HIPPARCOS CATALOGUE, a high-precision catalogue of more than 118,200 stars, was published in 1997. The lower-precision Tycho Catalogue of more than a million stars was published at the same time, while the enhanced Tycho-2 Catalogue of 2.5 million stars was published in 2000. Hipparcos' follow-up mission, Gaia , was launched in 2013
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Stellar Evolution
STELLAR EVOLUTION is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the age of the universe . The table shows the lifetimes of stars as a function of their masses. All stars are born from collapsing clouds of gas and dust, often called nebulae or molecular clouds . Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star. Nuclear fusion powers a star for most of its life. Initially the energy is generated by the fusion of hydrogen atoms at the core of the main-sequence star. Later, as the preponderance of atoms at the core becomes helium , stars like the Sun
Sun
begin to fuse hydrogen along a spherical shell surrounding the core
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Projected Rotational Velocity
STELLAR ROTATION is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface. The rotation of a star produces an equatorial bulge due to centrifugal force . As stars are not solid bodies, they can also undergo differential rotation . Thus the equator of the star can rotate at a different angular velocity than the higher latitudes . These differences in the rate of rotation within a star may have a significant role in the generation of a stellar magnetic field . The magnetic field of a star interacts with the stellar wind . As the wind moves away from the star its rate of angular velocity slows. The magnetic field of the star interacts with the wind, which applies a drag to the stellar rotation. As a result, angular momentum is transferred from the star to the wind, and over time this gradually slows the star's rate of rotation
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Centimetre–gram–second System Of Units
The CENTIMETRE–GRAM–SECOND SYSTEM OF UNITS (abbreviated CGS or CGS) is a variant of the metric system based on the centimetre as the unit of length , the gram as the unit of mass , and the second as the unit of time . All CGS mechanical units are unambiguously derived from these three base units, but there are several different ways of extending the CGS system to cover electromagnetism . The CGS system has been largely supplanted by the MKS system based on the metre , kilogram , and second, which was in turn extended and replaced by the International System of Units (SI). In many fields of science and engineering, SI is the only system of units in use but there remain certain subfields where CGS is prevalent
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Effective Temperature
The EFFECTIVE TEMPERATURE of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation . Effective temperature
Effective temperature
is often used as an estimate of a body's surface temperature when the body's emissivity curve (as a function of wavelength ) is not known. When the star's or planet's net emissivity in the relevant wavelength band is less than unity (less than that of a black body ), the actual temperature of the body will be higher than the effective temperature. The net emissivity may be low due to surface or atmospheric properties, including greenhouse effect
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Kelvin
The KELVIN SCALE is an absolute thermodynamic temperature scale using as its null point absolute zero , the temperature at which all thermal motion ceases in the classical description of thermodynamics . The KELVIN (symbol: K) is the base unit of temperature in the International System of Units (SI). The kelvin is defined as the fraction  1⁄273.16 of the thermodynamic temperature of the triple point of water (exactly 0.01 °C or 32.018 °F). In other words, it is defined such that the triple point of water is exactly 273.16 K. The Kelvin
Kelvin
scale is named after the Belfast-born, Glasgow University engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907), who wrote of the need for an "absolute thermometric scale". Unlike the degree Fahrenheit
Fahrenheit
and degree Celsius
Celsius
, the kelvin is not referred to or typeset as a degree
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Stellar Rotation
STELLAR ROTATION is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface. The rotation of a star produces an equatorial bulge due to centrifugal force . As stars are not solid bodies, they can also undergo differential rotation . Thus the equator of the star can rotate at a different angular velocity than the higher latitudes . These differences in the rate of rotation within a star may have a significant role in the generation of a stellar magnetic field . The magnetic field of a star interacts with the stellar wind . As the wind moves away from the star its rate of angular velocity slows. The magnetic field of the star interacts with the wind, which applies a drag to the stellar rotation. As a result, angular momentum is transferred from the star to the wind, and over time this gradually slows the star's rate of rotation
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Harvard Revised Catalogue
The BRIGHT STAR CATALOGUE, also known as the YALE CATALOGUE OF BRIGHT STARS or YALE BRIGHT STAR CATALOGUE, is a star catalogue that lists all stars of stellar magnitude 6.5 or brighter, which is roughly every star visible to the naked eye from Earth. The catalog contains 9,110 objects, of which 9,095 are stars, 11 are novae or supernovae , and 4 are non-stellar objects; the non-stellar objects are the globular clusters 47 Tucanae (designated HR 95) and NGC 2808
NGC 2808
(HR 3671), and the open clusters NGC 2281
NGC 2281
(HR 2496) and Messier 67 (HR 3515). The catalogue is fixed in number of entries, but its data is maintained, and it is appended with a comments section about the objects that has been steadily enhanced since the first version in 1908
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Smithsonian Astrophysical Observatory Star Catalog
The SMITHSONIAN ASTROPHYSICAL OBSERVATORY STAR CATALOG is an astrometric star catalogue . It was published by the Smithsonian Astrophysical Observatory in 1966 and contains 258,997 stars. The catalogue was compiled from various previous astrometric catalogues, and contains only stars to about ninth magnitude for which accurate proper motions were known. Names in the SAO catalogue start with the letters SAO, followed by a number. The numbers are assigned following 18 ten-degree bands of declination , with stars sorted by right ascension within each band. EXAMPLES OF SAO CATALOGUE ENTRIES * SAO 67174
SAO 67174
is Vega
Vega
. * SAO 113271
SAO 113271
is Betelgeuse . * SAO 40012 is HD 277559. * SAO 158687
SAO 158687
is the star that was occulted by Uranus
Uranus
in March 1977, leading to the discovery of rings around Uranus
Uranus

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Sun
The SUN is the star at the center of the Solar System
Solar System
. It is a nearly perfect sphere of hot plasma , with internal convective motion that generates a magnetic field via a dynamo process . It is by far the most important source of energy for life on Earth
Earth
. Its diameter is about 109 times that of Earth, and its mass is about 330,000 times that of Earth, accounting for about 99.86% of the total mass of the Solar System. About three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen , carbon , neon , and iron . The Sun
Sun
is a G-type main-sequence star (G2V) based on its spectral class . As such, it is informally referred to as a yellow dwarf
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Earth
EARTH is the third planet from the Sun
Sun
and the only object in the Universe
Universe
known to harbor life . According to radiometric dating and other sources of evidence, Earth
Earth
formed over 4 billion years ago . Earth\'s gravity interacts with other objects in space, especially the Sun
Sun
and the Moon
Moon
, Earth's only natural satellite . Earth
Earth
revolves around the Sun
Sun
in 365.26 days, a period known as an Earth
Earth
year . During this time, Earth
Earth
rotates about its axis about 366.26 times. Earth's axis of rotation is tilted, producing seasonal variations on the planet's surface
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