White Main Sequence Star
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White Main Sequence Star
An A-type main-sequence star (A) or A dwarf star is a main-sequence (hydrogen burning) star of spectral type A and luminosity class (five). These stars have spectra defined by strong hydrogen Balmer absorption lines. They measure between 1.7 and 2.1 solar masses (), have surface temperatures between 7,600 and 10,000  K, and live for about a quarter of the lifetime of our Sun. Bright and nearby examples are Altair (A7), Sirius A (A1), and Vega (A0). A-type stars do not have convective zones and thus are not expected to harbor magnetic dynamos. As a consequence, because they do not have strong stellar winds, they lack a means to generate X-ray emissions. Spectral standard stars The revised Yerkes Atlas system listed a dense grid of A-type dwarf spectral standard stars, but not all of these have survived to this day as standards. The "anchor points" and "dagger standards" of the MK spectral classification system among the A-type main-sequence dwarf stars, i.e. tho ...
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Sirius A And B Artwork
Sirius is the brightest star in the night sky. Its name is derived from the Greek word (Latin script: ), meaning 'glowing' or 'scorching'. The star is designated  Canis Majoris, Latinized to Alpha Canis Majoris, and abbreviated  CMa or Alpha CMa. With a visual apparent magnitude of −1.46, Sirius is almost twice as bright as Canopus, the next brightest star. Sirius is a binary star consisting of a main-sequence star of spectral type A0 or A1, termed Sirius A, and a faint white dwarf companion of spectral type DA2, termed Sirius B. The distance between the two varies between 8.2 and 31.5 astronomical units as they orbit every 50 years. Sirius appears bright because of its intrinsic luminosity and its proximity to the Solar System. At a distance of , the Sirius system is one of Earth's nearest neighbours. Sirius is gradually moving closer to the Solar System and it is expected to increase in brightness slightly over the ...
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X-ray Astronomy
X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by Balloon-borne telescope, balloons, sounding rockets, and X-ray astronomy satellite, satellites. X-ray astronomy uses a type of space telescope that can see x-ray radiation which standard optical telescopes, such as the Mauna Kea Observatories, cannot. X-ray generation, X-ray emission is expected from astronomical objects that contain extremely hot gases at temperatures from about a million kelvin (K) to hundreds of millions of kelvin (MK). Moreover, the maintenance of the E-layer of ionized gas high in the Earth's thermosphere also suggested a strong extraterrestrial source of X-rays. Although theory predicted that the Sun and the stars would be prominent X-ray sources, there was no way to verify this because Earth's atmo ...
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HD 88955
HD 88955 is a single, white-hued star in the southern constellation of Vela. It can be viewed with the naked eye, having an apparent visual magnitude of 3.85. The distance to HD 88955 can be determined from its annual parallax shift of , which yields a separation of 100 light years from the Sun. It is moving further from the Earth with a heliocentric radial velocity of +7 km/s. Bayesian analysis suggests HD 88955 is a member of the Argus Association, a group of co-moving stars usually associated with the IC 2391 open cluster. This is an A-type main-sequence star with a stellar classification of A2 V. It is about 410 million years old with a projected rotational velocity of 100 km/s. The star has 2.17 times the mass of the Sun and 2.11 times the Sun's radius. It is radiating 23 times the Sun's luminosity from its photosphere at an effective temperature of 9,451 K. An infrared excess An infrared excess is a measurement of an astronomical so ...
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HD 45320
HD may refer to: Business * H-D or Harley-Davidson, a motorcycle manufacturer * The Home Depot, NYSE stock symbol: HD Chemistry * Hydrogen deuteride, a diatomic compound of hydrogen and deuterium * Mustard gas Codes * Air Do, formerly Hokkaido International Airlines, IATA designator * HD postcode area, covering Huddersfield, Brighouse and Holmfirth in England, UK * Heidelberg's vehicle registration plate code * Hunedoara County (Romania)'s ISO 3166 code Medicine * Hansen's disease or leprosy * Hirschsprung's disease, a disorder of the abdomen * Huntington's disease, a genetic disorder affecting the central nervous system ** HD (gene) or huntingtin, the IT15 gene, which codes for the huntingtin protein People * H.D. or Hilda Doolittle (1886–1961), American poet and novelist Other uses * ''Helsingborgs Dagblad'', a Swedish newspaper * Henry Draper Catalogue, an astronomical catalogue often used to designate stars * Department of Highways (other) * '' ...
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Fomalhaut
Fomalhaut (, ) is the brightest star in the southern constellation of Piscis Austrinus, the Southern Fish, and one of the brightest stars in the night sky. It has the Bayer designation Alpha Piscis Austrini, which is an alternative form of α Piscis Austrini, and is abbreviated Alpha PsA or α PsA. This is a class A star on the main sequence approximately from the Sun as measured by the Hipparcos astrometry satellite. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. It is classified as a Vega-like star that emits excess infrared radiation, indicating it is surrounded by a circumstellar disk. Together with the K-type main-sequence star TW Piscis Austrini, and the red dwarf star LP 876-10, Fomalhaut constitute a triple star system, even though the companions are separated by approximately 8 degrees. Fomalhaut was the first stellar system with an extrasolar planet candidate im ...
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Phecda
Phecda , also called Gamma Ursae Majoris (γ Ursae Majoris, abbreviated Gamma UMa, γ UMa), is a star in the constellation of Ursa Major. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. Based upon parallax measurements with the ''Hipparcos'' astrometry satellite, it is located at a distance of around from the Sun. It is more familiar to most observers in the northern hemisphere as the lower-left star forming the bowl of the Big Dipper, together with Alpha Ursae Majoris (Dubhe, upper-right), Beta Ursae Majoris (Merak, lower-right) and Delta Ursae Majoris (Megrez, upper-left). Along with four other stars in this well-known asterism, Phecda forms a loose association of stars known as the Ursa Major moving group. Like the other stars in the group, it is a main sequence star, as the Sun is, although somewhat hotter, brighter and larger. Phecda is located in relatively close physical pro ...
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MK Spectral Classification
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines. Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary mainly due to the temperature of the photosphere, although in some cases there are true abundance differences. The ''spectral class'' of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature. Most stars are currently classified under the Morgan–Keenan (MK) system using the letters ''O'', ''B'', ''A'', ''F'', ''G'', ''K'', and ''M'', a sequence from the hottest (''O'' type) to the coolest (''M'' type). Each letter class is then subdivided ...
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Color Index
In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. The lower the color index, the more blue (or hotter) the object is. Conversely, the larger the color index, the more red (or cooler) the object is. This is a consequence of the logarithmic magnitude scale, in which brighter objects have smaller (more negative) magnitudes than dimmer ones. For comparison, the whitish Sun has a B−V index of , whereas the bluish Rigel has a B−V of −0.03 (its B magnitude is 0.09 and its V magnitude is 0.12, B−V = −0.03). Traditionally, the color index uses Vega as a zero point. The blue supergiant Theta Muscae has one of the lowest B−V indices at −0.41, while the red giant and carbon star R Leporis has one of the largest, at +5.74. To measure the index, one observes the magnitude of an object successively through two different filters, such as U and B, or B and V, where U is ...
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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 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, such as the greenhouse effect. Star The effective temperature of a star is the temperature of a black body with the same luminosity per ''surface area'' () as the star and is defined according to the Stefan–Boltzmann law . Notice that the total ( bolometric) luminosity of a star is then , where is the stellar radius. The definition of the stellar radius is obviously not ...
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Luminosity
Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a star, galaxy, or other astronomical object, astronomical objects. In SI units, luminosity is measured in joules per second, or watts. In astronomy, values for luminosity are often given in the terms of the Solar luminosity, luminosity of the Sun, ''L''⊙. Luminosity can also be given in terms of the astronomical Magnitude (astronomy), magnitude system: the Absolute magnitude#Bolometric magnitude, absolute bolometric magnitude (''M''bol) of an object is a logarithmic measure of its total energy emission rate, while absolute magnitude is a logarithmic measure of the luminosity within some specific wavelength range or Passband, filter band. In contrast, the term ''brightness'' in astronomy is gene ...
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