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BL Herculis Variable
BL Herculis variables are a subclass of type II Cepheids with low luminosity and mass, that have a period of less than eight days. They are pulsating stars with light curves that frequently show a bump on the descending side for stars of the shortest periods and on the ascending side for longer period stars. Like other type II Cepheids, they are very old population II stars found in the galaxy’s halo and globular clusters. Also, compared to other type II Cepheids, BL Herculis variables have shorter periods and are fainter than W Virginis variables. Pulsating stars vary in spectral class as they vary in brightness and BL Herculis variables are normally class A at their brightest and class F when most dim. When plotted on the Hertzsprung–Russell diagram they fall in-between W Virginis and RR Lyrae variables. The prototype star, BL Herculis, varies between magnitude 9.7 and 10.6 in a period of 1.3 days. The brightest BL Herculis variables, with their maximum magnitudes, are: * ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Standard Candles
The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A ''direct'' distance measurement of an astronomical object is possible only for those objects that are "close enough" (within about a thousand parsecs) to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances. Several methods rely on a standard candle, which is an astronomical object that has a known luminosity. The ladder analogy arises because no single technique can measure distances at all ranges encountered in astronomy. Instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, and so on. Each rung of the ladder provides information that can be used to determine the distances at the next ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Astrometry
Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way. History The history of astrometry is linked to the history of star catalogues, which gave astronomers reference points for objects in the sky so they could track their movements. This can be dated back to Hipparchus, who around 190 BC used the catalogue of his predecessors Timocharis and Aristillus to discover Earth's precession. In doing so, he also developed the brightness scale still in use today. Hipparchus compiled a catalogue with at least 850 stars and their positions. Hipparchus's successor, Ptolemy, included a catalogue of 1,022 stars in his work the '' Almagest'', giving their location, coordinates, and brightness. In the 10th century, Abd al-Rahman al-Sufi carried out observations on the stars and described their positions, ma ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
W Virginis Variables
W Virginis variables are a subclass of Type II Cepheids which exhibit pulsation periods between 10–20 days,Wallerstein, G."The Cepheids of Population II and Related Stars" ''Publications of the Astronomical Society of the Pacific'', 114 p.689–699 (2002) and are of spectral class 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 ... F6 – K2.W. Strohmeier, ''Variable Stars'', Pergamon (1972)Soszyński, I.; Udalski, A.; Szymański, M. K.; Kubiak, M.; Pietrzyński, G.; Wyrzykowski, Ł.; Szewczyk, O.; Ulaczyk, K.; Poleski, R"The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. II.Type II Cepheids and Anomalous Cepheids in the Large Magellanic Cloud" ''Acta A.'', vol 58 (2008) They were first recognized as being distinct from classical Cepheids by Wilhel ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Metallicity
In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a convenient short term for ''"all elements except hydrogen and helium"''. This word-use is distinct from the conventional chemical or physical definition of a metal as an electrically conducting solid. Stars and nebulae with relatively high abundances of heavier elements are called "metal-rich" in astrophysical terms, even though many of those elements are nonmetals in chemistry. The presence of heavier elements hails from stellar nucleosynthesis, where the majority of elements heavier than hydrogen and helium in the Universe (''metals'', hereafter) are formed in the cores of stars as they evolve. Over time, stellar winds and supernovae deposit the metals into the surrounding environment, enriching the interstellar medium and providing ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
BD Cassiopeiae
BD, Bd or bd may refer to: In arts and entertainment * B. D. (Doonesbury), a major character in the ''Doonesbury'' comic strip * ''Bande dessinée'' (or "bédé"), a French term for comics * Bass drum, in sheet music notation * Brahe Djäknar, a Finnish choir * Broder Daniel, a Swedish indie pop band * ''Ben Drowned'', a web serial and web series, focused on the character of the same name * ВD, shorthand name for the Russian gaming magazine, ''Velikij Drakon'', where the "В" character is actually the Russian letter "ve". * Bette Davis's production company In business Business / Technology * B&D Australia, manufacturing company * Big data, a marketing term for technology of large data sets * Broker-dealer * Business day, a day of the week on which business is conducted * Business development, techniques aimed at attracting customers and penetrating markets * Business directory, a website or printed listing of information which lists all businesses within some category Busin ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
RT Trianguli Australis
RT Trianguli Australis, or RT TrA, is a BL Herculis variable (type II Cepheid) in the constellation of Triangulum Australe. Variable RT TrA varies between apparent magnitudes 9.4 and 10.2 over a period of 1.95 days. It was first discovered to be variable by Annie Jump Cannon in 1910 and initially classified as an RR Lyrae variable. Later authors segregated it and the similar V533 Centauri as RW Aurigae stars. In time it became clear that RT TrA was unrelated to RW Aur, instead being a member of a group of stars on the instability strip somewhat above the horizontal branch. These stars were then named as a group after BL Herculis, the brightest known member. BL Her stars have periods shorter than eight days. Like other BL Her variables, the light curve of RT TrA has a hump, in this case on the descending branch. The light curve is slightly asymmetric, with the minimum occurring at phase 0.6. Properties RT TrA is a cool giant star with a radius of , although its spectral ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
