The solar luminosity, L☉, is a unit of radiant flux conventionally used by astronomers to measure the luminosity of stars. It is defined in terms of the Suns output, one solar luminosity is 3. 828×1026 W. This does not include the solar luminosity, which would add 0.023 L☉. The Sun is a variable star, and its luminosity therefore fluctuates. The major fluctuation is the solar cycle that causes a periodic variation of about ±0. 1%. Other variations over the last 200–300 years are thought to be smaller than this. Solar luminosity is related to solar irradiance, Solar irradiance is responsible for the orbital forcing that causes the Milankovitch cycles, which determine Earthly glacial cycles. The mean irradiance at the top of the Earths atmosphere is known as the solar constant. Solar mass Solar radius Nuclear fusion Triple-alpha process Sackmann, I. -J, a Bright Young Sun Consistent with Helioseismology and Warm Temperatures on Ancient Earth and Mars, Astrophys. J.583, 1024–39, arXiv, astro-ph/0210128, Bibcode, 2003ApJ.583. 1024S, doi,10.
1086/345408 Foukal, P. Fröhlich, spruit, H. Wigley, T. M. L. Variations in solar luminosity and their effect on the Earths climate, Nature,443, 161–66, Bibcode, 2006Natur.443. 161F, doi,10. 1038/nature05072, PMID16971941 Pelletier, variations in Solar Luminosity from Timescales of Minutes to Months, Astrophys. J.463, L41–L45, arXiv, astro-ph/9510026, Bibcode, 1996ApJ. 463L. 41P, doi,10. 1086/310049 Stoykova, D. A. Shopov, ford, D. Georgiev, L. N. et al
A constellation is formally defined as a region of the celestial sphere, with boundaries laid down by the International Astronomical Union. The constellation areas mostly had their origins in Western-traditional patterns of stars from which the constellations take their names, in 1922, the International Astronomical Union officially recognized the 88 modern constellations, which cover the entire sky. They began as the 48 classical Greek constellations laid down by Ptolemy in the Almagest, Constellations in the far southern sky are late 16th- and mid 18th-century constructions. 12 of the 88 constellations compose the zodiac signs, though the positions of the constellations only loosely match the dates assigned to them in astrology. The term constellation can refer to the stars within the boundaries of that constellation. Notable groupings of stars that do not form a constellation are called asterisms, when astronomers say something is “in” a given constellation they mean it is within those official boundaries.
Any given point in a coordinate system can unambiguously be assigned to a single constellation. Many astronomical naming systems give the constellation in which an object is found along with a designation in order to convey a rough idea in which part of the sky it is located. For example, the Flamsteed designation for bright stars consists of a number, the word constellation seems to come from the Late Latin term cōnstellātiō, which can be translated as set of stars, and came into use in English during the 14th century. It denotes 88 named groups of stars in the shape of stellar-patterns, the Ancient Greek word for constellation was ἄστρον. Colloquial usage does not draw a distinction between constellation in the sense of an asterism and constellation in the sense of an area surrounding an asterism. The modern system of constellations used in astronomy employs the latter concept, the term circumpolar constellation is used for any constellation that, from a particular latitude on Earth, never sets below the horizon.
From the North Pole or South Pole, all constellations south or north of the equator are circumpolar constellations. In the equatorial or temperate latitudes, the term equatorial constellation has sometimes been used for constellations that lie to the opposite the circumpolar constellations. They generally include all constellations that intersect the celestial equator or part of the zodiac, usually the only thing the stars in a constellation have in common is that they appear near each other in the sky when viewed from the Earth. In galactic space, the stars of a constellation usually lie at a variety of distances, since stars travel on their own orbits through the Milky Way, the star patterns of the constellations change slowly over time. After tens to hundreds of thousands of years, their familiar outlines will become unrecognisable, the terms chosen for the constellation themselves, together with the appearance of a constellation, may reveal where and when its constellation makers lived.
The earliest direct evidence for the constellations comes from inscribed stones and it seems that the bulk of the Mesopotamian constellations were created within a relatively short interval from around 1300 to 1000 BC
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 used as an estimate of a bodys surface temperature when the bodys emissivity curve is not known. When the stars or planets net emissivity in the relevant wavelength band is less than unity, the net emissivity may be low due to surface or atmospheric properties, including greenhouse effect. Notice that the luminosity of a star is L =4 π R2 σ T e f f 4. The definition of the radius is obviously not straightforward. More rigorously the effective temperature corresponds to the temperature at the radius that is defined by a value of the Rosseland optical depth within the stellar atmosphere. The effective temperature and the bolometric luminosity are the two fundamental physical parameters needed to place a star on the Hertzsprung–Russell diagram, both effective temperature and bolometric luminosity depend on the chemical composition of a star.
The effective temperature of our Sun is around 5780 kelvin, stars have a decreasing temperature gradient, going from their central core up to the atmosphere. The core temperature of the temperature at the centre of the sun where nuclear reactions take place—is estimated to be 15,000,000 K. The effective temperature of a star indicates the amount of heat that the star radiates per unit of surface area, from the warmest surfaces to the coolest is the sequence of star types known as O, B, A, F, G, K, and M. The effective temperature of a planet can be calculated by equating the power received by the planet with the emitted by a blackbody of temperature T. Take the case of a planet at a distance D from the star and we allow the planet to reflect some of the incoming radiation by incorporating a parameter called the albedo. An albedo of 1 means that all the radiation is reflected, the effective temperature for Jupiter from this calculation is 112 K and 51 Pegasi b is 1258 K. A better estimate of effective temperature for some planets, such as Jupiter, the actual temperature depends on albedo and atmosphere effects.
The actual temperature from spectroscopic analysis for HD209458 b is 1130 K, the internal heating within Jupiter raises the effective temperature to about 152 K. The surface temperature of a planet can be estimated by modifying the effective-temperature calculation to account for emissivity and this area intercepts some of the power which is spread over the surface of a sphere of radius D. We allow the planet to some of the incoming radiation by incorporating a parameter a called the albedo. An albedo of 1 means that all the radiation is reflected, there is a factor ε, which is the emissivity and represents atmospheric effects
Monoceros is a faint constellation on the celestial equator. Its name is Greek for unicorn and its definition is attributed to the 17th-century Dutch cartographer Petrus Plancius. It is bordered by Orion to the west, Gemini to the north, Canis Major to the south, other bordering constellations include Canis Minor and Puppis. Monoceros is not easily seen with the eye, containing only a few fourth magnitude stars. Alpha Monocerotis has a magnitude of 3.93, slightly brighter than Gamma Monocerotis at 3.98. Beta Monocerotis is a star system, the three stars forming a triangle which seems to be fixed. The visual magnitudes of the stars are 4.7,5.2 and 6.1, William Herschel discovered it in 1781 and commented that it is one of the most beautiful sights in the heavens. Epsilon Monocerotis is a binary, with visual magnitudes of 4.5 and 6.5. S Monocerotis, or 15 Monocerotis, is a bluish white variable star and is located at the center of NGC2264, the variation in its magnitude is slight. It has a star of visual magnitude 8.
V838 Monocerotis, a red supergiant star, had an outburst starting on January 6,2002, in February of that year. After the outburst was over, the Hubble Space Telescope was able to observe a light echo, Monoceros contains Plasketts Star, which is a massive binary system whose combined mass is estimated to be that of almost 100 Suns put together. Monoceros contains two super-Earth exoplanets in one system, COROT-7b was detected by the COROT satellite and COROT-7c was detected by HARPS from ground-based telescopes. Until the announcement of Kepler-10b in January 2011, COROT-7b was the smallest exoplanet to have its diameter measured, both planets in this system were discovered in 2009. Monoceros contains many clusters and nebulae, most notable among them, M50 and it has an overall magnitude of 6.0 and is 4900 light-years from Earth. The Rosette Nebula, over 100 light-years in diameter, has a star cluster. It was independently discovered in the 1880s by Lewis Swift and Edward Emerson Barnard as they hunted for comets, the Christmas Tree Cluster is another open cluster in Monoceros.
Named for its resemblance to a Christmas tree, it is bright at an overall magnitude of 3.9
A variable star is a star whose brightness as seen from Earth fluctuates. Many, possibly most, stars have at least some variation in luminosity, an ancient Egyptian calendar of lucky and unlucky days composed some 3,200 years ago may be the oldest preserved historical document of the discovery of a variable star, the eclipsing binary Algol. This discovery, combined with supernovae observed in 1572 and 1604, proved that the sky was not eternally invariable as Aristotle. In this way, the discovery of variable stars contributed to the revolution of the sixteenth. The second variable star to be described was the eclipsing variable Algol, by Geminiano Montanari in 1669, chi Cygni was identified in 1686 by G. Kirch, R Hydrae in 1704 by G. D. Maraldi. By 1786 ten variable stars were known, John Goodricke himself discovered Delta Cephei and Beta Lyrae. Since 1850 the number of variable stars has increased rapidly, especially after 1890 when it became possible to identify variable stars by means of photography.
The latest edition of the General Catalogue of Variable Stars lists more than 46,000 variable stars in the Milky Way, as well as 10,000 in other galaxies, and over 10,000 suspected variables. The most common kinds of variability involve changes in brightness, but other types of variability occur, by combining light curve data with observed spectral changes, astronomers are often able to explain why a particular star is variable. Variable stars are generally analysed using photometry, spectrophotometry and spectroscopy, measurements of their changes in brightness can be plotted to produce light curves. Peak brightnesses in the curve are known as maxima, while troughs are known as minima. By estimating the magnitude and noting the time of observation a visual lightcurve can be constructed. The American Association of Variable Star Observers collects such observations from participants around the world, from the light curve the following data are derived, are the brightness variations periodical, irregular, or unique.
What is the period of the brightness fluctuations, what is the shape of the light curve. From the spectrum the following data are derived, what kind of star is it, what is its temperature, is it a single star, or a binary. does the spectrum change with time. In very few cases it is possible to make pictures of a stellar disk and these may show darker spots on its surface. Combining light curves with spectral data often gives a clue as to the changes occur in a variable star. For example, evidence for a star is found in its shifting spectrum because its surface periodically moves toward and away from us
In astronomy, declination is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declinations angle is measured north or south of the celestial equator, the root of the word declination means a bending away or a bending down. It comes from the root as the words incline and recline. Declination in astronomy is comparable to geographic latitude, projected onto the celestial sphere, points north of the celestial equator have positive declinations, while those south have negative declinations. Any units of measure can be used for declination, but it is customarily measured in the degrees, minutes. Declinations with magnitudes greater than 90° do not occur, because the poles are the northernmost and southernmost points of the celestial sphere, the Earths axis rotates slowly westward about the poles of the ecliptic, completing one circuit in about 26,000 years. This effect, known as precession, causes the coordinates of stationary celestial objects to change continuously, equatorial coordinates are inherently relative to the year of their observation, and astronomers specify them with reference to a particular year, known as an epoch.
Coordinates from different epochs must be rotated to match each other. The currently used standard epoch is J2000.0, which is January 1,2000 at 12,00 TT, the prefix J indicates that it is a Julian epoch. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, the declinations of Solar System objects change very rapidly compared to those of stars, due to orbital motion and close proximity. This similarly occurs in the Southern Hemisphere for objects with less than −90° − φ. An extreme example is the star which has a declination near to +90°. Circumpolar stars never dip below the horizon, there are other stars that never rise above the horizon, as seen from any given point on the Earths surface. Generally, if a star whose declination is δ is circumpolar for some observer, a star whose declination is −δ never rises above the horizon, as seen by the same observer. Likewise, if a star is circumpolar for an observer at latitude φ, neglecting atmospheric refraction, declination is always 0° at east and west points of the horizon.
At the north point, it is 90° − |φ|, and at the south point, from the poles, declination is uniform around the entire horizon, approximately 0°. Non-circumpolar stars are visible only during certain days or seasons of the year, the Suns declination varies with the seasons. As seen from arctic or antarctic latitudes, the Sun is circumpolar near the summer solstice, leading to the phenomenon of it being above the horizon at midnight
The components of proper motion in the equatorial coordinate system are measured in seconds of time for right ascension and seconds of arc in declination. Their combined value is computed as the proper motion, which is expressed in seconds of arc per year or per century. Knowledge of the motion and radial velocity allow approximate calculations of a stars true motion in space in respect to the Sun. Proper motion is not entirely proper, because it includes a component due to the motion of the Solar System itself, over the course of centuries, stars appear to maintain nearly fixed positions with respect to each other, so that they form the same constellations over historical time. Ursa Major or Crux, for example, looks nearly the same now as they did hundreds of years ago, precise long-term observations show that the constellations change shape, albeit very slowly, and that each star has an independent motion. This motion is caused by the movement of the relative to the Sun. The proper motion is a vector and is thus defined by two quantities, its position angle and its magnitude.
The first quantity indicates the direction of the motion on the celestial sphere. Proper motion may alternatively be defined by the changes per year in the stars right ascension and declination. The components of motion by convention are arrived at as follows. Suppose in a year an object moves from coordinates to coordinates, the changes of angle in seconds of arc per year are, The magnitude of the proper motion μ is given by vector addition of its components, where δ is the declination. The factor in cos δ accounts for the fact that the radius from the axis of the sphere to its surface varies as cos δ, for example, zero at the pole. Thus, the component of velocity parallel to the corresponding to a given angular change in α is smaller the further north the objects location. The change μα, which must be multiplied by cos δ to become a component of the motion, is sometimes called the proper motion in right ascension. Hence, the proper motions in right ascension and declination are made equivalent for straightforward calculations of various other stellar motions.
Position angle θ is related to these components by, Motions in equatorial coordinates can be converted to motions in galactic coordinates, for the majority of stars seen in the sky, the observed proper motions are usually small and unremarkable. Such stars are either faint or are significantly distant, have changes of below 10 milliarcseconds per year. A few do have significant motions, and are usually called high-proper motion stars, Motions can be in almost seemingly random directions
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 absorption lines, each line indicates an ion of a certain chemical element, with the line strength indicating the abundance of that ion. The relative abundance of the different ions varies with the temperature of the photosphere, the spectral class of a star is a short code summarizing the ionization state, giving an objective measure of the photospheres temperature and density. Most stars are classified under the Morgan–Keenan system using the letters O, B, A, F, G, K, and M. Each letter class is subdivided using a numeric digit with 0 being hottest and 9 being coolest. The sequence has been expanded with classes for other stars and star-like objects that do not fit in the system, such as class D for white dwarfs. In the MK system, a luminosity class is added to the class using Roman numerals.
This is based on the width of absorption lines in the stars spectrum. The full spectral class for the Sun is G2V, indicating a main-sequence star with a temperature around 5,800 K, the conventional color description takes into account only the peak of the stellar spectrum. This means that the assignment of colors of the spectrum can be misleading. There are no green, indigo, or violet stars, the brown dwarfs do not literally appear brown. The modern classification system is known as the Morgan–Keenan classification, each star is assigned a spectral class from the older Harvard spectral classification and a luminosity class using Roman numerals as explained below, forming the stars spectral type. The spectral classes O through M, as well as more specialized classes discussed later, are subdivided by Arabic numerals. For example, A0 denotes the hottest stars in the A class, fractional numbers are allowed, for example, the star Mu Normae is classified as O9.7. The Sun is classified as G2, the conventional color descriptions are traditional in astronomy, and represent colors relative to the mean color of an A-class star, which is considered to be white.
The apparent color descriptions are what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars. However, most stars in the sky, except the brightest ones, red supergiants are cooler and redder than dwarfs of the same spectral type, and stars with particular spectral features such as carbon stars may be far redder than any black body. O-, B-, and A-type stars are called early type
The unicorn is a legendary creature that has been described since antiquity as a beast with a single large, spiraling horn projecting from its forehead. The Bible describes an animal, the reem, which some versions translate as unicorn, in European folklore, the unicorn is often depicted as a white horse-like or goat-like animal with a long horn and cloven hooves. In the Middle Ages and Renaissance, it was described as an extremely wild woodland creature, a symbol of purity and grace. In the encyclopedias its horn was said to have the power to render poisoned water potable, in medieval and Renaissance times, the tusk of the narwhal was sometimes sold as unicorn horn. The earliest description is from Ctesias, who in his book Indika described them as wild asses, fleet of foot, having a horn a cubit and a half in length, and colored white and black. Aristotle must be following Ctesias when he mentions two one-horned animals, the oryx and the so-called Indian ass, strabo says that in the Caucasus there were one-horned horses with stag-like heads.
Cosmas Indicopleustes, a merchant of Alexandria who lived in the 6th century, made a voyage to India and he gives a description of a unicorn based on four brass figures in the palace of the King of Ethiopia. He states, from report, that it is impossible to take this ferocious beast alive, and that all its strength lies in its horn. When it finds itself pursued and in danger of capture, it throws itself from a precipice, and turns so aptly in falling, that it all the shock upon the horn. A one-horned animal is found on seals from the Indus Valley Civilization. Seals with such a design are thought to be a mark of social rank. Medieval knowledge of the fabulous beast stemmed from biblical and ancient sources, the predecessor of the medieval bestiary, compiled in Late Antiquity and known as Physiologus, popularized an elaborate allegory in which a unicorn, trapped by a maiden, stood for the Incarnation. As soon as the unicorn sees her, it lays its head on her lap and this became a basic emblematic tag that underlies medieval notions of the unicorn, justifying its appearance in every form of religious art.
Interpretations of the myth focus on the medieval lore of beguiled lovers, whereas some religious writers interpret the unicorn. The myths refer to a beast with one horn that can only be tamed by a virgin, with the rise of humanism, the unicorn acquired more orthodox secular meanings, emblematic of chaste love and faithful marriage. The Throne Chair of Denmark is made of unicorn horns – almost certainly narwhal tusks, the same material was used for ceremonial cups because the unicorns horn continued to be believed to neutralize poison, following classical authors. The unicorn, tamable only by a woman, was well established in medieval lore by the time Marco Polo described them as scarcely smaller than elephants. They have the hair of a buffalo and feet like an elephants and they have a single large black horn in the middle of the forehead
Scandium and yttrium are considered rare earth elements because they tend to occur in the same ore deposits as the lanthanides and exhibit similar chemical properties. They are not especially rare, but they tend to occur together in nature and are difficult to separate from one another, because of their geochemical properties, rare earth elements are typically dispersed and not often found concentrated as rare earth minerals in economically exploitable ore deposits. The first such mineral discovered was gadolinite, a composed of cerium, iron, silicon. This mineral was extracted from a mine in the village of Ytterby in Sweden, a table listing the seventeen rare earth elements, their atomic number and symbol, the etymology of their names, and their main usages is provided here. The distinction between the groups is more to do with atomic volume and geological behavior, Rare earth elements became known to the world with the discovery of the black mineral Ytterbite by Lieutenant Carl Axel Arrhenius in 1787, at a quarry in the village of Ytterby, Sweden.
Arrheniuss ytterbite reached Johan Gadolin, a Royal Academy of Turku professor, anders Gustav Ekeberg isolated beryllium from the gadolinite but failed to recognize other elements that the ore contained. After this discovery in 1794 a mineral from Bastnäs near Riddarhyttan, Sweden, in 1803 they obtained a white oxide and called it ceria. Martin Heinrich Klaproth independently discovered the same oxide and called it ochroia, in 1839 Carl Gustav Mosander, an assistant of Berzelius, separated ceria by heating the nitrate and dissolving the product in nitric acid. He called the oxide of the soluble salt lanthana and it took him three more years to separate the lanthana further into didymia and pure lanthana. Didymia, although not further separable by Mosanders techniques, was a mixture of oxides, in 1842 Mosander separated the yttria into three oxides, pure yttria and erbia. The earth giving pink salts he called terbium, the one that yielded yellow peroxide he called erbium, so in 1842 the number of known rare earth elements had reached six, cerium, didymium and terbium.
This confusion led to false claims of new elements, such as the mosandrium of J. Lawrence Smith. There were no further discoveries for 30 years, and the element didymium was listed in the table of elements with a molecular mass of 138. In 1879 Delafontaine used the new process of optical-flame spectroscopy. Also in 1879, the new element samarium was isolated by Paul Émile Lecoq de Boisbaudran from the mineral samarskite, the samaria earth was further separated by Lecoq de Boisbaudran in 1886 and a similar result was obtained by Jean Charles Galissard de Marignac by direct isolation from samarskite. They named the element gadolinium after Johan Gadolin, and its oxide was named gadolinia, the fractional crystallization of the oxides yielded europium in 1901. In 1839 the third source for rare earths became available and this is a mineral similar to gadolinite, uranotantalum. This mineral from Miass in the southern Ural Mountains was documented by Gustave Rose, the exact number of rare earth elements that existed was highly unclear, and a maximum number of 25 was estimated