1.
Messier object
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The Messier objects are a set of over 100 astronomical objects first listed by French astronomer Charles Messier in 1771. The number of objects in the lists he published reached 103, a similar list had been published in 1654 by Giovanni Hodierna, but attracted attention only recently and was probably not known to Messier. The first edition covered 45 objects numbered M1 to M45, the first such addition came from Nicolas Camille Flammarion in 1921, who added Messier 104 after finding a note Messier made in a copy of the 1781 edition of the catalogue. M105 to M107 were added by Helen Sawyer Hogg in 1947, M108 and M109 by Owen Gingerich in 1960, M102 was observed by Méchain, who communicated his notes to Messier. Méchain later concluded that this object was simply a re-observation of M101, though sources suggest that the object Méchain observed was the galaxy NGC5866. Messiers final catalogue was included in the Connaissance des Temps for 1784 and these objects are still known by their Messier number from this list. Messier lived and did his work at the Hôtel de Cluny. The list he compiled contains only objects found in the sky area he could observe and he did not observe or list objects visible only from farther south, such as the Large and Small Magellanic Clouds. A summary of the astrophysics of each Messier object can be found in the Concise Catalog of Deep-sky Objects, in early spring, astronomers sometimes gather for Messier marathons, when all of the objects can be viewed over a single night
2.
Constellation
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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 also 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 also 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
3.
Canes Venatici (constellation)
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Canes Venatici /ˈkeɪniːz vᵻˈnætᵻsaɪ/ is one of the 88 official modern constellations. It is a northern constellation that was created by Johannes Hevelius in the 17th century. Its name is Latin for hunting dogs, and the constellation is depicted in illustrations as representing the dogs of Boötes the Herdsman. The stars of Canes Venatici are not bright, in classical times, they were listed by Ptolemy as unfigured stars below the constellation Ursa Major in his star catalogue. In medieval times, the identification of these stars with the dogs of Boötes arose through a mistranslation, some of Boötess stars were traditionally described as representing the club of Boötes. When the Arabic text was translated into Latin, the translator Gerard of Cremona mistook the Arabic word كلاب for kilāb, meaning dogs. In 1533, the German astronomer Peter Apian depicted Boötes as having two dogs with him and these spurious dogs floated about the astronomical literature until Hevelius decided to specify their presence in the sky by making them a separate constellation in 1687. Hevelius chose the name Asterion for the dog and Chara for the southern dog, as Canes Venatici. In his star catalogue, the Czech astronomer Bečvář assigned the names Asterion to β CVn, Canes Venatici is bordered by Ursa Major to the north and west, Coma Berenices to the south, and Boötes to the east. The three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is CVn, the official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 14 sides. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 12h 06. 2m and 14h 07. 3m, while the coordinates are between +27. 84° and +52. 36°. Covering 465 square degrees, it ranks 38th of the 88 constellations in size, Canes Venatici contains no bright stars, α and β CVn being only of 3rd and 4th magnitude respectively. Flamsteed catalogued 25 stars in the constellation, labelling them 1 to 25 Canum Venaticorum, α CVn is the constellations brightest star, named by Sir Charles Scarborough in memory of King Charles I, the deposed king of Britain. Legend has it that α CVn was brighter than usual during the Restoration, cor Caroli is a wide double star, with a primary of magnitude 2.9 and a secondary of magnitude 5.6, the primary is 110 light-years from Earth. The primary also has an unusually strong variable magnetic field, β CVn is a yellow-hued main sequence star of magnitude 4.2,27 light-years from Earth. Its common name comes from the word for joy, Y CVn is a semiregular variable star that varies between magnitudes 5.0 and 6.5 over a period of around 158 days. It is a star and is being deep red in color. AM CVn, a blue star of magnitude 14, is the prototype of a special class of cataclysmic variable stars, in which the companion star is a white dwarf
4.
Right ascension
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Right ascension is the angular distance measured eastward along the celestial equator from the vernal equinox to the hour circle of the point in question. When combined with declination, these astronomical coordinates specify the direction of a point on the sphere in the equatorial coordinate system. Right ascension is the equivalent of terrestrial longitude. Both right ascension and longitude measure an angle from a direction on an equator. Right ascension is measured continuously in a circle from that equinox towards the east. Any units of measure could have been chosen for right ascension, but it is customarily measured in hours, minutes. Astronomers have chosen this unit to measure right ascension because they measure a stars location by timing its passage through the highest point in the sky as the Earth rotates. The highest point in the sky, called meridian, is the projection of a line onto the celestial sphere. A full circle, measured in units, contains 24 × 60 × 60 = 86 400s, or 24 × 60 = 1 440m. Because right ascensions are measured in hours, they can be used to time the positions of objects in the sky. For example, if a star with RA = 01h 30m 00s is on the meridian, sidereal hour angle, used in celestial navigation, is similar to right ascension, but increases westward rather than eastward. Usually measured in degrees, it is the complement of right ascension with respect to 24h and it is important not to confuse sidereal hour angle with the astronomical concept of hour angle, which measures angular distance of an object westward from the local meridian. The Earths axis rotates slowly westward about the poles of the ecliptic and this effect, known as precession, causes the coordinates of stationary celestial objects to change continuously, if rather slowly. Therefore, equatorial coordinates are inherently relative to the year of their observation, coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch. The right ascension of Polaris is increasing quickly, the North Ecliptic Pole in Draco and the South Ecliptic Pole in Dorado are always at right ascension 18h and 6h respectively. 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 concept of right ascension has been known at least as far back as Hipparchus who measured stars in equatorial coordinates in the 2nd century BC. But Hipparchus and his successors made their star catalogs in ecliptic coordinates, the easiest way to do that is to use an equatorial mount, which allows the telescope to be aligned with one of its two pivots parallel to the Earths axis
5.
Declination
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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, therefore, 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, conversely, 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, then 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
6.
Angular diameter
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The angular diameter or apparent size is an angular measurement describing how large a sphere or circle appears from a given point of view. In the vision sciences it is called the angle and in optics it is the angular aperture. The angular diameter can alternatively be thought of as the angle through which an eye or camera must rotate to look from one side of an apparent circle to the opposite side, Angular radius equals half the angular diameter. When D ≫ d, we have δ ≈ d / D, for practical use, the distinction is only significant for spherical objects that are relatively close, since the small-angle approximation holds for x ≪1, arcsin x ≈ arctan x ≈ x. Estimates of angular diameter may be obtained by holding the hand at right angles to an extended arm. In astronomy the sizes of objects in the sky are given in terms of their angular diameter as seen from Earth. Since these angular diameters are typically small, it is common to present them in arcseconds, an arcsecond is 1/3600th of one degree, and a radian is 180/ π degrees, so one radian equals 3600*180/ π arcseconds, which is about 206265 arcseconds. Therefore, the diameter of an object with physical diameter d at a distance D, expressed in arcseconds, is given by. These objects have a diameter of one arcsecond, an object of diameter 725. The angular diameter of the Sun, from a distance of one light-year, is 0. 03″, the angular diameter 0. 03″ of the Sun given above is approximately the same as that of a person at a distance of the diameter of the Earth. Thus the angular diameter of the Sun is about 250,000 times that of Sirius, the angular diameter of the Sun is also about 250,000 times that of Alpha Centauri A. The angular diameter of the Sun is about the same as that of the Moon, even though Pluto is physically larger than Ceres, when viewed from Earth Ceres has a much larger apparent size. While angular sizes measured in degrees are useful for larger patches of sky, we need much finer units when talking about the size of galaxies. The Moons motion across the sky can be measured in size, approximately 15 degrees every hour. A one-mile-long line painted on the face of the Moon would appear to us to be about one arc-second in length, in astronomy, it is typically difficult to directly measure the distance to an object. But the object may have a physical size and a measurable angular diameter. In that case, the angular diameter formula can be inverted to yield the Angular diameter distance to distant objects as d ≡2 D tan . In non-Euclidean space, such as our universe, the angular diameter distance is only one of several definitions of distance
7.
Minute and second of arc
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A minute of arc, arcminute, arc minute, or minute arc is a unit of angular measurement equal to 1/60 of one degree. Since one degree is 1/360 of a turn, one minute of arc is 1/21600 of a turn, a second of arc, arcsecond, or arc second is 1/60 of an arcminute, 1/3600 of a degree, 1/1296000 of a turn, and π/648000 of a radian. To express even smaller angles, standard SI prefixes can be employed, the number of square arcminutes in a complete sphere is 4 π2 =466560000 π ≈148510660 square arcminutes. The standard symbol for marking the arcminute is the prime, though a single quote is used where only ASCII characters are permitted. One arcminute is thus written 1′ and it is also abbreviated as arcmin or amin or, less commonly, the prime with a circumflex over it. The standard symbol for the arcsecond is the prime, though a double quote is commonly used where only ASCII characters are permitted. One arcsecond is thus written 1″ and it is also abbreviated as arcsec or asec. In celestial navigation, seconds of arc are used in calculations. This notation has been carried over into marine GPS receivers, which normally display latitude and longitude in the format by default. An arcsecond is approximately the angle subtended by a U. S. dime coin at a distance of 4 kilometres, a milliarcsecond is about the size of a dime atop the Eiffel Tower as seen from New York City. A microarcsecond is about the size of a period at the end of a sentence in the Apollo mission manuals left on the Moon as seen from Earth, since antiquity the arcminute and arcsecond have been used in astronomy. The principal exception is Right ascension in equatorial coordinates, which is measured in units of hours, minutes. These small angles may also be written in milliarcseconds, or thousandths of an arcsecond, the unit of distance, the parsec, named from the parallax of one arcsecond, was developed for such parallax measurements. It is the distance at which the radius of the Earths orbit would subtend an angle of one arcsecond. The ESA astrometric space probe Gaia is hoped to measure star positions to 20 microarcseconds when it begins producing catalog positions sometime after 2016, there are about 1.3 trillion µas in a turn. Currently the best catalog positions of stars actually measured are in terms of milliarcseconds, apart from the Sun, the star with the largest angular diameter from Earth is R Doradus, a red supergiant with a diameter of 0.05 arcsecond. The dwarf planet Pluto has proven difficult to resolve because its angular diameter is about 0.1 arcsecond, space telescopes are not affected by the Earths atmosphere but are diffraction limited. For example, the Hubble space telescope can reach a size of stars down to about 0. 1″
8.
Apparent magnitude
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The apparent magnitude of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth. The brighter an object appears, the lower its magnitude value, the Sun, at apparent magnitude of −27, is the brightest object in the sky. It is adjusted to the value it would have in the absence of the atmosphere, furthermore, the magnitude scale is logarithmic, a difference of one in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry, apparent magnitudes are used to quantify the brightness of sources at ultraviolet, visible, and infrared wavelengths. An apparent magnitude is measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V filter band would be denoted either as mV or often simply as V, the scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes. The brightest stars in the sky were said to be of first magnitude, whereas the faintest were of sixth magnitude. Each grade of magnitude was considered twice the brightness of the following grade and this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest, and is generally believed to have originated with Hipparchus. This implies that a star of magnitude m is 2.512 times as bright as a star of magnitude m +1 and this figure, the fifth root of 100, became known as Pogsons Ratio. The zero point of Pogsons scale was defined by assigning Polaris a magnitude of exactly 2. However, with the advent of infrared astronomy it was revealed that Vegas radiation includes an Infrared excess presumably due to a disk consisting of dust at warm temperatures. At shorter wavelengths, there is negligible emission from dust at these temperatures, however, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the scale was extrapolated to all wavelengths on the basis of the black body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance for the zero magnitude point, with the modern magnitude systems, brightness over a very wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30, astronomers have developed other photometric zeropoint systems as alternatives to the Vega system. The AB magnitude zeropoint is defined such that an objects AB, the dimmer an object appears, the higher the numerical value given to its apparent magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of exactly 100. Since an increase of 5 magnitudes corresponds to a decrease in brightness by a factor of exactly 100, each magnitude increase implies a decrease in brightness by the factor 5√100 ≈2.512. Inverting the above formula, a magnitude difference m1 − m2 = Δm implies a brightness factor of F2 F1 =100 Δ m 5 =100.4 Δ m ≈2.512 Δ m
9.
Galaxy morphological classification
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Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. The Hubble sequence is a classification scheme for galaxies invented by Edwin Hubble in 1926. It is often known colloquially as the “Hubble tuning-fork” because of the shape in which it is traditionally represented, Hubble’s scheme divides galaxies into three broad classes based on their visual appearance, Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in images. They are denoted by the letter E, followed by an integer n representing their degree of ellipticity on the sky. Spiral galaxies consist of a disk, with stars forming a spiral structure, and a central concentration of stars known as the bulge. They are given the symbol S, roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge. These barred spirals are given the symbol SB and these broad classes can be extended to enable finer distinctions of appearance and to encompass other types of galaxies, such as irregular galaxies, which have no obvious regular structure. The Hubble sequence is represented in the form of a two-pronged fork, with the ellipticals on the left. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the two meet the “handle”. To this day, the Hubble sequence is the most commonly used system for classifying galaxies, the de Vaucouleurs system for classifying galaxies is a widely used extension to the Hubble sequence, first described by Gérard de Vaucouleurs in 1959. In particular, he argued that rings and lenses are important structural components of spiral galaxies, the de Vaucouleurs system retains Hubble’s basic division of galaxies into ellipticals, lenticulars, spirals and irregulars. For example, a barred spiral galaxy with loosely wound arms. Visually, the de Vaucouleurs system can be represented as a version of Hubble’s tuning fork, with stage on the x-axis, family on the y-axis. De Vaucouleurs also assigned numerical values to each class of galaxy in his scheme, values of the numerical Hubble stage T run from −6 to +10, with negative numbers corresponding to early-type galaxies and positive numbers to late types. Elliptical galaxies are divided into three stages, compact ellipticals, normal ellipticals and late types, lenticulars are similarly subdivided into early, intermediate and late types. Irregular galaxies can be of type magellanic irregulars or compact, the use of numerical stages allows for more quantitative studies of galaxy morphology. Created by American astronomer William Wilson Morgan, together with Philip Keenan, Morgan developed the MK system for the classification of stars through their spectra. The Yerkes scheme uses the spectra of stars in the galaxy, the shape, real and apparent, thus, for example, the Andromeda Galaxy is classified as kS5. H
10.
Astrometry
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Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of the Solar System and our galaxy, 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, 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, hipparchuss successor, Ptolemy, included a catalogue of 1,022 stars in his work the Almagest, giving their location, coordinates, and brightness. Ibn Yunus observed more than 10,000 entries for the Suns position for years using a large astrolabe with a diameter of nearly 1.4 metres. In the 15th century, the Timurid astronomer Ulugh Beg compiled the Zij-i-Sultani, like the earlier catalogs of Hipparchus and Ptolemy, Ulugh Begs catalogue is estimated to have been precise to within approximately 20 minutes of arc. In the 16th century, Tycho Brahe used improved instruments, including large mural instruments, to measure star positions more accurately than previously, Taqi al-Din measured the right ascension of the stars at the Istanbul observatory of Taqi al-Din using the observational clock he invented. When telescopes became commonplace, setting circles sped measurements James Bradley first tried to measure stellar parallaxes in 1729, the stellar movement proved too insignificant for his telescope, but he instead discovered the aberration of light and the nutation of the Earths axis. His cataloguing of 3222 stars was refined in 1807 by Friedrich Bessel and he made the first measurement of stellar parallax,0.3 arcsec for the binary star 61 Cygni. Being very difficult to measure, only about 60 stellar parallaxes had been obtained by the end of the 19th century, astrographs using astronomical photographic plates sped the process in the early 20th century. Automated plate-measuring machines and more sophisticated technology of the 1960s allowed more efficient compilation of star catalogues. In the 1980s, charge-coupled devices replaced photographic plates and reduced optical uncertainties to one milliarcsecond and this technology made astrometry less expensive, opening the field to an amateur audience. In 1989, the European Space Agencys Hipparcos satellite took astrometry into orbit, operated from 1989 to 1993, Hipparcos measured large and small angles on the sky with much greater precision than any previous optical telescopes. During its 4-year run, the positions, parallaxes, and proper motions of 118,218 stars were determined with a degree of accuracy. A new Tycho catalog drew together a database of 1,058,332 to within 20-30 mas, additional catalogues were compiled for the 23,882 double/multiple stars and 11,597 variable stars also analyzed during the Hipparcos mission. Today, the catalogue most often used is USNO-B1.0, during the past 50 years,7,435 Schmidt camera plates were used to complete several sky surveys that make the data in USNO-B1.0 accurate to within 0.2 arcsec. In observational astronomy, astrometric techniques help identify stellar objects by their unique motions and it is instrumental for keeping time, in that UTC is basically the atomic time synchronized to Earths rotation by means of exact observations. Astrometry is an important step in the distance ladder because it establishes parallax distance estimates for stars in the Milky Way
11.
Radial velocity
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The radial velocity of an object with respect to a given point is the rate of change of the distance between the object and the point. That is, the velocity is the component of the objects velocity that points in the direction of the radius connecting the object. In astronomy, the point is taken to be the observer on Earth. In astronomy, radial velocity is measured to the first order of approximation by Doppler spectroscopy. The quantity obtained by this method may be called the barycentric radial-velocity measure or spectroscopic radial velocity, by contrast, astrometric radial velocity is determined by astrometric observations. A positive radial velocity indicates the distance between the objects is or was increasing, a radial velocity indicates the distance between the source and observer is or was decreasing. In many binary stars, the orbital motion usually causes radial velocity variations of several kilometers per second, as the spectra of these stars vary due to the Doppler effect, they are called spectroscopic binaries. Radial velocity can be used to estimate the ratio of the masses of the stars and it has been suggested that planets with high eccentricities calculated by this method may in fact be two-planet systems of circular or near-circular resonant orbit. When the star moves towards us, its spectrum is blueshifted, by regularly looking at the spectrum of a star—and so, measuring its velocity—it can be determined, if it moves periodically due to the influence of a companion. From the instrumental perspective, velocities are measured relative to the telescopes motion, in the case of spectroscopic measurements corrections of the order of ±20 cm/s with respect to aberration. Proper motion Peculiar velocity Relative velocity The Radial Velocity Equation in the Search for Exoplanets
12.
Redshift
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In physics, redshift happens when light or other electromagnetic radiation from an object is increased in wavelength, or shifted to the red end of the spectrum. Some redshifts are an example of the Doppler effect, familiar in the change of apparent pitches of sirens, a redshift occurs whenever a light source moves away from an observer. Finally, gravitational redshift is an effect observed in electromagnetic radiation moving out of gravitational fields. However, redshift is a common term and sometimes blueshift is referred to as negative redshift. Knowledge of redshifts and blueshifts has been applied to develop several terrestrial technologies such as Doppler radar and radar guns, Redshifts are also seen in the spectroscopic observations of astronomical objects. Its value is represented by the letter z, a special relativistic redshift formula can be used to calculate the redshift of a nearby object when spacetime is flat. However, in contexts, such as black holes and Big Bang cosmology. Special relativistic, gravitational, and cosmological redshifts can be understood under the umbrella of frame transformation laws, the history of the subject began with the development in the 19th century of wave mechanics and the exploration of phenomena associated with the Doppler effect. The effect is named after Christian Doppler, who offered the first known physical explanation for the phenomenon in 1842, the hypothesis was tested and confirmed for sound waves by the Dutch scientist Christophorus Buys Ballot in 1845. Doppler correctly predicted that the phenomenon should apply to all waves, before this was verified, however, it was found that stellar colors were primarily due to a stars temperature, not motion. Only later was Doppler vindicated by verified redshift observations, the first Doppler redshift was described by French physicist Hippolyte Fizeau in 1848, who pointed to the shift in spectral lines seen in stars as being due to the Doppler effect. The effect is called the Doppler–Fizeau effect. In 1868, British astronomer William Huggins was the first to determine the velocity of a moving away from the Earth by this method. In 1871, optical redshift was confirmed when the phenomenon was observed in Fraunhofer lines using solar rotation, about 0.1 Å in the red. In 1887, Vogel and Scheiner discovered the annual Doppler effect, in 1901, Aristarkh Belopolsky verified optical redshift in the laboratory using a system of rotating mirrors. The word does not appear unhyphenated until about 1934 by Willem de Sitter, perhaps indicating that up to point its German equivalent. Beginning with observations in 1912, Vesto Slipher discovered that most spiral galaxies, Slipher first reports on his measurement in the inaugural volume of the Lowell Observatory Bulletin. Three years later, he wrote a review in the journal Popular Astronomy, Slipher reported the velocities for 15 spiral nebulae spread across the entire celestial sphere, all but three having observable positive velocities
13.
Cosmic distance ladder
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The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an object is possible only for those objects that are close enough to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at distances and methods that work at larger distances. Several methods rely on a 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 higher rung, at the base of the ladder are fundamental distance measurements, in which distances are determined directly, with no physical assumptions about the nature of the object in question. The precise measurement of stellar positions is part of the discipline of astrometry, direct distance measurements are based upon the astronomical unit, which is the distance between the Earth and the Sun. Historically, observations of transits of Venus were crucial in determining the AU, in the first half of the 20th century, observations of asteroids were also important. Keplers laws provide precise ratios of the sizes of the orbits of objects orbiting the Sun, radar is used to measure the distance between the orbits of the Earth and of a second body. From that measurement and the ratio of the two sizes, the size of Earths orbit is calculated. The Earths orbit is known with a precision of a few meters, the most important fundamental distance measurements come from trigonometric parallax. As the Earth orbits the Sun, the position of stars will appear to shift slightly against the more distant background. These shifts are angles in a triangle, with 2 AU making the base leg of the triangle. The amount of shift is small, measuring 1 arcsecond for an object at the 1 parsec distance of the nearest stars. Astronomers usually express distances in units of parsecs, light-years are used in popular media, because parallax becomes smaller for a greater stellar distance, useful distances can be measured only for stars whose parallax is larger than a few times the precision of the measurement. Parallax measurements typically have an accuracy measured in milliarcseconds, the Hubble telescope WFC3 now has the potential to provide a precision of 20 to 40 microarcseconds, enabling reliable distance measurements up to 5,000 parsecs for small numbers of stars. By the early 2020s, the GAIA space mission will provide similarly accurate distances to all bright stars. Stars have a velocity relative to the Sun that causes proper motion, for a group of stars with the same spectral class and a similar magnitude range, a mean parallax can be derived from statistical analysis of the proper motions relative to their radial velocities
14.
Parsec
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The parsec is a unit of length used to measure large distances to objects outside the Solar System. One parsec is the distance at which one astronomical unit subtends an angle of one arcsecond, a parsec is equal to about 3.26 light-years in length. The nearest star, Proxima Centauri, is about 1.3 parsecs from the Sun, most of the stars visible to the unaided eye in the nighttime sky are within 500 parsecs of the Sun. The parsec unit was likely first suggested in 1913 by the British astronomer Herbert Hall Turner, named from an abbreviation of the parallax of one arcsecond, it was defined so as to make calculations of astronomical distances quick and easy for astronomers from only their raw observational data. Partly for this reason, it is still the unit preferred in astronomy and astrophysics, though the light-year remains prominent in science texts. This corresponds to the definition of the parsec found in many contemporary astronomical references. Derivation, create a triangle with one leg being from the Earth to the Sun. As that point in space away, the angle between the Sun and Earth decreases. A parsec is the length of that leg when the angle between the Sun and Earth is one arc-second. One of the oldest methods used by astronomers to calculate the distance to a star is to record the difference in angle between two measurements of the position of the star in the sky. The first measurement is taken from the Earth on one side of the Sun, and the second is approximately half a year later. The distance between the two positions of the Earth when the two measurements were taken is twice the distance between the Earth and the Sun. The difference in angle between the two measurements is twice the angle, which is formed by lines from the Sun. Then the distance to the star could be calculated using trigonometry. 5-parsec distance of 61 Cygni, the parallax of a star is defined as half of the angular distance that a star appears to move relative to the celestial sphere as Earth orbits the Sun. Equivalently, it is the angle, from that stars perspective. The star, the Sun and the Earth form the corners of a right triangle in space, the right angle is the corner at the Sun. Therefore, given a measurement of the angle, along with the rules of trigonometry. A parsec is defined as the length of the adjacent to the vertex occupied by a star whose parallax angle is one arcsecond
15.
Star catalogue
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A star catalogue or star catalog, 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 catalogues were compiled by many different ancient peoples, including the Babylonians, Greeks, Chinese, Persians, 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, from their existing records, it is known that the ancient Egyptians recorded the names of only a few identifiable constellations and a list of thirty-six decans that were used as a star clock. They are better known by their Assyrian-era name Three Stars Each and these star catalogues, written on clay tablets, listed thirty-six stars, twelve for Anu along the celestial equator, twelve for Ea south of that, and twelve for Enlil to the north. In Ancient Greece, the astronomer and mathematician Eudoxus laid down a set of the classical constellations around 370 BC. His catalogue Phaenomena, rewritten by Aratus of Soli between 275 and 250 BC as a poem, became one of the most consulted astronomical texts in antiquity. It contains descriptions of the positions of the stars, the shapes of the constellations, approximately in the 3rd century BC, the Greek astronomers Timocharis of Alexandria and Aristillus created another star catalogue. Hipparchus completed his star catalogue in 129 BC, which he compared to Timocharis and this led him to determine the first value of the precession of the equinoxes. In the 2nd century, Ptolemy of Roman Egypt published a star catalogue as part of his Almagest, ptolemys catalogue was based almost entirely on an earlier one by Hipparchus. It remained the star catalogue in the Western and Arab worlds for over eight centuries. The earliest known inscriptions for Chinese star names were written on oracle bones, sources dating from the Zhou Dynasty which provide star names include the Zuo Zhuan, the Shi Jing, and the Canon of Yao in the Book of Documents. The Lüshi Chunqiu written by the Qin statesman Lü Buwei provides most of the names for the twenty-eight mansions, an earlier lacquerware chest found in the Tomb of Marquis Yi of Zeng contains a complete list of the names of the twenty-eight mansions. Star catalogues are traditionally attributed to Shi Shen and Gan De, the Shi Shen astronomy is attributed to Shi Shen, and the Astronomic star observation to Gan De. It was not until the Han Dynasty that astronomers started to observe and record names for all the stars that were apparent in the night sky, not just those around the ecliptic. A star catalogue is featured in one of the chapters of the late 2nd-century-BC history work Records of the Grand Historian by Sima Qian and contains the schools of Shi Shen and Gan Des work. For his Spiritual Constitution of the Universe of 120 AD, the astronomer Zhang Heng compiled a star catalogue comprising 124 constellations, Chinese constellation names were later adopted by the Koreans and Japanese. A large number of star catalogues were published by Muslim astronomers in the medieval Islamic world and these were mainly Zij treatises, including Arzachels Tables of Toledo, the Maragheh observatorys Zij-i Ilkhani and Ulugh Begs Zij-i-Sultani
16.
New General Catalogue
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The NGC contains 7,840 objects, known as the NGC objects. It is one of the largest comprehensive catalogues, as it includes all types of space objects and is not confined to, for example. Dreyer also published two supplements to the NGC in 1895 and 1908, known as the Index Catalogues, describing a further 5,386 astronomical objects. Objects in the sky of the southern hemisphere are catalogued somewhat less thoroughly, the Revised New General Catalogue and Index Catalogue was compiled in 2009 by Wolfgang Steinicke. The original New General Catalogue was compiled during the 1880s by John Louis Emil Dreyer using observations from William Herschel and his son John, Dreyer had already published a supplement to Herschels General Catalogue of Nebulae and Clusters, containing about 1,000 new objects. In 1886, he suggested building a second supplement to the General Catalogue and this led to the publication of the New General Catalogue in the Memoirs of the Royal Astronomical Society in 1888. Assembling the NGC was a challenge, as Dreyer had to deal with many contradicting and unclear reports, while he did check some himself, the sheer number of objects meant Dreyer had to accept them as published by others for the purpose of his compilation. Dreyer was a careful transcriber and made few errors himself, and he was very thorough in his referencing, which allowed future astronomers to review the original references and publish corrections to the original NGC. The first major update to the NGC is the Index Catalogue of Nebulae and Clusters of Stars and it serves as a supplement to the NGC, and contains an additional 5,386 objects, collectively known as the IC objects. It summarizes the discoveries of galaxies, clusters and nebulae between 1888 and 1907, most of them made possible by photography, a list of corrections to the IC was published in 1912. The Revised New Catalogue of Nonstellar Astronomical Objects was compiled by Jack W. Sulentic and William G. Tifft in the early 1970s, and was published in 1973, as an update to the NGC. However, because the update had to be completed in just three summers, it failed to incorporate several previously-published corrections to the NGC data, and even introduced new errors. NGC2000.0 is a 1988 compilation of the NGC and IC made by Roger W. Sinnott and it incorporates several corrections and errata made by astronomers over the years. However, it too ignored the original publications and favoured modern corrections, the NGC/IC Project is a collaboration formed in 1993. It aims to identify all NGC and IC objects, and collect images, the Revised New General Catalogue and Index Catalogue is a compilation made by Wolfgang Steinicke in 2009. It is considered one of the most comprehensive and authoritative treatments of the NGC, messier object Catalogue of Nebulae and Clusters of Stars The Interactive NGC Catalog Online Adventures in Deep Space, Challenging Observing Projects for Amateur Astronomers
17.
Uppsala General Catalogue
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The Uppsala General Catalogue of Galaxies is a catalogue of 12,921 galaxies visible from the northern hemisphere. It was first published in 1973, the catalogue includes essentially all galaxies north of declination -02°30 and to a limiting diameter of 1.0 arcminute or to a limiting apparent magnitude of 14.5. The primary source of data is the blue prints of the Palomar Observatory Sky Survey and it also includes galaxies smaller than 1.0 arcminute in diameter but brighter than 14.5 magnitude from the Catalogue of Galaxies and of Clusters of Galaxies. The catalogue contains descriptions of the galaxies and their areas, plus conventional system classifications. Galaxy diameters are included and the classifications and descriptions are given in such a way as to provide as accurate an account as possible of the appearance of the galaxies on the prints, the accuracy of coordinates is only what is necessary for identifications purposes. There is an addendum to the catalogue called Uppsala General Catalogue Addendum which is abbreviated as UGCA, Uppsala Uppsala Astronomical Observatory Royal Society of Sciences in Uppsala
18.
Principal Galaxies Catalogue
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The Catalogue of Principal Galaxies is an astronomical catalog published in 1989 that lists B1950 and J2000 equatorial coordinates and cross-identifications for 73,197 galaxies. It is based on the Lyon-Meudon Extragalactic Database, which was started in 1983. 40,932 coordinates have standard deviations smaller than 10″, a total of 131,601 names from the 38 most common sources are listed. The Lyon-Meudon Extragalactic Database was eventually expanded into HyperLEDA, a database of a few million galaxies, Galaxies in the original PGC catalogue are numbered with their original PGC number in HyperLEDA. Numbers have also assigned for the other galaxies, although for those galaxies not in the original PGC catalogue. PGC6240 is a lenticular galaxy in the constellation Hydrus. It is located about 106 million parsecs away from Earth, PGC39058 is a dwarf galaxy which is located approximately 14 million light years away in the constellation of Draco. It is relatively nearby, however it is obscured by a star which is in front of the galaxy. Astronomical catalogue PGC info at ESOs archive of astronomical catalogues PGC readme at Centre de Données astronomiques de Strasbourg
19.
Spiral galaxy
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A spiral galaxy is a type of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and these are surrounded by a much fainter halo of stars, many of which reside in globular clusters. Spiral galaxies are named for the structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them. Roughly two-thirds of all spirals are observed to have a component in the form of a bar-like structure, extending from the central bulge. Our own Milky Way has recently confirmed to be a barred spiral. The most convincing evidence for its existence comes from a recent survey, performed by the Spitzer Space Telescope, together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in the local Universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters, Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name, naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very loose arms, whereas Sa, either way, spiral arms contain many young, blue stars, which make the arms so bright. A bulge is a huge, tightly packed group of stars, the term commonly refers to the central group of stars found in most spiral galaxies. Using the Hubble classification, the bulge of Sa galaxies is usually composed of Population II stars, further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller and are composed of young, some bulges have similar properties to those of elliptical galaxies, others simply appear as higher density centers of disks, with properties similar to disk galaxies. Many bulges are thought to host a supermassive black hole at their centers, such black holes have never been directly observed, but many indirect proofs exist. In our own galaxy, for instance, the object called Sagittarius A* is believed to be a black hole. There is a correlation between the mass of the black hole and the velocity dispersion of the stars in the bulge. However, some stars inhabit a spheroidal halo or galactic spheroid, the orbital behaviour of these stars is disputed, but they may describe retrograde and/or highly inclined orbits, or not move in regular orbits at all. The galactic halo also contains many globular clusters, due to their irregular movement around the center of the galaxy—if they do so at all—these stars often display unusually high proper motion
20.
Canes Venatici
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Canes Venatici /ˈkeɪniːz vᵻˈnætᵻsaɪ/ is one of the 88 official modern constellations. It is a northern constellation that was created by Johannes Hevelius in the 17th century. Its name is Latin for hunting dogs, and the constellation is depicted in illustrations as representing the dogs of Boötes the Herdsman. The stars of Canes Venatici are not bright, in classical times, they were listed by Ptolemy as unfigured stars below the constellation Ursa Major in his star catalogue. In medieval times, the identification of these stars with the dogs of Boötes arose through a mistranslation, some of Boötess stars were traditionally described as representing the club of Boötes. When the Arabic text was translated into Latin, the translator Gerard of Cremona mistook the Arabic word كلاب for kilāb, meaning dogs. In 1533, the German astronomer Peter Apian depicted Boötes as having two dogs with him and these spurious dogs floated about the astronomical literature until Hevelius decided to specify their presence in the sky by making them a separate constellation in 1687. Hevelius chose the name Asterion for the dog and Chara for the southern dog, as Canes Venatici. In his star catalogue, the Czech astronomer Bečvář assigned the names Asterion to β CVn, Canes Venatici is bordered by Ursa Major to the north and west, Coma Berenices to the south, and Boötes to the east. The three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is CVn, the official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 14 sides. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 12h 06. 2m and 14h 07. 3m, while the coordinates are between +27. 84° and +52. 36°. Covering 465 square degrees, it ranks 38th of the 88 constellations in size, Canes Venatici contains no bright stars, α and β CVn being only of 3rd and 4th magnitude respectively. Flamsteed catalogued 25 stars in the constellation, labelling them 1 to 25 Canum Venaticorum, α CVn is the constellations brightest star, named by Sir Charles Scarborough in memory of King Charles I, the deposed king of Britain. Legend has it that α CVn was brighter than usual during the Restoration, cor Caroli is a wide double star, with a primary of magnitude 2.9 and a secondary of magnitude 5.6, the primary is 110 light-years from Earth. The primary also has an unusually strong variable magnetic field, β CVn is a yellow-hued main sequence star of magnitude 4.2,27 light-years from Earth. Its common name comes from the word for joy, Y CVn is a semiregular variable star that varies between magnitudes 5.0 and 6.5 over a period of around 158 days. It is a star and is being deep red in color. AM CVn, a blue star of magnitude 14, is the prototype of a special class of cataclysmic variable stars, in which the companion star is a white dwarf
21.
Charles Messier
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Charles Messier was a French astronomer most notable for publishing an astronomical catalogue consisting of nebulae and star clusters that came to be known as the 110 Messier objects. The purpose of the catalogue was to help astronomical observers, in particular comet hunters such as himself, Messier was born in Badonviller in the Lorraine region of France, being the tenth of twelve children of Françoise B. Grandblaise and Nicolas Messier, a Court usher, six of his brothers and sisters died while young and in 1741, his father died. Charles interest in astronomy was stimulated by the appearance of the spectacular, great six-tailed comet in 1744, in 1751 he entered the employ of Joseph Nicolas Delisle, the astronomer of the French Navy, who instructed him to keep careful records of his observations. Messiers first documented observation was that of the Mercury transit of 6 May 1753, according to Meyer, Messier is buried in Père Lachaise Cemetery, Paris, in Section 11. Messiers occupation as a comet hunter led him to come across fixed diffuse objects in the night sky which could be mistaken for comets. He compiled a list of them, in collaboration with his friend and assistant Pierre Méchain, the entries are now known to be galaxies, planetary nebulae, other types of nebulae, and star clusters. Messier did his observing with a 100 mm refracting telescope from Hôtel de Cluny, in downtown Paris, the list he compiled contains only objects found in the area of the sky he could observe, from the north celestial pole to a declination of about −35. 7°. They are not organized scientifically by object type, or even by location, the first version of Messiers catalogue contained 45 objects and was published in 1774 in the journal of the French Academy of Sciences in Paris. In addition to his own discoveries, this version included objects previously observed by other astronomers, by 1780 the catalog had increased to 80 objects. The final version of the catalogue was published in 1781, in the 1784 issue of Connaissance des Temps, the final list of Messier objects had grown to 103. These seven objects, M104 through M110, are accepted by astronomers as official Messier objects, the crater Messier on the Moon and the asteroid 7359 Messier were named in his honor. Deep sky object List of Messier objects Messier object Messier marathon Caldwell catalogue OMeara, deep Sky Companions, The Messier Objects. Charles Messier Biography at Students for the Exploration and Development of Space, retrieved July 2007 Short biography of Charles Messier and history of the Messier Object Catalog by Jon Zander at OurDarkSkies. com. Retrieved July 2007 Life of a Comet Hunter, Messier and Astrobiology Professor Mark Brake and Martin Griffiths, Astrobiology Magazine European Edition, retrieved July 2007 Interactive Messier Catalog Greenhawk Observatory Amateur Photos of Charles Messier Objects Biography - Messier website. Messier Marathon Attempts to find as many Messier objects as possible in one night, retrieved July 2007 Clickable table of Messier objects Charles Messier explains his catalog on YouTube
22.
Barred spiral galaxy
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A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in approximately two-thirds of all spiral galaxies, bars generally affect both the motions of stars and interstellar gas within spiral galaxies and can affect spiral arms as well. The Milky Way Galaxy, where our own Solar System is located, is classified as a spiral galaxy. Edwin Hubble classified spiral galaxies of this type as SB in his Hubble sequence, sBa types feature tightly bound arms, while SBc types are at the other extreme and have loosely bound arms. SB0 is a lenticular galaxy. Among other types in Hubbles classifications for the galaxies are, spiral galaxy, elliptical galaxy, Barred galaxies are apparently predominant, with surveys showing that up to two-thirds of all spiral galaxies contain a bar. The current hypothesis is that the bar structure acts as a type of stellar nursery, the bar is thought to act as a mechanism that channels gas inwards from the spiral arms through orbital resonance, in effect funneling the flow to create new stars. This process is thought to explain why many barred spiral galaxies have active galactic nuclei. The creation of the bar is thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting further out, which creates a bar structure. Bars are thought to be temporary phenomena in the lives of spiral galaxies, past a certain size the accumulated mass of the bar compromises the stability of the overall bar structure. Barred spiral galaxies with high mass accumulated in their center tend to have short, since so many spiral galaxies have bar structures, it is likely that they are recurring phenomena in spiral galaxy development. The oscillating evolutionary cycle from spiral galaxy to barred spiral galaxy is thought to take on the average about two billion years, recent studies have confirmed the idea that bars are a sign of galaxies reaching full maturity as the formative years end. The sub-categories are based on how open or tight the arms of the spiral are, sBa types feature tightly bound arms. SBc types are at the extreme and have loosely bound arms. SBm describes somewhat irregular barred spirals, sB0 is a barred lenticular galaxy. Galaxy morphological classification Galaxy formation and evolution Lenticular galaxy Spiral galaxy Firehose instability Britt, Milky Way’s Central Structure Seen with Fresh Clarity. An article about the Spitzer Space Telescopes Milky Way discovery Devitt, galactic survey reveals a new look for the Milky Way
23.
Low-ionization nuclear emission-line region
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A low-ionization nuclear emission-line region is a type of galactic nucleus that is defined by its spectral line emission. The class of galactic nuclei was first identified by Timothy Heckman in the third of a series of papers on the spectra of nuclei that were published in 1980. Galaxies that contain LINERs are often referred to as LINER galaxies, LINER galaxies are very common, approximately one-third of all nearby galaxies may be classified as LINER galaxies. Approximately 75% of LINER galaxies are elliptical galaxies, lenticular galaxies. LINERs are found frequently in Sb-Scd galaxies, and they are very rare in nearby irregular galaxies. LINERs also may be found in luminous infrared galaxies, a class of galaxies defined by their infrared luminosities that are frequently formed when two galaxies collide with each other. Approximately one-quarter of LIRGs may contain LINERs, LINERs have been at the center of two major debates. First, astronomers have debated the source of energy that excites the gas in the centers of these galaxies. Some astronomers have proposed that active galactic nuclei with supermassive black holes are responsible for the LINER spectral emission, other astronomers have asserted that the emission is powered by star formation regions. The other major issue is related to how the ions are excited, some astronomers have suggested that shock waves propagating through the gas may ionize the gas, while others have suggested that photoionization may be responsible. These debates are complicated by the fact that LINERs are found in a variety of objects with different brightnesses and morphologies. Although both the energy sources and the mechanisms for LINER emission are still being studied, many LINERs are frequently referred to as AGN. A number of surveys have been performed to explore the connection between star formation and LINER activity, if a connection can be found between star formation activity and LINER activity, then this strengthens the possibility that LINERs are powered by the hot gas found in star formation regions. However, if star formation cannot be found in LINERs, then this definitively excludes star formation as powering LINER emission, recent observations with the Spitzer Space Telescope show a clear connection between LINER emission in luminous infrared galaxies and star formation activity. However, some mid-infrared spectral line emission from AGN have also detected in these galaxies. Normal nearby galaxies with LINERs, however, appear to be different, a few near-infrared spectroscopic surveys have identified some LINERs in normal galaxies that may be powered by star formation. However, most LINERs in nearby galaxies have low levels of star formation activity and these results demonstrate that most LINER in nearby normal galaxies may not be powered by star formation, although a few exceptions clearly exist. Messier 94 NGC5005 NGC5195 Sombrero Galaxy Seyfert galaxy - Another class of galaxies that contain AGN
24.
Astronomical spectroscopy
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Astronomical spectroscopy is used to measure three major bands of radiation, visible spectrum, radio, and X-ray. While all spectroscopy looks at areas of the spectrum, different methods are required to acquire the signal depending on the frequency. Ozone and molecular oxygen absorb light with wavelengths under 300 nm, meaning that X-ray, radio signals have much longer wavelengths than optical signals, and require the use of antennas or radio dishes. Physicists have been looking at the solar spectrum since Isaac Newton first used a prism to observe the refractive properties of light. In the early 1800s Joseph von Fraunhofer used his skills as a maker to create very pure prisms. The resolution of a prism is limited by its size, a prism will provide a more detailed spectrum. This issue was resolved in the early 1900s with the development of high-quality reflection gratings by J. S, plaskett at the Dominion Observatory in Ottawa, Canada. By creating a blazed grating which utilizes a number of parallel mirrors. These new spectroscopes were more detailed than a prism, required less light, the limitation to a blazed grating is the width of the mirrors, which can only be ground a finite amount before focus is lost, the maximum is around 1000 lines/mm. In order to overcome this limitation holographic gratings were developed, volume phase holographic gratings use a thin film of dichromated gelatin on a glass surface, which is subsequently exposed to a wave pattern created by an interferometer. The holographic gratings can have up to 6000 lines/mm and can be up to twice as efficient in collecting light as blazed gratings, because they are sealed between two sheets of glass, the holographic gratings are very versatile, potentially lasting decades before needing replacement. Light dispersed by the grating or prism in a spectrograph can be recorded by a detector, historically, photographic plates were widely used to record spectra until electronic detectors were developed, and today optical spectrographs most often employ charge-coupled devices. The wavelength scale of a spectrum can be calibrated by observing the spectrum of emission lines of known wavelength from a gas-discharge lamp, radio astronomy was founded with the work of Karl Jansky in the early 1930s, while working for Bell Labs. He built an antenna to look at potential sources of interference for transatlantic radio transmissions. One of the sources of noise discovered came not from Earth, in 1942, JS Hey captured the suns radio frequency using military radar receivers. Radio spectroscopy started with the discovery of the 21-centimeter H I line in 1951, radio interferometry was pioneered in 1946, when Joseph Lade Pawsey, Ruby Payne-Scott and Lindsay McCready used a single antenna atop a sea cliff to observe 200 MHz solar radiation. Two incident beams, one directly from the sun and the other reflected from the sea surface, the first multi-receiver interferometer was built in the same year by Martin Ryle and Vonberg. In 1960, Ryle and Antony Hewish published the technique of aperture synthesis to analyze interferometer data, the aperture synthesis process, which involves autocorrelating and discrete Fourier transforming the incoming signal, recovers both the spatial and frequency variation in flux
25.
Ion
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An ion is an atom or a molecule in which the total number of electrons is not equal to the total number of protons, giving the atom or molecule a net positive or negative electrical charge. Ions can be created, by chemical or physical means. In chemical terms, if an atom loses one or more electrons. If an atom gains electrons, it has a net charge and is known as an anion. Ions consisting of only a single atom are atomic or monatomic ions, because of their electric charges, cations and anions attract each other and readily form ionic compounds, such as salts. In the case of ionization of a medium, such as a gas, which are known as ion pairs are created by ion impact, and each pair consists of a free electron. The word ion comes from the Greek word ἰόν, ion, going and this term was introduced by English physicist and chemist Michael Faraday in 1834 for the then-unknown species that goes from one electrode to the other through an aqueous medium. Faraday also introduced the words anion for a charged ion. In Faradays nomenclature, cations were named because they were attracted to the cathode in a galvanic device, arrhenius explanation was that in forming a solution, the salt dissociates into Faradays ions. Arrhenius proposed that ions formed even in the absence of an electric current, ions in their gas-like state are highly reactive, and do not occur in large amounts on Earth, except in flames, lightning, electrical sparks, and other plasmas. These gas-like ions rapidly interact with ions of charge to give neutral molecules or ionic salts. These stabilized species are commonly found in the environment at low temperatures. A common example is the present in seawater, which are derived from the dissolved salts. Electrons, due to their mass and thus larger space-filling properties as matter waves, determine the size of atoms. Thus, anions are larger than the parent molecule or atom, as the excess electron repel each other, as such, in general, cations are smaller than the corresponding parent atom or molecule due to the smaller size of its electron cloud. One particular cation contains no electrons, and thus consists of a single proton - very much smaller than the parent hydrogen atom. Since the electric charge on a proton is equal in magnitude to the charge on an electron, an anion, from the Greek word ἄνω, meaning up, is an ion with more electrons than protons, giving it a net negative charge. A cation, from the Greek word κατά, meaning down, is an ion with fewer electrons than protons, there are additional names used for ions with multiple charges
26.
Resonance
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In physics, resonance is a phenomenon in which a vibrating system or external force drives another system to oscillate with greater amplitude at a specific preferential frequency. Frequencies at which the amplitude is a relative maximum are known as the systems resonant frequencies or resonance frequencies. At resonant frequencies, small periodic driving forces have the ability to produce large amplitude oscillations, Resonance occurs when a system is able to store and easily transfer energy between two or more different storage modes. However, there are some losses from cycle to cycle, called damping, when damping is small, the resonant frequency is approximately equal to the natural frequency of the system, which is a frequency of unforced vibrations. Some systems have multiple, distinct, resonant frequencies, resonant systems can be used to generate vibrations of a specific frequency, or pick out specific frequencies from a complex vibration containing many frequencies. Resonance occurs widely in nature, and is exploited in many manmade devices and it is the mechanism by which virtually all sine waves and vibrations are generated. Many sounds we hear, such as when hard objects of metal, glass, light and other short wavelength electromagnetic radiation is produced by resonance on an atomic scale, such as electrons in atoms. A familiar example is a swing, which acts as a pendulum. Pushing a person in a swing in time with the interval of the swing makes the swing go higher and higher. This is because the energy the swing absorbs is maximized when the match the swings natural oscillations. It may cause violent swaying motions and even catastrophic failure in improperly constructed structures including bridges, buildings, trains, avoiding resonance disasters is a major concern in every building, tower, and bridge construction project. As a countermeasure, shock mounts can be installed to absorb resonant frequencies, the Taipei 101 building relies on a 660-tonne pendulum —a tuned mass damper—to cancel resonance. Furthermore, the structure is designed to resonate at a frequency that does not typically occur, buildings in seismic zones are often constructed to take into account the oscillating frequencies of expected ground motion. Clocks keep time by mechanical resonance in a wheel, pendulum. The cadence of runners has been hypothesized to be energetically favorable due to resonance between the energy stored in the lower limb and the mass of the runner. Acoustic resonance is a branch of mechanical resonance that is concerned with the mechanical vibrations across the range of human hearing. Like mechanical resonance, acoustic resonance can result in failure of the object at resonance. The classic example of this is breaking a glass with sound at the precise resonant frequency of the glass
27.
Star formation
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Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as stellar nurseries or star-forming regions, fuse to form stars. It is closely related to formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a star, must also account for the statistics of binary stars. In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z =6.60, a spiral galaxy like the Milky Way contains stars, stellar remnants, and a diffuse interstellar medium of gas and dust. The interstellar medium consists of 10−4 to 106 particles per cm3 and is composed of roughly 70% hydrogen by mass. This medium has been enriched by trace amounts of heavier elements that were ejected from stars as they passed beyond the end of their main sequence lifetime. Higher density regions of the interstellar medium form clouds, or diffuse nebulae, in the dense nebulae where stars are produced, much of the hydrogen is in the molecular form, so these nebulae are called molecular clouds. These giant molecular clouds have densities of 100 particles per cm3, diameters of 100 light-years, masses of up to 6 million solar masses. About half the mass of the galactic ISM is found in molecular clouds and in the Milky Way there are an estimated 6,000 molecular clouds. The nearest nebula to the Sun where massive stars are being formed is the Orion nebula,1,300 ly away, however, lower mass star formation is occurring about 400–450 light years distant in the ρ Ophiuchi cloud complex. A more compact site of formation is the opaque clouds of dense gas and dust known as Bok globules. These can form in association with collapsing molecular clouds or possibly independently, the Bok globules are typically up to a light year across and contain a few solar masses. They can be observed as dark clouds silhouetted against bright emission nebulae or background stars, over half the known Bok globules have been found to contain newly forming stars. An interstellar cloud of gas will remain in equilibrium as long as the kinetic energy of the gas pressure is in balance with the potential energy of the internal gravitational force. Mathematically this is expressed using the theorem, which states that, to maintain equilibrium. If a cloud is massive enough that the gas pressure is insufficient to support it, the mass above which a cloud will undergo such collapse is called the Jeans mass. The Jeans mass depends on the temperature and density of the cloud and this coincides with the typical mass of an open cluster of stars, which is the end product of a collapsing cloud. In triggered star formation, one of several events occur to compress a molecular cloud
28.
Starburst region
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A starburst is an astrophysical event which involves star formation occurring at a rate that is large compared to the rate that is normally observed. A starburst will consume the gas supply in a timespan that is shorter than the lifetime of the galaxy. For example NGC6334 has a star formation rate estimated to be 3600 Solar Masses per year while the formation rate of the Milky Way is only about seven solar masses per year. Starburst can occur in entire galaxies or just regions of space, a starburst region is a region of space that is undergoing a large amount of star formation. For example the Tarantula Nebula is a Nebula in the Large Magellanic Cloud which has one of the highest star formation rates in the Local Group, by contrast a starburst galaxy is an entire galaxy that is experiencing a very high star formation rate. At this rate M82 will consume its 200 million Solar Masses of atomic, Starburst regions can occur in different shapes, for example in Messier 94 the inner ring is a star burst region. Messier 82 has a starburst core of about 600 parsec in diameter, starbursts are common during galaxy mergers such as the Antennae Galaxies. In the case of mergers the starburst can either be local or galaxy wide depending on the galaxies, Tarantula Nebula Starburst galaxy Pea galaxy Messier 82 Super star cluster
29.
Spiral arm
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A spiral galaxy is a type of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and these are surrounded by a much fainter halo of stars, many of which reside in globular clusters. Spiral galaxies are named for the structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them. Roughly two-thirds of all spirals are observed to have a component in the form of a bar-like structure, extending from the central bulge. Our own Milky Way has recently confirmed to be a barred spiral. The most convincing evidence for its existence comes from a recent survey, performed by the Spitzer Space Telescope, together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in the local Universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters, Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name, naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very loose arms, whereas Sa, either way, spiral arms contain many young, blue stars, which make the arms so bright. A bulge is a huge, tightly packed group of stars, the term commonly refers to the central group of stars found in most spiral galaxies. Using the Hubble classification, the bulge of Sa galaxies is usually composed of Population II stars, further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller and are composed of young, some bulges have similar properties to those of elliptical galaxies, others simply appear as higher density centers of disks, with properties similar to disk galaxies. Many bulges are thought to host a supermassive black hole at their centers, such black holes have never been directly observed, but many indirect proofs exist. In our own galaxy, for instance, the object called Sagittarius A* is believed to be a black hole. There is a correlation between the mass of the black hole and the velocity dispersion of the stars in the bulge. However, some stars inhabit a spheroidal halo or galactic spheroid, the orbital behaviour of these stars is disputed, but they may describe retrograde and/or highly inclined orbits, or not move in regular orbits at all. The galactic halo also contains many globular clusters, due to their irregular movement around the center of the galaxy—if they do so at all—these stars often display unusually high proper motion
30.
Robert Kennicutt
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Robert Charles Kennicutt, Jr. FRS is an American astronomer. He is the Plumian Professor of Astronomy at the Institute of Astronomy in the University of Cambridge and he was formerly Editor-in-Chief of the Astrophysical Journal. His research interests include the structure and evolution of galaxies and star formation in galaxies and he received his bachelors degree in physics from Rensselaer Polytechnic Institute in 1973. He was a student in astronomy at the University of Washington. He was awarded the Dannie Heineman Prize for Astrophysics in 2007 by the American Astronomical Society and he was made a fellow of the American Academy of Arts and Sciences in 2001 and appointed a Fellow of the Royal Society in 2011. Kennicutt is most well known for the Kennicutt–Schmidt Law which is a relation between the gas density and star formation rate in a given region
31.
Bulge (astronomy)
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In astronomy, a bulge is a tightly packed group of stars within a larger formation. The term almost exclusively refers to the group of stars found in most spiral galaxies. It is now thought there are at least two types of bulges, bulges that are like ellipticals and bulges that are like spiral galaxies. Bulges that have similar to those of elliptical galaxies are often called classical bulges due to their similarity to the historic view of bulges. These bulges are composed primarily of stars that are older, Population II stars and these stars are also in orbits that are essentially random compared to the plane of the galaxy, giving the bulge a distinct spherical form. Due to the lack of dust and gasses, bulges tend to have almost no star formation, the distribution of light is described by de Vaucouleurs law. Classical bulges are thought to be the result of collisions of smaller structures and these disrupt the paths of stars, resulting in the randomness of bulge orbits. Following such a merger, gas clouds are more likely to be converted into stars, many bulges have properties more similar to those of the central regions of spiral galaxies than elliptical galaxies. They are often referred to as pseudobulges or disky-bulges and these bulges have stars that are not orbiting randomly, but rather orbit in an ordered fashion in the same plane as the stars in the outer disk. This contrasts greatly with elliptical galaxies, subsequent studies show that the bulges of many galaxies are not devoid of dust, but rather show a varied and complex structure. This structure often looks similar to a galaxy, but is much smaller. Giant spiral galaxies are typically 2–100 times the size of those spirals that exist in bulges, where they exist, these central spirals dominate the light of the bulge in which they reside. Typically the rate at which new stars are formed in pseudobulges is similar to the rate at which stars form in disk galaxies, sometimes bulges contain nuclear rings that are forming stars at much higher rate than is typically found in outer disks, as shown in NGC4314. Properties such as structure and young stars suggest that some bulges did not form through the same process that made elliptical galaxies. Yet the theories for the formation of pseudobulges are less certain than those for classical bulges, pseudobulges may be the result of extremely gas-rich mergers that happened more recently than those mergers that formed classical bulges. However, it is difficult for disks to survive the merging process, many astronomers suggest that bulges that appear similar to disks form outside of the disk, and are not the product of a merging process. When left alone, disk galaxies can rearrange their stars and gas, the products of this process are often observed in such galaxies, both spiral disks and galactic bars can result from secular evolution of galaxy disks. Secular evolution is expected to send gas and stars to the center of a galaxy
32.
Hubble Space Telescope
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The Hubble Space Telescope is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. Although not the first space telescope, Hubble is one of the largest and most versatile, with a 2. 4-meter mirror, Hubbles four main instruments observe in the near ultraviolet, visible, and near infrared spectra. Hubbles orbit outside the distortion of Earths atmosphere allows it to take extremely high-resolution images, Hubble has recorded some of the most detailed visible light images ever, allowing a deep view into space and time. Many Hubble observations have led to breakthroughs in astrophysics, such as determining the rate of expansion of the universe. The HST was built by the United States space agency NASA, the Space Telescope Science Institute selects Hubbles targets and processes the resulting data, while the Goddard Space Flight Center controls the spacecraft. Space telescopes were proposed as early as 1923, Hubble was funded in the 1970s, with a proposed launch in 1983, but the project was beset by technical delays, budget problems, and the Challenger disaster. When finally launched in 1990, Hubbles main mirror was found to have been ground incorrectly, the optics were corrected to their intended quality by a servicing mission in 1993. Hubble is the telescope designed to be serviced in space by astronauts. After launch by Space Shuttle Discovery in 1990, five subsequent Space Shuttle missions repaired, upgraded, the fifth mission was canceled on safety grounds following the Columbia disaster. However, after spirited public discussion, NASA administrator Mike Griffin approved the fifth servicing mission, the telescope is operating as of 2017, and could last until 2030–2040. Its scientific successor, the James Webb Space Telescope, is scheduled for launch in 2018, the history of the Hubble Space Telescope can be traced back as far as 1946, to the astronomer Lyman Spitzers paper Astronomical advantages of an extraterrestrial observatory. In it, he discussed the two advantages that a space-based observatory would have over ground-based telescopes. First, the resolution would be limited only by diffraction, rather than by the turbulence in the atmosphere. Second, a telescope could observe infrared and ultraviolet light. Spitzer devoted much of his career to pushing for the development of a space telescope, space-based astronomy had begun on a very small scale following World War II, as scientists made use of developments that had taken place in rocket technology. An orbiting solar telescope was launched in 1962 by the United Kingdom as part of the Ariel space program, oAO-1s battery failed after three days, terminating the mission. It was followed by OAO-2, which carried out observations of stars and galaxies from its launch in 1968 until 1972. The continuing success of the OAO program encouraged increasingly strong consensus within the community that the LST should be a major goal
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Milky Way
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The Milky Way is the galaxy that contains our Solar System. The descriptive milky is derived from the appearance from Earth of the galaxy – a band of light seen in the night sky formed from stars that cannot be distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610, until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies, the Milky Way is a barred spiral galaxy with a diameter between 100,000 light-years and 180,000 light-years. The Milky Way is estimated to contain 100–400 billion stars, there are probably at least 100 billion planets in the Milky Way. The Solar System is located within the disk, about 26,000 light-years from the Galactic Center, on the edge of one of the spiral-shaped concentrations of gas. The stars in the inner ≈10,000 light-years form a bulge, the very center is marked by an intense radio source, named Sagittarius A*, which is likely to be a supermassive black hole. Stars and gases at a range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been termed dark matter, the rotational period is about 240 million years at the position of the Sun. The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself, the Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which is a component of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster. The Milky Way can be seen as a band of white light some 30 degrees wide arcing across the sky. Dark regions within the band, such as the Great Rift, the area of the sky obscured by the Milky Way is called the Zone of Avoidance. The Milky Way has a low surface brightness. Its visibility can be reduced by background light such as light pollution or stray light from the Moon. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be seen and it should be visible when the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1
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Dark matter
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Dark matter is an unidentified type of matter distinct from dark energy, baryonic matter, and neutrinos whose existence would explain a number of otherwise puzzling astronomical observations. The name refers to the fact that it does not emit or interact with radiation, such as light. The standard model of cosmology indicates that the total mass–energy of the universe contains 4. 9% ordinary matter,26. 8% dark matter and 68. 3% dark energy. Thus, dark matter constitutes 84. 5% of total mass, the most widely accepted hypothesis on the form for dark matter is that it is composed of weakly interacting massive particles that interact only through gravity and the weak force. All these lines of evidence suggest that galaxies, galaxy clusters, and the universe as a whole contain far more matter than that which is observable via electromagnetic signals. Many experiments to detect proposed dark matter particles through non-gravitational means are under way, however, no dark matter particle has been conclusively identified. ”In 1906 Henri Poincaré in the “The Milky Way and Theory of Gases” used dark matter, ” or “matière obscure” in French in discussing Kelvins work. The first to suggest the existence of matter was Dutch astronomer Jacobus Kapteyn in 1922. Fellow Dutchman and radio astronomy pioneer Jan Oort also hypothesized the existence of matter in 1932. In 1933, Swiss astrophysicist Fritz Zwicky, who studied galactic clusters while working at the California Institute of Technology, Zwicky applied the virial theorem to the Coma galaxy cluster and obtained evidence of unseen mass that he called dunkle Materie dark matter. Zwicky estimated its mass based on the motions of galaxies near its edge and compared that to an estimate based on its brightness and he estimated that the cluster had about 400 times more mass than was visually observable. The gravity effect of the galaxies was far too small for such fast orbits. Based on these conclusions, Zwicky inferred that some unseen matter provided the mass and this was the first formal inference about the existence of dark matter. However, Zwicky did correctly infer that the bulk of the matter was dark, the first robust indications that the mass to light ratio was anything other than unity came from measurements of galaxy rotation curves. In 1939, Horace W. Babcock reported the rotation curve for the Andromeda nebula and he attributed it to either light absorption within the galaxy or modified dynamics in the outer portions of the spiral and not to missing matter. Vera Rubin and Kent Ford in the 1960s–1970s provided further strong evidence, Rubin worked with a new spectrograph to measure the velocity curve of edge-on spiral galaxies with greater accuracy. This result was confirmed in 1978, an influential paper presented Rubins results in 1980. According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle, the search for this particle, by a variety of means, is one of the major efforts in particle physics. In cosmology, the CMB is explained as relic radiation which has travelled freely since the era of recombination, the CMB’s anisotropies are explained as the result of small primordial density fluctuations, and subsequent acoustic oscillations in the photon-baryon plasma whose restoring force is gravity
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Modified Newtonian dynamics
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In physics, Modified Newtonian Dynamics is a theory that proposes a modification of Newtons laws to account for observed properties of galaxies. In MOND, violation of Newtons Laws occurs at small accelerations. MOND is an example of a class of known as modified gravity. Since Milgroms original proposal, MOND has successfully predicted a variety of phenomena that are difficult to understand from a dark matter perspective. However, MOND and its generalisations do not adequately account for observed properties of galaxy clusters, several independent observations point to the fact that the visible mass in galaxies and galaxy clusters is insufficient to account for their dynamics, when analysed using Newtons laws. The former leads to the dark matter hypothesis, the leads to MOND. Agreement with Newtonian mechanics requires μ →1 for x ≫1, beyond these limits, the interpolating function is not specified by the theory, although it is possible to weakly constrain it empirically. Two common choices are the simple interpolating function, μ =11 + a 0 a, thus, in the deep-MOND regime, F N = m a 2 a 0. By fitting his law to rotation curve data, Milgrom found a0 ≈1.2 x 10−10 m s−2 to be optimal and this simple law is sufficient to make predictions for a broad range of galactic phenomena. Milgroms law can be interpreted in two different ways, one possibility is to treat it as a modification to the classical law of inertia, so that the force on an object is not proportional to the particles acceleration a but rather to μa. In this case, the dynamics would apply not only to gravitational phenomena. In this interpretation, Milgroms modification would apply exclusively to gravitational phenomena, by itself, Milgroms law is not a complete and self-contained physical theory, but rather an ad-hoc empirically motivated variant of one of the several equations that constitute classical mechanics. Several complete classical theories have proposed, which generally yield Milgroms law exactly in situations of high symmetry. A subset of these theories have been further embedded within relativistic theories. Distinguishing both theoretically and observationally between these alternatives is a subject of current research, the majority of astronomers, astrophysicists and cosmologists accept ΛCDM, and are committed to a dark matter solution of the missing-mass problem. MOND, by contrast, is studied by only a handful of researchers. Since MOND was specifically designed to produce flat rotation curves, these do not constitute evidence for the theory, nevertheless, a broad range of astrophysical phenomena are neatly accounted for within the MOND framework. Many of these came to light after the publication of Milgroms original papers and are difficult to explain using the dark matter hypothesis
36.
Group of galaxies
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The groups and clusters of galaxies can themselves be clustered, into superclusters of galaxies. The Milky Way galaxy is part of a group of galaxies called the Local Group, Groups of galaxies are the smallest aggregates of galaxies. They typically contain no more than 50 galaxies in a diameter of 1 to 2 megaparsecs and their mass is approximately 1013 solar masses. The spread of velocities for the galaxies is about 150 km/s. However, this definition should be used as a guide only, as larger, Groups are the most common structures of galaxies in the universe, comprising at least 50% of the galaxies in the local universe. Groups have a range between those of the very large elliptical galaxies and clusters of galaxies. In the local universe, about half of the groups exhibit diffuse X-ray emissions from their intracluster media and those that emit X-rays appear to have early-type galaxies as members. The diffuse X-ray emissions come from zones within the inner 10-50% of the virial radius. There are several subtypes of groups, Compact groups are small groups in closely grouped in a small area. They typically have around 5 galaxies in close proximity relatively isolated from other galaxies, the first discovered, Stephans Quintet, was discovered in 1877. Though Stephans Quintet itself is a group of 4 and a foreground galaxy. Astronomer Paul Hickson created a catalogue of such groups in 1982, Compact groups of galaxies readily show the effect of dark matter, as the visible mass is greatly less than that needed to dynamically bind the galaxies into a bound group. Compact galaxy groups are also not dynamically stable over Hubble time, fossil galaxy groups, fossil groups, or fossil clusters are believed to be the end-result of galaxy merging within a normal galaxy group, leaving behind the X-ray halo of the progenitor group. Galaxies within a group interact and merge, the physical process behind this galaxy-galaxy merger is dynamical friction. The time-scales for dynamical friction on luminous galaxies suggest that groups are old. Fossil groups are thus an important laboratory for studying the formation and evolution of galaxies, fossil groups may still contain unmerged dwarf galaxies, but the more massive members of the group have condensed into the central galaxy. The closest fossil group to the Milky Way is NGC6482, proto-groups are groups that are in the process of formation. They are the form of protoclusters
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Virgo Supercluster
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At least 100 galaxy groups and clusters are located within its diameter of 33 megaparsecs. It is one of about 10 million superclusters in the observable universe, a 2014 study indicates that the Virgo Supercluster is only a lobe of a greater supercluster, Laniakea, which is centered on the Great Attractor. Beginning with the first large sample of published by William and John Herschel in 1863. Debate went on during the 1960s and 1970s as to whether the Local Supercluster was actually a structure or an alignment of galaxies. The issue was resolved with the redshift surveys of the late 1970s and early 1980s. In a comprehensive 1982 paper, R. Brent Tully presented the conclusions of his research concerning the structure of the LS. It consists of two components, a flattened disk containing two-thirds of the superclusters luminous galaxies, and a roughly spherical halo containing the remaining one-third. The disk itself is an ellipsoid with a long axis / short axis ratio of at least 6 to 1. Data released in June 2003 from the 5-year Two-degree-Field Galaxy Redshift Survey has allowed astronomers to compare the LS to other superclusters, the LS represents a typical poor supercluster of rather small size. It has one rich galaxy cluster in the center, surrounded by filaments of galaxies, the Local Group is located on the outskirts of the LS in a small filament extending from the Fornax Cluster to the Virgo Cluster. The Virgo Superclusters volume is very approximately 7000 times that of the Local Group or 100 billion times that of the Milky Way, see volumes of similar orders of magnitude. The number density of galaxies in the LS falls off with the square of the distance from its center near the Virgo Cluster, overall, the vast majority of the luminous galaxies are concentrated in a small number of clouds. Ninety-eight percent can be found in the following 11 clouds, Canes Venatici, Virgo Cluster, Virgo II, Leo II, Virgo III, Crater, Leo I, Leo Minor, Draco, Antlia and NGC5643. The luminous galaxies in the halo are also concentrated in a number of clouds. This distribution indicates that most of the volume of the plane is a great void. A helpful analogy that matches the distribution is that of soap bubbles. Flattish clusters and superclusters are found at the intersection of bubbles, long filamentary structures seem to predominate. An example of this is the Hydra-Centaurus Supercluster, the nearest supercluster to the LS, lynden-Bell et al. dubbed the cause of this the Great Attractor
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ArXiv
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In many fields of mathematics and physics, almost all scientific papers are self-archived on the arXiv repository. Begun on August 14,1991, arXiv. org passed the half-million article milestone on October 3,2008, by 2014 the submission rate had grown to more than 8,000 per month. The arXiv was made possible by the low-bandwidth TeX file format, around 1990, Joanne Cohn began emailing physics preprints to colleagues as TeX files, but the number of papers being sent soon filled mailboxes to capacity. Additional modes of access were added, FTP in 1991, Gopher in 1992. The term e-print was quickly adopted to describe the articles and its original domain name was xxx. lanl. gov. Due to LANLs lack of interest in the rapidly expanding technology, in 1999 Ginsparg changed institutions to Cornell University and it is now hosted principally by Cornell, with 8 mirrors around the world. Its existence was one of the factors that led to the current movement in scientific publishing known as open access. Mathematicians and scientists regularly upload their papers to arXiv. org for worldwide access, Ginsparg was awarded a MacArthur Fellowship in 2002 for his establishment of arXiv. The annual budget for arXiv is approximately $826,000 for 2013 to 2017, funded jointly by Cornell University Library, annual donations were envisaged to vary in size between $2,300 to $4,000, based on each institution’s usage. As of 14 January 2014,174 institutions have pledged support for the period 2013–2017 on this basis, in September 2011, Cornell University Library took overall administrative and financial responsibility for arXivs operation and development. Ginsparg was quoted in the Chronicle of Higher Education as saying it was supposed to be a three-hour tour, however, Ginsparg remains on the arXiv Scientific Advisory Board and on the arXiv Physics Advisory Committee. The lists of moderators for many sections of the arXiv are publicly available, additionally, an endorsement system was introduced in 2004 as part of an effort to ensure content that is relevant and of interest to current research in the specified disciplines. Under the system, for categories that use it, an author must be endorsed by an established arXiv author before being allowed to submit papers to those categories. Endorsers are not asked to review the paper for errors, new authors from recognized academic institutions generally receive automatic endorsement, which in practice means that they do not need to deal with the endorsement system at all. However, the endorsement system has attracted criticism for allegedly restricting scientific inquiry, perelman appears content to forgo the traditional peer-reviewed journal process, stating, If anybody is interested in my way of solving the problem, its all there – let them go and read about it. The arXiv generally re-classifies these works, e. g. in General mathematics, papers can be submitted in any of several formats, including LaTeX, and PDF printed from a word processor other than TeX or LaTeX. The submission is rejected by the software if generating the final PDF file fails, if any image file is too large. ArXiv now allows one to store and modify an incomplete submission, the time stamp on the article is set when the submission is finalized
. Bibcode:2001ApJ...546..681T. doi:10.1086/318301.
