Sagittarius is one of the constellations of the zodiac. It is one of the 48 constellations listed by the 2nd-century astronomer Ptolemy and remains one of the 88 modern constellations, its name is Latin for the archer, its symbol is, a stylized arrow. Sagittarius is represented as a centaur pulling back a bow, it lies between Ophiuchus to the west and Capricornus and Microscopium to the east. The center of the Milky Way lies in the westernmost part of Sagittarius; as seen from the northern hemisphere, the constellation's brighter stars form an recognizable asterism known as "the Teapot". The stars δ Sgr, ε Sgr, ζ Sgr, φ Sgr form the body of the pot; these same stars formed the bow and arrow of Sagittarius. Marking the bottom of the teapot's "handle", is the bright star Zeta Sagittarii, named Ascella, the fainter Tau Sagittarii. To complete the teapot metaphor, under good conditions, a dense area of the Milky Way can be seen rising in a north-westerly arc above the spout, like a puff of steam rising from a boiling kettle.
The constellation as a whole is depicted as having the rough appearance of a stick-figure archer drawing its bow, with the fainter stars providing the outline of the horse's body. Sagittarius famously points its arrow at the heart of Scorpius, represented by the reddish star Antares, as the two constellations race around the sky. Following the direct line formed by Delta Sagittarii and Gamma2 Sagittarii leads nearly directly to Antares. Fittingly, Gamma2 Sagittarii is Alnasl, the Arabic word for "arrowhead", Delta Sagittarii is called Kaus Media, the "center of the bow," from which the arrow protrudes. Kaus Media bisects Lambda Sagittarii and Epsilon Sagittarii, whose names Kaus Borealis and Kaus Australis refer to the northern and southern portions of the bow, respectively. Α Sgr despite having the "alpha" designation, is not the brightest star of the constellation, having a magnitude of only 3.96. It is towards the bottom center of the map. Instead, the brightest star is Epsilon Sagittarii, at magnitude 1.85.
Sigma Sagittarii is the constellation's second-brightest star at magnitude 2.08. Nunki is a B2V star 260 light years away. "Nunki" is a Babylonian name of uncertain origin, but thought to represent the sacred Babylonian city of Eridu on the Euphrates, which would make Nunki the oldest star name in use. Zeta Sagittarii, with apparent magnitude 2.61 of A2 spectra, is a double star whose two components have magnitudes 3.3 and 3.5. Delta Sagittarii, is a K2 spectra star with magnitude 2.71 about 350 light years from Earth. Eta Sagittarii is a double star with component magnitudes of 3.18 and 10, while Pi Sagittarii is a triple system whose components have magnitudes 3.7, 3.8, 6.0. The Bayer designation Beta Sagittarii is shared by two star systems, β¹ Sagittarii, with apparent magnitude 3.96, β² Sagittarii, magnitude 7.4. The two stars are 378 light years from earth. Beta Sagittarii, located at a position associated with the forelegs of the centaur, has the traditional name Arkab, meaning "achilles tendon."
Nova Sagittarii 2015 No. 2 was discovered on 15 March 2015, by John Seach of Chatsworth Island, NSW, Australia. It lies near the center of the constellation, it reached a peak magnitude of 4.3 before fading. The Milky Way is at its densest near Sagittarius; as a result, Sagittarius contains many star clusters and nebulae. Sagittarius contains several well-known nebulae, near λ Sagittarii; the Lagoon Nebula] is an emission nebula, located 5,000 light-years from Earth and measures 140 light-years by 60 light-years. Though it appears grey in telescopes to the unaided eye, long-exposure photographs reveal its pink hue, common to emission nebulae, it is bright, with an integrated magnitude of 3.0. The Lagoon Nebula was discovered independently by John Flamsteed in 1680, Guillaume Le Gentil in 1747, Charles Messier in 1764; the central area of the Lagoon Nebula is known as the Hourglass Nebula, so named for its distinctive shape. The Hourglass Nebula has its shape because of matter propelled by Herschel 36.
The Lagoon Nebula features three dark nebulae catalogued in Barnard's Catalog. The Lagoon Nebula was instrumental in the discovery of Bok globules, as Bart Bok studied prints of the nebula intensively in 1947. 17,000 Bok globules were discovered in the nebula nine years as a part of the Palomar Sky Survey. The Omega Nebula is a bright nebula, sometimes called the Horseshoe Nebula or Swan Nebula, it is 4890 light-years from Earth. It was discovered in 1746 by Philippe Loys de Chésaux. Most viewed as a checkmark, it was seen as a swan by George F. Chambers in 1889, a loon by Roy Bishop, as a curl of smoke by Camille Flammarion; the Trifid N
NGC 6712 is a globular cluster, discovered by Le Gentil on July 9, 1749 when investigating the Milky Way star cloud in Aquila. He described it as a "true nebula," in contrast to the open star cluster M11. Independently discovered by William Herschel on June 16, 1784 and cataloged as H I.47. John Herschel was the first to describe it as a globular star cluster during his observations in the 1830s. NGC 6712 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images Simbad Webda NGC 6712
A meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point in the night sky. These meteors are caused by streams of cosmic debris called meteoroids entering Earth's atmosphere at high speeds on parallel trajectories. Most meteors are smaller than a grain of sand, so all of them disintegrate and never hit the Earth's surface. Intense or unusual meteor showers are known as meteor outbursts and meteor storms, which produce at least 1,000 meteors an hour, most notably from the Leonids; the Meteor Data Centre lists over 900 suspected meteor showers of which about 100 are well established. Several organizations point to viewing opportunities on the Internet; the first great meteor storm in the modern era was the Leonids of November 1833. One estimate is a peak rate of over one hundred thousand meteors an hour, but another, done as the storm abated, estimated in excess of two hundred thousand meteors during the 9 hours of storm, over the entire region of North America east of the Rocky Mountains.
American Denison Olmsted explained the event most accurately. After spending the last weeks of 1833 collecting information, he presented his findings in January 1834 to the American Journal of Science and Arts, published in January–April 1834, January 1836, he noted the shower was of short duration and was not seen in Europe, that the meteors radiated from a point in the constellation of Leo and he speculated the meteors had originated from a cloud of particles in space. Work continued, yet coming to understand the annual nature of showers though the occurrences of storms perplexed researchers; the actual nature of meteors was still debated during the XIX century. Meteors were conceived as an atmospheric phenomenon by many scientists until the Italian astronomer Giovanni Schiaparelli ascertained the relation between meteors and comets in his work "Notes upon the astronomical theory of the falling stars". In the 1890s, Irish astronomer George Johnstone Stoney and British astronomer Arthur Matthew Weld Downing, were the first to attempt to calculate the position of the dust at Earth's orbit.
They studied the dust ejected in 1866 by comet 55P/Tempel-Tuttle in advance of the anticipated Leonid shower return of 1898 and 1899. Meteor storms were anticipated, but the final calculations showed that most of the dust would be far inside of Earth's orbit; the same results were independently arrived at by Adolf Berberich of the Königliches Astronomisches Rechen Institut in Berlin, Germany. Although the absence of meteor storms that season confirmed the calculations, the advance of much better computing tools was needed to arrive at reliable predictions. In 1981 Donald K. Yeomans of the Jet Propulsion Laboratory reviewed the history of meteor showers for the Leonids and the history of the dynamic orbit of Comet Tempel-Tuttle. A graph from it was re-published in Sky and Telescope, it showed relative positions of the Earth and Tempel-Tuttle and marks where Earth encountered dense dust. This showed that the meteoroids are behind and outside the path of the comet, but paths of the Earth through the cloud of particles resulting in powerful storms were near paths of nearly no activity.
In 1985, E. D. Kondrat'eva and E. A. Reznikov of Kazan State University first identified the years when dust was released, responsible for several past Leonid meteor storms. In 1995, Peter Jenniskens predicted the 1995 Alpha Monocerotids outburst from dust trails. In anticipation of the 1999 Leonid storm, Robert H. McNaught, David Asher, Finland's Esko Lyytinen were the first to apply this method in the West. In 2006 Jenniskens published predictions for future dust trail encounters covering the next 50 years. Jérémie Vaubaillon continues to update predictions based on observations each year for the Institut de Mécanique Céleste et de Calcul des Éphémérides; because meteor shower particles are all traveling in parallel paths, at the same velocity, they will all appear to an observer below to radiate away from a single point in the sky. This radiant point is caused by the effect of perspective, similar to parallel railroad tracks converging at a single vanishing point on the horizon when viewed from the middle of the tracks.
Meteor showers are always named after the constellation from which the meteors appear to originate. This "fixed point" moves across the sky during the night due to the Earth turning on its axis, the same reason the stars appear to march across the sky; the radiant moves from night to night against the background stars due to the Earth moving in its orbit around the sun. See IMO Meteor Shower Calendar 2017 for maps of drifting "fixed points." When the moving radiant is at the highest point it will reach in the observer's sky that night, the sun will be just clearing the eastern horizon. For this reason, the best viewing time for a meteor shower is slightly before dawn — a compromise between the maximum number of meteors available for viewing, the lightening sky which makes them harder to see. Meteor showers are named after the nearest constellation or bright star with a Greek or Roman letter assigned, close to the radiant position at the peak of the shower, whereby the grammatical declension of the Latin possessive form is replaced by "id" or "ids".
Hence, meteors radiating from near the star delta Aquarii are called delta Aquariids. The International Astronomical Union's Task Group on Meteor Shower Nomenclature and the IAU's Meteor Data Center keep track of meteor shower nomenclature and which showers are e
Pioneer 11 is a 259-kilogram robotic space probe launched by NASA on April 6, 1973 to study the asteroid belt, the environment around Jupiter and Saturn, solar wind and cosmic rays. It was the first probe to encounter Saturn and the second to fly through the asteroid belt and by Jupiter. Thereafter, Pioneer 11 became the second of five artificial objects to achieve the escape velocity that will allow them to leave the Solar System. Due to power constraints and the vast distance to the probe, the last routine contact with the spacecraft was on September 30, 1995, the last good engineering data was received on November 24, 1995. Approved in February 1969, Pioneer 11 and its twin probe, Pioneer 10, were the first to be designed for exploring the outer Solar System. Yielding to multiple proposals throughout the 1960s, early mission objectives were defined as: Explore the interplanetary medium beyond the orbit of Mars Investigate the nature of the asteroid belt from the scientific standpoint and assess the belt's possible hazard to missions to the outer planets.
Explore the environment of Jupiter. Subsequent planning for an encounter with Saturn added many more goals: Map the magnetic field of Saturn and determine its intensity and structure. Determine how many electrons and protons of various energies are distributed along the trajectory of the spacecraft through the Saturn system. Map the interaction of the Saturn system with the solar wind. Measure the temperature of Saturn's atmosphere and that of Titan, the largest satellite of Saturn. Determine the structure of the upper atmosphere of Saturn where molecules are expected to be electrically charged and form an ionosphere. Map the thermal structure of Saturn's atmosphere by infrared observations coupled with radio occultation data. Obtain spin-scan images of the Saturnian system in two colors during the encounter sequence and polarimetry measurements of the planet. Probe the ring system and the atmosphere of Saturn with S-band radio occultation. Determine more the masses of Saturn and its larger satellites by accurate observations of the effects of their gravitational fields on the motion of the spacecraft.
As a precursor to the Mariner Jupiter/Saturn mission, verify the environment of the ring plane to find out where it may be safely crossed by the Mariner spacecraft without serious damage. Pioneer 11 was managed as part of the Pioneer program by NASA Ames Research Center. A backup unit, Pioneer H, is on display in the "Milestones of Flight" exhibit at the National Air and Space Museum in Washington, D. C.. Many elements of the mission proved to be critical in the planning of the Voyager program; the Pioneer 11 bus measured 36 centimeters deep and with six 76-centimeter-long panels forming the hexagonal structure. The bus housed propellant to control the orientation of the probe and eight of the twelve scientific instruments; the spacecraft had a mass of 260 kilograms. Orientation of the spacecraft was maintained with six 4.5-N, hydrazine monopropellant thrusters: pair one maintained a constant spin-rate of 4.8 rpm, pair two controlled the forward thrust, pair three controlled attitude. Information for the orientation was provided by performing conical scanning maneuvers to track Earth in its orbit, a star sensor able to reference Canopus, two Sun sensors.
The space probe included a redundant system transceivers, one attached to the high-gain antenna, the other to an omni-antenna and medium-gain antenna. Each transceiver was 8 watts and transmitted data across the S-band using 2110 MHz for the uplink from Earth and 2292 MHz for the downlink to Earth with the Deep Space Network tracking the signal. Prior to transmitting data, the probe used a convolutional encoder to allow correction of errors in the received data on Earth. Pioneer 11 used, they were positioned on each 3 meters in length and 120 degrees apart. This was expected to be a safe distance from the sensitive scientific experiments carried on board. Combined, the RTGs provided 155 watts at launch, decayed to 140 W in transit to Jupiter; the spacecraft required 100 W to power all systems. Much of the computation for the mission was performed on Earth and transmitted to the probe, where it was able to retain in memory, up to five commands of the 222 possible entries by ground controllers; the spacecraft included two command decoders and a command distribution unit, a limited form of processor, to direct operations on the spacecraft.
This system required that mission operators prepare commands long in advance of transmitting them to the probe. A data storage unit was included to record up to 6,144 bytes of information gathered by the instruments; the digital telemetry unit would be used to prepare the collected data in one of the thirteen possible formats before transmitting it back to Earth. Pioneer had one additional instrument more than a flux-gate magnetometer; the Pioneer 11 probe was launched on April 6, 1973 at 02:11:00 UTC, by the National Aeronautics and Space Administration from Space Launch Complex 36A at Cape Canaveral, Florida aboard an Atlas-Centaur launch vehicle. Its twin probe, Pioneer 10, had launched a year earlier on March 3, 1972. Pioneer 11 was launched on a trajectory directly aimed at Jupiter without any prior gravitational assists. In May 1974, Pioneer was retargeted to fly past Jupiter on a north-south trajectory enabling a Saturn flyby in 1979; the maneuver used 17 pounds of propellant, lasted 42 minutes and 36 seconds and increased Pioneer 11's speed by 230 km/h.
It made two mid-course corrections, on April 11, 1973 and November 7, 1974. Pioneer 11 flew past Jupiter in November and December 1974. During its c
Aquila is a constellation on the celestial equator. Its name is Latin for'eagle' and it represents the bird that carried Zeus/Jupiter's thunderbolts in Greco-Roman mythology, its brightest star, Altair, is one vertex of the Summer Triangle asterism. The constellation is best seen in the northern summer; because of this location, many clusters and nebulae are found within its borders, but they are dim and galaxies are few. Aquila was one of the 48 constellations described by the second-century astronomer Ptolemy, it had been earlier mentioned by Eudoxus in the fourth century BC and Aratus in the third century BC. It is now one of the 88 constellations defined by the International Astronomical Union; the constellation was known as Vultur volans to the Romans, not to be confused with Vultur cadens, their name for Lyra. It is held to represent the eagle which held Zeus's/Jupiter's thunderbolts in Greco-Roman mythology. Aquila is associated with the eagle that kidnapped Ganymede, a son of one of the kings of Troy, to Mount Olympus to serve as cup-bearer to the gods.
Ptolemy catalogued 19 stars jointly in this constellation and in the now obsolete constellation of Antinous, named in the reign of the emperor Hadrian, but sometimes erroneously attributed to Tycho Brahe, who catalogued 12 stars in Aquila and seven in Antinous. Hevelius determined 23 stars in 19 in the second; the Greek Aquila is based on the Babylonian constellation of the Eagle, located in the same area as the Greek constellation. Aquila, which lies in the Milky Way, contains many rich starfields and has been the location of many novae. Α Aql is the brightest star in this constellation and one of the closest naked-eye stars to Earth at a distance of 17 light-years. Its name comes from the Arabic phrase al-nasr al-tair, meaning "the flying eagle". Altair has a magnitude of 0.76. Β Aql is a yellow-hued star of 45 light-years from Earth. Its name comes from the Arabic phrase shahin-i tarazu, meaning "the balance". Γ Aql is an orange-hued giant star of 460 light-years from Earth. Its name, like that of Alshain, comes from the Arabic for "the balance".
Ζ Aql is a blue-white-hued star of 83 light-years from Earth. Η Aql is 1200 light-years from Earth. Among the brightest Cepheid variable stars, it has a minimum magnitude of 4.4 and a maximum magnitude of 3.5 with a period of 7.2 days. 15 Aql is an optical double star. The primary is an orange-hued giant of 325 light-years from Earth; the secondary is a purple-hued star of 550 light-years from Earth. The pair is resolved in small amateur telescopes. 57 Aql is a binary star. The primary is a blue-hued star of magnitude 5.7 and the secondary is a white star of magnitude 6.5. The system is 350 light-years from Earth. R Aql is a red-hued giant star 690 light-years from Earth, it is a Mira variable with a minimum magnitude of 12.0, a maximum magnitude of 6.0, a period around 9 months. It has a diameter of 400 D☉. FF Aql is a yellow-white-hued supergiant star, 2500 light-years from Earth, it is a Cepheid variable star with a minimum magnitude of 5.7, a maximum magnitude of 5.2, a period of 4.5 days. Ρ Aql moved across the border into neighboring Delphinus in 1992.
Two major novae have been observed in Aquila. Three interesting planetary nebulae lie in Aquila: NGC 6804 shows a small but bright ring. NGC 6781 bears some resemblance with the Owl Nebula in Ursa Major. NGC 6751 known as the Glowing Eye, is a planetary nebula. More deep-sky objects: NGC 6709 is a loose open cluster containing 40 stars, which range in magnitude from 9 to 11, it is about 3000 light-years from Earth. It is about 9100 light-years from Earth. NGC 6709 appears in a rich Milky Way star field and is classified as a Shapley class d and Trumpler class III 2 m cluster; these designations mean that it does not have many stars, is loose, does not show greater concentration at the center, has a moderate range of star magnitudes. NGC 6755 is an open cluster of 7.5 m. NGC 6760 is a globular cluster of 9.1 m. NGC 6749 is an open cluster. NGC 6778 is a planetary nebula. NGC 6741 is a planetary nebula. NGC 6772 is a planetary nebula. Aquila holds some extragalactic objects. One of them is what may be the largest single mass concentration of galaxies in the Universe known, the Hercules–Corona Borealis Great Wall.
It was discovered in November 2013, has the size of 10 billion light years. It is the most massive structure in the Universe known. NASA's Pioneer 11 space probe, which flew by Jupiter and Saturn in the 1970s, is expected to pass near the star Lambda Aquilae in about 4 million years. In illustrations of Aquila that represent it as an eagle, a nearly straight line of three stars symbolizes part of the wings; the center and brightest of these three stars is Altair. The tips of the wings extend further to the southeast and northwest; the head of the eagle stretches off to the southwest. According to Gavin White, the Babylonian Eagle carried the constellation called the Dead Man in its talons; the author draws a comparison to the classical stories of Antinous and Ganymede. In classical Greek mythology, Aquila was identified as Αετός Δίας, the eagle that
An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, many more are thought to exist, they are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center. This can result in a migration to the main body of the galaxy and a loss of cluster members through internal close encounters. Open clusters survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, can survive for longer. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring. Young open clusters may be contained within the molecular cloud from which they formed, illuminating it to create an H II region.
Over time, radiation pressure from the cluster will disperse the molecular cloud. About 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest of the gas away. Open clusters are key objects in the study of stellar evolution; because the cluster members are of similar age and chemical composition, their properties are more determined than they are for isolated stars. A number of open clusters, such as the Pleiades, Hyades or the Alpha Persei Cluster are visible with the naked eye; some others, such as the Double Cluster, are perceptible without instruments, while many more can be seen using binoculars or telescopes. The Wild Duck Cluster, M11, is an example; the prominent open cluster the Pleiades has been recognized as a group of stars since antiquity, while the Hyades forms part of Taurus, one of the oldest constellations. Other open clusters were noted by early astronomers as unresolved fuzzy patches of light; the Roman astronomer Ptolemy mentions the Praesepe, the Double Cluster in Perseus, the Ptolemy Cluster, while the Persian astronomer Al-Sufi wrote of the Omicron Velorum cluster.
However, it would require the invention of the telescope to resolve these nebulae into their constituent stars. Indeed, in 1603 Johann Bayer gave three of these clusters designations; the first person to use a telescope to observe the night sky and record his observations was the Italian scientist Galileo Galilei in 1609. When he turned the telescope toward some of the nebulous patches recorded by Ptolemy, he found they were not a single star, but groupings of many stars. For Praesepe, he found more than 40 stars. Where observers had noted only 6-7 stars in the Pleiades, he found 50. In his 1610 treatise Sidereus Nuncius, Galileo Galilei wrote, "the galaxy is nothing else but a mass of innumerable stars planted together in clusters." Influenced by Galileo's work, the Sicilian astronomer Giovanni Hodierna became the first astronomer to use a telescope to find undiscovered open clusters. In 1654, he identified the objects now designated Messier 41, Messier 47, NGC 2362 and NGC 2451, it was realised as early as 1767 that the stars in a cluster were physically related, when the English naturalist Reverend John Michell calculated that the probability of just one group of stars like the Pleiades being the result of a chance alignment as seen from Earth was just 1 in 496,000.
Between 1774–1781, French astronomer Charles Messier published a catalogue of celestial objects that had a nebulous appearance similar to comets. This catalogue included 26 open clusters. In the 1790s, English astronomer William Herschel began an extensive study of nebulous celestial objects, he discovered. Herschel conceived the idea that stars were scattered across space, but became clustered together as star systems because of gravitational attraction, he divided the nebulae into eight classes, with classes VI through VIII being used to classify clusters of stars. The number of clusters known continued to increase under the efforts of astronomers. Hundreds of open clusters were listed in the New General Catalogue, first published in 1888 by the Danish-Irish astronomer J. L. E. Dreyer, the two supplemental Index Catalogues, published in 1896 and 1905. Telescopic observations revealed two distinct types of clusters, one of which contained thousands of stars in a regular spherical distribution and was found all across the sky but preferentially towards the centre of the Milky Way.
The other type consisted of a sparser population of stars in a more irregular shape. These were found in or near the galactic plane of the Milky Way. Astronomers dubbed the former globular clusters, the latter open clusters; because of their location, open clusters are referred to as galactic clusters, a term, introduced in 1925 by the Swiss-American astronomer Robert Julius Trumpler. Micrometer measurements of the positions of stars in clusters were made as early as 1877 by the German astronomer E. Schönfeld and further pursued by the American astronomer E. E. Barnard prior to his death in 1923. No indication of stellar motion was detected by these efforts. However, in 1918 the Dutch-American astronomer Adriaan van Maanen was able to measure the proper motion of stars in part of the Pleiades cluster by comparing photographic plates taken at different times; as astrometry became more accurate, cluster stars were found to share a common proper motion through space. By comparing the photographic plates of the Pleiades cluster taken in 1918 with images taken in 1943, van
The apparent magnitude of an astronomical object is a number, a measure of its brightness as seen by an observer on Earth. The magnitude scale is logarithmic. A difference of 1 in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The brighter an object appears, the lower its magnitude value, with the brightest astronomical objects having negative apparent magnitudes: for example Sirius at −1.46. 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 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 simply as V, as in "mV = 15" or "V = 15" to describe a 15th-magnitude object; 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 night sky were said to be of first magnitude, whereas the faintest were of sixth magnitude, the limit of human visual perception. Each grade of magnitude was considered twice the brightness of the following grade, although that ratio was subjective as no photodetectors existed; this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest and is believed to have originated with Hipparchus. In 1856, Norman Robert Pogson formalized the system by defining a first magnitude star as a star, 100 times as bright as a sixth-magnitude star, thereby establishing the logarithmic scale still in use today; this implies that a star of magnitude m is about 2.512 times as bright as a star of magnitude m + 1. This figure, the fifth root of 100, became known as Pogson's Ratio; the zero point of Pogson's scale was defined by assigning Polaris a magnitude of 2. Astronomers discovered that Polaris is variable, so they switched to Vega as the standard reference star, assigning the brightness of Vega as the definition of zero magnitude at any specified wavelength.
Apart from small corrections, the brightness of Vega still serves as the definition of zero magnitude for visible and near infrared wavelengths, where its spectral energy distribution approximates that of a black body for a temperature of 11000 K. However, with the advent of infrared astronomy it was revealed that Vega's radiation includes an Infrared excess due to a circumstellar 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 magnitude 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, as a function of wavelength, can be computed. Small deviations are specified between systems using measurement apparatuses developed independently so that data obtained by different astronomers can be properly compared, but of greater practical importance is the definition of magnitude not at a single wavelength but applying to the response of standard spectral filters used in photometry over various wavelength bands.
With the modern magnitude systems, brightness over a wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30; the brightness of Vega is exceeded by four stars in the night sky at visible wavelengths as well as the bright planets Venus and Jupiter, these must be described by negative magnitudes. For example, the brightest star of the celestial sphere, has an apparent magnitude of −1.4 in the visible. Negative magnitudes for other bright astronomical objects can be found in the table below. Astronomers have developed other photometric zeropoint systems as alternatives to the Vega system; the most used is the AB magnitude system, in which photometric zeropoints are based on a hypothetical reference spectrum having constant flux per unit frequency interval, rather than using a stellar spectrum or blackbody curve as the reference. The AB magnitude zeropoint is defined such that an object's AB and Vega-based magnitudes will be equal in the V filter band.
As the amount of light received by a telescope is reduced by transmission through the Earth's atmosphere, any measurement of apparent magnitude is corrected for what it would have been as seen from above the atmosphere. 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 100. Therefore, the apparent magnitude m, in the spectral band x, would be given by m x = − 5 log 100 , more expressed in terms of common logarithms as m x