1.
90377 Sedna
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90377 Sedna is a large minor planet in the outer reaches of the Solar System that was, as of 2015, at a distance of about 86 astronomical units from the Sun, about three times as far as Neptune. Spectroscopy has revealed that Sednas surface composition is similar to that of some other objects, being largely a mixture of water, methane. Its surface is one of the reddest among Solar System objects and it is most likely a dwarf planet. Sedna has a long and elongated orbit, taking approximately 11,400 years to complete. These facts have led to speculation about its origin. The Minor Planet Center currently places Sedna in the scattered disc, others speculate that it might have been tugged into its current orbit by a passing star, perhaps one within the Suns birth cluster, or even that it was captured from another star system. Another hypothesis suggests that its orbit may be evidence for a planet beyond the orbit of Neptune. Astronomer Michael E. Sedna was discovered by Michael Brown, Chad Trujillo, the discovery formed part of a survey begun in 2001 with the Samuel Oschin telescope at Palomar Observatory near San Diego, California using Yales 160 megapixel Palomar Quest camera. On that day, an object was observed to move by 4.6 arcseconds over 3.1 hours relative to stars, later, the object was precovered on older images made by the Samuel Oschin telescope as well as on images from the Near-Earth Asteroid Tracking consortium. These previous positions expanded its known orbital arc and allowed a precise calculation of its orbit. The team made the name Sedna public before the object had been officially numbered, brian Marsden, the head of the Minor Planet Center, said that such an action was a violation of protocol, and that some members of the IAU might vote against it. However, no objection was raised to the name, and no competing names were suggested, Sedna has the longest orbital period of any known object in the Solar System of comparable size or larger, calculated at around 11,400 years. Its orbit is eccentric, with an aphelion estimated at 937 AU. This perihelion was the largest of that of any known Solar System object until the discovery of 2012 VP113, at its aphelion, Sedna orbits the Sun at a mere 4% of Earths orbital speed. When Sedna was discovered it was 89.6 AU from the Sun approaching perihelion, Eris was later detected by the same survey near aphelion at 97 AU. Only the orbits of some long-period comets extend farther than that of Sedna, when first discovered, Sedna was thought to have an unusually long rotational period. It was initially speculated that Sednas rotation was slowed by the pull of a large binary companion. Sedna has a V-band absolute magnitude of about 1.8, at the time of its discovery it was the intrinsically brightest object found in the Solar System since Pluto in 1930
2.
Trans-Neptunian object
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A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune,30 astronomical units. Twelve minor planets with a semi-major axis greater than 150 AU and perihelion greater than 30 AU are known, the first trans-Neptunian object to be discovered was Pluto in 1930. It took until 1992 to discover a second trans-Neptunian object orbiting the Sun directly,1992 QB1, as of February 2017 over 2,300 trans-Neptunian objects appear on the Minor Planet Centers List of Transneptunian Objects. Of these TNOs,2,000 have a perihelion farther out than Neptune, as of November 2016,242 of these have their orbits well-enough determined that they have been given a permanent minor planet designation. The largest known object is Pluto, followed by Eris,2007 OR10, Makemake. The Kuiper belt, scattered disk, and Oort cloud are three divisions of this volume of space, though treatments vary and a few objects such as Sedna do not fit easily into any division. The orbit of each of the planets is slightly affected by the influences of the other planets. Discrepancies in the early 1900s between the observed and expected orbits of Uranus and Neptune suggested that there were one or more additional planets beyond Neptune, the search for these led to the discovery of Pluto in February 1930, which was too small to explain the discrepancies. Revised estimates of Neptunes mass from the Voyager 2 flyby in 1989 showed that the problem was spurious, Pluto was easiest to find because it has the highest apparent magnitude of all known trans-Neptunian objects. It also has an inclination to the ecliptic than most other large TNOs. After Plutos discovery, American astronomer Clyde Tombaugh continued searching for years for similar objects. For a long time, no one searched for other TNOs as it was believed that Pluto. Only after the 1992 discovery of a second TNO,1992 QB1, a broad strip of the sky around the ecliptic was photographed and digitally evaluated for slowly moving objects. Hundreds of TNOs were found, with diameters in the range of 50 to 2,500 kilometers, Pluto and Eris were eventually classified as dwarf planets by the International Astronomical Union. Kuiper belt objects are classified into the following two groups, Resonant objects are locked in an orbital resonance with Neptune. Objects with a 1,2 resonance are called twotinos, and objects with a 2,3 resonance are called plutinos, after their most prominent member, classical Kuiper belt objects have no such resonance, moving on almost circular orbits, unperturbed by Neptune. Examples are 1992 QB1,50000 Quaoar and Makemake, the scattered disc contains objects farther from the Sun, usually with very irregular orbits. A typical example is the most massive known TNO, Eris, scattered-extended —Scattered-extended objects have a Tisserand parameter greater than 3 and have a time-averaged eccentricity greater than 0
3.
Perihelion and aphelion
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The perihelion is the point in the orbit of a celestial body where it is nearest to its orbital focus, generally a star. It is the opposite of aphelion, which is the point in the orbit where the body is farthest from its focus. The word perihelion stems from the Ancient Greek words peri, meaning around or surrounding, aphelion derives from the preposition apo, meaning away, off, apart. According to Keplers first law of motion, all planets, comets. Hence, a body has a closest and a farthest point from its parent object, that is, a perihelion. Each extreme is known as an apsis, orbital eccentricity measures the flatness of the orbit. Because of the distance at aphelion, only 93. 55% of the solar radiation from the Sun falls on a given area of land as does at perihelion. However, this fluctuation does not account for the seasons, as it is summer in the northern hemisphere when it is winter in the southern hemisphere and vice versa. Instead, seasons result from the tilt of Earths axis, which is 23.4 degrees away from perpendicular to the plane of Earths orbit around the sun. Winter falls on the hemisphere where sunlight strikes least directly, and summer falls where sunlight strikes most directly, in the northern hemisphere, summer occurs at the same time as aphelion. Despite this, there are larger land masses in the northern hemisphere, consequently, summers are 2.3 °C warmer in the northern hemisphere than in the southern hemisphere under similar conditions. Apsis Ellipse Solstice Dates and times of Earths perihelion and aphelion, 2000–2025 from the United States Naval Observatory
4.
Astronomical unit
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The astronomical unit is a unit of length, roughly the distance from Earth to the Sun. However, that varies as Earth orbits the Sun, from a maximum to a minimum. Originally conceived as the average of Earths aphelion and perihelion, it is now defined as exactly 149597870700 metres, the astronomical unit is used primarily as a convenient yardstick for measuring distances within the Solar System or around other stars. However, it is also a component in the definition of another unit of astronomical length. A variety of symbols and abbreviations have been in use for the astronomical unit. In a 1976 resolution, the International Astronomical Union used the symbol A for the astronomical unit, in 2006, the International Bureau of Weights and Measures recommended ua as the symbol for the unit. In 2012, the IAU, noting that various symbols are presently in use for the astronomical unit, in the 2014 revision of the SI Brochure, the BIPM used the unit symbol au. In ISO 80000-3, the symbol of the unit is ua. Earths orbit around the Sun is an ellipse, the semi-major axis of this ellipse is defined to be half of the straight line segment that joins the aphelion and perihelion. The centre of the sun lies on this line segment. In addition, it mapped out exactly the largest straight-line distance that Earth traverses over the course of a year, knowing Earths shift and a stars shift enabled the stars distance to be calculated. But all measurements are subject to some degree of error or uncertainty, improvements in precision have always been a key to improving astronomical understanding. Improving measurements were continually checked and cross-checked by means of our understanding of the laws of celestial mechanics, the expected positions and distances of objects at an established time are calculated from these laws, and assembled into a collection of data called an ephemeris. NASAs Jet Propulsion Laboratory provides one of several ephemeris computation services, in 1976, in order to establish a yet more precise measure for the astronomical unit, the IAU formally adopted a new definition. Equivalently, by definition, one AU is the radius of an unperturbed circular Newtonian orbit about the sun of a particle having infinitesimal mass. As with all measurements, these rely on measuring the time taken for photons to be reflected from an object. However, for precision the calculations require adjustment for such as the motions of the probe. In addition, the measurement of the time itself must be translated to a scale that accounts for relativistic time dilation
5.
Semi-major and semi-minor axes
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In geometry, the major axis of an ellipse is its longest diameter, a line segment that runs through the center and both foci, with ends at the widest points of the perimeter. The semi-major axis is one half of the axis, and thus runs from the centre, through a focus. Essentially, it is the radius of an orbit at the two most distant points. For the special case of a circle, the axis is the radius. One can think of the axis as an ellipses long radius. The semi-major axis of a hyperbola is, depending on the convention, thus it is the distance from the center to either vertex of the hyperbola. A parabola can be obtained as the limit of a sequence of ellipses where one focus is fixed as the other is allowed to move arbitrarily far away in one direction. Thus a and b tend to infinity, a faster than b, the semi-minor axis is a line segment associated with most conic sections that is at right angles with the semi-major axis and has one end at the center of the conic section. It is one of the axes of symmetry for the curve, in an ellipse, the one, in a hyperbola. The semi-major axis is the value of the maximum and minimum distances r max and r min of the ellipse from a focus — that is. In astronomy these extreme points are called apsis, the semi-minor axis of an ellipse is the geometric mean of these distances, b = r max r min. The eccentricity of an ellipse is defined as e =1 − b 2 a 2 so r min = a, r max = a. Now consider the equation in polar coordinates, with one focus at the origin, the mean value of r = ℓ / and r = ℓ /, for θ = π and θ =0 is a = ℓ1 − e 2. In an ellipse, the axis is the geometric mean of the distance from the center to either focus. The semi-minor axis of an ellipse runs from the center of the ellipse to the edge of the ellipse, the semi-minor axis is half of the minor axis. The minor axis is the longest line segment perpendicular to the axis that connects two points on the ellipses edge. The semi-minor axis b is related to the axis a through the eccentricity e. A parabola can be obtained as the limit of a sequence of ellipses where one focus is fixed as the other is allowed to move arbitrarily far away in one direction
6.
2012 VP113
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2012 VP113 is a planetoid in the outer reaches of the Solar System. It is the object with the farthest known perihelion in the Solar System and its discovery was announced on 26 March 2014. It has a magnitude of 4.0, which makes it likely to be a dwarf planet. It is expected to be half the size of Sedna. Its surface is thought to have a pink tinge, resulting from chemical changes produced by the effect of radiation on frozen water, methane and this optical color is consistent with formation in the gas-giant region and not the classical Kuiper belt, which is dominated by ultra-red colored objects. 2012 VP113 was first observed on 5 November 2012 with NOAOs 4-meter Víctor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory, carnegie’s 6. 5-meter Magellan telescope at Las Campanas Observatory in Chile was used to determine its orbit and surface properties. Before being announced to the public, it was tracked by Cerro Tololo Inter-American Observatory. It has an arc of about 2 years. Two precovery measurements from 22 October 2011 have been reported, a primary issue with observing it and finding precovery observations of it is that at an apparent magnitude of 23, it is too faint for most telescopes to easily observe. 2012 VP113 was abbreviated VP and nicknamed Biden by the team, after Joe Biden. 2012 VP113 has the largest perihelion distance of any object in the Solar System. Its last perihelion was around 1979, at a distance of 80 AU, it is currently 83 AU from the Sun. Only eight other Solar System objects are known to have larger than 47 AU. The paucity of bodies with perihelia at 50–75 AU appears not to be an observational artifact and it is possibly a member of a hypothesized Hills cloud. It has a perihelion, argument of perihelion, and current position in the sky similar to those of Sedna, in fact, all known Solar System bodies with semi-major axes over 150 AU and perihelia greater than Neptunes have arguments of perihelion clustered near 340 ± 55°. This could indicate a similar mechanism for these bodies. 2000 CR105 was the first such object discovered and it is currently unknown how 2012 VP113 acquired a perihelion distance beyond the Kuiper belt. The orbital architecture of the region may signal the presence of more than one planet
7.
Solar System
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The Solar System is the gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly. Of those objects that orbit the Sun directly, the largest eight are the planets, with the remainder being significantly smaller objects, such as dwarf planets, of the objects that orbit the Sun indirectly, the moons, two are larger than the smallest planet, Mercury. The Solar System formed 4.6 billion years ago from the collapse of a giant interstellar molecular cloud. The vast majority of the mass is in the Sun. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being composed of rock. The four outer planets are giant planets, being more massive than the terrestrials. All planets have almost circular orbits that lie within a flat disc called the ecliptic. The Solar System also contains smaller objects, the asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptunes orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity. Such objects are categorized as dwarf planets, identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds. Six of the planets, at least four of the dwarf planets, each of the outer planets is encircled by planetary rings of dust and other small objects. The solar wind, a stream of charged particles flowing outwards from the Sun, the heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium, it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, the Solar System is located in the Orion Arm,26,000 light-years from the center of the Milky Way. For most of history, humanity did not recognize or understand the concept of the Solar System, the invention of the telescope led to the discovery of further planets and moons. The principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99. 86% of the known mass. The Suns four largest orbiting bodies, the giant planets, account for 99% of the mass, with Jupiter. The remaining objects of the Solar System together comprise less than 0. 002% of the Solar Systems total mass, most large objects in orbit around the Sun lie near the plane of Earths orbit, known as the ecliptic
8.
Detached object
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Detached objects are a dynamical class of minor planets in the outer reaches of the Solar System and belong to the broader family of trans-Neptunian objects. This reflects the dynamical gradation that can exist between the parameters of the scattered disk and the detached population. At least nine such bodies have been identified, of which the largest, most distant. Those with perihelia greater than 75 AU are termed sednoids, as of 2016, there are two known sednoids, Sedna and 2012 VP113. Detached objects have much larger than Neptunes aphelion. They often have highly elliptical, very large orbits with semi-major axes of up to a few hundred astronomical units, such orbits cannot have been created by gravitational scattering by the giant planets. Instead, a number of explanations have been put forward, including an encounter with a star or a distant planet-sized object. The classification suggested by the Deep Ecliptic Survey team introduces a distinction between scattered-near objects and scattered-extended objects using a Tisserands parameter value of 3. Detached objects are one of five distinct classes of TNO, the other four classes are classical Kuiper-belt objects, resonant objects, scattered-disc objects. Detached objects generally have a distance greater than 40 AU, deterring strong interactions with Neptune. However, there are no boundaries between the scattered and detached regions, since both can coexist as TNOs in an intermediate region with perihelion distance between 37 and 40 AU. One such intermediate body with a well determined orbit is 2003 FY128, although Sedna is officially considered a scattered-disc object by the MPC, its discoverer Michael E. This classification of Sedna as an object is accepted in recent publications. They have orbital periods of more than 300 years and most have only observed over a short observation arc of a couple years. Further improvement in the orbit and potential resonance of objects will help to understand the migration of the giant planets. For example, simulations by Emel’yanenko and Kiseleva in 2007 show that many distant objects could be in resonance with Neptune. They show a 10% likelihood that 2000 CR105 is in a 20,1 resonance, a 38% likelihood that 2003 QK91 is in a 10,3 resonance, and an 84% likelihood that 2000 YW134 is in an 8,3 resonance. The likely dwarf planet 2005 TB190 appears to have less than a 1% likelihood of being in a 4,1 resonance
9.
Oort cloud
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The Oort cloud, sometimes called the Öpik–Oort cloud, is a theoretical cloud of predominantly icy planetesimals believed to surround the Sun to as far as somewhere between 50,000 and 200,000 AU. It is divided into two regions, a disc-shaped inner Oort cloud and a spherical outer Oort cloud, both regions lie beyond the heliosphere and in interstellar space. The Kuiper belt and the disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth as far from the Sun as the Oort cloud. The outer limit of the Oort cloud defines the boundary of the Solar System. The outer Oort cloud is only bound to the Solar System. These forces occasionally dislodge comets from their orbits within the cloud, based on their orbits, most of the short-period comets may come from the scattered disc, but some may still have originated from the Oort cloud. In 1932, the Estonian astronomer Ernst Öpik postulated that long-period comets originated in a cloud at the outermost edge of the Solar System. The idea was revived by Dutch astronomer Jan Oort as a means to resolve a paradox. Thus, Oort reasoned, a comet could not have formed while in its current orbit, there are two main classes of comet, short-period comets and long-period comets. Ecliptic comets have relatively small orbits, below 10 AU, and follow the ecliptic plane, all long-period comets have very large orbits, on the order of thousands of AU, and appear from every direction in the sky. Oort noted that there was a peak in numbers of comets with aphelia of roughly 20,000 AU. The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 AU to as far as 50,000 AU from the Sun, some estimates place the outer edge at between 100,000 and 200,000 AU. The region can be subdivided into a spherical outer Oort cloud of 20, 000–50,000 AU, the outer cloud is only weakly bound to the Sun and supplies the long-period comets to inside the orbit of Neptune. The inner Oort cloud is known as the Hills cloud, named after Jack G. Hills. The Hills cloud explains the existence of the Oort cloud after billions of years. The outer Oort cloud may have trillions of objects larger than 1 km, earlier it was thought to be more massive, but improved knowledge of the size distribution of long-period comets led to lower estimates. The mass of the inner Oort cloud has not been characterized, if analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of ices such as water, methane, ethane, carbon monoxide and hydrogen cyanide. The Oort cloud is thought to be a remnant of the protoplanetary disc that formed around the Sun approximately 4.6 billion years ago
10.
Perturbation (astronomy)
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In astronomy, perturbation is the complex motion of a massive body subject to forces other than the gravitational attraction of a single other massive body. The other forces can include a body, resistance, as from an atmosphere. The study of perturbations began with the first attempts to predict planetary motions in the sky, in ancient times the causes were a mystery. Newton, at the time he formulated his laws of motion and of gravitation, applied them to the first analysis of perturbations, the complex motions of gravitational perturbations can be broken down. The hypothetical motion that the body follows under the effect of one other body only is typically a conic section. This is called a problem, or an unperturbed Keplerian orbit. The differences between that and the motion of the body are perturbations due to the additional gravitational effects of the remaining body or bodies. If there is one other significant body then the perturbed motion is a three-body problem. A general analytical solution exists for the problem, when more than two bodies are considered analytic solutions exist only for special cases. Even the two-body problem becomes insoluble if one of the bodies is irregular in shape, most systems that involve multiple gravitational attractions present one primary body which is dominant in its effects. The gravitational effects of the bodies can be treated as perturbations of the hypothetical unperturbed motion of the planet or satellite around its primary body. In methods of general perturbations, general differential equations, either of motion or of change in the elements, are solved analytically, usually by series expansions. The result is expressed in terms of algebraic and trigonometric functions of the orbital elements of the body in question. This can be applied generally to many different sets of conditions, historically, general perturbations were investigated first. The classical methods are known as variation of the elements, variation of parameters or variation of the constants of integration, in these methods, it is considered that the body is always moving in a conic section, however the conic section is constantly changing due to the perturbations. General perturbations is applicable if the perturbing forces are about one order of magnitude smaller, or less. In the Solar System, this is usually the case, Jupiter, general perturbation methods are preferred for some types of problems, as the source of certain observed motions are readily found. In effect, the positions and velocities are perturbed directly, special perturbations can be applied to any problem in celestial mechanics, as it is not limited to cases where the perturbing forces are small
11.
Giant planet
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A giant planet is any massive planet. They are usually composed of low-boiling-point materials, rather than rock or other solid matter. There are four giant planets in the Solar System, Jupiter, Saturn, Uranus. Many extrasolar giant planets have been identified orbiting other stars, giant planets are also sometimes called jovian planets, after Jupiter. They are also known as gas giants. However, many astronomers apply the term only to Jupiter and Saturn, classifying Uranus and Neptune. Both names are misleading, all of the giant planets consist primarily of fluids above their critical points. The principal components are hydrogen and helium in the case of Jupiter and Saturn, the defining differences between a very low-mass brown dwarf and a gas giant are debated. One school of thought is based on formation, the other, part of the debate concerns whether brown dwarfs must, by definition, have experienced nuclear fusion at some point in their history. The term gas giant was coined in 1952 by the fiction writer James Blish and was originally used to refer to all giant planets. Arguably it is something of a misnomer, because throughout most of the volume of these planets the pressure is so high that matter is not in gaseous form. Other than solids in the core and the layers of the atmosphere, all matter is above the critical point. Fluid planet would be an accurate term. Jupiter also has metallic hydrogen near its center, but much of its volume is hydrogen, helium, the observable atmospheres of all these planets are quite thin compared to their radii, only extending perhaps one percent of the way to the center. Thus the observable portions are gaseous, in the outer Solar System, hydrogen and helium are referred to as gases, water, methane, and ammonia as ices, and silicates and metals as rock. When deep planetary interiors are considered, it may not be far off to say that, by ice astronomers mean oxygen and carbon, by rock they mean silicon, and by gas they mean hydrogen and helium. The many ways in which Uranus and Neptune differ from Jupiter, with this terminology in mind, some astronomers have started referring to Uranus and Neptune as ice giants to indicate the predominance of the ices in their interior composition. The alternative term jovian planet refers to the Roman god Jupiter—the genitive form of which is Jovis, objects large enough to start deuterium fusion are called brown dwarfs, and these occupy the mass range between that of large giant planets and the lowest-mass stars
12.
Galactic tide
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A galactic tide is tidal force experienced by objects subject to the gravitational field of a galaxy such as the Milky Way. Tidal forces are dependent on the gradient of a field, rather than its strength. Two interacting galaxies will not always collide head-on, and the tidal forces will distort each galaxy along an axis pointing roughly towards, such tails are typically strongly curved. If a tail appears to be straight, it is probably being viewed edge-on, the stars and gas that comprise the tails will have been pulled from the easily distorted galactic discs of one or both bodies, rather than the gravitationally bound galactic centres. Two very prominent examples of collisions producing tidal tails are the Mice Galaxies, just as the Moon raises two water tides on opposite sides of the Earth, so a galactic tide produces two arms in its galactic companion. Secondly, if one of the two galaxies is in the foreground, then the second galaxy — and the bridge between them — may be partially obscured, together, these effects can make it hard to see where one galaxy ends and the next begins. Tidal loops, where a tail joins with its parent galaxy at both ends, are rarer still, because tidal effects are strongest in the immediate vicinity of a galaxy, satellite galaxies are particularly likely to be affected. The stripping mechanism is the same as between two galaxies, although its comparatively weak gravitational field ensures that only the satellite, not the host galaxy, is affected. If the satellite is very small compared to the host, the tidal tails produced are likely to be symmetric. It has been suggested that the discs of gas and stars around some galaxies, such as Andromeda. Tidal effects are present within a galaxy, where their gradients are likely to be steepest. This can have consequences for the formation of stars and planetary systems, typically a stars gravity will dominate within its own system, with only the passage of other stars substantially affecting dynamics. However, at the outer reaches of the system, the gravity is weak. In the Solar System, the hypothetical Oort cloud, believed to be the source of long-period comets, the Oort cloud is believed to be a vast shell surrounding the Solar System, possibly over a light-year in radius. Across such a vast distance, the gradient of the Milky Ways gravitational field plays a far more noticeable role, such a body, being composed of a rock and ice mixture, would become a comet when subjected to the increased solar radiation present in the inner Solar System. It has been suggested that the tide may also contribute to the formation of an Oort cloud. This shows that the effects of the tide are quite complex. Cumulatively the effect can be significant, however, up to 90% of all comets originating from an Oort cloud may be the result of the galactic tide
13.
Accretion (astrophysics)
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In astrophysics, accretion is the accumulation of particles into a massive object by gravitationally attracting more matter, typically gaseous matter, in an accretion disk. Most astronomical objects, such as galaxies, stars, and planets, are formed by accretion processes, the idea proposed in the 19th century that Earth and the other terrestrial planets formed from meteoric material was developed in a quantitative way in 1969 by Viktor Safronov. He calculated, in detail, the different stages of planet formation. Since then, the theory has been developed using intensive numerical simulations to study planetesimal accumulation. Stars form by the collapse of interstellar gas. Prior to collapse, this gas is mostly in the form of molecular clouds, as the cloud collapses, losing potential energy, it heats up, gaining kinetic energy, and the conservation of angular momentum ensures that the cloud forms a flatted disk—the accretion disk. A few hundred years after the Big Bang, the Universe cooled to the point where atoms could form. As the Universe continued to expand and cool, the atoms lost enough kinetic energy, as further accretion occurred, galaxies formed. Galaxies grow through mergers and smooth gas accretion, accretion also occurs inside galaxies, forming stars. Stars are thought to form inside giant clouds of cold molecular hydrogen—giant molecular clouds of roughly 300,000 M☉ and 65 light-years in diameter, over millions of years, giant molecular clouds are prone to collapse and fragmentation. These fragments then form small, dense cores, which in turn collapse into stars, the cores range in mass from a fraction to several times that of the Sun and are called protostellar nebulae. They possess diameters of 2, 000–20,000 astronomical units, compare it with the particle number density of the air at the sea level—2. 8×1019/cm3. The initial collapse of a solar-mass protostellar nebula takes around 100,000 years, every nebula begins with a certain amount of angular momentum. This core forms the seed of what will become a star, as the collapse continues, conservation of angular momentum dictates that the rotation of the infalling envelope accelerates, which eventually forms a disk. Around this time the protostar begins to fuse deuterium, if the protostar is sufficiently massive, hydrogen fusion follows. Otherwise, if its mass is too low, the object becomes a brown dwarf and this birth of a new star occurs approximately 100,000 years after the collapse begins. Objects at this stage are known as Class I protostars, which are also called young T Tauri stars, evolved protostars, or young stellar objects. By this time, the star has already accreted much of its mass
14.
Apsis
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An apsis is an extreme point in an objects orbit. The word comes via Latin from Greek and is cognate with apse, for elliptic orbits about a larger body, there are two apsides, named with the prefixes peri- and ap-, or apo- added to a reference to the thing being orbited. For a body orbiting the Sun, the point of least distance is the perihelion, the terms become periastron and apastron when discussing orbits around other stars. For any satellite of Earth including the Moon the point of least distance is the perigee, for objects in Lunar orbit, the point of least distance is the pericynthion and the greatest distance the apocynthion. For any orbits around a center of mass, there are the terms pericenter and apocenter, periapsis and apoapsis are equivalent alternatives. A straight line connecting the pericenter and apocenter is the line of apsides and this is the major axis of the ellipse, its greatest diameter. For a two-body system the center of mass of the lies on this line at one of the two foci of the ellipse. When one body is larger than the other it may be taken to be at this focus. Historically, in systems, apsides were measured from the center of the Earth. In orbital mechanics, the apsis technically refers to the distance measured between the centers of mass of the central and orbiting body. However, in the case of spacecraft, the family of terms are used to refer to the orbital altitude of the spacecraft from the surface of the central body. The arithmetic mean of the two limiting distances is the length of the axis a. The geometric mean of the two distances is the length of the semi-minor axis b, the geometric mean of the two limiting speeds is −2 ε = μ a which is the speed of a body in a circular orbit whose radius is a. The words pericenter and apocenter are often seen, although periapsis/apoapsis are preferred in technical usage, various related terms are used for other celestial objects. The -gee, -helion and -astron and -galacticon forms are used in the astronomical literature when referring to the Earth, Sun, stars. The suffix -jove is occasionally used for Jupiter, while -saturnium has very rarely used in the last 50 years for Saturn. The -gee form is used as a generic closest approach to planet term instead of specifically applying to the Earth. During the Apollo program, the terms pericynthion and apocynthion were used when referring to the Moon, regarding black holes, the term peri/apomelasma was used by physicist Geoffrey A. Landis in 1998 before peri/aponigricon appeared in the scientific literature in 2002
15.
Sun
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The Sun is the star at the center of the Solar System. It is a perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 109 times that of Earth, and its mass is about 330,000 times that of Earth, accounting for about 99. 86% of the total mass of the Solar System. About three quarters of the Suns mass consists of hydrogen, the rest is mostly helium, with smaller quantities of heavier elements, including oxygen, carbon, neon. The Sun is a G-type main-sequence star based on its spectral class and it formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into a disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core and it is thought that almost all stars form by this process. The Sun is roughly middle-aged, it has not changed dramatically for more than four billion years and it is calculated that the Sun will become sufficiently large enough to engulf the current orbits of Mercury, Venus, and probably Earth. The enormous effect of the Sun on Earth has been recognized since prehistoric times, the synodic rotation of Earth and its orbit around the Sun are the basis of the solar calendar, which is the predominant calendar in use today. The English proper name Sun developed from Old English sunne and may be related to south, all Germanic terms for the Sun stem from Proto-Germanic *sunnōn. The English weekday name Sunday stems from Old English and is ultimately a result of a Germanic interpretation of Latin dies solis, the Latin name for the Sun, Sol, is not common in general English language use, the adjectival form is the related word solar. The term sol is used by planetary astronomers to refer to the duration of a solar day on another planet. A mean Earth solar day is approximately 24 hours, whereas a mean Martian sol is 24 hours,39 minutes, and 35.244 seconds. From at least the 4th Dynasty of Ancient Egypt, the Sun was worshipped as the god Ra, portrayed as a falcon-headed divinity surmounted by the solar disk, and surrounded by a serpent. In the New Empire period, the Sun became identified with the dung beetle, in the form of the Sun disc Aten, the Sun had a brief resurgence during the Amarna Period when it again became the preeminent, if not only, divinity for the Pharaoh Akhenaton. The Sun is viewed as a goddess in Germanic paganism, Sól/Sunna, in ancient Roman culture, Sunday was the day of the Sun god. It was adopted as the Sabbath day by Christians who did not have a Jewish background, the symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions
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Open cluster
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An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy and 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 body of the galaxy. Open clusters generally survive for a few hundred years, with the most massive ones surviving for a few billion years. In contrast, the more massive clusters of stars exert a stronger gravitational attraction on their members. Open clusters have been only in spiral and irregular galaxies. Young open clusters may not be contained within the cloud from which they formed. Over time, radiation pressure from the cluster will disperse the molecular cloud, typically, 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 easily determined than they are for isolated stars. A number of 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 barely perceptible without instruments, 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, 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, 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 as if they were single stars, 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, for Praesepe, he found more than 40 stars. Where previously observers had noted only 6-7 stars in the Pleiades, 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 Galileos work, the Sicilian astronomer Giovanni Hodierna became possibly the first astronomer to use a telescope to find previously undiscovered open clusters, in 1654, he identified the objects now designated Messier 41, Messier 47, NGC2362 and NGC2451. Between 1774–1781, French astronomer Charles Messier published a catalogue of objects that had a nebulous appearance similar to comets
17.
Planets beyond Neptune
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Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. In 1978, Pluto was conclusively determined to be too small for its gravity to affect the giant planets, resulting in a brief search for a tenth planet. As of March 2014, observations with the WISE telescope have ruled out the possibility of a Saturn-sized object out to 10,000 AU, and a Jupiter-sized or larger object out to 26,000 AU. In 2016 further work showed this unknown distant planet is likely on an inclined, eccentric orbit that goes no closer than about 200 AU, the orbit is predicted to be anti-aligned to the clustered extreme trans-Neptunian objects. Because Pluto is no longer considered a planet by the International Astronomical Union, Le Verrier predicted the position of this new planet and sent his calculations to German astronomer Johann Gottfried Galle. On 23 September 1846, the following his receipt of the letter, Galle and his student Heinrich dArrest discovered Neptune. There remained some slight discrepancies in the giant planets orbits and these were taken to indicate the existence of yet another planet orbiting beyond Neptune. Even before Neptunes discovery, some speculated that one alone was not enough to explain the discrepancy. Hansens opinion was that a body could not adequately explain the motion of Uranus. In 1848, Jacques Babinet raised an objection to Le Verriers calculations, claiming that Neptunes observed mass was smaller and its orbit larger than Le Verrier had initially predicted. He postulated, based largely on simple subtraction from Le Verriers calculations, that another planet of roughly 12 Earth masses, which he named Hyperion, must exist beyond Neptune. Le Verrier denounced Babinets hypothesis, saying, absolutely nothing by which one could determine the position of another planet, astronomer George Forbes concluded on the basis of this evidence that two planets must exist beyond Neptune. These elements concorded suggestively with those made independently by another astronomer named David Peck Todd, however, sceptics argued that the orbits of the comets involved were still too uncertain to produce meaningful results. In 1900 and 1901, Harvard College Observatory director William Henry Pickering led two searches for trans-Neptunian planets, the second was launched when Gabriel Dallet suggested that a single trans-Neptunian planet lying at 47 AU could account for the motion of Uranus. Pickering agreed to examine plates for any suspected planets, in neither case were any found. In 1909, Thomas Jefferson Jackson See, an astronomer with a reputation as a contrarian, opined that there is certainly one. Tentatively naming the first planet Oceanus, he placed their respective distances at 42,56 and 72 AU from the Sun and he gave no indication as to how he determined their existence, and no known searches were mounted to locate them. In 1911, Indian astronomer Venkatesh P, the three inner Galilean moons of Jupiter, Io, Europa and Ganymede, are locked in a complicated 1,2,4 resonance called a Laplace resonance
18.
Kuiper belt
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It is similar to the asteroid belt, but it is far larger—20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists mainly of small bodies, although many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles, such as methane, ammonia and water. The Kuiper belt is home to three officially recognized dwarf planets, Pluto, Haumea, and Makemake, some of the Solar Systems moons, such as Neptunes Triton and Saturns Phoebe, are also thought to have originated in the region. The Kuiper belt was named after Dutch-American astronomer Gerard Kuiper, though he did not actually predict its existence, in 1992,1992 QB1 was discovered, the first Kuiper belt object since Pluto. Since its discovery, the number of known KBOs has increased to over a thousand, the Kuiper belt should not be confused with the theorized Oort cloud, which is a thousand times more distant and is mostly spherical. The objects within the Kuiper belt, together with the members of the scattered disc, Pluto is the largest and most-massive member of the Kuiper belt and the largest and the second-most-massive known TNO, surpassed only by Eris in the scattered disc. Originally considered a planet, Plutos status as part of the Kuiper belt caused it to be reclassified as a planet in 2006. It is compositionally similar to other objects of the Kuiper belt, and its orbital period is characteristic of a class of KBOs, known as plutinos. After the discovery of Pluto in 1930, many speculated that it not be alone. The region now called the Kuiper belt was hypothesized in various forms for decades and it was only in 1992 that the first direct evidence for its existence was found. The number and variety of speculations on the nature of the Kuiper belt have led to continued uncertainty as to who deserves credit for first proposing it. The first astronomer to suggest the existence of a population was Frederick C. That same year, astronomer Armin O. Leuschner suggested that Pluto may be one of many long-period planetary objects yet to be discovered. Kuiper was operating on the common in his time that Pluto was the size of Earth and had therefore scattered these bodies out toward the Oort cloud or out of the Solar System. Were Kuipers hypothesis correct, there would not be a Kuiper belt today, the hypothesis took many other forms in the following decades. Cameron postulated the existence of a mass of small material on the outskirts of the solar system. Observation, however, ruled out this hypothesis, in 1977, Charles Kowal discovered 2060 Chiron, an icy planetoid with an orbit between Saturn and Uranus. He used a blink comparator, the device that had allowed Clyde Tombaugh to discover Pluto nearly 50 years before
19.
Planet Nine
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Planet Nine does not have an official name, and it will not receive one unless its existence is confirmed, typically through optical imaging. Once confirmed, the IAU will certify a name, with priority given to a name proposed by its discoverers. It will likely be a name chosen from Roman or Greek mythology, in their original paper, Batygin and Brown simply referred to the object as perturber, and only in later press releases did they use Planet Nine. They have also used the names Jehoshaphat and George for Planet Nine, Brown has stated, We actually call it Phattie when were just talking to each other. Planet Nine is hypothesized to follow an elliptical orbit around the Sun, with an orbital period of 10. The period with which it goes around the sun is a multiple of the periods of all the furthest Kuiper Belt objects. The high eccentricity of Planet Nines orbit could take it as far away as 1,200 AU at its aphelion. Brown thinks that if Planet Nine is confirmed to exist, a probe could fly by it in as little as 20 years, the planet is estimated to have 10 times the mass and two to four times the diameter of Earth. An object with the diameter as Neptune has not been excluded by previous surveys. An infrared survey by the Wide-field Infrared Survey Explorer in 2009 allowed for a Neptune-sized object beyond 700 AU. Using a metric based on work by Jean-Luc Margot, Brown calculated that only at the smallest size and farthest distance was it on the border of being called a dwarf planet. Brown speculates that the planet is most likely an ejected ice giant, similar in composition to Uranus and Neptune. Attempts to detect planets beyond Neptune by indirect means such as orbital perturbation date back beyond the discovery of Pluto. That could have been another planet, it could have been a star that came close to the Sun, later in 2015, they also proposed that the Centaurs with large semi-major axis are also being pushed around by the putative planet. Gomes argued that a new planet was the probable of the possible explanations. The initial argument for the existence of a planet beyond Neptune was published in 2014 by astronomers Chad Trujillo, in a later work announcing the discovery of several more distant objects Trujillo and Sheppard noted a correlation between the longitude of perihelion and the argument of perihelion of these objects. Those with a longitude of perihelion of 0–120° have arguments of perihelion between 280–360°, and those with longitude of perihelion of 180–340° have argument of perihelion 0–40° and they found a statistical significance of this correlation of 99. 99%. It was a proof of concept simulation that did not obtain a unique orbit for the planet as they state there are many possible orbital configurations the planet could have
20.
Argument of periapsis
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The argument of periapsis, symbolized as ω, is one of the orbital elements of an orbiting body. Parametrically, ω is the angle from the ascending node to its periapsis. For specific types of orbits, words such as perihelion, perigee, periastron, an argument of periapsis of 0° means that the orbiting body will be at its closest approach to the central body at the same moment that it crosses the plane of reference from South to North. An argument of periapsis of 90° means that the body will reach periapsis at its northmost distance from the plane of reference. Adding the argument of periapsis to the longitude of the ascending node gives the longitude of the periapsis, however, especially in discussions of binary stars and exoplanets, the terms longitude of periapsis or longitude of periastron are often used synonymously with argument of periapsis. In the case of equatorial orbits, the argument is strictly undefined, where, ex and ey are the x- and y-components of the eccentricity vector e. In the case of circular orbits it is assumed that the periapsis is placed at the ascending node. Kepler orbit Orbital mechanics Orbital node
21.
Neptune
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Neptune is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the planet. Neptune is 17 times the mass of Earth and is more massive than its near-twin Uranus. Neptune orbits the Sun once every 164.8 years at a distance of 30.1 astronomical units. It is named after the Roman god of the sea and has the astronomical symbol ♆, Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to perturbation by an unknown planet. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the predicted by Urbain Le Verrier. Its largest moon, Triton, was discovered shortly thereafter, though none of the remaining known 14 moons were located telescopically until the 20th century. The planets distance from Earth gives it a small apparent size. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989, the advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has recently allowed for additional detailed observations from afar. Neptunes composition can be compared and contrasted with the Solar Systems other giant planets, however, its interior, like that of Uranus, is primarily composed of ices and rock, which is why Uranus and Neptune are normally considered ice giants to emphasise this distinction. Traces of methane in the outermost regions in part account for the blue appearance. In contrast to the hazy, relatively featureless atmosphere of Uranus, Neptunes atmosphere has active, for example, at the time of the Voyager 2 flyby in 1989, the planets southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, because of its great distance from the Sun, Neptunes outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching 55 K. Temperatures at the centre are approximately 5,400 K. Neptune has a faint and fragmented ring system. On both occasions, Galileo seems to have mistaken Neptune for a star when it appeared close—in conjunction—to Jupiter in the night sky, hence. At his first observation in December 1612, Neptune was almost stationary in the sky because it had just turned retrograde that day and this apparent backward motion is created when Earths orbit takes it past an outer planet. Because Neptune was only beginning its yearly cycle, the motion of the planet was far too slight to be detected with Galileos small telescope
22.
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
. Bibcode:2004ApJ...617..645B. doi:10.1086/422095. Archived from the original (PDF) on 2006-06-27. Retrieved 2008-04-02.
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