A quasar is an luminous active galactic nucleus. It has been theorized that most large galaxies contain a supermassive central black hole with mass ranging from millions to billions of times the mass of the Sun. In quasars and other types of AGN, the black hole is surrounded by a gaseous accretion disk; as gas falls toward the black hole, energy is released in the form of electromagnetic radiation, which can be observed across the electromagnetic spectrum. The power radiated by quasars is enormous: the most powerful quasars have luminosities thousands of times greater than a galaxy such as the Milky Way; the term "quasar" originated as a contraction of quasi-stellar radio source, because quasars were first identified during the 1950s as sources of radio-wave emission of unknown physical origin, when identified in photographic images at visible wavelengths they resembled faint star-like points of light. High-resolution images of quasars from the Hubble Space Telescope, have demonstrated that quasars occur in the centers of galaxies, that some host-galaxies are interacting or merging galaxies.
As with other categories of AGN, the observed properties of a quasar depend on many factors including the mass of the black hole, the rate of gas accretion, the orientation of the accretion disk relative to the observer, the presence or absence of a jet, the degree of obscuration by gas and dust within the host galaxy. Quasars are found over a broad range of distances, quasar discovery surveys have demonstrated that quasar activity was more common in the distant past; the peak epoch of quasar activity was 10 billion years ago. As of 2017, the most distant known quasar is ULAS J1342+0928 at redshift z = 7.54. The supermassive black hole in this quasar, estimated at 800 million solar masses, is the most distant black hole identified to date; the term "quasar" was first used in a paper by Chinese-born U. S. astrophysicist Hong-Yee Chiu in May 1964, in Physics Today, to describe certain astronomically-puzzling objects: So far, the clumsily long name'quasi-stellar radio sources' is used to describe these objects.
Because the nature of these objects is unknown, it is hard to prepare a short, appropriate nomenclature for them so that their essential properties are obvious from their name. For convenience, the abbreviated form'quasar' will be used throughout this paper. Between 1917 and 1922, it became clear from work by Heber Curtis, Ernst Öpik and others, that some objects seen by astronomers were in fact distant galaxies like our own, but when radio astronomy commenced in the 1950s, astronomers detected, among the galaxies, a small number of anomalous objects with properties that defied explanation. The objects emitted large amounts of radiation of many frequencies, but no source could be located optically, or in some cases only a faint and point-like object somewhat like a distant star; the spectral lines of these objects, which identify the chemical elements of which the object is composed, were extremely strange and defied explanation. Some of them changed their luminosity rapidly in the optical range and more in the X-ray range, suggesting an upper limit on their size no larger than our own Solar System.
This implies an high power density. Considerable discussion took place over, they were described as "quasi-stellar radio sources", or "quasi-stellar objects", a name which reflected their unknown nature, this became shortened to "quasar". The first quasars were discovered as radio sources in all-sky radio surveys, they were first noted as radio sources with no corresponding visible object. Using small telescopes and the Lovell Telescope as an interferometer, they were shown to have a small angular size. Hundreds of these objects were recorded by 1960 and published in the Third Cambridge Catalogue as astronomers scanned the skies for their optical counterparts. In 1963, a definite identification of the radio source 3C 48 with an optical object was published by Allan Sandage and Thomas A. Matthews. Astronomers had detected what appeared to be a faint blue star at the location of the radio source and obtained its spectrum, which contained many unknown broad emission lines; the anomalous spectrum defied interpretation.
British-Australian astronomer John Bolton made many early observations of quasars, including a breakthrough in 1962. Another radio source, 3C 273, was predicted to undergo five occultations by the Moon. Measurements taken by Cyril Hazard and John Bolton during one of the occultations using the Parkes Radio Telescope allowed Maarten Schmidt to find a visible counterpart to the radio source and obtain an optical spectrum using the 200-inch Hale Telescope on Mount Palomar; this spectrum revealed the same strange emission lines. Schmidt was able to demonstrate that these were to be the ordinary spectral lines of hydrogen redshifted by 15.8 percent - an extreme redshift never seen in astronomy before. If this was due to the physical motion of the "star" 3C 273 was receding at an enormous velocity, around 47,000 km/s, far beyond the speed of any known star and defying any obvious explanation. Nor would an extreme velocity help to explain 3C 273's huge radio emissions. Although it raised many questions, Schmidt's discovery revolutionized quasar observation.
The strange spectrum of 3C 48 was identified by Schmidt and Oke as hydrogen and magnesium redshifted by 37%. Shortly afterwards, two more quasar spectra in 1964 and five more in 1965, were confirmed as ordinary
The Kuiper belt called the Edgeworth–Kuiper belt, is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune to 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists of small bodies or remnants from when the Solar System formed. While many asteroids are composed of rock and metal, most Kuiper belt objects are composed of frozen volatiles, such as methane and water; the Kuiper belt is home to three recognized dwarf planets: Pluto and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, may have originated in the region; the Kuiper belt was named after Dutch-American astronomer Gerard Kuiper, though he did not predict its existence. In 1992, Albion was discovered, the first Kuiper belt object since Charon. Since its discovery, the number of known KBOs has increased to over a thousand, more than 100,000 KBOs over 100 km in diameter are thought to exist.
The Kuiper belt was thought to be the main repository for periodic comets, those with orbits lasting less than 200 years. Studies since the mid-1990s have shown that the belt is dynamically stable and that comets' true place of origin is the scattered disc, a dynamically active zone created by the outward motion of Neptune 4.5 billion years ago. The Kuiper belt is distinct from the theoretical Oort cloud, a thousand times more distant and is spherical; the objects within the Kuiper belt, together with the members of the scattered disc and any potential Hills cloud or Oort cloud objects, are collectively referred to as trans-Neptunian objects. Pluto is the largest and most massive member of the Kuiper belt, the largest and the second-most-massive known TNO, surpassed only by Eris in the scattered disc. Considered a planet, Pluto's status as part of the Kuiper belt caused it to be reclassified as a dwarf planet in 2006, it is compositionally similar to many other objects of the Kuiper belt and its orbital period is characteristic of a class of KBOs, known as "plutinos", that share the same 2:3 resonance with Neptune.
After the discovery of Pluto in 1930, many speculated. The region now called, it was only in 1992. The number and variety of prior 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 trans-Neptunian population was Frederick C. Leonard. Soon after Pluto's discovery by Clyde Tombaugh in 1930, Leonard pondered whether it was "not that in Pluto there has come to light the first of a series of ultra-Neptunian bodies, the remaining members of which still await discovery but which are destined to be detected"; that same year, astronomer Armin O. Leuschner suggested that Pluto "may be one of many long-period planetary objects yet to be discovered." In 1943, in the Journal of the British Astronomical Association, Kenneth Edgeworth hypothesized that, in the region beyond Neptune, the material within the primordial solar nebula was too spaced to condense into planets, so rather condensed into a myriad of smaller bodies.
From this he concluded that "the outer region of the solar system, beyond the orbits of the planets, is occupied by a large number of comparatively small bodies" and that, from time to time, one of their number "wanders from its own sphere and appears as an occasional visitor to the inner solar system", becoming a comet. In 1951, in a paper in Astrophysics: A Topical Symposium, Gerard Kuiper speculated on a similar disc having formed early in the Solar System's evolution, but he did not think that such a belt still existed today. Kuiper was operating on the assumption, 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 Kuiper's hypothesis correct, there would not be a Kuiper belt today; the hypothesis took many other forms in the following decades. In 1962, physicist Al G. W. Cameron postulated the existence of "a tremendous mass of small material on the outskirts of the solar system". In 1964, Fred Whipple, who popularised the famous "dirty snowball" hypothesis for cometary structure, thought that a "comet belt" might be massive enough to cause the purported discrepancies in the orbit of Uranus that had sparked the search for Planet X, or, at the least, massive enough to affect the orbits of known comets.
Observation 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 same device that had allowed Clyde Tombaugh to discover Pluto nearly 50 years before. In 1992, another object, 5145 Pholus, was discovered in a similar orbit. Today, an entire population of comet-like bodies, called the centaurs, is known to exist in the region between Jupiter and Neptune; the centaurs' orbits have dynamical lifetimes of a few million years. From the time of Chiron's discovery in 1977, astronomers have speculated that the centaurs therefore must be replenished by some outer reservoir. Further evidence for the existence of the Kuiper belt emerged from the study of comets; that comets have finite lifespans. As they approach the Sun, its heat causes their volatile surfaces to sublimate into space d
The Oort cloud, named after the Dutch astronomer Jan Oort, sometimes called the Öpik–Oort cloud, is a hypothetical cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2,000 to 200,000 AU. It is divided into two regions: a spherical outer Oort cloud. Both regions lie in interstellar space; the Kuiper belt and the scattered 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 cosmographical boundary of the Solar System and the extent of the Sun's Hill sphere; the outer Oort cloud is only loosely bound to the Solar System, thus is affected by the gravitational pull both of passing stars and of the Milky Way itself. These forces dislodge comets from their orbits within the cloud and send them toward the inner Solar System. 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.
Astronomers conjecture that the matter composing the Oort cloud formed closer to the Sun and was scattered far into space by the gravitational effects of the giant planets early in the Solar System's evolution. Although no confirmed direct observations of the Oort cloud have been made, it may be the source of all long-period and Halley-type comets entering the inner Solar System, many of the centaurs and Jupiter-family comets as well; the existence of the Oort cloud was first postulated by Estonian astronomer Ernst Öpik in 1932. Oort independently proposed it in 1950. There are two main classes of comet: long-period comets. Ecliptic comets have small orbits, below 10 AU, follow the ecliptic plane, the same plane in which the planets lie. All long-period comets have large orbits, on the order of thousands of AU, appear from every direction in the sky. A. O. Leuschner in 1907 suggested that many comets believed to have parabolic orbits, thus making single visits to the solar system had elliptical orbits and would return after long periods.
In 1932 Estonian astronomer Ernst Öpik postulated that long-period comets originated in an orbiting cloud at the outermost edge of the Solar System. Dutch astronomer Jan Oort independently revived the idea in 1950 as a means to resolve a paradox: Over the course of the Solar System's existence the orbits of comets are unstable, dynamics dictate that a comet must either collide with the Sun or a planet or else be ejected from the Solar System by planetary perturbations. Moreover, their volatile composition means that as they approach the Sun, radiation boils the volatiles off until the comet splits or develops an insulating crust that prevents further outgassing. Thus, Oort reasoned, a comet could not have formed while in its current orbit and must have been held in an outer reservoir for all of its existence, he noted that there was a peak in numbers of long-period comets with aphelia of 20,000 AU, which suggested a reservoir at that distance with a spherical, isotropic distribution. Those rare comets with orbits of about 10,000 AU have gone through one or more orbits through the Solar System and have had their orbits drawn inward by the gravity of the planets.
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, a torus-shaped inner Oort cloud of 2,000–20,000 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, who proposed its existence in 1981. Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo; the Hills cloud explains the continued existence of the Oort cloud after billions of years. The outer Oort cloud may have trillions of objects larger than 1 km, billions with absolute magnitudes brighter than 11, with neighboring objects tens of millions of kilometres apart, its total mass is not known, assuming that Halley's Comet is a suitable prototype for comets within the outer Oort cloud the combined mass is 3×1025 kilograms, or five times that of Earth.
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 estimated. If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of ices such as water, ethane, carbon monoxide and hydrogen cyanide. However, the discovery of the object 1996 PW, an object whose appearance was consistent with a D-type asteroid in an orbit typical of a long-period comet, prompted theoretical research that suggests that the Oort cloud population consists of one to two percent asteroids. Analysis of the carbon and nitrogen isotope ratios in both the long-period and Jupiter-family comets shows little difference between the two, despite their vastly s
A charge-coupled device is a device for the movement of electrical charge from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time. CCDs move charge between capacitive bins in the device, with the shift allowing for the transfer of charge between bins. In recent years CCD has become a major technology for digital imaging. In a CCD image sensor, pixels are represented by p-doped metal-oxide-semiconductors capacitors; these capacitors are biased above the threshold for inversion when image acquisition begins, allowing the conversion of incoming photons into electron charges at the semiconductor-oxide interface. Although CCDs are not the only technology to allow for light detection, CCD image sensors are used in professional and scientific applications where high-quality image data are required. In applications with less exacting quality demands, such as consumer and professional digital cameras, active pixel sensors known as complementary metal-oxide-semiconductors are used.
The charge-coupled device was invented in 1969 in the United States at AT&T Bell Labs by Willard Boyle and George E. Smith; the lab was working on semiconductor bubble memory when Boyle and Smith conceived of the design of what they termed, in their notebook, "Charge'Bubble' Devices". The device could be used as a shift register; the essence of the design was the ability to transfer charge along the surface of a semiconductor from one storage capacitor to the next. The concept was similar in principle to the bucket-brigade device, developed at Philips Research Labs during the late 1960s; the first patent on the application of CCDs to imaging was assigned to Michael Tompsett. The initial paper describing the concept listed possible uses as a memory, a delay line, an imaging device; the first experimental device demonstrating the principle was a row of spaced metal squares on an oxidized silicon surface electrically accessed by wire bonds. The first working CCD made with integrated circuit technology was a simple 8-bit shift register.
This device had input and output circuits and was used to demonstrate its use as a shift register and as a crude eight pixel linear imaging device. Development of the device progressed at a rapid rate. By 1971, Bell researchers led by Michael Tompsett were able to capture images with simple linear devices. Several companies, including Fairchild Semiconductor, RCA and Texas Instruments, picked up on the invention and began development programs. Fairchild's effort, led by ex-Bell researcher Gil Amelio, was the first with commercial devices, by 1974 had a linear 500-element device and a 2-D 100 x 100 pixel device. Steven Sasson, an electrical engineer working for Kodak, invented the first digital still camera using a Fairchild 100 x 100 CCD in 1975; the first KH-11 KENNEN reconnaissance satellite equipped with charge-coupled device array technology for imaging was launched in December 1976. Under the leadership of Kazuo Iwama, Sony started a large development effort on CCDs involving a significant investment.
Sony managed to mass-produce CCDs for their camcorders. Before this happened, Iwama died in August 1982. In January 2006, Boyle and Smith were awarded the National Academy of Engineering Charles Stark Draper Prize, in 2009 they were awarded the Nobel Prize for Physics, for their invention of the CCD concept. Michael Tompsett was awarded the 2010 National Medal of Technology and Innovation for pioneering work and electronic technologies including the design and development of the first charge coupled device imagers, he was awarded the 2012 IEEE Edison Medal "For pioneering contributions to imaging devices including CCD Imagers and thermal imagers". In a CCD for capturing images, there is a photoactive region, a transmission region made out of a shift register. An image is projected through a lens onto the capacitor array, causing each capacitor to accumulate an electric charge proportional to the light intensity at that location. A one-dimensional array, used in line-scan cameras, captures a single slice of the image, whereas a two-dimensional array, used in video and still cameras, captures a two-dimensional picture corresponding to the scene projected onto the focal plane of the sensor.
Once the array has been exposed to the image, a control circuit causes each capacitor to transfer its contents to its neighbor. The last capacitor in the array dumps its charge into a charge amplifier, which converts the charge into a voltage. By repeating this process, the controlling circuit converts the entire contents of the array in the semiconductor to a sequence of voltages. In a digital device, these voltages are sampled and stored in memory. Before the MOS capacitors are exposed to light, they are biased into the depletion region; the gate is biased at a positive potential, above the threshold for strong inversion, which will result in the creation
Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It is the largest known plutoid. Pluto was discovered by Clyde Tombaugh in 1930 and was considered to be the ninth planet from the Sun. After 1992, its status as a planet was questioned following the discovery of several objects of similar size in the Kuiper belt. In 2005, Eris, a dwarf planet in the scattered disc, 27% more massive than Pluto, was discovered; this led the International Astronomical Union to define the term "planet" formally in 2006, during their 26th General Assembly. That definition excluded reclassified it as a dwarf planet. Pluto is the largest and second-most-massive known dwarf planet in the Solar System, the ninth-largest and tenth-most-massive known object directly orbiting the Sun, it is less massive than Eris. Like other Kuiper belt objects, Pluto is made of ice and rock and is small—about one-sixth the mass of the Moon and one-third its volume, it has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU from the Sun.
This means that Pluto periodically comes closer to the Sun than Neptune, but a stable orbital resonance with Neptune prevents them from colliding. Light from the Sun takes about 5.5 hours to reach Pluto at its average distance. Pluto has five known moons: Charon, Nix and Hydra. Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body; the New Horizons spacecraft performed a flyby of Pluto on July 14, 2015, becoming the first spacecraft to do so. During its brief flyby, New Horizons made detailed measurements and observations of Pluto and its moons. In September 2016, astronomers announced that the reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be ingredients for the emergence of life, produced from methane and other gases released from the atmosphere of Pluto and transferred about 19,000 km to the orbiting moon. In the 1840s, Urbain Le Verrier used Newtonian mechanics to predict the position of the then-undiscovered planet Neptune after analyzing perturbations in the orbit of Uranus.
Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranus's orbit was being disturbed by another planet besides Neptune. In 1906, Percival Lowell—a wealthy Bostonian who had founded Lowell Observatory in Flagstaff, Arizona, in 1894—started an extensive project in search of a possible ninth planet, which he termed "Planet X". By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search until his death to no avail. Unknown to Lowell, his surveys had captured two faint images of Pluto on March 19 and April 7, 1915, but they were not recognized for what they were. There are fourteen other known precovery observations, with the earliest made by the Yerkes Observatory on August 20, 1909. Percival's widow, Constance Lowell, entered into a ten-year legal battle with the Lowell Observatory over her husband's legacy, the search for Planet X did not resume until 1929.
Vesto Melvin Slipher, the observatory director, gave the job of locating Planet X to 23-year-old Clyde Tombaugh, who had just arrived at the observatory after Slipher had been impressed by a sample of his astronomical drawings. Tombaugh's task was to systematically image the night sky in pairs of photographs examine each pair and determine whether any objects had shifted position. Using a blink comparator, he shifted back and forth between views of each of the plates to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and 29. A lesser-quality photograph taken on January 21 helped confirm the movement. After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. Pluto has yet to complete a full orbit of the Sun since its discovery because one Plutonian year is 247.68 years long.
The discovery made headlines around the globe. Lowell Observatory, which had the right to name the new object, received more than 1,000 suggestions from all over the world, ranging from Atlas to Zymal. Tombaugh urged Slipher to suggest a name for the new object before someone else did. Constance Lowell proposed Zeus Percival and Constance; these suggestions were disregarded. The name Pluto, after the god of the underworld, was proposed by Venetia Burney, an eleven-year-old schoolgirl in Oxford, interested in classical mythology, she suggested it in a conversation with her grandfather Falconer Madan, a former librarian at the University of Oxford's Bodleian Library, who passed the name to astronomy professor Herbert Hall Turner, who cabled it to colleagues in the United States. Each member of the Lowell Observatory was allowed to vote on a short-list of three potential names: Minerva and Pluto. Pluto received every vote; the name was announced on May 1, 1930. Upon the announcement, Madan gave Venetia £5 as
An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, moons and galaxies – in either observational or theoretical astronomy. Examples of topics or fields astronomers study include planetary science, solar astronomy, the origin or evolution of stars, or the formation of galaxies. Related but distinct subjects like physical cosmology. Astronomers fall under either of two main types: observational and theoretical. Observational astronomers analyze the data. In contrast, theoretical astronomers create and investigate models of things that cannot be observed; because it takes millions to billions of years for a system of stars or a galaxy to complete a life cycle, astronomers must observe snapshots of different systems at unique points in their evolution to determine how they form and die. They use these data to create models or simulations to theorize how different celestial objects work.
Further subcategories under these two main branches of astronomy include planetary astronomy, galactic astronomy, or physical cosmology. Astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using physical laws. Today, that distinction has disappeared and the terms "astronomer" and "astrophysicist" are interchangeable. Professional astronomers are educated individuals who have a Ph. D. in physics or astronomy and are employed by research institutions or universities. They spend the majority of their time working on research, although they quite have other duties such as teaching, building instruments, or aiding in the operation of an observatory; the number of professional astronomers in the United States is quite small. The American Astronomical Society, the major organization of professional astronomers in North America, has 7,000 members; this number includes scientists from other fields such as physics and engineering, whose research interests are related to astronomy.
The International Astronomical Union comprises 10,145 members from 70 different countries who are involved in astronomical research at the Ph. D. beyond. Contrary to the classical image of an old astronomer peering through a telescope through the dark hours of the night, it is far more common to use a charge-coupled device camera to record a long, deep exposure, allowing a more sensitive image to be created because the light is added over time. Before CCDs, photographic plates were a common method of observation. Modern astronomers spend little time at telescopes just a few weeks per year. Analysis of observed phenomena, along with making predictions as to the causes of what they observe, takes the majority of observational astronomers' time. Astronomers who serve as faculty spend much of their time teaching undergraduate and graduate classes. Most universities have outreach programs including public telescope time and sometimes planetariums as a public service to encourage interest in the field.
Those who become astronomers have a broad background in maths and computing in high school. Taking courses that teach how to research and present papers are invaluable. In college/university most astronomers get a Ph. D. in astronomy or physics. While there is a low number of professional astronomers, the field is popular among amateurs. Most cities have amateur astronomy clubs that meet on a regular basis and host star parties; the Astronomical Society of the Pacific is the largest general astronomical society in the world, comprising both professional and amateur astronomers as well as educators from 70 different nations. Like any hobby, most people who think of themselves as amateur astronomers may devote a few hours a month to stargazing and reading the latest developments in research. However, amateurs span the range from so-called "armchair astronomers" to the ambitious, who own science-grade telescopes and instruments with which they are able to make their own discoveries and assist professional astronomers in research.
List of astronomers List of women astronomers List of Muslim astronomers List of French astronomers List of Hungarian astronomers List of Russian astronomers and astrophysicists List of Slovenian astronomers Dallal, Ahmad. "Science and Technology". In Esposito, John; the Oxford History of Islam. Oxford University Press, New York. ISBN 0-300-15911-0. Kennedy, E. S.. "A Survey of Islamic Astronomical Tables. 46. Philadelphia: American Philosophical Society. Toomer, Gerald. "Al-Khwārizmī, Abu Jaʿfar Muḥammad ibn Mūsā". In Gillispie, Charles Coulston. Dictionary of Scientific Biography. 7. New York: Charles Scribner's Sons. ISBN 0-684-16962-2. American Astronomical Society European Astronomical Society International Astronomical Union Astronomical Society of the Pacific Space's astronomy news
The Solar System is the gravitationally bound planetary system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, such as the five dwarf planets and small Solar System bodies. 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 gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's mass is in the Sun, with the majority of the remaining mass contained in Jupiter; the four smaller inner planets, Venus and Mars, are terrestrial planets, being composed of rock and metal. The four outer planets are giant planets, being more massive than the terrestrials; the two largest and Saturn, are gas giants, being composed of hydrogen and helium. All eight planets have circular orbits that lie within a nearly flat disc called the ecliptic.
The Solar System contains smaller objects. The asteroid belt, which lies between the orbits of Mars and Jupiter contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed of ices, beyond them a newly discovered population of sednoids. Within these populations are several dozen to 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 trans-Neptunian objects Pluto and Eris. In addition to these two regions, various other small-body populations, including comets and interplanetary dust clouds travel between regions. Six of the planets, at least four of the dwarf planets, many of the smaller bodies are orbited by natural satellites termed "moons" after the Moon; 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, creates a bubble-like region in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium; the Oort cloud, thought to be the source for long-period comets, may exist at a distance a thousand times further than the heliosphere. The Solar System is located in the Orion Arm, 26,000 light-years from the center of the Milky Way galaxy. For most of history, humanity did not understand the concept of the Solar System. Most people up to the Late Middle Ages–Renaissance believed Earth to be stationary at the centre of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system.
In the 17th century, Galileo discovered that the Sun was marked with sunspots, that Jupiter had four satellites in orbit around it. Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn. Edmond Halley realised in 1705 that repeated sightings of a comet were recording the same object, returning once every 75–76 years; this was the first evidence that anything other than the planets orbited the Sun. Around this time, the term "Solar System" first appeared in English. In 1838, Friedrich Bessel measured a stellar parallax, an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism. Improvements in observational astronomy and the use of unmanned spacecraft have since enabled the detailed investigation of other bodies orbiting the Sun; the principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99.86% of the system's known mass and dominates it gravitationally.
The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System together comprise less than 0.002% of the Solar System's total mass. Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic; the planets are close to the ecliptic, whereas comets and Kuiper belt objects are at greater angles to it. All the planets, most other objects, orbit the Sun in the same direction that the Sun is rotating. There are exceptions, such as Halley's Comet; the overall structure of the charted regions of the Solar System consists of the Sun, four small inner planets surrounded by a belt of rocky asteroids, four giant planets surrounded by the Kuiper belt of icy objects. Astronomers sometimes informally divide this structure into separate regions; the inner Solar System includes the asteroid belt. The outer Solar System is including the four giant planets.
Since the discovery of the Kuiper belt, the outermost parts of the Solar Sys