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Astronomy (Greek: αστρονομία = άστρον + νόμος, astronomia = astron + nomos, literally, "law of the stars") is the science of celestial objects (e.g., stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (e.g., auroras and cosmic background radiation). It is concerned with the evolution, physics, chemistry, and motion of celestial objects, as well as the formation and development of the universe. Astronomy is one of the oldest sciences. Astronomers of early civilizations performed methodical observations of the night sky, and astronomical artifacts have been found from much earlier periods. However, it required the invention of the telescope before astronomy developed into a modern science. Since the 20th century, the field of professional astronomy has split into observational astronomy and theoretical astrophysics. Observational astronomy is concerned with acquiring data, which involves building and maintaining instruments, as well as processing the results. Theoretical astrophysics is concerned with ascertaining the observational implications of computer or analytic models. The two fields complement each other, with theoretical astronomy seeking to explain the observational results. Astronomical observations can be used to test fundamental theories in physics, such as general relativity. Historically, amateur astronomers have contributed to many important astronomical discoveries, and astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and observation of transient phenomena. Modern astronomy is not to be confused with astrology, the belief system that claims human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin, most thinkers in both fields believe they are now distinct.• History In early times, astronomy only comprised the observation and predictions of the motions of the naked-eye objects. In some locations, such as Stonehenge, early cultures assembled massive artifacts that likely had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops, as well as the length of the year.• As civilizations developed, most notably Babylonia, Persia, Egypt, ancient Greece, India, and China, astronomical observatories were assembled and ideas on the nature of the universe began to be explored. Early ideas on the motions of the planets were developed, and the nature of the Sun, Moon and the Earth in the universe were explored philosophically. The Earth was believed to be the centre of the universe with the Sun, the Moon and the stars rotating around it. This is what is known as the geocentric model of the universe. A few notable astronomical discoveries were made prior to the application of the telescope. For example, the obliquity of the ecliptic was estimated as early as 1,000 B.C by the Chinese. The Chaldeans discovered that eclipses recurred in a repeating cycle known as a saros. In the second century B.C., the size and distance of the Moon were estimated by Hipparchus. During the Middle Ages, observational astronomy was mostly stagnant in medieval Europe until the 13th century. However, observational astronomy flourished in the Persian Empire and other parts of the Islamic world. Islamic astronomers introduced many names that are now used for individual stars.•• Scientific revolution During the Renaissance, Nicolaus Copernicus proposed a heliocentric model of the Solar System. His work was defended, expanded upon, and corrected by Galileo Galilei and Johannes Kepler. Galileo added the innovation of using telescopes to enhance his observations. Kepler was the first to devise a system that described correctly the details of the motion of the planets with the Sun at the center. However, Kepler did not succeed in formulating a theory behind the laws he wrote down. It was left to Newton's invention of celestial dynamics and his law of gravitation to finally explain the motions of the planets. Newton also developed the reflecting telescope. Further discoveries paralleled the improvements in size and quality of the telescope. More extensive star calatogues were produced by Lacaille. The astronomer William Herschel made an extensive catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first new planet found. The distance to a star was first announced in 1838 when the parallax of 61 Cygni was measured by Friedrich Bessel. During the nineteenth century, attention to the three body problem by Euler, Clairaut and D'Alembert led to more accurate predictions about the motions of the Moon and planets. This work was further refined by Lagrance and Laplace, allowing the masses of the planets and moons to be estimated from their perturbations. Significant advances in astronomy came about with the introduction of new technology, including the spectroscope and photography. Fraunhofer discovered about 600 bands in the spectrum of the Sun in 1814-15, which, in 1859, Kirchhoff ascribed to the presence of different elements. Stars were proven to be similar to Earth's own sun, but with a wide range of temperatures, masses, and sizes. The existence of Earth's galaxy, the Milky Way, as a separate group of stars was only proved in the 20th century, along with the existence of "external" galaxies, and soon after, the expansion of the universe, seen in the recession of most galaxies from us. Modern astronomy has also discovered many exotic objects such as quasars, pulsars, blazars and radio galaxies, and has used these observations to develop physical theories which describe some of these objects in terms of equally exotic objects such as black holes and neutron stars. Physical cosmology made huge advances during the 20th century, with the model of the Big Bang heavily supported by the evidence provided by astronomy and physics, such as the cosmic microwave background radiation, Hubble's law, and cosmological abundances of elements. Astronomical observations
Methods of data collection In astronomy, information is mainly received from the detection and analysis of light and other forms of electromagnetic radiation. Other cosmic rays are also observed, and several experiments are designed to detect gravitational waves in the near future. Neutrino detectors have been used to observe solar neutrinos, and neutrino emissions from supernovae have also been detected.•• A traditional division of astronomy is given by the region of the electromagnetic spectrum observed. At the low frequency end of the spectrum, radio astronomy detects radiation of millimeter to dekameter wavelength. The radio telescope receivers are similar to those used in radio broadcast transmission but much more sensitive. Microwaves form the millimeter end of the radio spectrum and are important for studies of the cosmic microwave background radiation. Infrared astronomy and far infrared astronomy deal with the detection and analysis of infrared radiation (wavelengths longer than red light). The most common tool is the telescope but using a detector which is sensitive to the infrared. Infrared radiation is heavily absorbed by atmospheric water vapor, so infrared observatories have to be located in high, dry places or in outer space. Space telescopes in particular avoid atmospheric thermal emission, atmospheric opacity, and the negative effects of astronomical seeing at infrared and other wavelengths. Infrared is particularly useful for observation of galactic regions cloaked by dust. Historically, most astronomical data has been collected through optical astronomy. This is the portion of the spectrum that uses optical components (mirrors, lenses, CCD detectors and photographic films) to observe light from near infrared to near ultraviolet wavelengths. Visible light astronomy (using wavelengths that can be detected with the eyes, about 400 - 700 nm) falls in the middle of this range. The most common tool is the telescope, with electronic imagers and spectrographs. More energetic sources are observed and studied in high-energy astronomy, which includes X-ray astronomy, gamma ray astronomy, and extreme UV (ultraviolet) astronomy, as well as studies of neutrinos and cosmic rays. Optical and radio astronomy can be performed with ground-based observatories, because the Earth's atmosphere is transparent at the wavelengths being detected. The atmosphere is opaque at the wavelengths of X-ray astronomy, gamma-ray astronomy, UV astronomy and (except for a few wavelength "windows") far infrared astronomy, so observations must be carried out mostly from balloons or space observatories. Powerful gamma rays can, however be detected by the large air showers they produce, and the study of cosmic rays can also be regarded as a branch of astronomy.• Planetary astronomy has benefited from direct observation in the form of spacecraft and sample return missions. These include fly-by missions with remote sensors, landing vehicles that can perform experiments on the surface materials, impactors that allow remote sensing of buried materials, and sample return missions that allow direct laboratory examination. Astrometry and celestial mechanics One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects in the sky. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in celestial navigation. Careful measurement of the positions of the planets has led to a solid understanding of gravitational perturbations and an ability to determine past and future positions of the planets with great accuracy, a field known as celestial mechanics. More recently the tracking of near-Earth objects will allow for predictions of close encounters, and potential collisions, with the Earth.• The measurement of stellar parallax of nearby stars provides a fundamental baseline in the cosmic distance ladder that is used to measure the scale of the universe. Parallax measurements of nearby stars provides an absolute baseline for the properties of more distant stars, because their properties can be compared. Measurements of radial velocity and proper motion show the kinematics of these systems through the Milky Way galaxy. Astrometric results are also used to measure the distribution of dark matter in the galaxy.• During the 1990s, the astrometric technique of measuring the stellar wobble was used to detect large extrasolar planets orbiting nearby stars.• Interdisciplinary studies Astronomy has developed significant interdisciplinary links with other major scientific fields. These include: Solar astronomy The most frequently studied star is the Sun, a typical main-sequence dwarf star of stellar class G2 V, and about 4.6 Gyr in age. The Sun is not considered a variable star, but it does undergo periodic changes in activity known as the sunspot cycle. This is an 11-year fluctuation in sunspot numbers. Sunspots are regions of lower than average temperature that are associated with intense magnetic activity.• A solar wind of plasma particles constantly streams outward from the Sun until it reaches the heliopause. This solar wind interacts with the magnetosphere of the Earth to create the Van Allen radiation belts, as well as the aurora where the lines of the Earth's magnetic field descend into the atmosphere.• Planetary science
Stellar astronomy
Galactic astronomy
Galaxies and clusters
Cosmology Observations of the large-scale structure of the universe, a branch known as physical cosmology, have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the big bang, wherein our universe began at a single point in time and thereafter expanded over the course of 13.7 Gyr to its present condition. The concept of the big bang can be traced back to the discovery of the microwave background radiation in 1965. In the course of this expansion, the universe underwent several evolutionary stages. In the very early moments, it is theorized that the universe underwent a very rapid cosmic inflation, which homogenized the starting conditions. Thereafter nucleosynthesis produced the elemental abundance of the early universe. When the first atoms formed space became transparent to radiation; releasing the energy viewed today as the microwave background radiation. The expanding universe then underwent a dark age due to the lack of stellar energy sources.• A hierarchical structure of matter began to form from minute variations in the mass density. Matter accumulated in the densest regions, forming clouds of gas and the earliest stars. These massive stars triggered the reionization process and are believed to have created many of the heavy elements in the early universe. Gravitational aggregations clustered into filaments, leaving voids in the gaps. Gradually organizations of gas and dust merged to form the first primitive galaxies. Over time these pulled in more matter, and were often organized into groups and clusters of galaxies, then into larger-scale superclusters.• Fundamental to the structure of the universe is the existence of dark matter and dark energy. These are now thought to be the dominant components, forming 96% of the density of the universe. So much effort is being spent to try and understand the physics of these components.• Amateur astronomy Collectively, amateur astronomers observe a variety of celestial objects and phenomena sometimes with equipment they build themselves. Common targets of amateur astronomers include the Moon, planets, stars, comets, meteor showers, and a variety of deep sky objects such as star clusters, galaxies, and nebulae. One branch of amateur astronomy, amateur astrophotography, involves the taking of photos of the night sky. Many amateurs like to specialise in observing particular objects, types of objects, or types of events which interest them.•• Most amateurs work at visible wavelengths, but a small minority experiment with wavelengths outside the visible spectrum. This includes the use of infrared filters on conventional telescopes, and also the use of radio telescopes. The pioneer of amateur radio astronomy was Karl Jansky who started observing the sky at radio wavelengths in the 1930s. A number of amateur astronomers use either homemade telescopes or use radio telescopes which were originally built for astronomy research but which are now available to amateurs (e.g. the One-Mile Telescope).•• Amateur astronomers continue to make scientific contributions to the field of astronomy. Indeed it is one of the few scientific disciplines where amateurs can still make significant contributions. Amateurs can make occultation measurements that are used to refine the orbits of minor planets. They can also discover comets and perform regular observations of variable stars. Improvements in digital technology have allowed amateurs to make impressive advances in the field of astrophotography. ••• Major questions in astronomy Although the scientific discipline of astronomy has made tremendous strides in understanding the nature of the universe and its contents, there remain some important unanswered questions. Answers to these may require the construction of new ground and space-based instruments, and possibly new developments in theoretical and experimental physics. See also | |||||||||||||||||
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