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Astronomy is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation). It is concerned with the evolution, physics, chemistry, meteorology, 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, the invention of the telescope was required before astronomy was able to develop into a modern science. Historically, astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, the making of calendars, and even astrology, but professional astronomy is nowadays often considered to be identical with astrophysics. Since the 20th century, the field of professional astronomy split into observational and theoretical branches. Observational astronomy is focused on acquiring and analyzing data, mainly using basic principles of physics. Theoretical astronomy is oriented towards the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain the observational results, and observations being used to confirm theoretical results.
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 that human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin and a part of their methods (namely, the use of ephemerides), they are distinct.[1]
[edit] Lexicology
The word astronomy literally means "law of the stars" and is derived from the Greek αστρονομία, astronomia, from the words άστρον (astron, "star") and νόμος (nomos, "laws of naming").
[edit] Use of terms "astronomy" and "astrophysics"
Generally, either the term "astronomy" or "astrophysics" may be used to refer to this subject.[2][3][4] Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the earth's atmosphere and of their physical and chemical properties"[5]and "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena".[6] In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject.[7] However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics.[2] Various departments that research this subject may use "astronomy" and "astrophysics", partly depending on whether the department is historically affiliated with a physics department,[3] and many professional astronomers actually have physics degrees.[4] Even the name of the scientific journal Astronomy & Astrophysics reveals the ambiguity of the use of the term.
[edit] History
- Further information: Archaeoastronomy
In early times, astronomy only comprised the observation and predictions of the motions of objects visible to the naked eye. 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 in understanding the length of the year.[8]
Before tools such as the telescope were invented early study of the stars had to be conducted from the only vantage points available, namely tall buildings, trees and high ground using the bare eye.
As civilizations developed, most notably Mesopotamia, Egypt, Persia, Maya, Greece, India, China, and the Islamic world, astronomical observatories were assembled, and ideas on the nature of the universe began to be explored. Most of early astronomy actually consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon and the Earth in the universe were explored philosophically. The Earth was believed to be the center of the universe with the Sun, the Moon and the stars rotating around it. This 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.[citation needed] 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, at least until the 13th century. However, observational astronomy flourished in the Persian Empire and other parts of the world. Astronomers during that time introduced many names that are now used for individual stars.[9][10]
[edit] 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 innovated by 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 the size and quality of the telescope. More extensive star catalogues were produced by Lacaille. The astronomer William Herschel made a detailed 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 Lagrange 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 the Earth's own Sun, but with a wide range of temperatures, masses, and sizes.[9]
The existence of the 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.
[edit] Observational astronomy
In astronomy, information is mainly received from the detection and analysis of light and other forms of electromagnetic radiation.[11] Observational astronomy may be divided according to the observed region of the electromagnetic spectrum. Some parts of the spectrum can be observed from the Earth's surface, while other parts are only observable from either high altitudes or space. Specific information on these subfields is given below.
[edit] Radio astronomy
Radio astronomy studies radiation with wavelengths greater than approximately one millimeter.[12] Radio astronomy is different from most other forms of observational astronomy in that the observed radio waves can be treated as waves rather than as discrete photons. Hence, it is relatively easier to measure both the amplitude and phase of radio waves, whereas this is not as easily done at shorter wavelengths.[12]
Although some radio waves are produced by astronomical objects in the form of thermal emission, most of the radio emission that is observed from Earth is seen in the form of synchrotron radiation, which is produced when electrons oscillate around magnetic fields.[12] Additionally, a number of spectral lines produced by interstellar gas, particularly the hydrogen spectral line at 21 cm, are observable at radio wavelengths.[7][12]
A wide variety of objects are observable at radio wavelengths, including supernovae, interstellar gas, pulsars, and active galactic nuclei.[7][12]
[edit] Infrared astronomy
Infrared astronomy deals with the detection and analysis of infrared radiation (wavelengths longer than red light). Except at wavelengths close to visible light, infrared radiation is heavily absorbed by the atmosphere, and the atmosphere produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places or in space. Infrared astronomy is particularly useful for observation of galactic regions cloaked by dust, and for studies of molecular gases.
[edit] Optical astronomy
Historically, optical astronomy, also called visible light astronomy, is the oldest form of astronomy.[13] Optical images were originally drawn by hand. In the late nineteenth century and most of the twentieth century, images were made using photographic equipment. Modern images are made using digital detectors, particularly detectors using charge-coupled devices (CCDs). Although visible light itself extends from approximately 4000 Å (400 nm) to 7000 Å (700 nm),[13] the same equipment used at these wavelengths is also used to observe some near-ultraviolet and near-infrared radiation.
[edit] Ultraviolet astronomy
Ultraviolet astronomy is generally used to refer to observations at ultraviolet wavelengths between approximately 100 and 3200 Å.[12] Light at these wavelengths is absorbed by the Earth's atmosphere, so observations at these wavelengths must be performed from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue stars (O stars and B stars) that are very bright in this wave band. This includes the blue stars in other galaxies, which have been the targets of several ultraviolet surveys. Other objects commonly observed in ultraviolet light include planetary nebulae, supernova remnants, and active galactic nuclei.[12] However, ultraviolet light is easily absorbed by interstellar dust, and measurement of the ultraviolet light from objects need to be corrected for extinction.[12]
[edit] X-ray astronomy
X-ray astronomy is the study of astronomical objects at X-ray wavelengths. Typically, objects emit X-ray radiation as synchrotron emission (produced by electrons oscillating around magnetic field lines), thermal emission from thin gases (called bremsstrahlung radiation) that is above 107 (10 million) Kelvin, and thermal emission from thick gases (called blackbody radiation) that are above 107 Kelvin.[12] Since X-rays are absorbed by the Earth's atmosphere, all X-ray observations must be done from high-altitude balloons, rockets, or spacecraft.[citation needed] Notable X-ray sources include X-ray binaries, pulsars, supernova remnants, elliptical galaxies, clusters of galaxies, and active galactic nuclei.[12]
[edit] Gamma-ray astronomy
Gamma ray astronomy is the study of astronomical objects at the shortest wavelengths of the electromagnetic spectrum. Gamma rays may be observed directly by satellites such as the Compton Gamma Ray Observatory or by specialized telescopes called atmospheric Cherenkov telescopes.[12] The Cherenkov telescopes do not actually detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere.[14]
Most gamma-ray emitting sources are actually gamma-ray bursts, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources. These steady gamma-ray emitters include pulsars, neutron stars, and black hole candidates such as active galactic nuclei.[12]
[edit] Fields of observational astronomy not based on the electromagnetic spectrum
Other than electromagnetic radiation, few things may be observed from the Earth that originate from great distances.
In neutrino astronomy, astronomers use special underground facilities such as SAGE, GALLEX, and Kamioka II/III for detecting neutrinos. These neutrinos originate primarily from the Sun but also from supernovae.[12]
Cosmic rays consisting of very high energy particles can be observed hitting the Earth's atmosphere.[citation needed] Additionally, some future neutrino detectors will also be sensitive to the neutrinos produced when cosmic rays hit the Earth's atmosphere.[12]
A few gravitational wave observatories have been constructed, but gravitational waves are extremely difficult to detect.[15]
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 material, and sample return missions that allow direct, laboratory examination.
[edit] 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. 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.[16]
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 provide 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.[17]
During the 1990s, the astrometric technique of measuring the stellar wobble was used to detect large extrasolar planets orbiting nearby stars.[18]
[edit] Theoretical astronomy
| Nucleosynthesis |
 |
| Related topics |
Theoretical astronomers use a wide variety of tools which include analytical models (for example, polytropes to approximate the behaviors of a star) and computational numerical simulations. Each has some advantages. Analytical models of a process are generally better for giving insight into the heart of what is going on. Numerical models can reveal the existence of phenomena and effects that would otherwise not be seen.[19][20]
Theorists in astronomy endeavor to create theoretical models and figure out the observational consequences of those models. This helps allow observers to look for data that can refute a model or help in choosing between several alternate or conflicting models.
Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model.
Topics studied by theoretical astronomers include: stellar dynamics and evolution; galaxy formation; large-scale structure of matter in the Universe; origin of cosmic rays; general relativity and physical cosmology, including string cosmology and astroparticle physics. Astrophysical relativity serves as a tool to gauge the properties of large scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis for black hole (astro)physics and the study of gravitational waves.
Some widely accepted and studied theories and models in astronomy, now included in the Lambda-CDM model are the Big Bang, Cosmic inflation, dark matter, and fundamental theories of physics.
A few examples of this process:
| Physical process | Experimental tool | Theoretical model | Explains/predicts |
| Gravitation | Radio telescopes | Self-gravitating system | Emergence of a star system |
| Nuclear fusion | Spectroscopy | Stellar evolution | How the stars shine and how metals formed |
| The Big Bang | Hubble Space Telescope, COBE | Expanding universe | Age of the Universe |
| Quantum fluctuations | Cosmic inflation | Flatness problem | |
| Gravitational collapse | X-ray astronomy | General relativity | Black holes at the center of Andromeda galaxy |
| CNO cycle in stars |
Dark matter and dark energy are the current leading topics in astronomy, as their discovery and controversy originated during the study of the galaxies.
[edit] Subfield of astronomy for specific astronomical objects
[edit] 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 temperatures that are associated with intense magnetic activity.[21]
The Sun has steadily increased in luminosity over the course of its life, increasing by 40% since it first became a main-sequence star. The Sun has also undergone periodic changes in luminosity that can have a significant impact on the Earth.[22] The Maunder minimum, for example, is believed to have caused the Little Ice Age phenomenon during the Middle Ages.[23]
The visible outer surface of the Sun is called the photosphere. Above this layer is a thin region known as the chromosphere. This is surrounded by a transition region of rapidly increasing temperatures, then by the super-heated corona.
At the center of the Sun is the core region, a volume of sufficient temperature and pressure for nuclear fusion to occur. Above the core is the radiation zone, where the plasma conveys the energy flux by means of radiation. The outer layers form a convection zone where the gas material transports energy primarily through physical displacement of the gas. It is believed that this convection zone creates the magnetic activity that generates sun spots.[21]
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.[24]
[edit] Planetary science
This astronomical field examines the assemblage of planets, moons, dwarf planets, comets, asteroids, and other bodies orbiting the Sun, as well as extrasolar planets. The solar system has been relatively well-studied, initially through telescopes and then later by spacecraft. This has provided a good overall understanding of the formation and evolution of this planetary system, although many new discoveries are still being made.[25]
The solar system is subdivided into the inner planets, the asteroid belt, and the outer planets. The inner terrestrial planets consist of Mercury, Venus, Earth, and Mars. The outer gas giant planets are Jupiter, Saturn, Uranus and Neptune.[26] Beyond Neptune lie the Kuiper Belt, and finally the Oort Cloud, which may extend as far as a light-year.
The planets were formed by a protoplanetary disk that surrounded the early Sun. Through a process that included gravitational attraction, collision, and accretion, the disk formed clumps of matter that, with time, became protoplanets. The radiation pressure of the solar wind then expelled most of the unaccreted matter, and only those planets with sufficient mass retained their gaseous atmosphere. The planets continued to sweep up, or eject, the remaining matter during a period of intense bombardment, evidenced by the many impact craters on the Moon. During this period, some of the protoplanets may have collided, the leading hypothesis for how the Moon was formed.[27]
Once a planet reaches sufficient mass, the materials with different densities segregate within, during planetary differentiation. This process can form a stony or metallic core, surrounded by a mantle and an outer surface. The core may include solid and liquid regions, and some planetary cores generate their own magnetic field, which can protect their atmospheres from solar wind stripping.[28]
A planet or moon's interior heat is produced from the collisions that created the body, radioactive materials (e.g. uranium, thorium, and 26Al), or tidal heating. Some planets and moons accumulate enough heat to drive geologic processes such as volcanism and tectonics. Those that accumulate or retain an atmosphere can also undergo surface erosion from wind or water. Smaller bodies, without tidal heating, cool more quickly; and their geological activity ceases with the exception of impact cratering.[29]
[edit] Stellar astronomy
The study of stars and stellar evolution is fundamental to our understanding of the universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior.
Star formation occurs in dense regions of dust and gas, known as giant molecular clouds. When destabilized, cloud fragments can collapse under the influence of gravity, to form a protostar. A sufficiently dense, and hot, core region will trigger nuclear fusion, thus creating a main-sequence star.[30]
Almost all elements heavier than hydrogen and helium were created inside the cores of stars.
The characteristics of the resulting star depend primarily upon its starting mass. The more massive the star, the greater its luminosity, and the more rapidly it expends the hydrogen fuel in its core. Over time, this hydrogen fuel is completely converted into helium, and the star begins to evolve. The fusion of helium requires a higher core temperature, so that the star both expands in size, and increases in core density. The resulting red giant enjoys a brief life span, before the helium fuel is in turn consumed. Very massive stars can also undergo a series of decreasing evolutionary phases, as they fuse increasingly heavier elements.
The final fate of the star depends on its mass, with stars of mass greater than about eight times the Sun becoming core collapse supernovae; while smaller stars form planetary nebulae, and evolve into white dwarfs. The remnant of a supernova is a dense neutron star, or, if the stellar mass was at least three times that of the Sun, a black hole.[31] Close binary stars can follow more complex evolutionary paths, such as mass transfer onto a white dwarf companion that can potentially cause a supernova. Planetary nebulae and supernovae are necessary for the distribution of metals to the interstellar medium; without them, all new stars (and their planetary systems) would be formed from hydrogen and
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