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Astronomy

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   A giant Hubble mosaic of the Crab Nebula, a supernova remnant.
   Enlarge
   A giant Hubble mosaic of the Crab Nebula, a supernova remnant.

   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, 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, 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

   Galileo's sketches and observations of the Moon revealed that the
   surface was mountainous.
   Enlarge
   Galileo's sketches and observations of the Moon revealed that the
   surface was mountainous.

   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 centre.
   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

   In Babylon and ancient Greece, astronomy consisted largely of
   astrometry, measuring the positions of stars and planets in the sky.
   Later, the work of astronomers Kepler and Newton led to the development
   of celestial mechanics, and astronomy focused on mathematically
   predicting the motions of gravitationally interacting celestial bodies.
   This was applied to solar system objects in particular. Today, the
   motions and positions of objects are more easily determined, and modern
   astronomy concentrates on observing and understanding the physical
   nature of celestial objects.

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.
   Because of the altitude and isolation, the Mauna Kea Observatory has
   some of the best observation conditions on Earth.
   Enlarge
   Because of the altitude and isolation, the Mauna Kea Observatory has
   some of the best observation conditions on Earth.

   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, o.virginia.edu/~rjp0i/museum/engines.html | title = Hall of
   Precision Astrometry | publisher = University of Virginia Department of
   Astronomy | language = English | accessdate = 2006-08-10 }}</ref>

   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:
     * Astrophysics: the study of the physics of the universe, including
       the physical properties ( luminosity, density, temperature,
       chemical composition) of astronomical objects.
     * Astrobiology: the study of the advent and evolution of biological
       systems in the universe.
     * Archaeoastronomy: the study of ancient or traditional astronomies
       in their cultural context, utilising archaeological and
       anthropological evidence.
     * Astrochemistry: the study of the chemicals found in outer space,
       usually in molecular gas clouds, and their formation, interaction
       and destruction. As such, it represents an overlap of the
       disciplines of astronomy and chemistry.

Astronomical objects

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.
   An ultraviolet image of the Sun's active photosphere as viewed by the
   TRACE space telescope. NASA photo.
   Enlarge
   An ultraviolet image of the Sun's active photosphere as viewed by the
   TRACE space telescope. NASA photo.

   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. The Maunder minimum, for example, is
   believed to have caused the Little Ice Age phenomenon during the Middle
   Ages.

   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 centre 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.

   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

   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.
   The black spot at top is a dust devil climbing a crater wall on Mars.
   This moving, swirling column of Martian atmosphere (comparable to a
   terrestrial tornado) created the long, dark streak. NASA image.
   Enlarge
   The black spot at top is a dust devil climbing a crater wall on Mars.
   This moving, swirling column of Martian atmosphere (comparable to a
   terrestrial tornado) created the long, dark streak. NASA image.

   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.

   The planets formed from 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 protoplanets may have collided, the leading
   hypothesis for how the Moon was formed.

   Once a planet reaches sufficient mass, the materials with different
   densities segregate within its interior during planetary
   differentiation. This process can form a stony or metallic core
   surrounded by a mantle and outer surface. The core may include solid
   and liquid regions, and some planetary cores generate their own
   magnetic field, which can protect its atmosphere from solar wind
   stripping.

   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.

Stellar astronomy

   The Ant planetary nebula. Ejecting gas from the dying central star
   shows symmetrical patterns unlike the chaotic patterns of ordinary
   explosions.
   Enlarge
   The Ant planetary nebula. Ejecting gas from the dying central star
   shows symmetrical patterns unlike the chaotic patterns of ordinary
   explosions.

   The study of stars and stellar evolution is fundamental to our
   understanding of the universe. The astrophysics of stars has been
   determined through observation, 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 and it becomes a
   main-sequence star.

   The characteristics of the resulting star depend primarily on 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. Fusion of helium requires a higher core
   temperature, so the star both expands in size and increases in density
   at the core. 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 shorter and shorter evolutionary phases as they
   fuse increasingly heavier elements.

   The final fate of the star depends on its mass, with stars of mass
   greater than 1.4 times the Sun becoming supernovae, while smaller stars
   will 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.

Galactic astronomy

   Observed structure of the Milky Way's spiral arms
   Enlarge
   Observed structure of the Milky Way's spiral arms

   Our solar system orbits within the Milky Way, a barred spiral galaxy
   that is a prominent member of the Local Group of galaxies. It is a
   rotating mass of gas, dust, stars and other objects, held together by
   mutual gravitational attraction. As the Earth is located within the
   dusty outer arms, there are large portions of the Milky Way that are
   obscured from view.

   In the centre of the Milky Way is the core region, a bar-shaped bulge
   with what is believed to be a supermassive black hole at the centre.
   This is surrounded by four primary arms that spiral out from the core.
   This is a region of active star formation that contains many younger,
   population II stars. The disk is surrounded by a spheroid halo of
   older, population I stars, as well as relatively dense concentrations
   of stars known as globular clusters.

   Between the stars lies the interstellar medium, a region of sparse
   matter. In the densest regions, molecular clouds of molecular hydrogen
   and other elements create star-forming regions. These begin as
   irregular dark nebulae, which concentrate and collapse (in volumes
   determined by the Jeans length) to form compact protostars.

   As the more massive stars appear, they transform the cloud into an H II
   region of glowing gas and plasma. The stellar wind and supernova
   explosions from these stars eventually serve to disperse the cloud,
   often leaving behind one or more young open clusters of stars. These
   gradually disperse to join the population of stars in the Milky Way.

   Kinematic studies of matter in the Milky Way and other galaxies have
   demonstrated that there is more mass than can be accounted for by
   visible matter. A dark matter halo appears to dominate the mass,
   although the nature of this dark matter remains undetermined.

Galaxies and clusters

   The study of objects outside our galaxy is a branch of astronomy
   concerned with the formation and evolution of Galaxies, their
   morphology and classification, the examination of active galaxies and
   the groups and clusters of galaxies. The later is important for the
   understanding of the large-scale structure of the cosmos.
   This image shows several blue, loop-shaped objects that are multiple
   images of the same galaxy, duplicated by the gravitational lens effect
   of the cluster of yellow galaxies near the middle of the photograph.
   The lens is produced by the cluster's gravitational field that bends
   light to magnify and distort the image of a more distant object.
   Enlarge
   This image shows several blue, loop-shaped objects that are multiple
   images of the same galaxy, duplicated by the gravitational lens effect
   of the cluster of yellow galaxies near the middle of the photograph.
   The lens is produced by the cluster's gravitational field that bends
   light to magnify and distort the image of a more distant object.

   Most galaxies are organized into distinct shapes that allow for
   classification schemes. They are commonly divided into spiral,
   elliptical and Irregular galaxies.

   As the name suggests, an elliptical galaxy has the cross-sectional
   shape of an ellipse. The stars move along random orbits with no
   preferred direction. These galaxies contains little or no interstellar
   dust, few star-forming regions and generally older stars. Elliptical
   galaxies are more commonly found at the core of galactic clusters and
   may be formed through mergers of large galaxies.

   A spiral galaxy is organized into a flat, rotating disk, usually with a
   prominent bulge or bar at the centre, and trailing bright arms that
   spiral outward. The arms are dusty regions of star formation where
   massive young stars produce a blue tint. Spiral galaxies are typically
   surrounded by a halo of older stars. Both the Milky Way and the
   Andromeda Galaxy are spiral galaxies.

   Irregular galaxies are chaotic in appearance, and are neither spiral
   nor elliptical in form. About a quarter of all galaxies are irregular,
   and their peculiar shape may be the result of gravitational
   interaction.

   An active galaxy is a formation that is emitting a significant amount
   of its energy from a source other than stars, dust and gas. They are
   powered by a compact region at the core, usually thought to be a
   supermassive black hole that is emitting radiation from infalling
   material.

   A radio galaxy is an active galaxy that is very luminous in the radio
   portion of the spectrum, and is emitting immense plumes or lobes of
   gas. Active galaxies that emit high-energy radiation include Seyfert
   galaxies, Quasars, and Blazars. Quasars are believed to be the most
   consistently luminous objects in the known universe.

   The large-scale structure of the cosmos is represented by groups and
   clusters of galaxies. This structure is organized in a hierarchy of
   groupings, with the largest being the superclusters. The collective
   matter is formed into filaments and walls, leaving large voids in
   between.

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 because of
   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

   Amateur astronomers can build their own equipment and hold star parties
   and gatherings, such as Stellafane.
   Enlarge
   Amateur astronomers can build their own equipment and hold star parties
   and gatherings, such as Stellafane.

   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.
     * Are there Earth-like planets around other stars? Astronomers have
       found massive stars and disks of debris around other stars. So the
       existence of smaller, terrestrial planets seems likely.
     * Is there other life in the Universe? Especially, is there other
       intelligent life? If so, what is the explanation for the Fermi
       paradox? The existence of life elsewhere has important scientific
       and philosophical implications.
     * What is the nature of dark matter and dark energy? These dominate
       the evolution and fate of the cosmos, yet we are still uncertain
       about their true nature.
     * Why did the universe come to be? Why, for example, are the physical
       constants so finely tuned that they permit the existence of life?
       Could they be the result of cosmological natural selection? What
       caused the cosmic inflation that produced our homogeneous universe?

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