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[[Image:Astronomybluevortex.jpg|right|frame|<center>[[Vortex]]</center>]]

[[Image:Astronomybluevortex.jpg|right|frame|<center>[[Vortex]]</center>]]

'''Astronomy'''  is the [[science|scientific study]] of [[celestial]] objects (such as [[star]]s, [[planet]]s, [[comet]]s, and [[galaxy|galaxies]]) and [[Phenomenon|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 [http://institute.daynal.org/archives/extended_reference/cosmology/tufts.html formation and development of the [[universe]]].  

'''Astronomy'''  is the [[science|scientific study]] of [[celestial]] objects (such as [[star]]s, [[planet]]s, [[comet]]s, and [[galaxy|galaxies]]) and [[Phenomenon|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 [https://institute.daynal.org/archives/extended_reference/cosmology/tufts.html formation and development of the [[universe]]].  

Astronomy is one of the oldest sciences. Astronomers of early [[civilization]]s 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 [[calendar]]s, and even astrolog], but professional astronomy is nowadays often considered to be synonymous 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.

Astronomy is one of the oldest sciences. Astronomers of early [[civilization]]s 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 [[calendar]]s, and even astrolog], but professional astronomy is nowadays often considered to be synonymous 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.

==History==

==History==

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. (George Forbes, History of Astronomy, [http://www.gutenberg.org/etext/8172]

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. (George Forbes, History of Astronomy, [https://www.gutenberg.org/etext/8172]

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.

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 [[idea]]s 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.

As civilizations developed, most notably Mesopotamia, Egypt, Persia, Maya, Greece, India, China, and the Islamic world, astronomical observatories were assembled, and [[idea]]s 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 1000 BC by the Chinese. The [[Chaldeans]] discovered that [[lunar eclipses]] recurred in a repeating cycle known as a [http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros.html Eclipses and the Saros saros]. In the 2nd century BC, the size and distance of the Moon were estimated by [http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Hipparchus.html Hipparchus]].   

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 1000 BC by the Chinese. The [[Chaldeans]] discovered that [[lunar eclipses]] recurred in a repeating cycle known as a [https://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros.html Eclipses and the Saros saros]. In the 2nd century BC, the size and distance of the Moon were estimated by [https://www-groups.dcs.st-and.ac.uk/~history/Biographies/Hipparchus.html 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 Islamic world and other parts of the world. Some of the prominent Arab Astronomers, who made significant contributions to the science were [[Al-Battani]] and [[Thebit]]. Astronomers during that time introduced many Arabic names that are now used for individual stars. (Arthur Berry, A Short History of Astronomy From Earliest Times Through the Nineteenth Century, "Cambridge history", The Cambridge Concise History of Astronomy,  ISBN 0-521-57600-8)

During the Middle Ages, observational astronomy was mostly stagnant in medieval Europe, at least until the 13th century. However, observational astronomy flourished in the Islamic world and other parts of the world. Some of the prominent Arab Astronomers, who made significant contributions to the science were [[Al-Battani]] and [[Thebit]]. Astronomers during that time introduced many Arabic names that are now used for individual stars. (Arthur Berry, A Short History of Astronomy From Earliest Times Through the Nineteenth Century, "Cambridge history", The Cambridge Concise History of Astronomy,  ISBN 0-521-57600-8)

==Observational astronomy==

==Observational astronomy==

In astronomy, [[information]] is mainly received from the detection and [[analysis]] of visible [[light]] or other regions of the [http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html electromagnetic radiation]. 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.

In astronomy, [[information]] is mainly received from the detection and [[analysis]] of visible [[light]] or other regions of the [https://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html electromagnetic radiation]. 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.

===Radio astronomy===

===Radio astronomy===

===Gamma-ray astronomy===

===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.  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. [http://www.pparc.ac.uk/frontiers/latest/feature.asp?article].  

[[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.  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. [https://www.pparc.ac.uk/frontiers/latest/feature.asp?article].  

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 star]]s, and [[black hole]] candidates such as active galactic nuclei.

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 star]]s, and [[black hole]] candidates such as active galactic nuclei.

[[Cosmic ray]]s consisting of very high energy particles can be observed hitting the Earth's atmosphere. Additionally, some future neutrino detectors will also be sensitive to the neutrinos produced when cosmic rays hit the Earth's atmosphere.

[[Cosmic ray]]s consisting of very high energy particles can be observed hitting the Earth's atmosphere. Additionally, some future neutrino detectors will also be sensitive to the neutrinos produced when cosmic rays hit the Earth's atmosphere.

A few [[gravitational wave]] observatories have been constructed, but gravitational waves are extremely difficult to detect. [http://www.europhysicsnews.com/full/20/article8/article8.html]|  

A few [[gravitational wave]] observatories have been constructed, but gravitational waves are extremely difficult to detect. [https://www.europhysicsnews.com/full/20/article8/article8.html]|  

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.

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.

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]].

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 [[Perturbation theory|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 object]]s will allow for predictions of close encounters, and potential collisions, with the Earth. [http://www.du.edu/~jcalvert/phys/orbits.htm]

Careful measurement of the positions of the planets has led to a solid understanding of gravitational [[Perturbation theory|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 object]]s will allow for predictions of close encounters, and potential collisions, with the Earth. [https://www.du.edu/~jcalvert/phys/orbits.htm]

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.[http://www.astro.virginia.edu/~rjp0i/museum/engines.html]  

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.[https://www.astro.virginia.edu/~rjp0i/museum/engines.html]  

During the 1990s, the astrometric technique of measuring the stellar wobble was used to detect large [[extrasolar planet]]s orbiting nearby stars. [http://www.nature.com/nature/journal/v355/n6356/abs/355145a0.html]

During the 1990s, the astrometric technique of measuring the stellar wobble was used to detect large [[extrasolar planet]]s orbiting nearby stars. [https://www.nature.com/nature/journal/v355/n6356/abs/355145a0.html]

==Theoretical astronomy==

==Theoretical astronomy==

===Solar astronomy===

===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.[http://www.talkorigins.org/faqs/faq-solar.html]

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.[https://www.talkorigins.org/faqs/faq-solar.html]

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.[http://catalog.loc.gov/cgi-bin/Pwebrecon.cgi?v3=1&DB=local&CMD=010a+2006000857&CNT=10+records+per+page] The [[Maunder minimum]], for example, is believed to have caused the [[Little Ice Age]] phenomenon during the Middle Ages.[http://www-astronomy.mps.ohio-state.edu/~pogge/Lectures/vistas97.html] [http://www-astronomy.mps.ohio-state.edu/Vistas/]

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.[https://catalog.loc.gov/cgi-bin/Pwebrecon.cgi?v3=1&DB=local&CMD=010a+2006000857&CNT=10+records+per+page] The [[Maunder minimum]], for example, is believed to have caused the [[Little Ice Age]] phenomenon during the Middle Ages.[https://www-astronomy.mps.ohio-state.edu/~pogge/Lectures/vistas97.html] [https://www-astronomy.mps.ohio-state.edu/Vistas/]

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]].

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.

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.

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 belt]]s, as well as the [[aurora (astronomy)|aurora]] where the lines of the [[Earth's magnetic field]] descend into the [[Earth's atmosphere|atmosphere]].[http://www-istp.gsfc.nasa.gov/Education/Intro.html]  

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 belt]]s, as well as the [[aurora (astronomy)|aurora]] where the lines of the [[Earth's magnetic field]] descend into the [[Earth's atmosphere|atmosphere]].[https://www-istp.gsfc.nasa.gov/Education/Intro.html]  

===Planetary science===

===Planetary science===

This astronomical field examines the assemblage of [[planet]]s, [[natural satellite|moons]], [[dwarf planet]]s, [[comet]]s, [[asteroid]]s, 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.(Remote Sensing for the Earth Sciences: Manual of Remote Sensing [http://marswatch.tn.cornell.edu/rsm.html}

This astronomical field examines the assemblage of [[planet]]s, [[natural satellite|moons]], [[dwarf planet]]s, [[comet]]s, [[asteroid]]s, 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.(Remote Sensing for the Earth Sciences: Manual of Remote Sensing [https://marswatch.tn.cornell.edu/rsm.html}

The solar system is subdivided into the inner planets, the [[asteroid belt]], and the outer planets. The inner [[terrestrial planet]]s consist of [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]]. The outer [[gas giant]] planets are [[Jupiter]], [[Saturn]], [[Uranus]] and [[Neptune]].[http://nssdc.gsfc.nasa.gov/planetary/ ] Beyond Neptune lie the [[Kuiper Belt]], and finally the [[Oort Cloud]], which may extend as far as a light-year.

The solar system is subdivided into the inner planets, the [[asteroid belt]], and the outer planets. The inner [[terrestrial planet]]s consist of [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]]. The outer [[gas giant]] planets are [[Jupiter]], [[Saturn]], [[Uranus]] and [[Neptune]].[https://nssdc.gsfc.nasa.gov/planetary/ ] 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 crater]]s on the Moon. During this period, some of the protoplanets may have collided, the [[giant impact hypothesis|leading hypothesis]] for how the Moon was formed.[http://www.dtm.ciw.edu/akir/Seminar/seminar.html]

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 crater]]s on the Moon. During this period, some of the protoplanets may have collided, the [[giant impact hypothesis|leading hypothesis]] for how the Moon was formed.[https://www.dtm.ciw.edu/akir/Seminar/seminar.html]

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.[http://www.dtm.ciw.edu/akir/Seminar/internal.html]  

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.[https://www.dtm.ciw.edu/akir/Seminar/internal.html]  

A planet or moon's interior heat is produced from the collisions that created the body, radioactive materials (''e.g.'' [[uranium]], [[thorium]], and [[Aluminum]]), 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.<ref name="new solar system"(The New Solar System, ISBN 0-521-64587-5)

A planet or moon's interior heat is produced from the collisions that created the body, radioactive materials (''e.g.'' [[uranium]], [[thorium]], and [[Aluminum]]), 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.<ref name="new solar system"(The New Solar System, ISBN 0-521-64587-5)

The study of [[star]]s 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.

The study of [[star]]s 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.[http://observe.arc.nasa.gov/nasa/space/stellardeath/stellardeath_intro.html]  

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.[https://observe.arc.nasa.gov/nasa/space/stellardeath/stellardeath_intro.html]  

Almost all elements heavier than [[hydrogen]] and [[helium]] were created inside the cores of stars.

Almost all elements heavier than [[hydrogen]] and [[helium]] were created inside the cores of stars.

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.

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 center of the Milky Way is the core, a bar-shaped bulge with what is believed to be a [[supermassive black hole]] at the center. This is surrounded by four primary arms that spiral 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 cluster]]s.[http://www.mpe.mpg.de/ir/GC/index.php] [http://www.creationresearch.org/crsq/articles/30/30_1/StellarPop.html]  

In the center of the Milky Way is the core, a bar-shaped bulge with what is believed to be a [[supermassive black hole]] at the center. This is surrounded by four primary arms that spiral 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 cluster]]s.[https://www.mpe.mpg.de/ir/GC/index.php] [https://www.creationresearch.org/crsq/articles/30/30_1/StellarPop.html]  

Between the stars lies the [[interstellar medium]], a region of sparse matter. In the densest regions, [[molecular cloud]]s of molecular hydrogen and other elements create star-forming regions. These begin as irregular [[dark nebula]]e, which concentrate and collapse (in volumes determined by the Jeans length) to form compact protostars.[http://www.astro.queensu.ca/~hanes/p014/Notes/Topic_063.html]

Between the stars lies the [[interstellar medium]], a region of sparse matter. In the densest regions, [[molecular cloud]]s of molecular hydrogen and other elements create star-forming regions. These begin as irregular [[dark nebula]]e, which concentrate and collapse (in volumes determined by the Jeans length) to form compact protostars.[https://www.astro.queensu.ca/~hanes/p014/Notes/Topic_063.html]

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 cluster]]s of stars. These clusters gradually disperse, and the stars join the population of the Milky Way.

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 cluster]]s of stars. These clusters gradually disperse, and the stars join the population of 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. [http://www.journals.uchicago.edu/PASP/journal/issues/v111n760/990017/990017.html]

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. [https://www.journals.uchicago.edu/PASP/journal/issues/v111n760/990017/990017.html]

===Extragalactic astronomy===

===Extragalactic astronomy===

The study of objects outside of our galaxy is a branch of astronomy concerned with the formation and evolution of Galaxies; their morphology and classification; and the examination of active galaxies, and the groups and clusters of galaxies. The latter is important for the understanding of the [[large-scale structure]] of the [[cosmos]].

The study of objects outside of our galaxy is a branch of astronomy concerned with the formation and evolution of Galaxies; their morphology and classification; and the examination of active galaxies, and the groups and clusters of galaxies. The latter is important for the understanding of the [[large-scale structure]] of the [[cosmos]].

Most galaxies are organized into distinct shapes that allow for classification schemes. They are commonly divided into [[spiral galaxy|spiral]], elliptical]] and Irregular galaxies.[http://www.astr.ua.edu/keel/galaxies/classify.html]  

Most galaxies are organized into distinct shapes that allow for classification schemes. They are commonly divided into [[spiral galaxy|spiral]], elliptical]] and Irregular galaxies.[https://www.astr.ua.edu/keel/galaxies/classify.html]  

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 contain 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.

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 contain 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.

An active galaxy is a formation that is emitting a significant amount of its energy from a source other than stars, dust and gas; and is powered by a compact region at the core, usually thought to be a super-massive black hole that is emitting radiation from in-falling material.

An active galaxy is a formation that is emitting a significant amount of its energy from a source other than stars, dust and gas; and is powered by a compact region at the core, usually thought to be a super-massive black hole that is emitting radiation from in-falling 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]], [[Quasar]]s, and [[Blazar]]s. Quasars are believed to be the most consistently luminous objects in the known universe.[http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html]

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]], [[Quasar]]s, and [[Blazar]]s. Quasars are believed to be the most consistently luminous objects in the known universe.[https://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html]

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 [[supercluster]]s. The collective matter is formed into filaments and walls, leaving large voids in between. (Astronomy: The Evolving Universe, ISBN 0-521-80090-0)

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 [[supercluster]]s. The collective matter is formed into filaments and walls, leaving large voids in between. (Astronomy: The Evolving Universe, ISBN 0-521-80090-0)

In the course of this expansion, the universe underwent several evolutionary stages. In the very early moments, it is theorized that the universe experienced a very rapid cosmic inflation, which homogenized the starting conditions. Thereafter, nucleosynthesis produced the elemental abundance of the early universe.

In the course of this expansion, the universe underwent several evolutionary stages. In the very early moments, it is theorized that the universe experienced 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.[http://map.gsfc.nasa.gov/m_uni.html]  

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.[https://map.gsfc.nasa.gov/m_uni.html]  

A [[hierarchy|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.

A [[hierarchy|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]].[http://www.damtp.cam.ac.uk/user/gr/public/gal_lss.html]

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]].[https://www.damtp.cam.ac.uk/user/gr/public/gal_lss.html]

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. For this reason, much effort is expended in trying to understand the physics of these components.[http://www.lbl.gov/Science-Articles/Archive/dark-energy.html]

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. For this reason, much effort is expended in trying to understand the physics of these components.[https://www.lbl.gov/Science-Articles/Archive/dark-energy.html]

==Interdisciplinary studies==

==Interdisciplinary studies==

==Amateur astronomy==

==Amateur astronomy==

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

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

Collectively, amateur astronomers observe a variety of celestial objects and phenomena sometimes with equipment that they build themselves. Common targets of amateur astronomers include the Moon, planets, stars, comets, meteor showers, and a variety of [[deep-sky object]]s 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 specialize in the observation of particular objects, types of objects, or types of events which interest them.[http://www.amsmeteors.org/][http://www.astropix.com/]

Collectively, amateur astronomers observe a variety of celestial objects and phenomena sometimes with equipment that they build themselves. Common targets of amateur astronomers include the Moon, planets, stars, comets, meteor showers, and a variety of [[deep-sky object]]s 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 specialize in the observation of particular objects, types of objects, or types of events which interest them.[https://www.amsmeteors.org/][https://www.astropix.com/]

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]]).[http://www.nrao.edu/whatisra/hist_jansky.shtml][http://www.users.globalnet.co.uk/~arcus/cara/]

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]]).[https://www.nrao.edu/whatisra/hist_jansky.shtml][https://www.users.globalnet.co.uk/~arcus/cara/]

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.[http://www.lunar-occultations.com/iota/iotandx.htm][http://cfa-www.harvard.edu/ep/comet/comet6.html][http://www.aavso.org/]  

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.[https://www.lunar-occultations.com/iota/iotandx.htm][https://cfa-www.harvard.edu/ep/comet/comet6.html][https://www.aavso.org/]  

==Major questions in astronomy==

==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.

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.

* What is the origin of the stellar mass spectrum? That is, why do astronomers observe the same distribution of stellar masses - ;the [[initial mass function]];apparently regardless of the initial conditions? [http://www.sciencemag.org/cgi/content/full/295/5552/82?ijkey=3Dzzwlrn9nK7LUM&keytype=3Dref&siteid=3Dsci]

* What is the origin of the stellar mass spectrum? That is, why do astronomers observe the same distribution of stellar masses - ;the [[initial mass function]];apparently regardless of the initial conditions? [https://www.sciencemag.org/cgi/content/full/295/5552/82?ijkey=3Dzzwlrn9nK7LUM&keytype=3Dref&siteid=3Dsci]

A deeper understanding of the formation of stars and planets is needed.

A deeper understanding of the formation of stars and planets is needed.

* 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.

* 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.

[http://www.astrobio.net/news/article236.html]

[https://www.astrobio.net/news/article236.html]

* 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 natures.<ref name="physics questions" [http://web.archive.org/web/20060203152634/http://www.pnl.gov/energyscience/01-02/11-questions/11questions.htm]

* 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 natures.<ref name="physics questions" [https://web.archive.org/web/20060203152634/https://www.pnl.gov/energyscience/01-02/11-questions/11questions.htm]

* 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? [http://www.meta-library.net/cq-wein/index-frame.html]

* 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? [https://www.meta-library.net/cq-wein/index-frame.html]

* What will be the ultimate fate of the universe?[http://map.gsfc.nasa.gov/m_uni/uni_101fate.html]

* What will be the ultimate fate of the universe?[https://map.gsfc.nasa.gov/m_uni/uni_101fate.html]

==External links==

==External links==

*[http://www.astronomy2009.org International Year of Astronomy 2009] IYA2009 Main website

*[https://www.astronomy2009.org International Year of Astronomy 2009] IYA2009 Main website

* [http://www.aip.org/history/cosmology/index.htm Cosmic Journey: A History of Scientific Cosmology] from the American Institute of Physics

* [https://www.aip.org/history/cosmology/index.htm Cosmic Journey: A History of Scientific Cosmology] from the American Institute of Physics

*[http://antwrp.gsfc.nasa.gov/apod/ Astronomy Picture of the Day]

*[https://antwrp.gsfc.nasa.gov/apod/ Astronomy Picture of the Day]

*[http://www.astronomy.net.nz Southern Hemisphere Astronomy]

*[https://www.astronomy.net.nz Southern Hemisphere Astronomy]

*[http://www.skyandtelescope.com/ Sky & Telescope] publishers

*[https://www.skyandtelescope.com/ Sky & Telescope] publishers

*[http://www.iasa.ir Iran Astronomy Sciences Academy] IASA

*[https://www.iasa.ir Iran Astronomy Sciences Academy] IASA

*[http://www.astronomy.com/ Astronomy Magazine]

*[https://www.astronomy.com/ Astronomy Magazine]

*[http://www.universetoday.com/ Universe Today] for astronomy and space-related news

*[https://www.universetoday.com/ Universe Today] for astronomy and space-related news

*[http://www.celestiamotherlode.net/catalog/educational.php/ Celestia Motherlode] Educational site for Astronomical journeys through space

*[https://www.celestiamotherlode.net/catalog/educational.php/ Celestia Motherlode] Educational site for Astronomical journeys through space

*[http://www.hubblesite.org Hubblesite.org - home of NASA's Hubble Space Telescope]

*[https://www.hubblesite.org Hubblesite.org - home of NASA's Hubble Space Telescope]

*[http://www.literature.at/elib/index.php5?title=Astronomy_-_A_History_-_George_Forbes_-_1909 Astronomy - A History - G. Forbes - 1909 (eLibrary Project - eLib Text)]

*[https://www.literature.at/elib/index.php5?title=Astronomy_-_A_History_-_George_Forbes_-_1909 Astronomy - A History - G. Forbes - 1909 (eLibrary Project - eLib Text)]

*  (historical)

*  (historical)

* [http://www.vega.org.uk/video/subseries/16 Prof. Sir Harry Kroto, NL], Astrophysical Chemistry Lecture Series. 8 Freeview Lectures provided by the Vega Science Trust.

* [https://www.vega.org.uk/video/subseries/16 Prof. Sir Harry Kroto, NL], Astrophysical Chemistry Lecture Series. 8 Freeview Lectures provided by the Vega Science Trust.

* [http://ads.harvard.edu/books/clab/ Core books] and [http://ads.harvard.edu/books/claj/ core journals] in Astronomy, from the Smithsonian/NASA [[Astrophysics Data System]]

* [https://ads.harvard.edu/books/clab/ Core books] and [https://ads.harvard.edu/books/claj/ core journals] in Astronomy, from the Smithsonian/NASA [[Astrophysics Data System]]

[[Category: General Reference]]

[[Category: General Reference]]

[[Category: Astronomy]]

[[Category: Astronomy]]