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Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a
natural science Natural science is a Branches of science, branch of science concerned with the description, understanding and prediction of Phenomenon, natural phenomena, based on empirical evidence from observation and experimentation. Mechanisms such as peer r ...

natural science
that studies
celestial objects In astronomy, an astronomical object or celestial object is a naturally occurring physical object, physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often use ...
and
phenomena A phenomenon (; plural phenomena) is an observable fact or event. The term came into its modern philosophical Philosophy (from , ) is the study of general and fundamental questions, such as those about reason, existence, knowledge ...
. It uses
mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It has no generally ...
,
physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Physical scien ...

physics
, and
chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, molecules and i ...
in order to explain their origin and
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...
. Objects of interest include
planets A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or a natural satellite around a planet. Normally, orbit r ...

planets
,
moons A natural satellite, or moon, is, in the most common usage, an astronomical body that orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or ...

moons
,
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

star
s,
nebulae " from the Eagle Nebula. Evidence from the Spitzer Telescope suggests that the pillars may already have been destroyed by a supernova explosion, but the light showing us the destruction will not reach the Earth for another millennium. A nebula ...

nebulae
,
galaxies A galaxy is a gravitationally bound system of star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to ...

galaxies
, and
comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a Comet tail, tail. These ...
s. Relevant phenomena include
supernova (bright spot on the lower left), a type Ia supernova A type Ia supernova (read: "type one-A") is a type of supernova (bright spot on the lower left), a type Ia supernova within its host galaxy, NGC 4526 A supernova ( plural: supernova ...

supernova
explosions,
gamma ray burst In gamma-ray astronomy, gamma-ray bursts (GRBs) are immensely energetic explosions that have been observed in distant Galaxy, galaxies. They are the brightest and most energetic Electromagnetic radiation, electromagnetic events known to occur in ...

gamma ray burst
s,
quasar A quasar (; also known as a quasi-stellar object, abbreviated QSO) is an extremely luminous active galactic nucleus (AGN), in which a supermassive black hole with mass ranging from millions to billions of times the mass of the Sun is surround ...

quasar
s,
blazar A blazar is an active galactic nucleus (AGN) with a relativistic jet (a jet composed of ionized matter traveling at nearly the speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant important in ...
s,
pulsar Animation of a rotating pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines and the protruding cones represent the emission zones. A pulsar (from ''pulse'' and ''-ar'' as in “ quasar”) ...

pulsar
s, and
cosmic microwave background radiation The cosmic microwave background (CMB, CMBR), in Big Bang The Big Bang theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and ratio ...
. More generally, astronomy studies everything that originates outside
Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of the atmosphere (0.04402961% at April 2019 concentration ). Number ...
.
Cosmology Cosmology (from Ancient Greek, Greek κόσμος, ''kosmos'' "world" and -λογία, ''-logia'' "study of") is a branch of astronomy concerned with the study of the chronology of the universe. Physical cosmology is the study of the universe's ...
is a branch of astronomy that studies the
universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

universe
as a whole. Astronomy is one of the oldest natural sciences. The early civilizations in
recorded history Recorded history or written history is a History, historical narrative based on a written record or other documented communication. It contrasts with other narratives of the past, such as mythological, Oral history, oral or Archaeological recor ...
made methodical observations of the
night sky The term night sky, usually associated with astronomy from Earth, refers to the nighttime appearance of astronomical object, celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when ...

night sky
. These include the
Babylonians Babylonia () was an ancient Akkadian-speaking state and cultural area based in central-southern Mesopotamia Mesopotamia ( ar, بِلَاد ٱلرَّافِدَيْن '; grc, Μεσοποταμία; Syriac language, Classical Syriac: ...
,
Greeks The Greeks or Hellenes (; el, Έλληνες, ''Éllines'' ) are an ethnic group and nation native to the Eastern Mediterranean and the Black Sea regions, namely Greece, Greek Cypriots, Cyprus, Greeks in Albania, Albania, Greeks in Italy, Ital ...
,
Indians Indian or Indians refers to people or things related to India, or to the indigenous people of the Americas, or Aboriginal Australians until the 19th century. People South Asia * Indian people, people of Indian nationality, or people who come ...
,
Egyptians Egyptians are the people originating from the country of Egypt Egypt ( ; ar, مِصر ), officially the Arab Republic of Egypt, is a transcontinental country spanning the North Africa, northeast corner of Africa and Western Asia, southwe ...
,
Chinese Chinese can refer to: * Something related to China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the List of countries and dependencies by population, world's most populous country, with a populat ...
,
Maya Maya may refer to: Civilizations * Maya peoples The Maya peoples () are an ethnolinguistic group of indigenous peoples Indigenous peoples, also referred to as First people, Aboriginal people, Native people, or autochthonous people, are cu ...
, and many ancient
indigenous peoples of the Americas The Indigenous peoples of the Americas, also known as Amerindians or Indians, are the inhabitants of the Americas before the arrival of the European colonization of the Americas, European settlers in the 15th century, and the ethnic groups who no ...
. In the past, astronomy included disciplines as diverse as
astrometry in the optical wavelength range to determine precise positions of stars. ''Courtesy NASA/JPL-Caltech'' Astrometry is the branch of astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of ...
,
celestial navigation Celestial navigation, also known as astronavigation, is the ancient and modern practice of position fixing that enables a navigator to transition through a space without having to rely on estimated calculations, or dead reckoning, to know their p ...
,
observational astronomy Observation is the active acquisition of information from a primary source. In living beings, observation employs the senses. In science, observation can also involve the perception and recording of data (information), data via the use of scienti ...
, and the making of
calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day within such a system. A calendar is al ...
s. Nowadays, professional astronomy is often said to be the same as
astrophysics Astrophysics is a science that employs the methods and principles of physics in the study of astronomical objects and phenomena. Among the subjects studied are the Sun, other stars, galaxy, galaxies, extrasolar planets, the interstellar medium and ...
. Professional astronomy is split into
observational Observation is the active acquisition of information from a primary source. In living beings, observation employs the senses. In science, observation can also involve the perception and recording of data (information), data via the use of scienti ...
and
theoretical A theory is a rational type of abstract thinking about a phenomenon A phenomenon (; plural phenomena) is an observable fact or event. The term came into its modern philosophical usage through Immanuel Kant Immanuel Kant (, ; ; 22 April ...
branches. Observational astronomy is focused on acquiring data from observations of astronomical objects. This data is then analyzed using basic principles of physics. Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. These two fields complement each other. Theoretical astronomy seeks to explain observational results and observations are used to confirm theoretical results. Astronomy is one of the few sciences in which amateurs play an active role. This is especially true for the discovery and observation of transient events.
Amateur astronomers Image:Astronomy Amateur 3 V2.jpg, 250px, Amateur astronomers watch the night sky during the Perseids, Perseid meteor shower. Amateur astronomy is a hobby where participants enjoy observing or imaging celestial objects in the sky using the Naked e ...
have helped with many important discoveries, such as finding new comets.


Etymology

''Astronomy'' (from the
Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of 2018; Athens is ...
ἀστρονομία from ἄστρον ''astron'', "star" and -νομία '' -nomia'' from νόμος ''nomos'', "law" or "culture") means "law of the stars" (or "culture of the stars" depending on the translation). Astronomy should not be confused with
astrology Astrology is a pseudoscience that claims to divination, divine information about human affairs and terrestrial events by studying the movements and relative positions of Celestial objects in astrology, celestial objects. Astrology has bee ...
, the belief system which claims that human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin, they are now entirely distinct.


Use of terms "astronomy" and "astrophysics"

"Astronomy" and "astrophysics" are synonyms. 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," while "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena". In some cases, as in the introduction of the introductory textbook ''The Physical Universe'' by
Frank Shu Frank H. Shu (; born June2, 1943), is an American astrophysicist, astronomer and author An author is the creator or originator of any written work such as a book or play, and is also considered a writer. More broadly defined, an author is "t ...
, "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. However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics. Some fields, such as astrometry, are purely astronomy rather than also astrophysics. Various departments in which scientists carry out research on this subject may use "astronomy" and "astrophysics", partly depending on whether the department is historically affiliated with a physics department, and many professional
astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, natural satellite, moons, comets and galaxy, ga ...
s have physics rather than astronomy degrees. Some titles of the leading scientific journals in this field include ''
The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a Peer review, peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is ...
'', ''
The Astrophysical Journal ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a Peer review, peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery Hal ...
'', and ''
Astronomy & Astrophysics ''Astronomy & Astrophysics'' is a Peer review, peer-reviewed scientific journal covering theoretical, observational, and instrumental astronomy and astrophysics. The journal is run by a Board of Directors representing 27 sponsoring countries plus a ...
''.


History


Ancient times

In early historic times, astronomy only consisted of the observation and predictions of the motions of objects visible to the naked eye. In some locations, early cultures assembled massive artifacts that possibly had some astronomical purpose. In addition to their ceremonial uses, these
observatories An observatory is a location used for observing terrestrial, marine, or celestial events. Astronomy, climatology/meteorology, geophysics, geophysical, oceanography and volcanology are examples of disciplines for which observatories have been cons ...

observatories
could be employed to determine the seasons, an important factor in knowing when to plant crops and in understanding the length of the year.Forbes, 1909 Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye. As civilizations developed, most notably in
Mesopotamia Mesopotamia ( grc, Μεσοποταμία ''Mesopotamíā''; ar, بِلَاد ٱلرَّافِدَيْن ; syc, ܐܪܡ ܢܗܪ̈ܝܢ, or , ) is a historical region of Western Asia situated within the Tigris–Euphrates river system, in the ...
,
Greece Greece ( el, Ελλάδα, Elláda, ), officially the Hellenic Republic, is a country located in Southeastern Europe Southeast Europe or Southeastern Europe () is a geographical region of Europe Europe is a continent A contin ...
,
Persia Iran ( fa, ایران ), also called Persia, and officially the Islamic Republic of Iran, is a country in Western Asia Western Asia, also West Asia, is the westernmost subregion of Asia. It is entirely a part of the Greater Middle Ea ...
,
India India (Hindi: ), officially the Republic of India (Hindi: ), is a country in South Asia. It is the List of countries and dependencies by population, second-most populous country, the List of countries and dependencies by area, seventh-largest ...
,
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the List of countries and dependencies by population, world's most populous country, with a population of around 1.4 billion. Covering approximately 9.6& ...
,
Egypt Egypt ( ar, مِصر, Miṣr), officially the Arab Republic of Egypt, is a transcontinental country This is a list of countries located on more than one continent A continent is one of several large landmasses. Generally identif ...
, and
Central America Central America ( es, América Central, , ''Centroamérica'' ) is a region of the Americas. It is bordered by Mexico to the north, Colombia to the south, the Caribbean Sea to the east, and the Pacific Ocean to the west. Central America consist ...
, astronomical observatories were assembled and ideas on the nature of the Universe began to develop. Most early astronomy consisted of mapping the positions of the stars and planets, a science now referred to as
astrometry in the optical wavelength range to determine precise positions of stars. ''Courtesy NASA/JPL-Caltech'' Astrometry is the branch of astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of ...
. 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 In astronomy, the geocentric model (also known as geocentrism, often exemplified specifically by the Ptolemaic system) is a superseded description of the Universe with Earth at the center. Under the geocentric model, the Sun, Moon, stars, and ...
of the Universe, or the
Ptolemaic system In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses ma ...

Ptolemaic system
, named after
Ptolemy Claudius Ptolemy (; grc-koi, Κλαύδιος Πτολεμαῖος, , ; la, Claudius Ptolemaeus; AD) was a mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Ancient Greek, Greek ...
. A particularly important early development was the beginning of mathematical and scientific astronomy, which began among the Babylonians, who laid the foundations for the later astronomical traditions that developed in many other civilizations. The
Babylonians Babylonia () was an ancient Akkadian-speaking state and cultural area based in central-southern Mesopotamia Mesopotamia ( ar, بِلَاد ٱلرَّافِدَيْن '; grc, Μεσοποταμία; Syriac language, Classical Syriac: ...
discovered that lunar eclipses recurred in a repeating cycle known as a
saros Saros may refer to: * Saros (astronomy), an 18-year period, across which lunar and solar eclipses repeat * Saros (Nigeria), descendants of free slaves from Sierra Leone who migrated to Nigeria * Saros (software) an Eclipse plug-in for distributed co ...
. Following the Babylonians, significant advances in astronomy were made in
ancient Greece Ancient Greece ( el, Ἑλλάς, Hellás) was a civilization belonging to a period of History of Greece, Greek history from the Greek Dark Ages of the 12th–9th centuries BC to the end of Classical Antiquity, antiquity ( AD 600). This era was ...
and the
Hellenistic The Hellenistic period covers the period of Mediterranean history between the death of Alexander the Great in 323 BC and the emergence of the Roman Empire, as signified by the Battle of Actium The Battle of Actium was a naval battle in t ...
world.
Greek astronomy Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of 2018; Athens is ...
is characterized from the start by seeking a rational, physical explanation for celestial phenomena. In the 3rd century BC,
Aristarchus of Samos Aristarchus of Samos (; grc-gre, Ἀρίσταρχος ὁ Σάμιος, ''Aristarkhos ho Samios''; ) was an ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient ...
estimated the size and distance of the Moon and Sun, and he proposed a model of the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

Solar System
where the Earth and planets rotated around the Sun, now called the
heliocentric Heliocentrism is the astronomical model in which the Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of continent A continent ...
model. In the 2nd century BC,
Hipparchus Hipparchus of Nicaea (; el, Ἵππαρχος, ''Hipparkhos'';  BC) was a Ancient Greek astronomy, Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry but is most famous for his incidental discover ...
discovered
precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In other ...

precession
, calculated the size and distance of the Moon and invented the earliest known astronomical devices such as the
astrolabe An astrolabe ( grc, ἀστρολάβος ; ar, ٱلأَسْطُرلاب ; persian, ستاره‌یاب ) is an ancient astronomical instrument that was a handheld model of the universe. Its various functions also make it an elaborate inclinome ...
. Hipparchus also created a comprehensive catalog of 1020 stars, and most of the
constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived outline or pattern, typically representing an animal, mythological person or creature, or an inanimate object. The origins of the earliest ...
s of the northern hemisphere derive from Greek astronomy. The
Antikythera mechanism The Antikythera mechanism ( ) is an ancient Greek hand-powered orrery, described as the oldest example of an analogue computer, used to predict astronomy, astronomical positions and eclipses decades in advance. It could also be used to track th ...

Antikythera mechanism
(c. 150–80 BC) was an early analog computer designed to calculate the location of the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
,
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its planet ...

Moon
, and
planets A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or a natural satellite around a planet. Normally, orbit r ...

planets
for a given date. Technological artifacts of similar complexity did not reappear until the 14th century, when mechanical
astronomical clock An astronomical clock, horologium, or orloj is a clock with special mechanisms and dials to display astronomical information, such as the relative positions of the sun, moon The Moon is Earth's only proper natural satellite. At one ...
s appeared in Europe.


Middle Ages

Medieval Europe housed a number of important astronomers.
Richard of Wallingford The first or given name Richard originates, via Old French Old French (, , ; French language, Modern French: ) was the language spoken in Northern France from the 8th century to the 14th century. Rather than a unified Dialect#Dialect or lan ...

Richard of Wallingford
(1292–1336) made major contributions to astronomy and horology, including the invention of the first astronomical clock, the Rectangulus which allowed for the measurement of angles between planets and other astronomical bodies, as well as an
equatorium An equatorium (plural, equatoria) is an astronomical calculating instrument. It can be used for finding the positions of the Moon, Sun, and planets without calculation, using a geometrical model to represent the position of a given celestial bod ...
called the ''Albion'' which could be used for astronomical calculations such as
lunar Lunar most commonly means "of or relating to the Moon". Lunar may also refer to: Arts and entertainment * Lunar (series), ''Lunar'' (series), a series of video games * Lunar (song), "Lunar" (song), by David Guetta * "Lunar", a song by Priestess fr ...

lunar
,
solar Solar may refer to: Astronomy * Of or relating to the Sun. ** A solar telescope 175px, The Swedish 1-m Solar Telescope at Roque de los Muchachos Observatory, La Palma in the Canary Islands. A solar telescope is a special purpose telescope used ...

solar
and
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

planet
ary
longitude Longitude (, ), is a geographic coordinate that specifies the east East is one of the four cardinal direction The four cardinal directions, or cardinal points, are the directions north North is one of the four compass points or car ...

longitude
s and could predict
eclipse An eclipse is an astronomical event that occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three ce ...
s.
Nicole Oresme Nicole Oresme (; c. 1320–1325 – July 11, 1382), also known as Nicolas Oresme, Nicholas Oresme, or Nicolas d'Oresme, was a significant philosopher A philosopher is someone who practices philosophy. The term ''philosopher'' comes from the g ...
(1320–1382) and
Jean Buridan Jean Buridan (; Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman ...
(1300–1361) first discussed evidence for the rotation of the Earth, furthermore, Buridan also developed the theory of impetus (predecessor of the modern scientific theory of
inertia Inertia is the resistance of any physical object to any change in its velocity The velocity of an object is the rate of change of its position with respect to a frame of reference, and is a function of time. Velocity is equivalent to ...
) which was able to show planets were capable of motion without the intervention of angels.
Georg von Peuerbach Georg von Peuerbach (also Purbach, Peurbach; la, Purbachius; born May 30, 1423 – April 8, 1461) was an Austrian astronomer, mathematician and Scientific instrument, instrument maker, best known for his streamlined presentation of Ptolemaic astr ...

Georg von Peuerbach
(1423–1461) and
Regiomontanus Johannes Müller von Königsberg (6 June 1436 – 6 July 1476), better known as Regiomontanus (), was a mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Ancient Greek, Greek: ) inc ...

Regiomontanus
(1436–1476) helped make astronomical progress instrumental to Copernicus's development of the heliocentric model decades later. Astronomy flourished in the Islamic world and other parts of the world. This led to the emergence of the first astronomical
observatories An observatory is a location used for observing terrestrial, marine, or celestial events. Astronomy, climatology/meteorology, geophysics, geophysical, oceanography and volcanology are examples of disciplines for which observatories have been cons ...

observatories
in the
Muslim world The terms Muslim world and Islamic world commonly refer to the Islamic Islam (;There are ten pronunciations of ''Islam'' in English, differing in whether the first or second syllable has the stress, whether the ''s'' is or , and whether ...

Muslim world
by the early 9th century. In 964, the
Andromeda Galaxy The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula (see below), is a barred spiral galaxy approximately from Earth and the nearest major galaxy to the Milky Way The Milky Way is th ...

Andromeda Galaxy
, the largest
galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and concrete, abstract is what belongs to or ...

galaxy
in the
Local Group Distribution of the iron content (in logarithmic scale) in four neighbouring dwarf galaxies of the Milky Way The Local Group is the galaxy group that includes the Milky Way The Milky Way is the galaxy A galaxy is a gravitation ...

Local Group
, was described by the Persian Muslim astronomer
Abd al-Rahman al-Sufi 'Abd al-Rahman al-Sufi ( fa, عبدالرحمن صوفی (December 7, 903 in Rey, Iran – May 25, 986 in Shiraz Shiraz (; fa, شیراز, Šîrâz ) is the List of Iranian cities by population, fifth-most-populous city of Iran and the capit ...
in his ''
Book of Fixed Stars The ''Book of Fixed Stars'' ( ar, كتاب صور الكواكب ', literally ''The Book of the Shapes of Stars'') is an astronomical text written by Abd al-Rahman al-Sufi 'Abd al-Rahman al-Sufi ( fa, عبدالرحمن صوفی (December 7, 9 ...
''. The
SN 1006 SN 1006 was a supernova (bright spot on the lower left), a type Ia supernova within its host galaxy, NGC 4526 A supernova ( plural: supernovae or supernovas, abbreviations: SN and SNe) is a powerful and luminous stellar explosion An ...

SN 1006
supernova (bright spot on the lower left), a type Ia supernova A type Ia supernova (read: "type one-A") is a type of supernova (bright spot on the lower left), a type Ia supernova within its host galaxy, NGC 4526 A supernova ( plural: supernova ...

supernova
, the brightest apparent magnitude stellar event in recorded history, was observed by the Egyptian Arabic astronomer
Ali ibn Ridwan Abu'l Hassan Ali ibn Ridwan Al-Misri () (c. 988 - c. 1061) was an Arab The Arabs (singular Arab ; singular ar, عَرَبِيٌّ, ISO 233: , Arabic pronunciation: , plural ar, عَرَبٌ, ISO 233: , Arabic pronunciation: ) are an ethnic g ...
and
Chinese astronomers Astronomy in China has a long history stretching from the Shang Dynasty, being refined over a period of more than 3,000 years. The Ancient China, Ancient Chinese people have identified stars from 1300BCE, as Chinese star names later categorized in ...
in 1006. Some of the prominent Islamic (mostly Persian and Arab) astronomers who made significant contributions to the science include
Al-Battani Abū ʿAbd Allāh Muḥammad ibn Jābir ibn Sinān al-Raqqī al-Ḥarrānī aṣ-Ṣābiʾ al-Battānī ( ar, محمد بن جابر بن سنان البتاني) ( Latinized as Albategnius, Albategni or Albatenius) (c. 858 – 929) was a Syrian A ...

Al-Battani
, Thebit,
Abd al-Rahman al-Sufi 'Abd al-Rahman al-Sufi ( fa, عبدالرحمن صوفی (December 7, 903 in Rey, Iran – May 25, 986 in Shiraz Shiraz (; fa, شیراز, Šîrâz ) is the List of Iranian cities by population, fifth-most-populous city of Iran and the capit ...
,
Biruni Abu Rayhan al-Biruni (973 – after 1050) was an Iranian in scholar and polymath A polymath ( el, πολυμαθής, ', "having learned much"; Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic ...
, Abū Ishāq Ibrāhīm al-Zarqālī,
Al-Birjandi Abd Ali ibn Muhammad ibn Husayn Birjandi ( fa, عبدعلی محمد بن حسین بیرجندی) (died 1528) was a prominent 16th-century Persian astronomer, mathematician A mathematician is someone who uses an extensive knowledge of mathema ...
, and the astronomers of the
Maragheh Maragheh ( fa, مراغه, az, ماراغا ), also Romanized Romanization or romanisation, in linguistics Linguistics is the science, scientific study of language. It encompasses the analysis of every aspect of language, as well as th ...
and
Samarkand fa, سمرقند , native_name_lang = , settlement_type = City , image_skyline = , image_alt = , image_caption = , image_flag = ...
observatories. Astronomers during that time introduced many Arabic names now used for individual stars. It is also believed that the ruins at
Great Zimbabwe Great Zimbabwe is an ancient city in the south-eastern hills of Zimbabwe near Lake Mutirikwe and the town of Masvingo. It is thought to have been the capital of a great kingdom, although which kingdom is not certain, during the country's Late I ...

Great Zimbabwe
and
Timbuktu Timbuktu () (french: Tombouctou; Tuareg languages, Tuareg ⵜⵏⴱⴾⵜ ''Tin Buqt''; Koyra Chiini: ''Tumbutu'') is a city in Mali, situated north of the Niger River. The town is the capital of the Tombouctou Region, one of the eight Regions ...

Timbuktu
may have housed astronomical observatories. In Post-classical
West Africa West Africa or Western Africa is the westernmost region of Africa. The United Nations defines Western Africa as the 17 countries of Benin, Burkina Faso, Cape Verde, The Gambia, Ghana, Guinea, Guinea-Bissau, Ivory Coast, Liberia, Mali, Mauritania, ...

West Africa
, Astronomers studied the movement of stars and relation to seasons, crafting charts of the heavens as well as precise diagrams of orbits of the other planets based on complex mathematical calculations. Songhai historian
Mahmud KatiAl Hajj Mahmud Kati (or Mahmoud Kati) (1468? - 1552 or 1593) was an African Muslim scholar. He is traditionally held to be the author of the West Africa West Africa or Western Africa is the westernmost region of Africa. The United Nations ...
documented a
meteor shower A meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point in the night sky. These meteors are caused by streams of cosmic debris called meteoroids entering Earth's atmosphere at extr ...

meteor shower
in August 1583. Europeans had previously believed that there had been no astronomical observation in
sub-Saharan Africa Sub-Saharan Africa (commonly called Black Africa) is, geographically, the area of the continent of Africa that lies south of the Sahara. According to the United Nations, it consists of all list of sovereign states and dependent territories in ...

sub-Saharan Africa
during the pre-colonial Middle Ages, but modern discoveries show otherwise. For over six centuries (from the recovery of ancient learning during the late Middle Ages into the Enlightenment), the Roman Catholic Church gave more financial and social support to the study of astronomy than probably all other institutions. Among the Church's motives was finding the date for Easter.


Scientific revolution

During the
Renaissance The Renaissance ( , ) , from , with the same meanings. is a period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in m ...

Renaissance
,
Nicolaus Copernicus Nicolaus Copernicus (; pl, Mikołaj Kopernik; german: link=no, Niclas Koppernigk, modern: ''Nikolaus Kopernikus''; 19 February 1473 – 24 May 1543) was a Renaissance The Renaissance ( , ) , from , with the same meanings. was a ...

Nicolaus Copernicus
proposed a heliocentric model of the solar system. His work was defended by
Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei (; 15 February 1564 – 8 January 1642) was an Italian astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the ...

Galileo Galilei
and expanded upon by
Johannes Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe as ...

Johannes Kepler
. Kepler was the first to devise a system that correctly described the details of the motion of the planets around the Sun. However, Kepler did not succeed in formulating a theory behind the laws he wrote down. It was
Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March Old Style and New Style dates, 1726/27) was an English mathematician, physicist, astronomer, theologian, and author (described in his time as a "natural philosophy, natural philosopher") ...

Isaac Newton
, with his invention of celestial dynamics and his , who finally explained the motions of the planets. Newton also developed the reflecting telescope.Forbes, 1909, pp. 58–64 Improvements in the size and quality of the telescope led to further discoveries. The English astronomer
John Flamsteed John Flamsteed FRS (19 August 1646 – 31 December 1719) was an English astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astr ...

John Flamsteed
catalogued over 3000 stars, More extensive star catalogues were produced by
Nicolas Louis de Lacaille Abbé Nicolas-Louis de Lacaille, formerly sometimes spelled de la Caille, (; 15 March 1713 – 21 March 1762) was a French astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or fi ...

Nicolas Louis de Lacaille
. The astronomer
William Herschel Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer of music. He frequently collaborated with his younger sister and fellow astronomer Ca ...

William Herschel
made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

Uranus
, the first new planet found. During the 18–19th centuries, the study of the
three-body problem In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through S ...
by
Leonhard Euler Leonhard Euler ( ; ; 15 April 170718 September 1783) was a Swiss mathematician, physicist, astronomer, geographer, logician and engineer who made important and influential discoveries in many branches of mathematics, such as infinitesimal c ...

Leonhard Euler
,
Alexis Claude Clairaut Alexis Claude Clairaut (; 13 May 1713 – 17 May 1765) was a French mathematician, astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They obs ...
, and
Jean le Rond d'Alembert Jean-Baptiste le Rond d'Alembert (; ; 16 November 1717 – 29 October 1783) was a French mathematician, mechanics, mechanician, physicist, philosopher, and music theorist. Until 1759 he was, together with Denis Diderot, a co-editor of the ''Enc ...
led to more accurate predictions about the motions of the Moon and planets. This work was further refined by
Joseph-Louis Lagrange Joseph-Louis Lagrange (born Giuseppe Luigi LagrangiaPierre Simon Laplace Pierre-Simon, marquis de Laplace (; ; 23 March 1749 – 5 March 1827) was a French scholar and polymath A polymath ( el, πολυμαθής, ', "having learned much"; Latin Latin (, or , ) is a classical language belonging to the I ...
, 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 An optical spectrometer (spectrophotometer, spectrograph or spectroscope) is an instrument used to measure properties of light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that c ...

spectroscope
and
photography Photography is the art, application, and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employed in ...
.
Joseph von Fraunhofer Joseph Ritter von Fraunhofer (; ; 6 March 1787 – 7 June 1826) was a Bavarian physicist and optical lens manufacturer. He made optical glass and achromatic telescope objective lenses, invented the spectroscope, and developed diffraction gra ...

Joseph von Fraunhofer
discovered about 600 bands in the spectrum of the Sun in 1814–15, which, in 1859,
Gustav Kirchhoff Gustav Robert Kirchhoff (; 12 March 1824 – 17 October 1887) was a German physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science ...
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
temperature Temperature is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy, present in all matter, which is the source of the occurrence of heat, a flow of energy, when a body is in contact with another that is ...

temperature
s,
mass Mass is the physical quantity, quantity of ''matter'' in a physical body. It is also a measure (mathematics), measure of the body's ''inertia'', the resistance to acceleration (change of velocity) when a net force is applied. An object's mass ...
es, and sizes. The existence of the Earth's galaxy, the
Milky Way The Milky Way is the galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and concr ...

Milky Way
, as its own group of stars was only proved in the 20th century, along with the existence of "external" galaxies. The observed recession of those galaxies led to the discovery of the expansion of the
Universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

Universe
. Theoretical astronomy led to speculations on the existence of objects such as black holes and neutron stars, which have been used to explain such observed phenomena as
quasar A quasar (; also known as a quasi-stellar object, abbreviated QSO) is an extremely luminous active galactic nucleus (AGN), in which a supermassive black hole with mass ranging from millions to billions of times the mass of the Sun is surround ...

quasar
s,
pulsar Animation of a rotating pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines and the protruding cones represent the emission zones. A pulsar (from ''pulse'' and ''-ar'' as in “ quasar”) ...

pulsar
s,
blazar A blazar is an active galactic nucleus (AGN) with a relativistic jet (a jet composed of ionized matter traveling at nearly the speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant important in ...
s, and radio galaxy, radio galaxies. Physical cosmology made huge advances during the 20th century. In the early 1900s the model of the Big Bang theory was formulated, heavily evidenced by
cosmic microwave background radiation The cosmic microwave background (CMB, CMBR), in Big Bang The Big Bang theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and ratio ...
, Hubble's law, and the Big Bang nucleosynthesis, cosmological abundances of elements. Space telescopes have enabled measurements in parts of the electromagnetic spectrum normally blocked or blurred by the atmosphere. In February 2016, it was revealed that the LIGO project had first observation of gravitational waves, detected evidence of gravitational waves in the previous September.


Observational astronomy

The main source of information about celestial body, celestial bodies and other objects is visible light, or more generally electromagnetic radiation. Observational astronomy may be categorized according to the corresponding region of the electromagnetic spectrum on which the observations are made. Some parts of the spectrum can be observed from the Earth's surface, while other parts are only observable from either high altitudes or outside the Earth's atmosphere. Specific information on these subfields is given below.


Radio astronomy

Radio astronomy uses radiation with wavelengths greater than approximately one millimeter, outside the visible range. 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 (waves), phase of radio waves, whereas this is not as easily done at shorter wavelengths. Although some radio waves are emitted directly by astronomical objects, a product of black-body radiation, thermal emission, most of the radio emission that is observed is the result of synchrotron radiation, which is produced when electrons orbit magnetic fields. Additionally, a number of spectral lines produced by interstellar gas, notably the hydrogen spectral line at 21 cm, are observable at radio wavelengths. A wide variety of other objects are observable at radio wavelengths, including
supernova (bright spot on the lower left), a type Ia supernova A type Ia supernova (read: "type one-A") is a type of supernova (bright spot on the lower left), a type Ia supernova within its host galaxy, NGC 4526 A supernova ( plural: supernova ...

supernova
e, interstellar gas,
pulsar Animation of a rotating pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines and the protruding cones represent the emission zones. A pulsar (from ''pulse'' and ''-ar'' as in “ quasar”) ...

pulsar
s, and active galactic nuclei.


Infrared astronomy

Infrared astronomy is founded on the detection and analysis of infrared radiation, wavelengths longer than red light and outside the range of our vision. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets, circumstellar disks or nebulae whose light is blocked by dust. The longer wavelengths of infrared can penetrate clouds of dust that block visible light, allowing the observation of young stars embedded in molecular clouds and the cores of galaxies. Observations from the Wide-field Infrared Survey Explorer (WISE) have been particularly effective at unveiling numerous Galactic protostars and their host star clusters. With the exception of infrared wavelengths close to visible light, such radiation is heavily absorbed by the atmosphere, or masked, as the atmosphere itself produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places on Earth or in space. Some molecules radiate strongly in the infrared. This allows the study of the chemistry of space; more specifically it can detect water in comets.


Optical astronomy

Historically, optical astronomy, also called visible light astronomy, is the oldest form of astronomy. Images of observations were originally drawn by hand. In the late 19th century and most of the 20th century, images were made using photographic equipment. Modern images are made using digital detectors, particularly using charge-coupled devices (CCDs) and recorded on modern medium. Although visible light itself extends from approximately 4000 Ångstrom, Å to 7000 Å (400 nanometre, nm to 700 nm), that same equipment can be used to observe some near-ultraviolet and near-infrared radiation.


Ultraviolet astronomy

Ultraviolet astronomy employs ultraviolet wavelengths between approximately 100 and 3200 Å (10 to 320 nm). Light at those wavelengths is absorbed by the Earth's atmosphere, requiring observations at these wavelengths to 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
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

star
s (OB 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. However, as ultraviolet light is easily absorbed by interstellar dust, an adjustment of ultraviolet measurements is necessary.


X-ray astronomy

X-ray astronomy uses X-radiation, X-ray wavelengths. Typically, X-ray radiation is produced by synchrotron emission (the result of electrons orbiting magnetic field lines), bremsstrahlung radiation, thermal emission from thin gases above 107 (10 million) kelvins, and blackbody radiation, thermal emission from thick gases above 107 Kelvin. Since X-rays are absorbed by the Earth's atmosphere, all X-ray observations must be performed from high-altitude balloons, rockets, or X-ray astronomy satellites. Notable Astrophysical X-ray source, X-ray sources include X-ray binaries,
pulsar Animation of a rotating pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines and the protruding cones represent the emission zones. A pulsar (from ''pulse'' and ''-ar'' as in “ quasar”) ...

pulsar
s, supernova remnants, elliptical galaxies, clusters of galaxies, and active galactic nuclei.


Gamma-ray astronomy

Gamma ray astronomy observes 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 detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere. Most Gamma ray, 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.


Fields not based on the electromagnetic spectrum

In addition to electromagnetic radiation, a few other events originating from great distances may be observed from the Earth. In neutrino astronomy, astronomers use heavily shielded Neutrino observatory, underground facilities such as SAGE (ruSsian American Gallium Experiment), SAGE, GALLEX, and Kamioka Observatory, Kamioka II/III for the detection of neutrinos. The vast majority of the neutrinos streaming through the Earth originate from the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
, but 24 neutrinos were also detected from supernova 1987A. Cosmic rays, which consist of very high energy particles (atomic nuclei) that can decay or be absorbed when they enter the Earth's atmosphere, result in a cascade of secondary particles which can be detected by current observatories. Some future neutrino detectors may also be sensitive to the particles produced when cosmic rays hit the Earth's atmosphere. Gravitational-wave astronomy is an emerging field of astronomy that employs gravitational-wave detectors to collect observational data about distant massive objects. A few observatories have been constructed, such as the ''Laser Interferometer Gravitational Observatory'' LIGO. LIGO made its First observation of gravitational waves, first detection on 14 September 2015, observing gravitational waves from a binary black hole. A second gravitational wave was detected on 26 December 2015 and additional observations should continue but gravitational waves require extremely sensitive instruments. The combination of observations made using electromagnetic radiation, neutrinos or gravitational waves and other complementary information, is known as multi-messenger astronomy.


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 Celestial navigation, also known as astronavigation, is the ancient and modern practice of position fixing that enables a navigator to transition through a space without having to rely on estimated calculations, or dead reckoning, to know their p ...
(the use of celestial objects to guide navigation) and in the making of
calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day within such a system. A calendar is al ...
s. 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 objects will allow for predictions of close encounters or potential collisions of the Earth with those objects. 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, as their properties can be compared. Measurements of the radial velocity and proper motion of stars allow astronomers to plot the movement of these systems through the Milky Way galaxy. Astrometric results are the basis used to calculate the distribution of speculated dark matter in the galaxy. During the 1990s, the measurement of the stellar wobble of nearby stars was Methods of detecting extrasolar planets#Astrometry, used to detect large extrasolar planets orbiting those stars.


Theoretical astronomy

Theoretical astronomers use several tools including mathematical model, analytical models and computational Numerical analysis, numerical simulations; each has its particular advantages. Analytical models of a process are better for giving broader insight into the heart of what is going on. Numerical models reveal the existence of phenomena and effects otherwise unobserved. Theorists in astronomy endeavor to create theoretical models and from the results predict observational consequences of those models. The observation of a phenomenon predicted by a model allows astronomers to select between several alternate or conflicting models as the one best able to describe the phenomena. Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency between the data and the model's results, the general tendency is to try to make minimal modifications to the model so that it produces results that fit the data. In some cases, a large amount of inconsistent data over time may lead to the total abandonment of a model. Phenomena modeled by theoretical astronomers include: stellar dynamics and Stellar evolution, evolution; Galaxy formation and evolution, galaxy formation; large-scale structure of the universe, large-scale distribution of matter in the
Universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

Universe
; origin of cosmic rays; general relativity and physical cosmology, including string theory, 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 Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Physical scien ...

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, dark matter and fundamental theories of
physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Physical scien ...

physics
. A few examples of this process: Along with Cosmic inflation, dark matter and dark energy are the current leading topics in astronomy, as their discovery and controversy originated during the study of the galaxies.


Specific subfields


Astrophysics

Astrophysics is the branch of astronomy that employs the principles of physics and
chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, molecules and i ...
"to ascertain the nature of the astronomical objects, rather than their positions or motions in space". Among the objects studied are the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
, other
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

star
s,
galaxies A galaxy is a gravitationally bound system of star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to ...

galaxies
, extrasolar planets, the interstellar medium and the cosmic microwave background. Their emissions are examined across all parts of the electromagnetic spectrum, and the properties examined include luminosity, density,
temperature Temperature is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy, present in all matter, which is the source of the occurrence of heat, a flow of energy, when a body is in contact with another that is ...

temperature
, and chemistry, chemical composition. Because astrophysics is a very broad subject, ''astrophysicists'' typically apply many disciplines of physics, including mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, theory of relativity, relativity, nuclear physics, nuclear and particle physics, and atomic, molecular, and optical physics, atomic and molecular physics. In practice, modern astronomical research often involves a substantial amount of work in the realms of Theoretical physics, theoretical and observational physics. Some areas of study for astrophysicists include their attempts to determine the properties of dark matter, dark energy, and black holes; whether or not time travel is possible, wormholes can form, or the multiverse exists; and the Cosmogony, origin and ultimate fate of the universe. Topics also studied by theoretical astrophysicists include Formation and evolution of the Solar System, Solar System formation and evolution; stellar dynamics and Stellar evolution, evolution; galaxy formation and evolution; magnetohydrodynamics; large-scale structure of the universe, large-scale structure of matter in the universe; origin of cosmic rays; general relativity and physical cosmology, including string theory, string cosmology and astroparticle physics.


Astrochemistry

Astrochemistry is the study of the abundance and reactions of molecules in the
Universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

Universe
, and their interaction with radiation. The discipline is an overlap of astronomy and
chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, molecules and i ...
. The word "astrochemistry" may be applied to both the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

Solar System
and the interstellar medium. The study of the abundance of elements and isotope ratios in Solar System objects, such as meteorites, is also called cosmochemistry, while the study of interstellar atoms and molecules and their interaction with radiation is sometimes called molecular astrophysics. The formation, atomic and chemical composition, evolution and fate of molecular cloud, molecular gas clouds is of special interest, because it is from these clouds that solar systems form. Studies in this field contribute to the understanding of the formation of the Solar System, Earth's origin and geology, abiogenesis, and the origin of climate and oceans.


Astrobiology

Astrobiology is an interdisciplinary scientific field concerned with the abiogenesis, origins, Protocell, early evolution, distribution, and future of life in the
universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

universe
. Astrobiology considers the question of whether extraterrestrial life exists, and how humans can detect it if it does. The term exobiology is similar. Astrobiology makes use of molecular biology, biophysics, biochemistry,
chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, molecules and i ...
, astronomy, physical cosmology, exoplanetology and geology to investigate the possibility of life on other worlds and help recognize biospheres that might be different from that on Earth. Abiogenesis, The origin and early evolution of life is an inseparable part of the discipline of astrobiology. Astrobiology concerns itself with interpretation of existing Scientific method, scientific data, and although speculation is entertained to give context, astrobiology concerns itself primarily with hypotheses that fit firmly into existing Theory#Science, scientific theories. This interdisciplinary field encompasses research on the origin of planetary systems, origins of List of interstellar and circumstellar molecules, organic compounds in space, rock-water-carbon interactions, abiogenesis on Earth, planetary habitability, research on biosignatures for life detection, and studies on the potential for extremophile, life to adapt to challenges on Earth and in outer space.


Physical cosmology

Cosmology Cosmology (from Ancient Greek, Greek κόσμος, ''kosmos'' "world" and -λογία, ''-logia'' "study of") is a branch of astronomy concerned with the study of the chronology of the universe. Physical cosmology is the study of the universe's ...
(from the Greek κόσμος (''kosmos'') "world, universe" and λόγος (''logos'') "word, study" or literally "logic") could be considered the study of the Universe as a whole. 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 metric expansion of space, expanded over the course of 13.8 billion years to its present condition. The concept of the Big Bang can be traced back to the discovery of the Cosmic microwave background radiation, 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 experienced a very rapid cosmic inflation, which homogenized the starting conditions. Thereafter, Big Bang nucleosynthesis, nucleosynthesis produced the elemental abundance of the early Universe. (See also nucleocosmochronology.) When the first neutral atoms formed from a sea of primordial ions, space became transparent to radiation, releasing the energy viewed today as the microwave background radiation. The expanding Universe then underwent a Dark Age due to the lack of stellar energy sources. A hierarchical structure of matter began to form from minute variations in the mass density of space. Matter accumulated in the densest regions, forming clouds of gas and the earliest stars, the Population III stars. These massive stars triggered the reionization process and are believed to have created many of the heavy elements in the early Universe, which, through nuclear decay, create lighter elements, allowing the cycle of nucleosynthesis to continue longer. 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 Galaxy groups and clusters, groups and clusters of galaxies, then into larger-scale superclusters. Various fields of physics are crucial to studying the universe. Interdisciplinary studies involve the fields of quantum mechanics, particle physics, plasma physics, condensed matter physics, statistical mechanics, optics, and nuclear physics. Fundamental to the structure of the Universe is the existence of dark matter and dark energy. These are now thought to be its dominant components, forming 96% of the mass of the Universe. For this reason, much effort is expended in trying to understand the physics of these components.


Extragalactic astronomy

The study of objects outside our galaxy is a branch of astronomy concerned with the Galaxy formation and evolution, formation and evolution of Galaxies, their morphology (description) and Galaxy morphological classification, classification, the observation of Active galaxy, active galaxies, and at a larger scale, the Galaxy groups and clusters, groups and clusters of galaxies. Finally, the latter is important for the understanding of the large-scale structure of the cosmos. Most
galaxies A galaxy is a gravitationally bound system of star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to ...

galaxies
are organized into distinct shapes that allow for classification schemes. They are commonly divided into spiral galaxy, spiral, elliptical galaxy, elliptical and irregular galaxy, Irregular galaxies. As the name suggests, an elliptical galaxy has the cross-sectional shape of an ellipse. The stars move along randomness, random orbits with no preferred direction. These galaxies contain little or no interstellar dust, few star-forming regions, and older stars. Elliptical galaxies are more commonly found at the core of galactic clusters, and may have been 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 center, and trailing bright arms that spiral outward. The arms are dusty regions of star formation within which massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the
Milky Way The Milky Way is the galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and concr ...

Milky Way
and one of our nearest galaxy neighbors, the
Andromeda Galaxy The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula (see below), is a barred spiral galaxy approximately from Earth and the nearest major galaxy to the Milky Way The Milky Way is th ...

Andromeda Galaxy
, are spiral galaxies. Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical. About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction. An active galaxy is a formation that emits a significant amount of its energy from a source other than its stars, dust and gas. It is powered by a compact region at the core, thought to be a supermassive 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 shorter frequency, high-energy radiation include Seyfert galaxy, 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 into a hierarchy of groupings, with the largest being the superclusters. The collective matter is formed into Galaxy filament, filaments and walls, leaving large Void (astronomy), voids between.


Galactic astronomy

The
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

Solar System
orbits within the
Milky Way The Milky Way is the galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and concr ...

Milky Way
, a barred spiral galaxy that is a prominent member of the
Local Group Distribution of the iron content (in logarithmic scale) in four neighbouring dwarf galaxies of the Milky Way The Local Group is the galaxy group that includes the Milky Way The Milky Way is the galaxy A galaxy is a gravitation ...

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 its 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, Stellar population, population I stars. The disk is surrounded by a Galactic spheroid, spheroid halo of older, population II 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 Hydrogen, molecular hydrogen and other elements create star-forming regions. These begin as a compact pre-stellar core or 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 (ionized atomic hydrogen) of glowing gas and plasma. The Solar wind, stellar wind and supernova explosions from these stars eventually cause the cloud to disperse, often leaving behind one or more young open clusters 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.


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.Harpaz, 1994, pp. 7–18 Star formation occurs in dense regions of dust and gas, known as Dark nebula, 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. Almost all elements heavier than hydrogen and helium were nucleosynthesis, 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 fuses its hydrogen fuel into helium in its core. Over time, this hydrogen fuel is completely converted into helium, and the star begins to Stellar evolution, evolve. The fusion of helium requires a higher core temperature. A star with a high enough core temperature will push its outer layers outward while increasing its core density. The resulting red giant formed by the expanding outer layers enjoys a brief life span, before the helium fuel in the core is in turn consumed. Very massive stars can also undergo a series of evolutionary phases, as they fuse increasingly heavier elements.Harpaz, 1994 The final fate of the star depends on its mass, with stars of mass greater than about eight times the Sun becoming core collapse
supernova (bright spot on the lower left), a type Ia supernova A type Ia supernova (read: "type one-A") is a type of supernova (bright spot on the lower left), a type Ia supernova within its host galaxy, NGC 4526 A supernova ( plural: supernova ...

supernova
e; while smaller stars blow off their outer layers and leave behind the inert core in the form of a white dwarf. The ejection of the outer layers forms a planetary nebula. 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. Closely orbiting 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 distribute the "metallicity, metals" produced in the star by fusion to the interstellar medium; without them, all new stars (and their planetary systems) would be formed from hydrogen and helium alone.


Solar astronomy

At a distance of about eight light-minutes, the most frequently studied star is the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
, a typical main-sequence dwarf star of stellar class G2 V, and about 4.6 billion years (Gyr) old. 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 oscillation in Wolf number, sunspot number. Sunspots are regions of lower-than- average temperatures that are associated with intense magnetic activity. The Sun has steadily increased in luminosity 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, and finally by the super-heated solar corona, 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. Above that is the convection zone where the gas material transports energy primarily through physical displacement of the gas known as convection. It is believed that the movement of mass within the convection zone creates the magnetic activity that generates sunspots. A solar wind of plasma particles constantly streams outward from the Sun until, at the outermost limit of the Solar System, it reaches the heliopause (astronomy), heliopause. As the solar wind passes the Earth, it interacts with the Earth's magnetic field (magnetosphere) and deflects the solar wind, but traps some creating the Van Allen radiation belts that envelop the Earth. The aurora (astronomy), aurora are created when solar wind particles are guided by the magnetic flux lines into the Earth's polar regions where the lines then descend into the Earth's atmosphere, atmosphere.


Planetary science

Planetary science is the study of the assemblage of
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

planet
s,
moons A natural satellite, or moon, is, in the most common usage, an astronomical body that orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or ...

moons
, dwarf planets,
comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a Comet tail, tail. These ...
s, asteroids, and other bodies orbiting the Sun, as well as extrasolar planets. The
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

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 the Sun's planetary system, although many new discoveries are still being made. The Solar System is divided into the Inner solar system, inner Solar System (subdivided into the inner planets and the asteroid belt), the Outer solar system, outer Solar System (subdivided into the outer planets and Centaurs (minor planets), centaurs), comets, the trans-Neptunian region (subdivided into the Kuiper belt, and the scattered disc) and the farthest regions (e.g., boundaries of the heliosphere, and the Oort Cloud, which may extend as far as a light-year). The inner terrestrial planets consist of Mercury (planet), Mercury, Venus, Earth, and Mars. The outer Giant planet, giant planets are the Gas giant, gas giants (Jupiter and Saturn) and the Ice giant, ice giants (
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

Uranus
and Neptune). The planets were formed 4.6 billion years ago in the 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 and one such collision may have giant impact hypothesis, formed the Moon. Once a planet reaches sufficient mass, the materials of different densities segregate within, during planetary differentiation. This process can form a stony or metallic core, surrounded by a mantle and an outer crust. 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. A planet or moon's interior heat is produced from the collisions that created the body, by the decay of radioactive materials (''e.g.'' uranium, thorium, and Aluminium#Isotopes, 26Al), or tidal acceleration, tidal heating caused by interactions with other bodies. 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.


Interdisciplinary studies

Astronomy and astrophysics have developed significant interdisciplinary links with other major scientific fields. Archaeoastronomy is the study of ancient or traditional astronomies in their cultural context, utilizing archaeology, archaeological and anthropology, anthropological evidence. Astrobiology is the study of the advent and evolution of biological systems in the Universe, with particular emphasis on the possibility of non-terrestrial life. Astrostatistics is the application of statistics to astrophysics to the analysis of a vast amount of observational astrophysical data. The study of chemicals found in space, including their formation, interaction and destruction, is called astrochemistry. These substances are usually found in molecular clouds, although they may also appear in low-temperature stars, brown dwarfs and planets. Cosmochemistry is the study of the chemicals found within the Solar System, including the origins of the elements and variations in the isotope ratios. Both of these fields represent an overlap of the disciplines of astronomy and chemistry. As "forensic astronomy", finally, methods from astronomy have been used to solve problems of law and history.


Amateur astronomy

Astronomy is one of the sciences to which amateurs can contribute the most. Collectively, amateur astronomers observe a variety of celestial objects and phenomena sometimes with Amateur telescope making, equipment that they build themselves. Common targets of amateur astronomers include the Sun, the Moon, planets, stars, comets,
meteor shower A meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point in the night sky. These meteors are caused by streams of cosmic debris called meteoroids entering Earth's atmosphere at extr ...

meteor shower
s, and a variety of deep-sky objects such as star clusters, galaxies, and nebulae. Astronomy clubs are located throughout the world and many have programs to help their members set up and complete observational programs including those to observe all the objects in the Messier (110 objects) or Herschel 400 catalogues of points of interest in the night sky. 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 that 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 Guthe Jansky, 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 and 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.


Unsolved problems 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. * 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? A deeper understanding of the formation of stars and planets is needed. * Is there other Extraterrestrial life, 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 the Solar System normal or atypical? * What is the nature of dark matter and dark energy? These dominate the evolution and fate of the cosmos, yet their true nature remains unknown. * What will be the ultimate fate of the universe? * How did the first galaxies form? How did supermassive black holes form? * What is creating the ultra-high-energy cosmic rays? * Why is the abundance of lithium in the cosmos four times lower than predicted by the standard Big Bang model? * What really happens beyond the event horizon?


See also

* Airmass * Astronomical acronyms * Astronomical instruments * Cosmogony * International Year of Astronomy * List of astronomy acronyms * List of Russian astronomers and astrophysicists * List of software for astronomy research and education * Outline of space science * Science tourism * Space exploration * Starlight * Stellar collision * ''Universe: The Infinite Frontier'' (television series)


References


Bibliography

* * *


External links


NASA/IPAC Extragalactic Database (NED)NED-Distances

International Year of Astronomy 2009
IYA2009 Main website

from the American Institute of Physics
Southern Hemisphere Astronomy

Celestia Motherlode
Educational site for Astronomical journeys through space
Kroto, Harry
Astrophysical Chemistry Lecture Series.
Core books
an
Core journals
in Astronomy, from the Smithsonian/NASA Astrophysics Data System
A Journey with Fred Hoyle
by Wickramasinghe, Chandra.
Astronomy books from the History of Science Collection
at Linda Hall Library {{Use dmy dates, date=April 2019 Astronomy, Physical sciences