'Astronomy' is the
scientific study of
celestial objects (such as
stars,
planets,
comets, and
galaxies) and
phenomena that originate outside the
Earth's atmosphere (such as the
cosmic background radiation). It is concerned with the evolution,
physics,
chemistry,
meteorology, and
motion of celestial objects, as well as the
formation and development of the universe.
Astronomy is one of the youngest sciences. Astronomers of early civilizations performed methodical observations of the night sky, and astronomical artifacts have been found from much earlier periods. However, the invention of the
telescope was required before astronomy was able to develop into a modern science. Historically, astronomy has included disciplines as diverse as
astrometry,
celestial navigation, observational astronomy, the making of
calendars, and even
astrology, but professional astronomy is nowadays often considered to be identical with 'astrophysics'. Since the 20th century, the field of professional astronomy split into
observational and
theoretical branches. Observational astronomy is focused on acquiring and analyzing data, mainly using basic principles of physics. Theoretical astronomy is oriented towards the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain the observational results, and observations being used to confirm theoretical results.
Amateur astronomers have contributed to many important astronomical discoveries, and astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and observation of transient
phenomena.
Old or even ancient astronomy is not to be confused with
astrology, the belief system that claims that human affairs are correlated with the positions of celestial objects. Although the
two fields share a common origin and a part of their methods (namely, the use of
ephemerides), they are distinct.
[1]
Lexicology
The word ''astronomy'' literally means "law of the stars" (or "culture of the stars" depending on the translation) and is derived from the
Greek αστρονομία, ''astronomia'', from the words άστρον (''astron'', "star") and νόμος (''nomos'', "laws or cultures").
Use of terms "astronomy" and "astrophysics"
Generally, either the term "astronomy" or "astrophysics" may be used to refer to this subject.
[2][3][4] Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the earth's atmosphere and of their physical and chemical properties"
[5]and "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena".
[6] In some cases, as in the introduction of the introductory textbook '''The Physical Universe''' by
Frank Shu, "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject.
[7] However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics.
Various departments that research this subject may use "astronomy" and "astrophysics", partly depending on whether the department is historically affiliated with a physics department,
and many professional astronomers actually have physics degrees.
Even the name of the scientific journal
Astronomy & Astrophysics reveals the ambiguity of the use of the term.
History
Main articles: History of astronomy
A celestial map from the 17th century, by the Dutch cartographer Frederik de Wit.
In early times, astronomy only comprised the observation and predictions of the motions of objects visible to the naked eye. In some locations, such as
Stonehenge, early cultures assembled massive artifacts that likely had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops, as well as in understanding the length of the year.
[8]
Before tools such as the telescope were invented early study of the stars had to be conducted from the only vantage points available, namely tall buildings, trees and high ground using the bare eye.
As civilizations developed, most notably
Mesopotamia,
Egypt,
Persia,
Maya,
Greece,
India,
China, and the
Islamic world, astronomical observatories were assembled, and ideas on the nature of the universe began to be explored. Most of early astronomy actually consisted of mapping the positions of the stars and planets, a science now referred to as
astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon and the Earth in the universe were explored philosophically. The Earth was believed to be the center of the universe with the Sun, the Moon and the stars rotating around it. This is known as the geocentric model of the universe.
A few notable astronomical discoveries were made prior to the application of the telescope. For example, the
obliquity of the ecliptic was estimated as early as 1,000 B.C by the Chinese. The
Chaldeans discovered that
eclipses recurred in a repeating cycle known as a
saros. In the second century B.C., the size and distance of the Moon were estimated by
Hipparchus.
During the Middle Ages, observational astronomy was mostly stagnant in
medieval Europe, at least until the
13th century. However, observational astronomy flourished in the
Persian Empire and other parts of the world. Astronomers during that time introduced many names that are now used for individual stars.
[9][10]
Scientific revolution
Galileo's sketches and observations of the Moon revealed that the surface was mountainous
During the
Renaissance,
Nicolaus Copernicus proposed a
heliocentric model of the
solar system. His work was defended, expanded upon, and corrected by
Galileo Galilei and
Johannes Kepler. Galileo innovated by using telescopes to enhance his observations.
Kepler was the first to devise a system that described correctly the details of the motion of the planets with the Sun at the center. However, Kepler did not succeed in formulating a theory behind the laws he wrote down. It was left to
Newton's invention of
celestial dynamics and his
law of gravitation to finally explain the motions of the planets. Newton also developed the
reflecting telescope.
Further discoveries paralleled the improvements in the size and quality of the telescope. More extensive star catalogues were produced by
Lacaille. The astronomer
William Herschel made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet
Uranus, the first new planet found. The distance to a star was first announced in 1838 when the
parallax of
61 Cygni was measured by
Friedrich Bessel.
During the nineteenth century, attention to the
three body problem by
Euler,
Clairaut, and
D'Alembert led to more accurate predictions about the motions of the Moon and planets. This work was further refined by
Lagrange and
Laplace, allowing the masses of the planets and moons to be estimated from their perturbations.
Significant advances in astronomy came about with the introduction of new technology, including the
spectroscope and
photography.
Fraunhofer discovered about 600 bands in the spectrum of the Sun in 1814-15, which, in 1859,
Kirchhoff ascribed to the presence of different elements. Stars were proven to be similar to the Earth's own Sun, but with a wide range of
temperatures,
masses, and sizes.
9
The existence of the Earth's galaxy, the
Milky Way, as a separate group of stars, was only proved in the
20th century, along with the existence of "external" galaxies, and soon after, the expansion of the
universe, seen in the recession of most galaxies from us. Modern astronomy has also discovered many exotic objects such as
quasars,
pulsars,
blazars, and
radio galaxies, and has used these observations to develop physical theories which describe some of these objects in terms of equally exotic objects such as
black holes and
neutron stars.
Physical cosmology made huge advances during the 20th century, with the model of the
Big Bang heavily supported by the evidence provided by astronomy and physics, such as the
cosmic microwave background radiation,
Hubble's law, and
cosmological abundances of elements.
Observational astronomy
Main articles: Observational astronomy
In astronomy,
information is mainly received from the detection and analysis of light and other forms of
electromagnetic radiation.
[11] Observational astronomy may be divided according to the observed region of the
electromagnetic spectrum. Some parts of the spectrum can be observed from the
Earth's surface, while other parts are only observable from either high altitudes or space. Specific information on these subfields is given below.
Radio astronomy
Radio astronomy studies radiation with
wavelengths greater than approximately one
millimeter.
[12] Radio astronomy is different from most other forms of observational astronomy in that the observed
radio waves can be treated as
waves rather than as discrete
photons. Hence, it is relatively easier to measure both the
amplitude and
phase of radio waves, whereas this is not as easily done at shorter wavelengths.
Although some
radio waves are produced by astronomical objects in the form of
thermal emission, most of the radio emission that is observed from Earth is seen in the form of
synchrotron radiation, which is produced when
electrons oscillate around
magnetic fields.
Additionally, a number of
spectral lines produced by
interstellar gas, particularly the
hydrogen spectral line at 21
cm, are observable at radio wavelengths.
A wide variety of objects are observable at radio wavelengths, including
supernovae, interstellar gas,
pulsars, and
active galactic nuclei.
Infrared astronomy
Infrared astronomy deals with the detection and analysis of
infrared radiation (wavelengths longer than red light). Except at wavelengths close to visible light, infrared radiation is heavily absorbed by the atmosphere, and the atmosphere produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places or in space. Infrared astronomy is particularly useful for observation of galactic regions cloaked by dust, and for studies of molecular gases.
Optical astronomy
Historically,
optical astronomy, also called visible light astronomy, is the oldest form of astronomy.
[13] Optical images were originally drawn by hand. In the late nineteenth century and most of the twentieth century, images were made using photographic equipment. Modern images are made using digital detectors, particularly detectors using
charge-coupled devices (CCDs). Although visible light itself extends from approximately 4000
Å (400
nm) to 7000 Å (700 nm),
the same equipment used at these wavelengths is also used to observe some
near-ultraviolet and
near-infrared radiation.
Ultraviolet astronomy
Ultraviolet astronomy is generally used to refer to observations at
ultraviolet wavelengths between approximately 100 and 3200 Å.
Light at these wavelengths is absorbed by the Earth's atmosphere, so observations at these wavelengths must be performed from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue
stars (
O stars and
B stars) that are very bright in this wave band. This includes the blue stars in other galaxies, which have been the targets of several ultraviolet surveys. Other objects commonly observed in ultraviolet light include
planetary nebulae,
supernova remnants, and active galactic nuclei.
However, ultraviolet light is easily absorbed by
interstellar dust, and measurement of the ultraviolet light from objects need to be corrected for extinction.
X-ray astronomy
X-ray astronomy is the study of astronomical objects at X-ray wavelengths. Typically, objects emit X-ray radiation as
synchrotron emission (produced by electrons oscillating around magnetic field lines), thermal emission from thin gases (called
bremsstrahlung radiation) that is above 10
7 (10 million)
Kelvin, and thermal emission from thick gases (called
blackbody radiation) that are above 10
7 Kelvin.
Since X-rays are absorbed by the Earth's atmosphere, all X-ray observations must be done from high-altitude balloons, rockets, or spacecraft. Notable X-ray sources include
X-ray binaries,
pulsars, supernova remnants,
elliptical galaxies,
clusters of galaxies, and active galactic nuclei.
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.
[14]
Most
gamma-ray emitting sources are actually
gamma-ray bursts, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources. These steady gamma-ray emitters include pulsars,
neutron stars, and
black hole candidates such as active galactic nuclei.
Fields of observational astronomy not based on the electromagnetic spectrum
Other than electromagnetic radiation, few things may be observed from the Earth that originate from great distances.
In
neutrino astronomy, astronomers use special underground facilities such as
SAGE,
GALLEX, and
Kamioka II/III for detecting
neutrinos. These neutrinos originate primarily from the
Sun but also from
supernovae.
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.
[15]
Planetary astronomy has benefited from direct observation in the form of spacecraft and sample return missions. These include fly-by missions with remote sensors; landing vehicles that can perform experiments on the surface materials; impactors that allow remote sensing of buried material, and sample return missions that allow direct, laboratory examination.
Astrometry and celestial mechanics
Main articles: Astrometry,
Celestial mechanics
One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in
celestial navigation.
Careful measurement of the positions of the planets has led to a solid understanding of gravitational
perturbations, and an ability to determine past and future positions of the planets with great accuracy, a field known as
celestial mechanics. More recently the tracking of
near-Earth objects will allow for predictions of close encounters, and potential collisions, with the Earth.
[16]
The measurement of
stellar parallax of nearby stars provides a fundamental baseline in the
cosmic distance ladder that is used to measure the scale of the universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, because their properties can be compared. Measurements of
radial velocity and
proper motion show the kinematics of these systems through the Milky Way galaxy. Astrometric results are also used to measure the distribution of
dark matter in the galaxy.
[17]
During the 1990s, the astrometric technique of measuring the
stellar wobble was
used to detect large
extrasolar planets orbiting nearby stars.
[18]
Theoretical astronomy
Theoretical astronomers use a wide variety of tools which include
analytical models (for example,
polytropes to approximate the behaviors of a
star) and
computational
numerical simulations. Each has some advantages. Analytical models of a process are generally better for giving insight into the heart of what is going on. Numerical models can reveal the existence of phenomena and effects that would otherwise not be seen.
[19][20]
Theorists in astronomy endeavor to create theoretical models and figure out the observational consequences of those models. This helps allow observers to look for data that can refute a model or help in choosing between several alternate or conflicting models.
Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model.
Topics studied by theoretical astronomers include:
stellar dynamics and
evolution;
galaxy formation;
large-scale structure of
matter in the
Universe; origin of
cosmic rays;
general relativity and
physical cosmology, including
string cosmology and
astroparticle physics. Astrophysical relativity serves as a tool to gauge the properties of large scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis for
black hole (''astro'')
physics and the study of
gravitational waves.
Some widely accepted and studied theories and models in astronomy, now included in the
Lambda-CDM model are the
Big Bang,
Cosmic inflation,
dark matter, and fundamental theories of
physics.
A few examples of this process:
Dark matter and
dark energy are the current leading topics in astronomy, as their discovery and controversy originated during the study of the galaxies.
Subfield of astronomy for specific astronomical objects
Solar astronomy
Main articles: Sun
The most frequently studied star is the
Sun, a typical main-sequence
dwarf star of
stellar class G2 V, and about 4.6 Gyr in age. The Sun is not considered a
variable star, but it does undergo periodic changes in activity known as the
sunspot cycle. This is an 11-year fluctuation in
sunspot numbers. Sunspots are regions of lower-than- average temperatures that are associated with intense magnetic activity.
[21]
The Sun has steadily increased in luminosity over the course of its life, increasing by 40% since it first became a main-sequence star. The Sun has also undergone periodic changes in luminosity that can have a significant impact on the Earth.
[22] The
Maunder minimum, for example, is believed to have caused the
Little Ice Age phenomenon during the
Middle Ages.
[23]
The visible outer surface of the Sun is called the
photosphere. Above this layer is a thin region known as the
chromosphere. This is surrounded by a transition region of rapidly increasing temperatures, then by the super-heated
corona.
At the center of the Sun is the core region, a volume of sufficient temperature and pressure for
nuclear fusion to occur. Above the core is the
radiation zone, where the plasma conveys the energy flux by means of radiation. The outer layers form a
convection zone where the gas material transports energy primarily through physical displacement of the gas. It is believed that this convection zone creates the magnetic activity that generates sun spots.
21
A solar wind of plasma particles constantly streams outward from the Sun until it reaches the
heliopause. This solar wind interacts with the
magnetosphere of the Earth to create the
Van Allen radiation belts, as well as the
aurora where the lines of the
Earth's magnetic field descend into the
atmosphere.
[24]
Planetary science
Main articles: Planetary science,
Planetary geology
This astronomical field examines the assemblage of
planets,
moons,
dwarf planets,
comets,
asteroids, and other bodies orbiting the Sun, as well as extrasolar planets. The
solar system has been relatively well-studied, initially through telescopes and then later by spacecraft. This has provided a good overall understanding of the formation and evolution of this planetary system, although many new discoveries are still being made.
[25]
The solar system is subdivided into the inner planets, the
asteroid belt, and the outer planets. The inner
terrestrial planets consist of
Mercury,
Venus,
Earth, and
Mars. The outer
gas giant planets are
Jupiter,
Saturn,
Uranus and
Neptune.
[26] Beyond Neptune lie the
Kuiper Belt, and finally the
Oort Cloud, which may extend as far as a light-year.
The planets were formed by a
protoplanetary disk that surrounded the early Sun. Through a process that included gravitational attraction, collision, and accretion, the disk formed clumps of matter that, with time, became protoplanets. The
radiation pressure of the
solar wind then expelled most of the unaccreted matter, and only those planets with sufficient mass retained their gaseous atmosphere. The planets continued to sweep up, or eject, the remaining matter during a period of intense bombardment, evidenced by the many
impact craters on the Moon. During this period, some of the protoplanets may have collided, the
leading hypothesis for how the Moon was formed.
[27]
Once a planet reaches sufficient mass, the materials with different densities segregate within, during
planetary differentiation. This process can form a stony or metallic core, surrounded by a mantle and an outer surface. The core may include solid and liquid regions, and some planetary cores generate their own
magnetic field, which can protect their atmospheres from solar wind stripping.
[28]
A planet or moon's interior heat is produced from the collisions that created the body, radioactive materials (''e.g.''
uranium,
thorium, and
26Al), or
tidal heating. Some planets and moons accumulate enough heat to drive geologic processes such as
volcanism and tectonics. Those that accumulate or retain an
atmosphere can also undergo surface
erosion from wind or water. Smaller bodies, without tidal heating, cool more quickly; and their geological activity ceases with the exception of impact cratering.
[29]
Stellar astronomy
The
Ant planetary nebula. Ejecting gas from the dying central star shows symmetrical patterns unlike the chaotic patterns of ordinary explosions.
The study of
stars and
stellar evolution is fundamental to our understanding of the universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior.
Star formation occurs in dense regions of dust and gas, known as
giant molecular clouds. When destabilized, cloud fragments can collapse under the influence of gravity, to form a
protostar. A sufficiently dense, and hot, core region will trigger
nuclear fusion, thus creating a
main-sequence star.
[30]
Almost all elements heavier than
hydrogen and
helium were
created inside the cores of stars.
The characteristics of the resulting star depend primarily upon its starting mass. The more massive the star, the greater its luminosity, and the more rapidly it expends the hydrogen fuel in its core. Over time, this hydrogen fuel is completely converted into helium, and the star begins to
evolve. The fusion of helium requires a higher core temperature, so that the star both expands in size, and increases in core density. The resulting
red giant enjoys a brief life span, before the helium fuel is in turn consumed. Very massive stars can also undergo a series of decreasing evolutionary phases, as they fuse increasingly heavier elements.
The final fate of the star depends on its mass, with stars of mass greater than about eight times the
Sun becoming core collapse
supernovae; while smaller stars form
planetary nebulae, and evolve into
white dwarfs. The remnant of a supernova is a dense
neutron star, or, if the stellar mass was at least three times that of the Sun, a
black hole.
[31] Close binary stars can follow more complex evolutionary paths, such as mass transfer onto a white dwarf companion that can potentially cause a supernova. Planetary nebulae and supernovae are necessary for the distribution of
metals to the interstellar medium; without them, all new stars (and their planetary systems) would be formed from hydrogen and helium alone.
Galactic astronomy
Main articles: Galactic astronomy
Observed structure of the Milky Way's spiral arms
Our
solar system orbits within the
Milky Way, a
barred spiral galaxy that is a prominent member of the
Local Group of galaxies. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is located within the dusty outer arms, there are large portions of the Milky Way that are obscured from view.
In the 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 clusters.
[32][33]
Between the stars lies the
interstellar medium, a region of sparse matter. In the densest regions,
molecular clouds of
molecular hydrogen and other elements create star-forming regions. These begin as irregular
dark nebulae, which concentrate and collapse (in volumes determined by the
Jeans length) to form compact protostars.
[34]
As the more massive stars appear, they transform the cloud into an
H II region of glowing gas and plasma. The
stellar wind and supernova explosions from these stars eventually serve to disperse the cloud, often leaving behind one or more young
open clusters of stars. These 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.
[35]
Extragalactic astronomy
Main articles: 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.
This image shows several blue, loop-shaped objects that are multiple images of the same galaxy, duplicated by the
gravitational lens effect of the cluster of yellow galaxies near the middle of the photograph. The lens is produced by the cluster's gravitational field that bends light to magnify and distort the image of a more distant object.
Most galaxies are organized into distinct shapes that allow for classification schemes. They are commonly divided into
spiral,
elliptical and
Irregular galaxies.
[36]
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.
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 where massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the
Milky Way and the
Andromeda Galaxy are spiral galaxies.
Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical. 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 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,
Quasars, and
Blazars. Quasars are believed to be the most consistently luminous objects in the known universe.
[37]
The
large-scale structure of the cosmos is represented by groups and clusters of galaxies. This structure is organized in a hierarchy of groupings, with the largest being the
superclusters. The collective matter is formed into
filaments and walls, leaving large
voids in between.
[38]
Cosmology
Main articles: Physical cosmology
Cosmology (from the Greek κοσμος "world, universe" and λογος "word, study") 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
expanded over the course of 13.7 Gyr to its present condition. The concept of the big bang can be traced back to the discovery of the
microwave background radiation in 1965.
In the course of this expansion, the universe underwent several evolutionary stages. In the very early moments, it is theorized that the universe experienced a very rapid
cosmic inflation, which homogenized the starting conditions. Thereafter,
nucleosynthesis produced the elemental abundance of the early universe. (See also
nucleocosmochronology.)
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.
[39]
A hierarchical structure of matter began to form from minute variations in the mass density. Matter accumulated in the densest regions, forming clouds of gas and the
earliest stars. These massive stars triggered the
reionization process and are believed to have created many of the heavy elements in the early universe.
Gravitational aggregations clustered into filaments, leaving voids in the gaps. Gradually, organizations of gas and dust merged to form the first primitive galaxies. Over time, these pulled in more matter, and were often organized into
groups and clusters of galaxies, then into larger-scale superclusters.
[40]
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.
[41]
Interdisciplinary studies
Astronomy and astrophysics have developed significant interdisciplinary links with other major scientific fields. These include:
★
Astrobiology: the study of the advent and evolution of biological systems in the universe.
★
Archaeoastronomy: the study of ancient or traditional astronomies in their cultural context, utilizing
archaeological and
anthropological evidence.
★
Astrochemistry: the study of the
chemicals found in space, usually in
molecular clouds, and their formation, interaction and destruction. It represents an overlap of the disciplines of astronomy and chemistry.
★
Cosmochemistry: the study of the chemicals found within the
Solar System, including the origins of the elements and variations in the
isotope ratios.
Amateur astronomy
Main articles: Amateur astronomy
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 objects such as star clusters, galaxies, and nebulae. One branch of amateur astronomy, amateur
astrophotography, involves the taking of photos of the night sky. Many amateurs like to specialize in the observation of particular objects, types of objects, or types of events which interest them.
[42][43]
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).
[44][45]
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.
[46][47][48]
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.
★ 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?
[49] 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.
[50][51]
★ 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.
[52]
★ 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?
[53]
★ What will be the
ultimate fate of the universe?
[54]
See also
Lists
★
List of basic astronomy topics
★
List of astronomy topics
★
★
★
★
Related articles
★
Astrology and astronomy
★
Astronomer
★
Astrophysics Data System
★
Cosmogony
★
International Year of Astronomy
★
Solar System
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Space exploration
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Space science
References
1. The New Cosmos: An Introduction to Astronomy and Astrophysics, Albrecht Unsöld, , , Springer, 2001, ISBN 3-540-67877-8
2. Curions About Astronomy: What's the difference between astronomy and astrophysics? B. Scharringhausen
3. Archive of Astronomy Questions and Answers: What is the difference between astronomy and astrophysics? S. Odenwald
4. Penn State Erie-School of Science-Astronomy and Astrophysics
5. Merriam-Webster Online
6. Merriam-Webster Online
7. The Physical Universe, F. H. Shu, , , University Science Books, , ISBN 0-935702-05-9
8. History of Astronomy, George Forbes, , , Watts & Co., 1909,
9. A Short History of Astronomy From Earliest Times Through the Nineteenth Century, Arthur Berry, , , Dover Publications, Inc., 1961,
10. The Cambridge Concise History of Astronomy, , , , Cambridge University Press, 1999, ISBN 0-521-57600-8
11. Electromagnetic Spectrum
12. Allen's Astrophysical Quantities, A. N. Cox, editor, , , Springer-Verlag, 2000, ISBN 0-387-98746-0
13. Philip's Atlas of the Universe, P. Moore, , , George Philis Limited, 1997, ISBN 0-540-07465-9
14. The electromagnetic spectrum
15. Opening new windows in observing the Universe G. A. Tammann, F. K. Thielemann, D. Trautmann
16. Celestial Mechanics
17. Hall of Precision Astrometry
18. A planetary system around the millisecond pulsar PSR1257+12, Wolszczan, A.; Frail, D. A., , , Nature, 1992
19. H. Roth, ''A Slowly Contracting or Expanding Fluid Sphere and its Stability'', ''Phys. Rev.'' ('39', p;525–529, 1932)
20. A.S. Eddington, ''Internal Constitution of the Stars''
21. The Solar FAQ
22. Environmental issues : essential primary sources."
23. The Once & Future Sun Pogge, Richard W.
24. The Exploration of the Earth's Magnetosphere D. P. Stern, M. Peredo
25. Remote Sensing for the Earth Sciences: Manual of Remote Sensing, J. F. Bell III, B. A. Campbell, M. S. Robinson, , , John Wiley & Sons, 2004,
26. Lunar and Planetary Science E. Grayzeck, D. R. Williams
27. Planetary Formation and Our Solar System
28. The Planets After Formation
29. The New Solar System, , , , Cambridge press, 1999, ISBN 0-521-64587-5
30. Stellar Evolution & Death
31. The Cambridge Atlas of Astronomy, , , , Cambridge University Press, 1994, ISBN 0-521-43438-6
32. The Galactic Centre
33. The Role Of Stellar Population Types In The Discussion Of Stellar Evolution, , Danny R., Faulkner, CRS Quarterly, 1993
34. Star Formation; The Interstellar Medium
35. The Early History of Dark Matter, Van den Bergh, Sidney, , , Publications of the Astronomy Society of the Pacific, 1999
36. Galaxy Classification
37. Active Galaxies and Quasars
38. Astronomy: The Evolving Universe, , Michael, Zeilik, Wiley, 2002, ISBN 0-521-80090-0
39. Cosmology 101: The Study of the Universe
40. Galaxy Clusters and Large-Scale Structure
41. Dark Energy Fills the Cosmos
42. The Americal Meteor Society
43. Catching the Light: Astrophotography
44. Karl Jansky and the Discovery of Cosmic Radio Waves F. Ghigo
45. Cambridge Amateur Radio Astronomers
46. The International Occultation Timing Association
47. Edgar Wilson Award
48. American Association of Variable Star Observers
49. The Initial Mass Function of Stars: Evidence for Uniformity in Variable Systems, , Pavel, Kroupa, Science, 2002
50. Complex Life Elsewhere in the Universe?
51. The Quest for Extraterrestrial Intelligence
52. 11 Physics Questions for the New Century
53. Was the Universe Designed?
54. What is the Ultimate Fate of the Universe?
External links
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Astronomy Guide For reviews on astronomy products, how-to's and current events.
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International Year of Astronomy 2009 IYA2009 Main website
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Cosmic Journey: A History of Scientific Cosmology from the American Institute of Physics
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Astronomy Picture of the Day
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Southern Hemisphere Astronomy
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Sky & Telescope publishers
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Astronomy Magazine
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Latest astronomy news in 11 languages
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Universe Today for astronomy and space-related news
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Celestia Motherlode Educational site for Astronomical journeys through space
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Search Engine for Astronomy
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Hubblesite.org - home of NASA's Hubble Space Telescope
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Astronomy - A History - G. Forbes - 1909 (eLibrary Project - eLib Text)
★ (historical)
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Cosmic Journey: A History of Scientific Cosmology from the American Institute of Physics
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Prof. Sir Harry Kroto, NL, Astrophysical Chemistry Lecture Series. 8 Freeview Lectures provided by the Vega Science Trust.
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Core books and
core journals in Astronomy, from the Smithsonian/NASA
Astrophysics Data System
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