Astronomy (from Greek: ἀστρονομία) is a natural science
that studies celestial objects and phenomena. It applies mathematics,
physics, and chemistry, in an effort to explain the origin of those
objects and phenomena and their evolution. Objects of interest include
planets, moons, stars, galaxies, and comets; the phenomena include
supernova explosions, gamma ray bursts, and cosmic microwave
background radiation. More generally, all phenomena that originate
Earth's atmosphere are within the purview of astronomy. A
related but distinct subject, physical cosmology, is concerned with
the study of the
Universe as a whole.
Astronomy is one of the oldest of the natural sciences. The early
civilizations in recorded history, such as the Babylonians, Greeks,
Indians, Egyptians, Nubians, Iranians, Chinese, Maya, and many ancient
indigenous peoples of the Americas performed methodical observations
of the night sky. Historically, astronomy has included disciplines as
diverse as astrometry, celestial navigation, observational astronomy
and the making of calendars, but professional astronomy is now often
considered to be synonymous with astrophysics.
Professional astronomy is split into observational and theoretical
Observational astronomy is focused on acquiring data from
observations of astronomical objects, which 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. The two fields complement each
other, with theoretical astronomy seeking to explain observational
results and observations being used to confirm theoretical results.
Astronomy is one of the few sciences where amateurs still play an
active role, especially in the discovery and observation of transient
phenomena. Amateur astronomers have made and contributed to many
important astronomical discoveries, such as finding new comets.
1.1 Use of terms "astronomy" and "astrophysics"
2.1 Ancient times
2.2 Middle Ages
2.3 Scientific revolution
3 Observational astronomy
3.3 Optical astronomy
3.5 X-ray astronomy
3.6 Gamma-ray astronomy
3.7 Fields not based on the electromagnetic spectrum
Astrometry and celestial mechanics
4 Theoretical astronomy
5 Specific subfields
5.1 Solar astronomy
5.2 Planetary science
5.3 Stellar astronomy
5.4 Galactic astronomy
5.5 Extragalactic astronomy
5.6 Physical cosmology
6 Interdisciplinary studies
7 Amateur astronomy
8 Unsolved problems in astronomy
9 See also
12 External links
19th century Sydney Observatory,
Australia (1873) 
Quito Astronomical Observatory
Quito Astronomical Observatory is located 12 minutes
south of the
Equator in Quito, Ecuador.
Astronomy (from the Greek 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, 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"
Generally, either the term "astronomy" or "astrophysics" may be used
to refer to this subject. Based on strict dictionary
definitions, "astronomy" refers to "the study of objects and matter
Earth's atmosphere and of their physical and chemical
properties" and "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
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. However, since most
modern astronomical research deals with subjects related to physics,
modern astronomy could actually be called astrophysics. Few 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 astronomers have
physics rather than astronomy degrees. Some titles of the leading
scientific journals in this field include The Astronomical Journal,
The Astrophysical Journal, and
Astronomy and Astrophysics.
Main article: History of astronomy
Archaeoastronomy and List of astronomers
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, early cultures assembled massive artifacts that
possibly 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.
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, Greece, Persia, India, China, Egypt, and
Central America, 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
Universe with the Sun, the
Moon and the stars rotating around
it. This is known as the geocentric model of the Universe, or the
Ptolemaic system, named after Ptolemy.
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 discovered that lunar eclipses recurred in a repeating
cycle known as a saros.
Greek equatorial sundial, Alexandria on the Oxus, present-day
Afghanistan 3rd–2nd century BCE
Following the Babylonians, significant advances in astronomy were made
in ancient Greece and the Hellenistic world.
Greek astronomy 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 estimated the size and distance of the
Sun, and was the first to propose a heliocentric model of the solar
system. In the 2nd century BC,
Hipparchus discovered precession,
calculated the size and distance of the
Moon and invented the earliest
known astronomical devices such as the astrolabe.
created a comprehensive catalog of 1020 stars, and most of the
constellations of the northern hemisphere derive from Greek
Antikythera mechanism (c. 150–80 BC) was an early
analog computer designed to calculate the location of the Sun, Moon,
and planets for a given date. Technological artifacts of similar
complexity did not reappear until the 14th century, when mechanical
astronomical clocks appeared in Europe.
During the Middle Ages, astronomy was mostly stagnant in medieval
Europe, at least until the 13th century. However, astronomy flourished
in the Islamic world and other parts of the world. This led to the
emergence of the first astronomical observatories in the Muslim world
by the early 9th century. In 964, the Andromeda Galaxy,
the largest galaxy in the Local Group, was discovered by the Persian
astronomer Azophi and first described in his
Book of Fixed Stars.
SN 1006 supernova, the brightest apparent magnitude stellar event
in recorded history, was observed by the Egyptian Arabic astronomer
Ali ibn Ridwan
Ali ibn Ridwan and the Chinese astronomers in 1006. Some of the
prominent Islamic (mostly Persian and Arab) astronomers who made
significant contributions to the science include Al-Battani, Thebit,
Azophi, Albumasar, Biruni, Arzachel, Al-Birjandi, and the astronomers
of the Maragheh and Samarkand observatories. Astronomers during that
time introduced many Arabic names now used for individual
stars. It is also believed that the ruins at Great Zimbabwe
and Timbuktu may have housed an astronomical observatory.
Europeans had previously believed that there had been no astronomical
observation in pre-colonial
Middle Ages sub-Saharan Africa but modern
discoveries show otherwise.
The Roman Catholic Church gave more financial and social support to
the study of astronomy for over six centuries, from the recovery of
ancient learning during the late
Middle Ages into the Enlightenment,
than any other, and, probably, all other, institutions. Among the
Church's motives was finding the date for Easter.
Galileo's sketches and observations of the
Moon revealed that the
surface was mountainous.
An astronomical chart from an early scientific manuscript, c. 1000
During the Renaissance,
Nicolaus Copernicus proposed a heliocentric
model of the solar system. His work was defended, expanded upon, and
Galileo Galilei and Johannes Kepler.
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.
The English astronomer
John Flamsteed catalogued over 3000 stars.
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
During the 18–19th centuries, the study of 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.
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. The observed recession of those
galaxies led to the discovery of the expansion of the 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 quasars, pulsars, blazars, and radio
Physical cosmology made huge advances during the 20th
century, with the model of the Big Bang, which is heavily supported by
evidence provided by cosmic microwave background radiation, Hubble's
law, and the 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 detected
evidence of gravitational waves in the previous September.
Main article: Observational astronomy
Our main source of information about celestial bodies and other
objects is visible light more generally electromagnetic radiation.
Observational astronomy may be divided according to the observed
region of the electromagnetic spectrum. Some parts of the spectrum can
be observed from the Earth's surface, while other parts are only
observable from either high altitudes or outside the Earth's
atmosphere. Specific information on these subfields is given below.
Very Large Array
Very Large Array in New Mexico, an example of a radio telescope
Radio astronomy uses radiation outside the visible range with
wavelengths greater than approximately one millimeter. 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 emitted directly by astronomical
objects, a product of 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
A wide variety of objects are observable at radio wavelengths,
including supernovae, interstellar gas, pulsars, and active galactic
Observatory is one of the highest observatory sites on Earth.
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
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
Subaru Telescope (left) and Keck
Observatory (center) on Mauna
Kea, both examples of an observatory that operates at near-infrared
and visible wavelengths. The
Infrared Telescope Facility (right)
is an example of a telescope that operates only at near-infrared
Main article: 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 Å to 7000 Å
(400 nm to 700 nm), that same equipment can be used to
observe some near-ultraviolet and near-infrared radiation.
Ultraviolet astronomy employs ultraviolet wavelengths between
approximately 100 and 3200 Å (10 to 320 nm).
those wavelengths are 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
stars (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.
Main article: X-ray astronomy
X-ray jet made from a supermassive black hole found by NASA's Chandra
X-ray Observatory, made visible by light from the early Universe
X-ray astronomy uses X-ray wavelengths. Typically, X-ray radiation is
produced by synchrotron emission (the result of electrons orbiting
magnetic field lines), thermal emission from thin gases above 107
(10 million) kelvins, and 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,
X-ray astronomy satellites. 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
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 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
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 underground
facilities such as SAGE, GALLEX, and Kamioka II/III for the detection
of neutrinos. The vast majority of the neutrinos streaming through the
Earth originate from the 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
LIGO made its 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
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
Astrometry and Celestial mechanics
Pismis 24 with a nebula
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 (the use of celestial
objects to guide navigation) and in the making of calendars.
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 or potential collisions of the
Earth with those
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 allows 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 used to detect large extrasolar planets orbiting those
Big Bang nucleosynthesis
Cosmic ray spallation
Main article: Theoretical astronomy
Theoretical astronomers use several tools including analytical models
and computational numerical simulations; each has its particular
advantages. Analytical models of a process are generally better for
giving broader insight into the heart of what is going on. Numerical
models reveal the existence of phenomena and effects otherwise
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 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 total
abandonment of a model.
Phenomena modeled by theoretical astronomers include: stellar dynamics
and evolution; galaxy formation; large-scale distribution 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:
Emergence of a star system
How the stars shine and how metals formed
The Big Bang
Hubble Space Telescope, COBE
Age of the Universe
Black holes at the center of Andromeda galaxy
CNO cycle in stars
The dominant source of energy for massive star.
Dark matter and dark energy are the current leading topics in
astronomy, as their discovery and controversy originated during
the study of the galaxies.
An ultraviolet image of the Sun's active photosphere as viewed by the
TRACE space telescope.
Lomnický štít (Slovakia) built in 1962
Main article: Sun
See also: Solar telescope
At a distance of about eight light-minutes, the most frequently
studied star is the 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
sunspot number. Sunspots are regions of lower-than- average
temperatures that are associated with intense magnetic activity.
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
Little Ice Age
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 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
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. As the solar wind passes the Earth, it interacts with the
Earth's magnetic field
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 are created when solar wind particles are guided by
the magnetic flux lines into the Earth's polar regions where the lines
the descend into the atmosphere.
Planetary science and Planetary geology
Planetary science is the study of the assemblage of planets, moons,
dwarf planets, comets, asteroids, and other bodies orbiting the Sun,
as well as extrasolar planets. The
Solar System has been relatively
well-studied, initially through telescopes and then later by
spacecraft. This has provided a good overall understanding of the
formation and evolution of this planetary system, although many new
discoveries are still being made.
The black spot at the top is a dust devil climbing a crater wall on
Mars. This moving, swirling column of Martian atmosphere (comparable
to a terrestrial tornado) created the long, dark streak.
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. Beyond
Neptune lies the
Kuiper Belt, and finally the Oort Cloud, which may extend as far as a
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 formed
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 26Al), or 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
The Ant planetary nebula. Ejecting gas from the dying central star
shows symmetrical patterns unlike the chaotic patterns of ordinary
Main article: Star
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
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 fuses its hydrogen fuel into helium 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. 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.
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 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
"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.
See also: Solar astronomy
Observed structure of the Milky Way's spiral arms
Main article: Galactic astronomy
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 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,
population I stars. The disk is surrounded by a 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 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
As the more massive stars appear, they transform the cloud into an H
II region (ionized atomic hydrogen) of glowing gas and plasma. The
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.
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.
Main article: Extragalactic astronomy
The study of objects outside our galaxy is a branch of astronomy
concerned with the formation and evolution of Galaxies, their
morphology (description) and classification, the observation of active
galaxies, and at a larger scale, the groups and clusters of galaxies.
Finally, the latter is important for the understanding of the
large-scale structure of the cosmos.
Most galaxies are organized into distinct shapes that allow for
classification schemes. They are commonly divided into spiral,
elliptical and Irregular galaxies.
As the name suggests, an elliptical galaxy has the cross-sectional
shape of an ellipse. The stars move along random orbits with no
preferred direction. These galaxies 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 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 and
one of our nearest galaxy neighbors, the Andromeda Galaxy, are spiral
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
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 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 shorter frequency, high-energy
radiation include Seyfert galaxies, Quasars, and Blazars. Quasars are
believed to be the most consistently luminous objects in the known
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 filaments and walls, leaving large voids
view • discuss • edit
Earliest universe (−13.80)
Omega Centauri forms
Andromeda Galaxy forms
Milky Way Galaxy
spiral arms form
Alpha Centauri forms
Earliest sexual reproduction
Axis scale: billion years
Human timeline and Life timeline
Main article: Physical cosmology
Cosmology (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.
Hubble Extreme Deep Field
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 13.8 billion years 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
Universe experienced a very rapid cosmic inflation, which
homogenized the starting conditions. Thereafter, nucleosynthesis
produced the elemental abundance of the early Universe. (See also
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
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
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
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
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 archaeological and anthropological
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 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.
Main article: Amateur astronomy
Amateur astronomers can build their own equipment, and hold star
parties and gatherings, such as Stellafane.
Astronomy is one of the sciences to which amateurs can contribute the
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 Sun, the
Moon, planets, stars, comets, meteor showers, 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 which interest them.
Most amateurs work at visible wavelengths, but a small minority
experiment with wavelengths outside the visible spectrum. This
includes the use of infrared filters on conventional telescopes, and
also the use of radio telescopes. The pioneer of amateur radio
astronomy was Karl Jansky, who started observing the sky at radio
wavelengths in the 1930s. A number of amateur astronomers use either
homemade telescopes or use radio telescopes which were originally
built for astronomy research but which are now available to amateurs
(e.g. the One-Mile Telescope).
Amateur astronomers continue to make scientific contributions to the
field of astronomy 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
Unsolved problems in astronomy
See also: List of unsolved problems in physics
Although the scientific discipline of astronomy has made tremendous
strides in understanding the nature of the
Universe and its contents,
there remain some important unanswered questions. Answers to these may
require the construction of new ground- and space-based instruments,
and possibly new developments in theoretical and experimental physics.
What is the origin of the stellar mass spectrum? That is, why do
astronomers observe the same distribution of stellar masses –
the initial mass function – apparently regardless of the
initial conditions? A deeper understanding of the formation of
stars and planets is needed.
Is there other life in the Universe? Especially, is there other
intelligent life? If so, what is the explanation for the Fermi
paradox? The existence of life elsewhere has important scientific and
philosophical implications. Is the
Solar System normal or
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
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?
Outline of astronomy
Outline of astronomy and Glossary of astronomy
International Year of Astronomy
List of astronomy acronyms
List of Russian astronomers and astrophysicists
Outline of space science
Universe: The Infinite Frontier (television series)
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