Astronomy
From Wikipedia, the free encyclopedia
Astronomy is a
natural science which is the study of
celestial objects (such as
stars,
galaxies,
planets,
moons, and
nebulae), the
physics,
chemistry, and
evolution of such objects, and phenomena that originate outside the
atmosphere of Earth, including
supernovae explosions,
gamma ray bursts, and
cosmic microwave background radiation. A related but distinct subject,
cosmology, is concerned with studying the
universe as a whole.
[1]
Astronomy is one of the oldest sciences. Prehistoric cultures have left astronomical artifacts such as the
Egyptian monuments and
Nubian monuments, and early civilizations such as the
Babylonians,
Greeks,
Chinese,
Indians,
Iranians and
Maya performed methodical observations of the
night sky. 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 and the making of
calendars, but professional astronomy is nowadays often considered to be synonymous with
astrophysics.
[2]
During the 20th century, the field of professional astronomy split into
observational and
theoretical
branches. 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 the observational results and
observations being used to confirm theoretical results.
Astronomy is one of the few sciences where amateurs can still play an
active role, especially in the discovery and observation of transient
phenomena and
Amateur astronomers have made and contributed to many important astronomical discoveries.
Etymology
Astronomy (from the
Greek words
astron (
ἄστρον), "star" and
-nomia from
nomos (
νόμος),
"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.
[3] Although the
two fields share a common origin they are now entirely distinct.
[4]
Use of terms "astronomy" and "astrophysics"
Generally, either the term "astronomy" or "astrophysics" may be used to refer to this subject.
[5][6][7]
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"
[8]
and "astrophysics" refers to the branch of astronomy dealing with "the
behavior, physical properties, and dynamic processes of celestial
objects and phenomena".
[9] 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.
[10]
However, since most modern astronomical research deals with subjects
related to physics, modern astronomy could actually be called
astrophysics.
[5]
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,
[6] and many professional astronomers have physics rather than astronomy degrees.
[7] One of the leading scientific journals in the field is the European journal named
Astronomy and Astrophysics. The leading American journals are
The Astrophysical Journal and
The Astronomical Journal.
History
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.
[11]
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,
China,
Egypt,
Greece,
India, 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 of the 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.
[12]
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.
[13] The
Babylonians discovered that
lunar eclipses recurred in a repeating cycle known as a
saros.
[14]
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.
[15] In the 3rd century BC,
Aristarchus of Samos estimated the
size and distance of the Moon and Sun, and was the first to propose a
heliocentric model of the solar system.
[16] 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.
[17]
Hipparchus also created a comprehensive catalog of 1020 stars, and most
of the constellations of the northern hemisphere derive from Greek
astronomy.
[18] The
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.
[19]
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.
[20][21][22] 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.
[23] The
SN 1006 supernova, the brightest
apparent magnitude stellar event in recorded history, was observed by the Egyptian Arabic astronomer
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.
[24][25] It is also believed that the ruins at
Great Zimbabwe and
Timbuktu[26] may have housed an astronomical observatory.
[27] Europeans had previously believed that there had been no astronomical observation in pre-colonial Middle Ages
sub-Saharan Africa but modern discoveries show otherwise.
[28][29][30][31]
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 used telescopes to enhance his observations.
[32]
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.
[33] 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.
[32]
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.
[34] The distance to a star was first announced in 1838 when the
parallax of
61 Cygni was measured by
Friedrich Bessel.
[35]
During the 18–19th centuries, 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.
[36]
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.
[24]
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.
[37] 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.
Space telescopes have enabled measurements in parts of the electromagnetic spectrum normally blocked or blurred by the atmosphere.
Observational astronomy
In astronomy, the main source of information about
celestial bodies and other objects is visible
light or more generally
electromagnetic radiation.
[38] 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.
Radio astronomy
Radio astronomy studies radiation with
wavelengths greater than approximately one millimeter.
[39] 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.
[39]
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 the result of
synchrotron radiation, which is produced when
electrons orbit
magnetic fields.
[39] Additionally, a number of
spectral lines produced by
interstellar gas, notably the
hydrogen spectral line at 21 cm, are observable at radio wavelengths.
[10][39]
A wide variety of objects are observable at radio wavelengths, including
supernovae, interstellar gas,
pulsars, and
active galactic nuclei.
[10][39]
Infrared astronomy
ALMA Observatory is one of the highest observatory sites on Earth.
[40]
Infrared astronomy is founded on the detection and analysis of
infrared
radiation (wavelengths longer than red light). 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. Longer infrared wavelengths
can penetrate clouds of dust that block visible light, allowing the
observation of young stars 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.
[41][42] With the exception of
wavelengths
close to visible light, infrared 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 or in space.
[43]
Some molecules radiate strongly in the infrared. This allows the study
the chemistry of space; more specifically it can detect water in comets.
[44]
Optical astronomy
Historically, optical astronomy, also called visible light astronomy, is the oldest form of astronomy.
[45]
Optical 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 detectors 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),
[45] that same equipment can be used to observe some
near-ultraviolet and
near-infrared radiation.
Ultraviolet astronomy
Ultraviolet astronomy refers to observations at
ultraviolet wavelengths between approximately 100 and 3200 Å (10 to 320 nm).
[39]
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 (
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.
[39] However, as ultraviolet light is easily absorbed by
interstellar dust, an appropriate adjustment of ultraviolet measurements is necessary.
[39]
X-ray astronomy
X-ray astronomy is the study of
astronomical objects at
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 10
7 (10 million)
kelvins, and
thermal emission from thick gases above 10
7 Kelvin.
[39] Since X-rays are absorbed by the
Earth's atmosphere, all X-ray observations must be performed 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.
[39]
X-rays were first observed and documented in 1895 by
Wilhelm Conrad Röntgen, a
German scientist who found them when experimenting with
vacuum tubes.
Through a series of experiments, Röntgen was able to discover the
beginning elements of radiation. The "X", in fact, holds its own
significance, as it represents Röntgen's inability to identify exactly
the type of radiation.
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.
[39]
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.
[46]
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.
[39]
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.
[39] Cosmic rays,
which consist of very high energy particles that can decay or be
absorbed when they enter the Earth's atmosphere, result in a cascade of
particles which can be detected by current observatories.
[47] Additionally, some future
neutrino detectors may also be sensitive to the particles produced when cosmic rays hit the Earth's atmosphere.
[39]
Gravitational wave astronomy is an emerging new field of astronomy which aims to use
gravitational wave detectors to collect observational data about compact objects. A few observatories have been constructed, such as the
Laser Interferometer Gravitational Observatory LIGO, but
gravitational waves are extremely difficult to detect.
[48]
Combining observations made using electromagnetic radiation,
neutrinos or gravitational waves with those made using a different
means, which shall give complementary information, is known as
multi-messenger astronomy.
[49]
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 (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, and potential collisions, with the Earth.
[50]
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
radial velocity and
proper motion
plot the movement of these systems through the Milky Way galaxy.
Astrometric results are the basis used to calculate the distribution of
dark matter in the galaxy.
[51]
During the 1990s, the measurement of the
stellar wobble of nearby stars was
used to detect large
extrasolar planets orbiting nearby stars.
[52]
Theoretical astronomy
Theoretical astronomers use several tools including
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 reveal the existence of phenomena and effects otherwise
unobserved.
[53][54]
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.
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 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.
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,
[55] as their discovery and controversy originated during the study of the galaxies.
Specific subfields
Solar astronomy
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 fluctuation in
sunspot numbers. Sunspots are regions of lower-than- average temperatures that are associated with intense magnetic activity.
[56]
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.
[57] The
Maunder minimum, for example, is believed to have caused the
Little Ice Age phenomenon during the
Middle Ages.
[58]
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 are the outer layers that 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.
[56]
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. This solar wind interacts with the
magnetosphere of the Earth to create the
Van Allen radiation belts about the Earth, as well as the
aurora where the lines of the
Earth's magnetic field descend into the
atmosphere.
[59]
Planetary science
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.
[60]
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.
[61] Beyond Neptune lies the
Kuiper Belt, and finally the
Oort Cloud, which may extend as far as a light-year.
The planets were formed 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, the
leading hypothesis for how the Moon was formed.
[62]
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 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.
[63]
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.
[64]
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.
[65] 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.
[66]
Almost all elements heavier than
hydrogen and
helium were
created inside the cores of stars.
[65]
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.
[67]
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;
[68] while smaller stars form a
white dwarf as it ejects matter that forms a
planetary nebulae.
[69] 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.
[70]
Close binary stars can follow more complex evolutionary paths, such as
mass transfer onto a white dwarf companion that can potentially cause a
supernova.
[71] 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.
[72]
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 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.
[73]
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 protostars.
[66]
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.
[74]
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.
[75]
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.
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; and 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.
[76]
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 where
massive young stars produce a blue tint. Spiral galaxies are typically
surrounded by a halo of older stars. Both the
Milky Way and our nearest galaxy neighbor, 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 emitts 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 universe.
[77]
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 between.
[78]
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.
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
[79] to its present condition.
[80] The concept of the big bang can be traced back to the discovery of the
microwave background radiation in 1965.
[80]
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.
[80] (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.
[81]
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.
[82]
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.
[83]
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.
[84]
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
archaeological and
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 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
Amateur astronomers can build their own equipment, and can hold star parties and gatherings, such as
Stellafane.
Astronomy is one of the sciences to which amateurs can contribute the most.
[85]
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. 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 Herschal 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.
[86][87]
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).
[88][89]
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. There are hundreds of local astronomy clubs
throughout the world and many help their members set up and complete
observational programs such as ones to observe all the Messier or
Hershel catalogue objects.Improvements in digital technology have
allowed amateurs to make impressive advances in the field of
astrophotography.
[90][91][92]
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?[93] 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.[94][95] Is the Solar System normal or atypical?
- What caused the Universe to form? Is the premise of the Fine-tuned universe hypothesis correct? If so, could this be the result of cosmological natural selection? What caused the cosmic inflation that produced our homogeneous universe? Why is there a baryon asymmetry?
- 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.[96] What will be the ultimate fate of the universe?[97]
- 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?[1]