Not to be confused with astrology
, a pseudoscience.
: ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science
that studies celestial objects
. It uses mathematics
, and chemistry
in order to explain their origin and evolution
. Objects of interest include planets
, and comets
. Relevant phenomena include supernova
explosions, gamma ray bursts
, and cosmic microwave background radiation
. More generally, astronomy studies everything that originates outside Earth's atmosphere
is a branch of astronomy that studies the universe
as a whole.
Astronomy is one of the oldest natural sciences. The early civilizations in recorded history
made methodical observations of the night sky
. These include the Babylonians
, and many ancient indigenous peoples of the Americas
. In the past, astronomy included disciplines as diverse as astrometry
, celestial navigation
, observational astronomy
, and the making of calendars
. Nowadays, professional astronomy is often said to be the same as astrophysics
Professional astronomy is split into observational
branches. Observational astronomy is focused on acquiring data from observations of astronomical objects. This data is then analyzed using basic principles of physics. Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. These two fields complement each other. Theoretical astronomy seeks to explain observational results and observations are used to confirm theoretical results.
Astronomy is one of the few sciences in which amateurs play an active role
. This is especially true for the discovery and observation of transient events
. Amateur astronomers
have helped with many important discoveries, such as finding new comets.
Astronomical Observatory, New South Wales, Australia 1873
(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"
"Astronomy" and "astrophysics" are synonyms.
Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties,"
while "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena".
In some cases, as in the introduction of the introductory textbook The Physical Universe
by Frank Shu
, "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject.
However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics.
Some fields, such as astrometry, are purely astronomy rather than also astrophysics. Various departments in which scientists carry out research on this subject may use "astronomy" and "astrophysics", partly depending on whether the department is historically affiliated with a physics department,
and many professional 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 & Astrophysics
A celestial map from the 17th century, by the Dutch cartographer Frederik de Wit
In early historic times, astronomy only consisted of the observation and predictions of the motions of objects visible to the naked eye. In some locations, early cultures assembled massive artifacts that possibly had some astronomical purpose. In addition to their ceremonial uses, these observatories
could be employed to determine the seasons, an important factor in knowing when to plant crops and 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
, and Central America
, astronomical observatories were assembled and ideas on the nature of the Universe began to develop. Most early astronomy consisted of mapping the positions of the stars and planets, a science now referred to as astrometry
. 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
The Suryaprajnaptisūtra, a 6th-century BC astronomy text of Jains
at The Schoyen Collection, London. Above: its manuscript from c.
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.
discovered that lunar eclipses
recurred in a repeating cycle known as a saros
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
estimated the size and distance of the Moon and Sun
, and he proposed a model of the Solar System
where the Earth and planets rotated around the Sun, now called the heliocentric
In the 2nd century BC, Hipparchus
, calculated the size and distance of the Moon and invented the earliest known astronomical devices such as the astrolabe
Hipparchus also created a comprehensive catalog of 1020 stars, and most of the constellations
of the northern hemisphere derive from Greek astronomy.
The Antikythera mechanism
(c. 150–80 BC) was an early analog computer
designed to calculate the location of the Sun
, 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.
Medieval Europe housed a number of important astronomers. Richard of Wallingford
(1292–1336) made major contributions to astronomy and horology, including the invention of the first astronomical clock, the Rectangulus
which allowed for the measurement of angles between planets and other astronomical bodies, as well as an equatorium
called the Albion
which could be used for astronomical calculations such as lunar
and planetary longitudes
and could predict eclipses
. Nicole Oresme
(1320–1382) and Jean Buridan
(1300–1361) first discussed evidence for the rotation of the Earth, furthermore, Buridan also developed the theory of impetus (predecessor of the modern scientific theory of inertia
) which was able to show planets were capable of motion without the intervention of angels. Georg von Peuerbach
(1423–1461) and Regiomontanus
(1436–1476) helped make astronomical progress instrumental to Copernicus's development of the heliocentric model decades later.
Astronomy flourished in the Islamic world
and other parts of the world. This led to the emergence of the first astronomical observatories
in the Muslim world
by the early 9th century.
In 964, the Andromeda Galaxy
, the largest galaxy
in the Local Group
, was described by the Persian Muslim astronomer Abd al-Rahman al-Sufi
in his Book of Fixed Stars
The SN 1006 supernova
, the brightest apparent magnitude
stellar event in recorded history, was observed by the Egyptian Arabic astronomer Ali ibn Ridwan
and Chinese astronomers
in 1006. Some of the prominent Islamic (mostly Persian and Arab) astronomers who made significant contributions to the science include Al-Battani
, Abd al-Rahman al-Sufi
, Abū Ishāq Ibrāhīm al-Zarqālī
, and the astronomers of the Maragheh
observatories. Astronomers during that time introduced many Arabic names now used for individual stars
For over six centuries (from the recovery of ancient learning during the late Middle Ages into the Enlightenment), the Roman Catholic Church gave more financial and social support to the study of astronomy than probably all other institutions. Among the Church's motives was finding the date for Easter.
's sketches and observations of the Moon
revealed that the surface was mountainous.
An astronomical chart from an early scientific manuscript, c. 1000
Improvements in the size and quality of the telescope led to further discoveries. The English astronomer John Flamsteed
catalogued over 3000 stars,
More extensive star catalogues were produced by Nicolas Louis de 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.
Significant advances in astronomy came about with the introduction of new technology, including the spectroscope
. Joseph von Fraunhofer
discovered about 600 bands in the spectrum of the Sun in 1814–15, which, in 1859, Gustav 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
, and sizes.
The existence of the Earth's galaxy, the Milky Way
, as its own group of stars was only proved in the 20th century, along with the existence of "external" galaxies. The observed recession of those galaxies led to the discovery of the expansion of the Universe
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
, and radio galaxies
. Physical cosmology
made huge advances during the 20th century. In the early 1900s the model of the Big Bang
theory was formulated, heavily evidenced by cosmic microwave background radiation
, 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.
The main source of information about celestial bodies
and other objects is visible light
, or more generally electromagnetic radiation
Observational astronomy may be categorized according to the corresponding region of the electromagnetic spectrum
on which the observations are made. Some parts of the spectrum can be observed from the Earth's surface, while other parts are only observable from either high altitudes or outside the Earth's atmosphere. Specific information on these subfields is given below.
Radio astronomy uses radiation with wavelengths
greater than approximately one millimeter, outside the visible range.
Radio astronomy is different from most other forms of observational astronomy in that the observed radio waves
can be treated as waves
rather than as discrete photons
. Hence, it is relatively easier to measure both the amplitude
of radio waves, whereas this is not as easily done at shorter wavelengths.
Observatory is one of the highest observatory sites on Earth. Atacama, Chile.
Infrared astronomy is founded on the detection and analysis of infrared
radiation, wavelengths longer than red light and outside the range of our vision. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets, circumstellar disks
or nebulae whose light is blocked by dust. The longer wavelengths of infrared can penetrate clouds of dust that block visible light, allowing the observation of young stars embedded in molecular clouds
and the cores of galaxies. Observations from the Wide-field Infrared Survey Explorer
(WISE) have been particularly effective at unveiling numerous Galactic protostars
and their host star clusters
With the exception of infrared wavelengths
close to visible light, such radiation is heavily absorbed by the atmosphere, or masked, as the atmosphere itself produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places on Earth or in space.
Some molecules radiate strongly in the infrared. This allows the study of the chemistry of space; more specifically it can detect water in comets.
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
Ultraviolet astronomy employs ultraviolet
wavelengths between approximately 100 and 3200 Å (10 to 320 nm).
Light at those wavelengths is absorbed by the Earth's atmosphere, requiring observations at these wavelengths to be performed from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue stars
) that are very bright in this wave band. This includes the blue stars in other galaxies, which have been the targets of several ultraviolet surveys. Other objects commonly observed in ultraviolet light include planetary nebulae
, supernova remnants
, and active galactic nuclei.
However, as ultraviolet light is easily absorbed by interstellar dust
, an adjustment of ultraviolet measurements is necessary.
X-ray 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
(10 million) kelvins
, and thermal emission from thick gases
Since X-rays are absorbed by the Earth's atmosphere
, all X-ray observations must be performed from high-altitude balloons
, or X-ray astronomy satellites
. Notable X-ray sources
include X-ray binaries
, supernova remnants
, elliptical galaxies
, clusters of galaxies
, and active galactic nuclei
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.
emitting sources are actually gamma-ray bursts
, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources. These steady gamma-ray emitters include pulsars, neutron stars
, and black hole
candidates such as active galactic nuclei.
Fields not based on the electromagnetic spectrum
In addition to electromagnetic radiation, a few other events originating from great distances may be observed from the Earth.
In neutrino astronomy
, astronomers use heavily shielded underground facilities
such as SAGE
, 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.
The combination of observations made using electromagnetic radiation, neutrinos or gravitational waves and other complementary information, is known as multi-messenger astronomy
Astrometry and celestial mechanics
One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in celestial navigation
(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 objects.
The measurement of stellar parallax
of nearby stars provides a fundamental baseline in the cosmic distance ladder
that is used to measure the scale of the Universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, as their properties can be compared. Measurements of the radial velocity
and proper motion
of stars allow astronomers to plot the movement of these systems through the Milky Way galaxy. Astrometric results are the basis used to calculate the distribution of speculated dark matter
in the galaxy.
Theoretical astronomers use several tools including analytical models
and computationalnumerical simulations
; each has its particular advantages. Analytical models of a process are better for giving broader insight into the heart of what is going on. Numerical models reveal the existence of phenomena and effects otherwise unobserved.
Theorists in astronomy endeavor to create theoretical models and from the results predict observational consequences of those models. The observation of a phenomenon predicted by a model allows astronomers to select between several alternate or conflicting models as the one best able to describe the phenomena.
Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency between the data and the model's results, the general tendency is to try to make minimal modifications to the model so that it produces results that fit the data. In some cases, a large amount of inconsistent data over time may lead to the total abandonment of a model.
A few examples of this process:
Astrophysics applies physics
to understand the measurements made by astronomy. Representation of the Observable Universe that includes images from Hubble
and other telescopes
is the branch of astronomy that employs the principles of physics and chemistry
"to ascertain the nature of the astronomical objects
, rather than their positions or motions in space".
Among the objects studied are the Sun
, other stars
, extrasolar planets
, the interstellar medium
and the cosmic microwave background
Their emissions are examined across all parts of the electromagnetic spectrum
, and the properties examined include luminosity
, and chemical
composition. Because astrophysics is a very broad subject, astrophysicists
typically apply many disciplines of physics, including mechanics
, statistical mechanics
, quantum mechanics
and particle physics
, and atomic and molecular physics
is the study of the abundance and reactions of molecules
in the Universe
, and their interaction with radiation
The discipline is an overlap of astronomy and chemistry
. The word "astrochemistry" may be applied to both the Solar System
and the interstellar medium
. The study of the abundance of elements and isotope
ratios in Solar System objects, such as meteorites
, is also called cosmochemistry
, while the study of interstellar atoms and molecules and their interaction with radiation is sometimes called molecular astrophysics. The formation, atomic and chemical composition, evolution and fate of molecular gas clouds
is of special interest, because it is from these clouds that solar systems form.
(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.
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 neutral atoms
formed from a sea of primordial ions, space became transparent to radiation, releasing the energy viewed today as the microwave background radiation. The expanding Universe then underwent a Dark Age due to the lack of stellar energy sources.
A hierarchical structure of matter began to form from minute variations in the mass density of space. Matter accumulated in the densest regions, forming clouds of gas and the earliest stars, the Population III stars
. These massive stars triggered the reionization
process and are believed to have created many of the heavy elements in the early Universe, which, through nuclear decay, create lighter elements, allowing the cycle of nucleosynthesis to continue longer.
Gravitational aggregations clustered into filaments, leaving voids in the gaps. Gradually, organizations of gas and dust merged to form the first primitive galaxies. Over time, these pulled in more matter, and were often organized into groups and clusters
of galaxies, then into larger-scale superclusters.
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.
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.
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 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 galaxies.
Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical. About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction.
An active galaxy is a formation that emits a significant amount of its energy from a source other than its stars, dust and gas. It is powered by a compact region at the core, thought to be a supermassive black hole that is emitting radiation from in-falling material.
A radio galaxy
is an active galaxy that is very luminous in the radio portion of the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies that emit shorter frequency, high-energy radiation include Seyfert galaxies
, and Blazars
. Quasars are believed to be the most consistently luminous objects in the known universe.
Observed structure of the Milky Way
's spiral arms
The 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
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.
, often referred to as 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
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.
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.
The Sun has steadily increased in luminosity by 40% since it first became a main-sequence star. The Sun has also undergone periodic changes in luminosity that can have a significant impact on the Earth.
The Maunder minimum
, for example, is believed to have caused the Little Ice Age
phenomenon during the Middle Ages
The visible outer surface of the Sun is called the photosphere
. Above this layer is a thin region known as the chromosphere
. This is surrounded by a transition region of rapidly increasing temperatures, and finally by the super-heated corona
At the center of the Sun is the core region, a volume of sufficient temperature and pressure for nuclear fusion
to occur. Above the core is the radiation zone
, where the plasma conveys the energy flux by means of radiation. Above that is the convection zone
where the gas material transports energy primarily through physical displacement of the gas known as convection. It is believed that the movement of mass within the convection zone creates the magnetic activity that generates sunspots.
A solar wind of plasma particles constantly streams outward from the Sun until, at the outermost limit of the Solar System, it reaches the heliopause
. As the solar wind passes the Earth, it interacts with the Earth's magnetic field
) 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 then descend into the atmosphere
Planetary science is the study of the assemblage of planets
, dwarf planets
, 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 the Sun's planetary system, although many new discoveries are still being made.
The Solar System is divided into the inner Solar System
(subdivided into the inner planets and the asteroid belt
), the outer Solar System
(subdivided into the outer planets and centaurs
), comets, the trans-Neptunian region (subdivided into the Kuiper belt
, and the scattered disc
) and the farthest regions (e.g., boundaries of the heliosphere
, and the Oort Cloud
, which may extend as far as a light-year). The inner terrestrial planets
consist of Mercury
, Earth, and Mars
. The outer giant planets
are the gas giants
) and the ice giants
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 the Moon
Once a planet reaches sufficient mass, the materials of different densities segregate within, during planetary differentiation
. This process can form a stony or metallic core, surrounded by a mantle and an outer crust. The core may include solid and liquid regions, and some planetary cores generate their own magnetic field
, which can protect their atmospheres from solar wind stripping.
A planet or moon's interior heat is produced from the collisions that created the body, by the decay of radioactive materials (e.g. uranium
, 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 impact cratering.
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
is the study of the advent and evolution of biological systems in the Universe, with particular emphasis on the possibility of non-terrestrial life. Astrostatistics
is the application of statistics to astrophysics to the analysis of a vast amount of observational astrophysical data.
The study of chemicals
found in space, including their formation, interaction and destruction, is called astrochemistry
. These substances are usually found in molecular clouds
, although they may also appear in low-temperature stars, brown dwarfs and planets. Cosmochemistry
is the study of the chemicals found within the Solar System, including the origins of the elements and variations in the isotope
ratios. Both of these fields represent an overlap of the disciplines of astronomy and chemistry. As "forensic astronomy
", finally, methods from astronomy have been used to solve problems of law and history.
Amateur 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 most.
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 that interest them.
Most amateurs work at visible wavelengths, but a small minority experiment with wavelengths outside the visible spectrum. This includes the use of infrared filters on conventional telescopes, and also the use of radio telescopes. The pioneer of amateur radio astronomy was Karl Jansky
, who started observing the sky at radio wavelengths in the 1930s. A number of amateur astronomers use either homemade telescopes or use radio telescopes which were originally built for astronomy research but which are now available to amateurs (e.g.
the One-Mile Telescope
Amateur astronomers continue to make scientific contributions to the field of astronomy and it is one of the few scientific disciplines where amateurs can still make significant contributions. Amateurs can make occultation measurements that are used to refine the orbits of minor planets. They can also discover comets, and perform regular observations of variable stars. Improvements in digital technology have allowed amateurs to make impressive advances in the field of astrophotography.
Unsolved problems in astronomy
Although the scientific discipline of astronomy has made tremendous strides in understanding the nature of the Universe and its contents, there remain some important unanswered questions. Answers to these may require the construction of new ground- and space-based instruments, and possibly new developments in theoretical and experimental physics.
- What is the origin of the stellar mass spectrum? That is, why do astronomers observe the same distribution of stellar masses—the initial mass function—apparently regardless of the initial conditions? A deeper understanding of the formation of stars and planets is needed.
- Is there other life in the Universe? Especially, is there other intelligent life? If so, what is the explanation for the Fermi paradox? The existence of life elsewhere has important scientific and philosophical implications. Is the Solar System normal or atypical?
- What is the nature of dark matter and dark energy? These dominate the evolution and fate of the cosmos, yet their true nature remains unknown.
- What will be the ultimate fate of the universe?
- How did the first galaxies form? How did supermassive black holes form?
- What is creating the ultra-high-energy cosmic rays?
- Why is the abundance of lithium in the cosmos four times lower than predicted by the standard Big Bang model?
- What really happens beyond the event horizon?
- ^ Unsöld, Albrecht; Baschek, Bodo (2001). Classical Astronomy and the Solar System – Introduction. p. 1.
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