Astronomy

Encyclopedia : A : AS : AST : Astronomy

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For other uses, see Astronomy (disambiguation)}}}.

Astronomy (Greek: αστρονομία = άστρον + νόμος, astronomia = astron + nomos, literally, "law of the stars") is the science of celestial objects (e.g., stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (e.g., auroras and cosmic background radiation). It is concerned with the evolution, physics, chemistry, and motion of celestial objects as well as the formation and development of the universe. Astronomical observations can be used to test fundamental theories in physics, such as general relativity. Theoretical astrophysics complements observational astronomy in that it seeks to explain astronomical phenomena.

Astronomy is one of the oldest sciences. Astronomers of ancient Greece practiced a scientific methodology, and advanced observation techniques may have been known much earlier (see archaeoastronomy). Historically, amateur astronomers have contributed to many important astronomical discoveries, and astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and observation of transient phenomena.

Modern astronomy is not to be confused with astrology, the belief system that claims human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin, they are fundamentally different: astronomers employ the scientific method, whereas astrologers do not.

Divisions

In Babylon and ancient Greece, astronomy consisted largely of astrometry, measuring the positions of stars and planets in the sky. Later, the work of astronomers Kepler and Newton led to the development of celestial mechanics, and astronomy focussed on mathematically predicting the motions of gravitationally interacting celestial bodies. This was applied to solar system objects in particular. Today, the motions and positions of objects are more easily determined, and modern astronomy concentrates on observing and understanding the physical nature of celestial objects.

Since the twentieth century, the field of professional astronomy has split into observational astronomy and theoretical astrophysics. Observational astronomy is concerned with acquiring data, which involves building and maintaining instruments as well as processing the results; this branch is sometimes referred to as "astrometry" or simply "astronomy". Theoretical astrophysics is concerned with ascertaining the observational implications of computer or analytic models.

The fields of astronomy can also be categorized by the subject or problem addressed, or by the method of data collection.

By subject or problem addressed

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 in the sky.

Careful measurement of the positions of the planets has led to a solid understanding of gravitational perturbations and an ability to predict the 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.

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 provides an absolute baseline for the properties of more distant stars, since their properties can be compared. Measurements of radial velocity and proper motion show the kinematics of these systems through the Milky Way galaxy. Astrometric results are also used to measure the distribution of dark matter in the galaxy.

During the 1990s, the astrometric technique of measuring the stellar wobble has led to the discovery of large extrasolar planets orbiting nearby stars.

Solar astronomy

The most frequently studied star is the Sun, a typical main sequence dwarf star of stellar class G2 V, and about 4.7 Gyr in age. The Sun is not considered a variable star, but it does undergo periodic changes in activity known as the sunspot cycle. This is an 11-year fluctuation in sunspot numbers. Sunspots are regions of lower than average temperature that are associated with intense magnetic activity.

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. 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, then by the super-heated corona.

At the center of the Sun is the core region, a volume of sufficient temperature and pressure for nuclear fusion to occur. Above the core is the radiation zone, where the plasma conveys the energy flux by means of radiation. The outer layers form a convection zone where the gas material transports energy primarily through physical displacement of the gas. It is believed that this convection zone creates the magnetic activity that generates sun spots.

A solar wind of plasma particles constantly streams outward from the Sun until it reaches the heliopause. This solar wind interacts with the magnetosphere of the Earth to create the Van Allen radiation belts, as well as the aurora where the lines of the Earth's magnetic field descend into the atmosphere.

Planetary science

This astronomical field examines the assemblage of planets, moons, minor planets, comets, asteroids, and other bodies orbiting the Sun. More recently it has been expanded to examine extrasolar planets.

The solar system has been relatively well-studied. In part this is due to its close proximity to us, as it permits close scrutiny by spacecraft. As a result astronomers have gained a good understand of the formation and evolution of this system of planets, although new discoveries are still being made.

The solar system is typically sub-divided into the four inner terrestrial planets, consisting of Mercury, Venus, Earth, Mars; the asteroid belt; and the four outer gas giants: Jupiter, Saturn, Uranus and Neptune. There is some debate as to whether the icy bodies in the outer solar system, such as Pluto, should be considered planets, and this issue remains unresolved.

The solar system is believed to have formed from a protoplanetary disk that surrounded the early Sun. Through a process that included accretion and gravitational attraction, the disk formed clumps of matter that assembled into protoplanets. The radiation pressure of the solar wind then expelled most of the remaining matter, while these larger components collided to form the planets and moons. This early period of intense bombardment created many of the impact crater features that are seen on the Moon. Those planets with sufficient mass also accumulated a gaseous atmosphere.

Planetary geology is the study of the geological processes on various planets. Above a certain radius, a planetary body will collapse under the force of its own gravity to form a spheroid. Internal heating will segregate the material constituents of a planet or large moon, forming a stony or metallic core surrounded by a mantle and outer surface. This core may include solid and liquid regions, and some planetary cores generate their own magnetic field.

Smaller bodies such as moons will cool more rapidly and their geological activity will cease, with the exception of impact cratering. The Earth and Venus, however, retain sufficient internal heat to maintain geological activity. This allows for volcanic activity and other tectonic forces.

Stellar astronomy

The study of stars and stellar evolution is a major field of astronomy, and is fundamental to our understanding of the universe. Astronomers have gained a deep understanding of the physics of stars through observation and by means of computer simulations of the interior.

Star formation occurs in dense regions of dust and gas, known as giant molecular clouds. These collapse under the influence of gravity to form a protostar. If the core region of the star acquires sufficient temperature and pressure, it begins to undergo nuclear fusion and becomes a main-sequence star.

The type of star that results is almost entirely dependent on its starting mass. The more massive the star, the greater its surface temperature 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. Fusion of helium requires a higher core temperature, so the star both expands in size and increases in temperature. The resulting red giant enjoys a much shorter life span than the hydrogen burning phase, and the star undergoes a series of shorter and shorter evolutionary phases as it fuses increasingly heavier elements.

The final fate of the star depends on its mass, with stars of mass greater than 1.4 times the Sun becoming supernovae, while smaller stars will form planetary nebulae and evolve into white dwarfs. The remnant of a supernova is a dense neutron star, or, if the stellar mass was at least three times that of the Sun, a black hole.

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 region, 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 out from the core. This is a region of active star formation that contains many younger, population II stars. The disk is surrounded by a speroid halo of older, population I 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 irregular dark nebulae, which concentrate and collapse (in volumes determined by the Jeans length) to form compact protostars.

As the more massive stars appear, they transform the cloud into an H II region of glowing gas and plasma. The stellar wind and supernova explosions from these stars eventually serve to disperse the cloud, often leaving behind one or more young open clusters of stars. These gradually disperse to join the population of stars in the Milky Way.

Kinematic studies of matter in the Milky Way have demonstrated that the galaxy has more mass than can be accounted for by the visible matter. A dark matter halo appears to dominate the mass, although the nature of this dark matter remains undetermined.

Galaxies and clusters

The study of objects outside our galaxy is a branch of astronomy concerned with the formation and evolution of Galaxies, their morphology and classification, the examination of active galaxies and the groups and clusters of galaxies. The later 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. 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. These arems are dusty regions of star formation, giving them a blue tint due to the presence of young, hot stars. These galaxies are typically surrounded by a halo of older, population II stars. The Andromeda Galaxy is an example of a spiral galaxy that is part of the Local Group of galaxies.

Another prominent type is the elliptical galaxy. As the name suggests, these are shaped in an ellipse. The motions of the stars within these galaxies is random, and there is little or no interstellar dust, few star-forming regions and generally older stars. Elliptical galaxies tend to lie near the cores of galactic clusters and are believed to have formed through mergers of large galaxies.

Irregular galaxies are neither spiral nor elliptical in form, and are generally chaotic in appearance. These form about a quarter of all galaxies, and are believed to have been deformed through some type of gravitational interaction.

An active galaxy is a formation that is emitting a significant amount of its energy from a source other than stars, dust and gas. Most such galaxies are powered by a compact region at the core, usually thought to be a supermassive black hole.

A radio galaxy is an active galaxy that is very luminous in the radio portion of the spectrum, and is emitting immense plumes or lobes of gas.

Active galaxies that emit high-energy radiation include Seyfert galaxies, Quasars, and Blazars. Most active galaxies are ellipticals and are believed to be powered by a supermassive black hole that are emitting radiation due to infalling material. Quasars are believed to be the most consistently luminous objects in the known universe.

The large-scale structure of the cosmos appears to be represented by groups and clusters of galaxies. This structure is organized in a hierarchy of groupings, with the largest known being the superclusters. The collective matter is formed into filaments and walls, leaving large voids in between.

Cosmology

Observations of the large-scale structure of the universe, a branch known as physical cosmology, have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the big bang, wherein our universe began at a single point in time and thereafter expanded over the course of 13.7 Gyr to its present condition. The concept of the big bang can be traced back to the discovery of the microwave background radiation in 1965.

In the course of this expansion, the universe underwent several evolutionary stages. In the very early moments, it is theorized that the universe underwent a very rapid cosmic inflation, which homogenized the starting conditions. Thereafter nucleosynthesis produced the primordial elements.

Once the universe had expanded sufficiently, the first atoms formed and thereafter space became transparent to radiation. The left-over radiation from this period formed the cosmic microwave background radiation. The expanding universe then underwent a dark age due to the lack of stellar energy sources.

A hierarchical structure of matter formed from minute variations in the mass density. Matter accumulated in the densest regions, forming clouds of gas and the earliest stars. These massive stars are believed to have created many of the heavy elements in the early universe.

Gravitational aggregations clustered into filaments, leaving voids in the gaps. Gradually organizations of gas and dust merged to form the first primitive galaxies. Over time these pulled in more matter, and were often organized into groups and clusters of galaxies, then into larger-scale superclusters.

Fundamental to the structure of the universe is the existence of dark matter and dark energy. These are now thought to be the dominant components, forming 96% of the density of the universe. So much effort is being spent to try and understand the physics of these components.

Interdisciplinary studies

Astronomy has developed significant interdisciplinary links with other major scientific fields. These include:

• Astrophysics: the study of the physics of the universe, including the physical properties (luminosity, density, temperature, chemical composition) of astronomical objects.
• Astrobiology: the study of the advent and evolution of biological systems in the universe.
• Archaeoastronomy: the study of ancient or traditional astronomies in their cultural context, utilising archaeological and anthropological evidence.
• Astrochemistry: the study of the chemicals found in outer space, usually in molecular gas clouds, and their formation, interaction and destruction. As such, it represents an overlap of the disciplines of astronomy and chemistry.

By methods of data collection

In astronomy, information is mainly received from the detection and analysis of light and other forms of electromagnetic radiation. Other cosmic rays are also observed, and several experiments are designed to detect gravitational waves in the near future. Neutrino detectors have been used to observe solar neutrinos, and neutrino emissions from supernovae have also been detected.

A traditional division of astronomy is given by the region of the electromagnetic spectrum observed. At the low frequency end of the spectrum, radio astronomy detects radiation of millimeter to dekameter wavelength. The radio telescope receivers are similar to those used in radio broadcast transmission but much more sensitive. Microwaves form the millimeter end of the radio spectrum and are important for studies of the cosmic microwave background radiation.

Infrared astronomy deals with the detection and analysis of infrared radiation (wavelengths longer than red light). The most common tool is the telescope but using a detector which is sensitive to the infrared. Infrared radiation is heavily absorbed by atmospheric water vapor, so infrared observatories have to be located in high, dry places or in outer space. Space telescopes in particular avoid atmospheric thermal emission, atmospheric opacity, and the negative effects of astronomical seeing at infrared and other wavelengths. Infrared is particularly useful for observation of galactic regions cloaked by dust.

Historically, most astronomical data has been collected through optical astronomy. This is the portion of the spectrum that uses optical components (mirrors, lenses, CCD detectors and photographic films) to observe light from near infrared to near ultraviolet wavelengths. Visible light astronomy (using wavelengths that can be detected with the eyes, about 400 - 700 nm) falls in the middle of this range. The most common tool is the telescope, with electronic imagers and spectrographs.

More energetic sources are observed and studied in high-energy astronomy, which includes X-ray astronomy, gamma ray astronomy, and extreme UV (ultraviolet) astronomy, as well as studies of neutrinos and cosmic rays.

Optical and radio astronomy can be performed with ground-based observatories, because the Earth's atmosphere is transparent at the wavelengths being detected. The atmosphere is opaque at the wavelengths of X-ray astronomy, gamma-ray astronomy, UV astronomy and (except for a few wavelength "windows") far infrared astronomy, so observations must be carried out mostly from balloons or space observatories. Powerful gamma rays can, however be detected by the large air showers they produce, and the study of cosmic rays can also be regarded as a branch of astronomy.

Planetary astronomy has benefited from direct observation in the form of spacecraft and sample return missions. These include fly-by missions with remote sensors, landing vehicles that can perform experiments on the surface materials, impactors that allow remote sensing of buried materials, and sample return missions that allow direct laboratory examination.

History

In early times, astronomy only comprised the observation and predictions of the motions of the naked-eye objects. Aristotle said that the Earth was the center of the Universe and everything rotated around it in orbits that were perfect circles. Aristotle had to be right because people thought that Earth had to be in the center with everything rotating around it because the wind would otherwise scatter leaves, and birds would only fly in one direction. For a long time, people thought that Aristotle was right, but it is probable that Aristotle accidentally did more to hinder our knowledge than help it.

The first ancestor of an astronomy computer, was the Antikythera mechanism, an ancient Greek device for calculating the movements of planets, dating from about 80 BC. It was discovered in an ancient shipwreck off the Greek island of Antikythera, between Kythera and Crete. The device became famous for its use of a differential gear, previously believed to have been invented in the 16th century and the miniaturization and complexity of its parts, comparable to a clock made in the 18th century. The original mechanism is displayed in the Bronze collection of the National Archaeological Museum of Athens, accompanied by a replica. Another replica is on display at the American Computer Museum in Bozeman, Montana.

The Hindu Vedic text, Rigveda refers to the 27 constellations associated with the motions of the sun and also the 12 zodiacal divisions of the sky. The ancient Greeks made important contributions to astronomy, among them the definition of the magnitude system. The Bible contains a number of statements on the position of the earth in the universe and the nature of the stars and planets, most of which are poetic rather than literal; see Biblical cosmology. In 500 AD, Aryabhata presented a mathematical system that described the earth as spinning on its axis and considered the motions of the planets with respect to the sun.

East Asia's first astronomical observatory was developed in Silla, one of Three Kingdoms of Korea, under the reign of Queen Seondeok of Silla. It was termed Cheomsongdae, and is one of the oldest surviving scientific installations on Earth. It means star-gazing tower in Korean.

Although classical astronomy was one of the seven key subjects taught at medieval universities in Europe, observational astronomy was mostly stagnant in medieval Europe until the XIII century, with astronomers and astrologers such as Johannes de Sacrobosco, in England, and Guido Bonatti from Forlì, in Italy.

Tycho Brahe's work is very famous, in the 16th Century. However, observational astronomy flourished in the Iranian world and other parts of Islamic realm. The late 9th century Persian astronomer al-Farghani wrote extensively on the motion of celestial bodies. His work was translated into Latin in the 12th century. In the late 10th century, a huge observatory was built near Tehran, Persia (now Iran), by the Persian astronomer al-Khujandi, who observed a series of meridian transits of the Sun, which allowed him to calculate the obliquity of the ecliptic. Also in Persia, Omar Khayyám performed a reformation of the calendar that was more accurate than the Julian and came close to the Gregorian. In the 15th century, Abraham Zacuto was responsible for the adaptations of astronomical theory to the practical needs of Portuguese caravel expeditions.

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 added the innovation of using telescopes to enhance his observations. Kepler was the first to devise a system that described correctly the details of the motion of the planets with the Sun at the center. However, Kepler did not succeed in formulating a theory behind the laws he wrote down, and parts of his discoveries are no longer considered significant, e.g., the spacing of planetary orbits to allow insertion of regular solids between adjacent planets' spheres. 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.

Stars were found to be faraway objects. With the advent of spectroscopy it was proved that they were similar to Earth's own sun, but with a wide range of temperatures, masses, and sizes. The existence of Earth's galaxy, the Milky Way, as a separate group of stars was only proven in the 20th century, along with the existence of "external" galaxies, and soon after, the expansion of the universe, seen in the recession of most galaxies from us. Modern astronomy has also discovered many exotic objects such as quasars, pulsars, blazars and radio galaxies, and has used these observations to develop physical theories which describe some of these objects in terms of equally exotic objects such as black holes and neutron stars. Physical cosmology made huge advances during the 20th century, with the model of the Big Bang heavily supported by the evidence provided by astronomy and physics, such as the cosmic microwave background radiation, Hubble's law, and cosmological abundances of elements.

Astronomical timelines

• Artificial satellites and space probes
• Astronomical maps, catalogues, and surveys
• Big Bang
• Black hole physics
• Cosmic microwave background astronomy
• Cosmology
• Galaxies, clusters of galaxies, and large scale structure
• Interstellar medium and intergalactic medium
• Discovery of natural satellites and planets
• Other background radiation fields
• Solar astronomy
• Solar system astronomy
• Stellar astronomy
• Telescopes, observatories, and observing technology
• White dwarfs, neutron stars, and supernovae

References and further reading

See also

• Astronomers and Astrophysicists
• Space science

External links

• [AbsoluteAstronomy.com]
• [The Amazing Sky]
• [Astronomy Picture of the Day]
• [Night Sky Info]
• [Sky & Telescope]
• [Southern Hemisphere Astronomy]

General subfields within the Natural sciences
Astronomy | Biology | Chemistry | Earth science | Ecology | Physical science | Physics

 

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