Subdisciplines Of Modern Astronomy

one relatively newer goal of modern astronomy is to describe and characterize objects in the distant universe, with the Milky Way galaxy being recognized as a distinct and related group of stars only in the 20th century. This realization was followed by recognition of the expansion of the universe as described by Hubble's law, as well as distant objects such as quasars, pulsars, radio galaxies, black holes, and neutron stars.

The field of observational astronomy is based on data received from electromagnetic radiation from celestial objects and is divided into different sub-fields based on the wavelengths being studied. Radio astronomy deals with interpreting radiation received from celestial objects where the radiation has a wavelength greater than one millimeter, and is commonly used to study supernovae, interstellar gas, pulsars, and galactic nuclei. Radio astronomy uses wave theory to interpret these signals, since these long wavelengths are more easily assigned wavelengths and amplitudes than shorter wavelength forms of radiation. Most radio emissions from space received on Earth are a form of synchrotron radiation, produced when electrons oscillate in a magnetic field, although some is also associated with thermal emission from celestial objects, and interstellar gas is typically associated with 21-cm radio waves.

Infrared astronomy works with infrared wavelengths (longer than the wavelength of red light) and is used primarily to study areas such as planets and circumstellar disks that are too cold to radiate in the visible wavelengths of the electromagnetic spectrum. The longer infrared wavelengths are able to penetrate dust clouds, so infrared astronomy is also useful for observing processes such as star formation in molecular clouds and galactic cores blocked from observations in the visible wavelengths. Infrared astronomy observatories must be located in outer space or in high dry locations since the Earth's atmosphere is associated with significant infrared emissions.

optical astronomy, the oldest form of observational astronomy, uses light recorded from the visible wavelengths. Most optical astronomy is now completed by using digital recording apparatus, speeding analysis. ultraviolet astronomy (observations in the ultraviolet wavelengths) is used to study thermal radiation and the emission of spectral lines from hot blue stars, planetary nebula, supernova, and active galactic nuclei. Like infrared observatories, ultraviolet observation stations must be located in the upper atmosphere or in space, since ultraviolet rays are strongly absorbed by Earth's atmosphere.

The study and analysis of celestial objects at X-ray wavelengths is known as X-ray astronomy. X-ray emitters include some binary star systems, pulsars, supernova remnants, elliptical galaxies, galaxy clusters, and active galactic nuclei. X-rays are produced by celestial objects by thermal and synchrotron emission (generated by the oscillation of electrons around magnetic fields), but are absorbed by the Earth's atmosphere, so they must be observed from high-altitude balloons, rockets, or space. The study of the shortest wavelengths of the electromagnetic spectrum, known as gamma-ray astronomy, can so far be observed only by indirect observations of gamma ray bursts from objects including pulsars, neutron stars, and black holes near galactic nuclei.

See also astrophysics; black holes; constellation; cosmology; galaxies; galaxy clusters; origin and evolution of the universe; universe.

FURTHER READING

Chaisson, Eric, and steve McMillan. Astronomy Today. 6th ed. upper saddle River, N.J.: Addison-Wesley, 2007.

Comins, Neil F. Discovering the Universe. 8th ed. New

York: W. H. Freeman, 2008. snow, Theodore P. Essentials of the Dynamic Universe: An Introduction to Astronomy. 4th ed. st. Paul, minn.: West Publishing Company, 1991.

astrophysics Astrophysics is the branch of astronomy that examines the behavior, physical properties, and dynamic processes of celestial objects and phenomena. Astrophysics includes study of the luminosity, temperature, density, chemical composition, and other characteristics of celestial objects and aims at understanding the physical laws that explain these characteristics and behavior of celestial systems. Astrophysics is related to observational astronomy, as well as cosmology, the study of the theories related to the very large-scale structure and evolution of the universe. Astrophysicists study these systems using principles from different subfields in physics and astronomy, including thermodynamics, mechanics, electromagnetism, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics.

much of astrophysics is founded on formulating theories based on observational astronomy using principles of quantum mechanics and relativity. Theoretical astrophysicists use analytical models and complex computational and numerical models of the behavior of celestial systems to understand better the origin and evolution of the universe and to test for unpredicted phenomena. In general theoretical models of celestial behavior are tested with the observations and constraints from astronomical studies, and the agreement (or lack thereof) between the model and the observed behavior is used to refine the models of celestial evolution.

Current topics of research in astrophysics include celestial and stellar dynamics and evolution, the large-scale structure of the universe, cosmology and the origin and evolution of the universe, models for galaxy formation, the physics of black holes, quasars, and phenomena such as gravity waves, and implications and tests of models of general relativity.

See also astronomy; black holes; constellation; cosmology; galaxies; galaxy clusters; origin and evolution of the universe; universe.

FURTHER READING

Chaisson, Eric, and Steve McMillan. Astronomy Today. 6th ed. Upper Saddle River, N.J.: Addison-Wesley,

2007.

Comins, Neil F. Discovering the Universe. 8th ed. New

York: W. H. Freeman, 2008. Encyclopedia of Astronomy and Astrophysics. CRC Press, Taylor and Francis Group. Available online. URL: http://eaa.crcpress.com/. Accessed October 24, 2008. ScienceDaily. "Astrophysics News." ScienceDaily LLC. Available online. URL: http://www.sciencedaily.com/ news/space_time/astrophysics/. Accessed October 24,

2008.

Snow, Theodore P. Essentials of the Dynamic Universe: An Introduction to Astronomy. 4th ed. St. Paul, Minn.: West Publishing Company, 1991.

atmosphere Thin sphere around the Earth consisting of the mixture of gases we call air, held in place by gravity. The most abundant gas is nitrogen (78 percent), followed by oxygen (21 percent), argon (0.9 percent), carbon dioxide (0.036 percent), and minor amounts of helium, krypton, neon, and xenon. Atmospheric (or air) pressure is the force per unit area (similar to weight) that the air above a certain point exerts on any object below it. Atmospheric pressure causes most of the volume of the atmosphere to be compressed to 3.4 miles (5.5 km) above the Earth's surface, even though the entire atmosphere is hundreds of kilometers thick.

The atmosphere is always moving, because the equator receives more of the Sun's heat per unit area than the poles. The heated air expands and rises to where it spreads out, then it cools and sinks, and gradually returns to the equator. This pattern of global air circulation forms Hadley cells that mix air between the equator and midlatitudes. similar circulation cells mix air in the middle to high latitudes, and between the poles and high latitudes. The effects of the Earth's rotation modify this simple picture of the atmosphere's circulation. The Coriolis effect causes any freely moving body in the Northern Hemisphere to veer to the right, and toward the left in the southern Hemisphere. The combination of these effects forms the familiar trade winds, easterlies and westerlies, and doldrums.

The atmosphere is divided into several layers, based mainly on the vertical temperature gradients that vary significantly with height. Atmospheric pressure and air density both decrease more uniformly with height, and therefore are not a useful way to differentiate between different atmospheric layers.

The lower 36,000 feet (11,000 m) of the atmosphere, the troposphere, is where the temperature generally decreases gradually, at about 70°F per mile

-100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30°C I.I.I_J_J_J_I_L_L_L_I , I.I.I

-100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30°C I.I.I_J_J_J_I_L_L_L_I , I.I.I

Temperature

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Structure of the atmosphere showing various layers and temperature profile with height

(6.4°C per km), with increasing height above the surface. This is because the Sun heats the surface, which in turn warms the lower part of the troposphere. Most of the familiar atmospheric and weather phenomena occur in the troposphere.

Above the troposphere is a boundary region known as the tropopause, marking the transition into the stratosphere. The stratosphere in turn continues to a height of about 31 miles (50 km). The base of the stratosphere contains a region known as an isothermal, where the temperature remains the same with increasing height. The tropopause is generally at higher elevations in summer than winter and is also the region where the jet streams are located. Jet streams are narrow, streamlike channels of air that flow at high velocities, often exceeding 115 miles per hour (100 knots). Above about 12.5 miles (20 km), the isothermal region gives way to the upper stratosphere, where temperatures increase with height, back to near surface temperatures at 31 miles (50 km). The heating of the stratosphere is due to ozone at this level absorbing ultraviolet radiation from the sun.

The mesosphere lies above the stratosphere, extending between 31 and 53 miles (50-85 km). An isothermal region known as the stratopause separates the stratosphere and mesosphere. The air temperature in the mesosphere decreases dramatically above the stratopause, reaching a low of -130°F (-90°C) at the top of the mesosphere. The mesopause separates the mesosphere from the thermosphere, a hot layer where temperatures rise to more than 150°F (80°C). The relatively few oxygen molecules at this level absorb solar energy and heat quickly, and temperatures may change dramatically in this region in response to changing solar activity. The thermosphere continues to thin upward, extending to about 311 miles (500 km) above the surface. Above this level, atoms dissociate from molecules and are able to shoot outward and escape the gravitational pull of Earth. This far region of the atmosphere is sometimes referred to as the exosphere.

In addition to the temperature-based division of the atmosphere, it is possible to divide the atmosphere into different regions based on their chemical and other properties. Using such a scheme, the lower 46.5-62 miles (75-100 km) of the atmosphere may be referred to as the homosphere, which contains a well-mixed atmosphere with a fairly uniform ratio of gases from base to top. In the overlying hetero-sphere, the denser gases (oxygen and nitrogen) have settled to the base, whereas lighter gases (hydrogen and helium) have risen to greater heights, resulting in chemical differences with height.

The upper parts of the homosphere and the heterosphere contain a large number of electrically charged particles known as ions. Also called the ionosphere, this region strongly influences radio transmissions and the formation of the aurora borealis and aurora australis.

The production and destruction or removal of gases from the atmospheric system occur at approximately equal rates, although some gases are gradually increasing or decreasing in abundance, as described below. soil bacteria and other biologic agents remove nitrogen from the atmosphere, whereas decay of organic material releases nitrogen back to the atmosphere. However, decaying organic material removes oxygen from the atmosphere by combining with other substances to produce oxides. Animals also remove oxygen from the atmosphere by breathing, whereas photosynthesis returns oxygen to the atmosphere.

Water vapor is an extremely important gas in the atmosphere, but it varies greatly in concentration (0-4 percent) from place to place and from time to time. Though water vapor is normally invisible, it becomes visible as clouds, fog, ice, and rain when the water molecules coalesce into larger groups. In the liquid or solid state, water constitutes the precipitation that falls to Earth and is the basis for the hydro-logic cycle. Water vapor is also a major factor in heat transfer in the atmosphere. A kind of heat known as latent heat is released when water vapor turns into solid ice or liquid water. This heat, a major source of atmospheric energy, is a major contributor to the formation of thunderstorms, hurricanes, and other weather phenomena. Water vapor may also play a longer-term role in atmospheric regulation, as it is a greenhouse gas that absorbs a significant portion of the outgoing radiation from the Earth, causing the atmosphere to warm.

Carbon dioxide (C02), although small in concentration, is another very important gas in the Earth's atmosphere. Carbon dioxide is produced during decay of organic material, from volcanic out-gassing, deforestation, burning of fossil fuels, and cow, termite, and other animal emissions. Plants take up carbon dioxide during photosynthesis, and many marine organisms use it for their shells, made of CaC03 (calcium carbonate). When these organisms (for instance, phytoplankton) die, their shells can sink to the bottom of the ocean and be buried, removing carbon dioxide from the atmospheric system. Like water vapor, carbon dioxide is a greenhouse gas that traps some of the outgoing solar radiation reflected from the earth, causing the atmosphere to warm up. Because carbon dioxide is released by the burning of fossil fuels, its concentration is increasing in the atmosphere as humans consume more fuel. The concentration of Co2 in the atmosphere has increased by 15 percent since 1958, enough to cause considerable global warming. Estimates predict that the concentration of C02 will increase by another 35 percent by the end of the 21st century, further enhancing global warming.

other gases also contribute to the greenhouse effect, notably methane (CH4), nitrous oxide (N02) and chlorofluorocarbons (CFCs). Methane concentration is increasing in the atmosphere and is produced by the breakdown of organic material by bacteria in rice paddies and other environments, termites, and the stomachs of cows. Produced by microbes in the soil, No2 is also increasing in concentration by 1 percent every few years, even though it is destroyed by ultraviolet radiation in the atmosphere. Chlorofluo-rocarbons have received much attention since they are long-lived greenhouse gases increasing in atmospheric concentration as a result of human activity. Chlorofluorocarbons trap heat like other greenhouse gases, and also destroy ozone (03), a protective blanket that shields the Earth from harmful ultraviolet radiation. Chlorofluorocarbons were used widely as refrigerants and as propellants in spray cans. Their use has been largely curtailed, but since they have such a long residence time in the atmosphere, they are still destroying ozone and contributing to global warming, and will continue to do so for many years.

ozone is found primarily in the upper atmosphere where free oxygen atoms combine with oxygen molecules (02) in the stratosphere. The loss of ozone has been dramatic in recent years, even leading to the formation of "ozone holes" with virtually no ozone present above the Arctic and Antarctic in the fall. There is currently debate about how much of the ozone loss is human-induced by chlorofluorocarbon production, and how much may be related to natural fluctuations in ozone concentration.

Many other gases and particulate matter play important roles in atmospheric phenomena. For instance, small amounts of sulfur dioxide (S02) produced by the burning of fossil fuels mixes with water to form sulfuric acid, the main harmful component of acid rain. Acid rain is killing the biota of many natural lake systems, particularly in the northeastern United States in areas underlain by granitic-type rocks, and it is causing a wide range of other environmental problems across the world. other pollutants are major causes of respiratory problems and environmental degradation, and the major increase in particulate matter in the atmosphere in the past century has increased the hazards and health effects from these atmospheric particles.

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