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Ultraviolet Astronomy

Ultraviolet astronomy

Matter in the universe emits radiation (energy in the form of subatomic particles or waves) from all parts of the electromagnetic spectrum. The electromagnetic spectrum is the range of wavelengths produced by the interaction of electricity and magnetism. The electromagnetic spectrum includes light waves, radio waves, infrared radiation, ultraviolet radiation, X rays, and gamma rays.

Ultraviolet astronomy is the study of celestial matter that emits ultraviolet radiation. Ultraviolet waves are just shorter than the violet end (shortest wavelength) of the visible light spectrum. This branch of astronomy has provided additional information about stars (including the Sun), galaxies, the solar system, the interstellar medium (the "empty" space between celestial bodies), and quasars.

Words to Know

Electromagnetic radiation: Radiation that transmits energy through the interaction of electricity and magnetism.

Gamma rays: Short-wavelength, high-energy radiation formed either by the decay of radioactive elements or by nuclear reactions.

Infrared radiation: Electromagnetic radiation of a wavelength shorter than radio waves but longer than visible light that takes the form of heat.

Quasars: Extremely bright, starlike sources of radio waves that are the oldest known objects in the universe.

Radiation: Energy transmitted in the form of subatomic particles or waves.

Radio waves: Longest form of electromagnetic radiation, measuring up to 6 miles (9.7 kilometers) from peak to peak.

Ultraviolet radiation: Electromagnetic radiation of a wavelength just shorter than the violet (shortest wavelength) end of the visible light spectrum.

Wavelength: The distance between two troughs or two peaks in any wave.

X rays: Electromagnetic radiation of a wavelength just shorter than ultraviolet radiation but longer than gamma rays that can penetrate solids and produce an electrical charge in gases.

An ultraviolet telescope is similar to an optical telescope, except for a special coating on the lens. Due to Earth's ozone layer, which filters out most ultraviolet rays, ultraviolet astronomy is impossible to conduct on the ground. In order to function, an ultraviolet telescope must be placed on a satellite orbiting beyond Earth's atmosphere.

Information collected by ultraviolet telescopes

Beginning in the 1960s, a series of ultraviolet telescopes have been launched on spacecraft. The first such instruments were the eight Orbiting Solar Observatories placed into orbit between 1962 and 1975. These satellites measured ultraviolet radiation from the Sun. The data collected from these telescopes provided scientists with a much more complete picture of the solar corona, the outermost part of the Sun's atmosphere.

The Orbiting Astronomical Observatories (OAO) were designed to provide information on a variety of subjects, including thousands of stars, a comet, a nova in the constellation Serpus, and some galaxies beyond

the Milky Way. Between 1972 and 1980, OAO Copernicus collected information on many stars as well as the composition, temperature, and structure of interstellar gas.

The most successful ultraviolet satellite to date was the International Ultraviolet Explorer (IUE) launched in 1978. The IUE was a joint project of the United States, Great Britain, and the European Space Agency. With very sensitive equipment, the IUE studied planets, stars, galaxies, nebulae, quasars, and comets. It recorded especially valuable information from novae and supernovae. Although intended to function for only three to five years, the IUE operated until September 30, 1996, making it the longest-lived astronomical satellite.

The IUE was succeeded by the Extreme Utraviolet Explorer (EUEV), which was launched on June 7, 1992. The EUEV was designed to extend the spectral coverage of the IUE by being able to observe much shorter wavelengths. A third ultraviolet satellite, the Far Ultraviolet Spectroscopic Explorer (FUSE), was launched on June 24, 1999. This satellite also was designed to look farther into the ultraviolet (meaning to shorter wavelengths) than the IUE. With FUSE, astronomers hope to study high-energy processes in stars and galaxies in addition to exploring conditions in the universe as they existed only shortly after the big bang (theory that explains the beginning of the universe as a tremendous explosion from a single point that occurred 12 to 15 billion years ago).

[See also Electromagnetic spectrum; Galaxy; International Ultra violet Explorer; Telescope ]

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ultraviolet astronomy

ultraviolet astronomy, study of celestial objects by means of the ultraviolet radiation they emit, in the wavelength range from about 90 to about 350 nanometers. Ultraviolet (UV) line spectrum measurements are used to discern the chemical composition, densities, and temperatures of interstellar gas and dust, and the temperature and composition of hot young stars. UV observations can also provide essential information about the evolution of galaxies. Because atmospheric interference from the ozone layer, oxygen, and nitrogen makes UV radiation difficult to observe from ground-based telescopes, high-altitude balloons, sounding rockets, and orbiting observatories are employed.

Although attempts to study the sun's UV spectrum from balloons were made during the 1920s, it was not until 1946 that rocket-borne instruments made this possible. Only limited additional progress was made until 1962, when the first Orbiting Solar Observatory (OSO) satellite was launched by the National Aeronautics and Space Administration (NASA). These returned thousands of UV spectra, including the first exteme-ultraviolet (wavelengths below 200 nanometers) observations of the solar corona. Through continuous monitoring of the sun over a 15-year period, this program enhanced our understanding of the solar atmosphere and of the 11-year sunspot cycle.

NASA's Orbiting Astronomical Observatory (OAO) satellites, the first of which was launched in 1966, returned UV data about stars and interstellar gas and dust and the first observations of the powerful UV radiation emitted by certain galaxies. Data from Copernicus (OAO-3), which was launched in 1972, led to the determination of the abundance of deuterium in interstellar matter; it also provided considerable information about the atmospheres of luminous hot stars. The Netherlands Astronomical Satellite (ANS) and the TD-1 satellite performed photometric and spectrophotometric surveys of stars in the UV wavelengths.

The International Ultraviolet Explorer (IUE)—a joint project of the United States, the European Space Agency, and Great Britain—was launched in 1978. In orbit for a decade, it monitored the UV spectrum of Halley's comet during its 1986 approach, provided data about the UV reflectivity of the major planets, and contributed to the understanding of quasars; its large telescope made possible the first UV observations of objects beyond the Milky Way, permitting the determination of temperature and structural changes of cool stars during their starspot cycles. The Extreme Ultraviolet Explorer (EUVE; 1992–2000) was the first orbiting observatory to focus on that part of the spectrum. In addition to data from these satellites, UV observations have also been made from two satellites launched in 1990 primarily for other purposes, the X-ray astronomy satellite ROSAT [ROentgen SATellite] and the Hubble Space Telescope.

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Ultraviolet Astronomy

Ultraviolet Astronomy

Ultraviolet radiation

Ultraviolet observatories

Research with UV telescopes

Resources

Ultraviolet astronomy, a part of the fields of astronomy and astrophysics, is the study of astronomical objects in the ultraviolet (UV) portion of the

electromagnetic spectrumspecifically, from the extreme UV (10 nanometers) to the near UV (400 nanometers), where one nanometer equals one-billionth of a meter. Because Earths atmosphere prevents ultraviolet radiation from reaching its surface, ground-based observatories cannot observe in the ultraviolet. Only with the advent of space-based telescopes has this area of astronomy become available for research. Ultraviolet radiation has a shorter wavelength and more energy than visual radiation. Much of ultraviolet astronomy, therefore, centers on energetic processes in stars and galaxies. Hot regions of stellar atmospheres, for example, invisible to optical telescopes, reveal a wealth of information to the ultraviolet telescope. The crowded, violent regions at the centers of some galaxies are also prime targets for ultraviolet telescopes.

Ultraviolet radiation

Scientists often refer to electromagnetic radiation in terms of its wavelength, the distance from one peak of a wave to the next peak. A convenient unit of wavelength is the angstrom (Å). One angstrom equals one ten-billionth (1 x 10-10) of a meter; and one angstrom equals 0.1 nanometer.

Visual light, the light human eyes are sensitive to, has wavelengths from about 4,000 to 7,000 angstroms. Beyond the visual is infrared lighthumans cannot see it, but can feel it as heat. On the short wavelength side of the visual part of the spectrum is the ultraviolet. Ultraviolet (often just called UV) light has wavelengths from 100 to 4,000 angstroms.

The Earths atmosphere is opaque to UV light, meaning it is difficult for UV radiation to penetrate it. This is fortunate for humans, since UV light is what causes sunburn and in sufficiently large doses, skin cancer. Optical telescopes cannot see wavelengths much shorter than 3,600 angstroms. Thus, to observe UV radiation from astronomical objects it is necessary to go above the atmosphere. Orbiting, space-based telescopes are needed, and only in the past few decades have they been available.

Ultraviolet observatories

Astronomers have developed many different kinds of telescopes besides the familiar optical instruments. Radio, infrared, ultraviolet, x-ray, and gamma-ray telescopes all have unique design requirements to maximize their efficiency in the part of the spectrum they intend to study.

Like gamma-ray and x-ray telescopes, UV telescopes have only been possible in the era of space flight, and the longest lived and most important of these so far has been the International Ultraviolet Explorer (IUE). Launched in 1978, IUE was designed to observe the UV sky for five years. Instead, the telescope was not shut down until September 30, 1996, and during its lifetime took tens of thousands of spectra of stars, nebulae, and galaxies.

IUE was a joint project of the United States (National Aeronautics and Space Administration), United Kingdom (UK Science Research Council), and the European Space Agency (ESA). It was operated for 16 hours each day at the Goddard Space Flight Center in Greenbelt, MD, and for eight hours each day at the Villafranca Satellite Tracking Station in Spain. Astronomers around the world used IUE for their research, and it has been one of the most productive missions in the history of spaceflight.

Despite its glowing track record, IUE had some important limitations. Its primary mirror was only 17 in (45 cm) in diameter. Therefore, IUE could not observe very faint objects. In addition, its instrumentation was developed in the 1970s and was not as technologically advanced as that available in the 1980s and 1990s. For this reason, a new generation of UV observatories was designed and built.

On June 7, 1992, the Extreme Ultraviolet Explorer (EUVE) was launched. This satellite was designed to extend the spectral coverage of IUE, which only went down to 1,100 Å. The EUVE telescope observed at wavelengths as short as 70 Å and extended the observing capability of space-based observatories throughout the UV range.

On June 24, 1999, the Far Ultraviolet Spectroscopic Explorer (FUSE) was launched. This satellite is also designed to look father into the ultravioleti.e., to shorter wavelengthsthan IUE, observing at wavelengths from 900 to 1,200 Å. With FUSE, astronomers will explore conditions in the universe as they existed only shortly after the big bang (the currently accepted theory on how the universe was created), in addition to myriad studies of high-energy processes in stars and galaxies.

These observatoriesIUE, EUVE, and FUSE are what NASA calls Explorer-class missions. These are smaller, less ambitious and expensive projects, designed to perform a specific task. This is in contrast to the great observatories such as the Hubble Space Telescope (HST), which includes a UV instrument called the Goddard High Resolution Spectrograph (GHRS). The GHRS can observe the same part of the spectrum as IUE, but the 8.5 ft (2.6 m) mirror of the HST is much larger than the 16 in (45 cm) mirror of IUE, and GHRS can observe much fainter objects than IUE.

On April 28, 2003, NASA launched the Galaxy Evolution Explorer (GALEX) for a 29-month mission to measure star formation approximately 10 billion year ago at ultraviolet wavelengths. Specifically, GALEX is studying hundreds of thousands of galaxies in order to determine their distances from the Earth and rate of star formation within them. As of October 2006, GALEX is continuing its mission of exploration.

Research with UV telescopes

UV telescopes reveal a wealth of information about hot and energetic processes in astronomical objects. This is because the hotter an object is the more energy it radiates at short wavelengths. UV radiation has shorter wavelengths than visual light, so hot objects are brighter in the UV than in the visual. For example, a hot star like Rigel (the blue-white star that forms Orions left foot) emits much more UV radiation than the sun.

UV telescopes have greatly enhanced astronomical understanding of the stars. It is well known that the temperature rises in the outer atmospheres of stars like the sun, but the causes of this temperature rise are poorly understood. Because the atmospheres get very

KEY TERMS

EUVE The Extreme Ultraviolet Explorer, launched in 1992. The EUVE telescope observes the short-wavelength end of UV radiation, from 70 to 760 Angstroms.

GHRS The Goddard High Resolution Spectrograph. An ultraviolet instrument that is part of the Hubble Space Telescope, GHRS extends the capabilities of IUE (International Ultraviolet Explorer) owing to its more modern instrumentation and the HSTs large mirror.

IUE The International Ultraviolet Explorer. Launched in 1978, IUE operated for nearly two decades despite its original five-year design lifetime. IUE was one of the most successful and productive of space-based observatories, and has been used to observe nearly every kind of astronomical object, from planets and comets to stars, nebulae, and galaxies.

UV radiation Radiation with wavelengths between 100 and 4,000 Angstroms. UV radiation causes sunburn and, in sufficient doses, skin cancer. Fortunately for humans, Earths atmosphere prevents most UV radiation from reaching the ground, but this also means that the ultraviolet radiation from astronomical objects can only be studied by telescopes orbiting above the atmosphere.

hot, they emit much of their radiation in the UV, and until the launch of IUE in 1978, the nature of these hot atmospheres was largely unknown. UV telescopes have also been used to study winds from hot stars, stars that are still in the process of forming, and hot, dead stars that orbit other stars, drawing matter off them and heating it until it emits large amounts of UV and x-ray radiation.

Another place where hot, high-energy conditions prevail is at the center of galaxies. The so-called active galaxies have intense high-energy sources at their centers. These galaxies often have huge jets of hot, high-energy material streaming out of them. A hypothesized source of the intense energy generation is an enormous black hole at the galactic center. IUE and other UV telescopes have been used to study galactic centers in an effort to understand the processes occurring in the crowded, violent environments thought to prevail there.

See also Galaxy.

%

Resources

BOOKS

Arny, Thomas. Explorations: An Introduction to Astronomy. Boston, MA: McGraw-Hill, 2006.

Bacon, Dennis Henry, and Percy Seymour. A Mechanical History of the Universe. London: Philip Wilson Publishing, Ltd., 2003.

Chaisson, Eric. Astronomy: A Beginners Guide to the Universe. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004.

Kaufmann, W., Discovering the Universe. 2nd ed. Freeman, 1991.

Mallary, Michael. Our Improbably Universe: A Physicist Considers How we Got Here. New York: Thunders Mouth Press, 2004.

Mark, Hans, Maureen Salkin, and Ahmed Yousef, eds. Encyclopedia of Space Science & Technology. New York: John Wiley & Sons, 2001.

Singh, Simon. Big Bang: The Origins of the Universe.New York: Harper Perennial, 2005.

Jeffrey C. Hall

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Ultraviolet Astronomy

Ultraviolet astronomy

Ultraviolet astronomy is the study of astronomical objects in the ultraviolet portion of the electromagnetic spectrum . Because Earth's atmosphere prevents ultraviolet radiation from reaching its surface, ground-based observatories cannot observe in the ultraviolet. Only with the advent of space-based telescopes has this area of astronomy become available for research. Ultraviolet radiation has a shorter wavelength and more energy than visual radiation, and much of ultraviolet astronomy therefore centers on energetic processes in stars and galaxies. Hot regions of stellar atmospheres, for example, invisible to optical telescopes, reveal a wealth of information to the ultraviolet telescope . The crowded, violent regions at the centers of some galaxies are also prime targets for ultraviolet telescopes.


Ultraviolet radiation

We often refer to electromagnetic radiation in terms of its wavelength, the distance from one peak of a light wave to the next peak. A convenient unit of wavelength is the Angstrom Å. One Angstrom equals one 10 billionth of a meter.

Visual light, the light our eyes are sensitive to, has wavelengths from about 4,000-7,000 Angstroms. Beyond the visual is infrared light—we cannot see it, but we can feel it as heat . On the short wavelength side of the visual part of the spectrum is the ultraviolet. Ultraviolet (often just called UV) light has wavelengths from 100-4,000 Angstroms.

Earth's atmosphere is opaque to UV light, meaning UV radiation cannot penetrate it. This is fortunate for us, since UV light is what causes sunburn and in sufficiently large doses, skin cancer . Optical telescopes cannot see wavelengths much shorter than 3,600 Angstroms, and to observe UV radiation from astronomical objects it is therefore necessary to go above the atmosphere. Orbiting, space-based telescopes are needed, and only in the past few decades have they been available.


Ultraviolet observatories

Astronomers have developed many different kinds of telescopes besides the familiar optical instruments. Radio , infrared, ultraviolet, x-ray, and gamma-ray telescopes all have unique design requirements to maximize their efficiency in the part of the spectrum they are intended to study.

Like gamma-ray and x–ray telescopes, UV telescopes have only been possible in the era of spaceflight, and the longest lived and most important of these so far has been the International Ultraviolet Explorer. Launched in 1978, IUE was designed to observe the UV sky for five years. Instead, the telescope was not shut down until September 30, 1996, and took tens of thousands of spectra of stars, nebulae, and galaxies.

IUE was a joint project of United States and European space agencies, and was operated for 16 hours each day at the Goddard Space Flight Center in Greenbelt, MD, and for eight hours each day at the Villafranca Satellite Tracking Station in Spain. Astronomers around the world used IUE for their research, and it has been one of the most productive missions in the history of spaceflight.

Despite its glowing track record, IUE had some important limitations. Its primary mirror was only 17 in (45 cm) in diameter, and IUE therefore could not observe very faint objects. Also, its instrumentation was developed in the 1970s and was not as technologically advanced as that available in the 1980s and 1990s. For this reason, a new generation of UV observatories was designed and built.

On June 7, 1992, the Extreme Ultraviolet Explorer (EUVE) was launched. This satellite was designed to extend the spectral coverage of IUE, which only went down to 1,100 Å. The EUVE telescope observed at wavelengths as short as 70 Å, and extended the observing capability of space-based observatories throughout the UV.

On June 24, 1999, the Far Ultraviolet Spectroscopic Explorer (FUSE) was launched. This satellite is also designed to look father into the ultraviolet—i.e., to shorter wavelengths—than IUE, observing at wavelengths from 900-1,200 Å. With FUSE, astronomers will explore conditions in the Universe as they existed only shortly after the big bang, in addition to myriad studies of high-energy processes in stars and galaxies.

These observatories—IUE, EUVE, and FUSE—are what NASA calls "Explorer-class" missions. These are smaller, less ambitious and expensive projects, designed to perform a specific task. This is in contrast to the "Great Observatories" such as the Hubble Space Telescope (HST), which includes a UV instrument called the Goddard High Resolution Spectrograph (GHRS). The GHRS can observe the same part of the spectrum as IUE, but the 8.5 ft (2.6 m) mirror of the HST is much larger than the 16 in (45 cm) mirror of IUE, and GHRS can observe much fainter objects than IUE.


Research with UV telescopes

UV telescopes reveal a wealth of information about hot and energetic processes in astronomical objects. This is because the hotter an object is, the more energy it radiates at short wavelengths. UV radiation has shorter wavelengths than visual light, so hot objects are brighter in the UV than in the visual. For example, a hot star like Rigel (the blue-white star that forms Orion's left foot) emits much more UV radiation than the Sun .

UV telescopes have greatly enhanced our understanding of the stars. It is well-known that the temperature rises in the outer atmospheres of stars like the Sun, but the causes of this temperature rise are poorly understood. Because the atmospheres get very hot, they emit much of their radiation in the UV, and until the launch of IUE in 1978, the nature of these hot atmospheres was largely unknown. UV telescopes have also been used to study winds from hot stars, stars that are still in the process of forming, and hot, dead stars that orbit other stars, drawing matter off them and heating it until it emits large amounts of UV and x–ray radiation.

Another place where hot, high-energy conditions prevail is at the center of galaxies. The so-called active galaxies have intense high-energy sources at their centers. These galaxies often have huge jets of hot, high-energy material streaming out of them. A hypothesized source of the intense energy generation is an enormous black hole at the galactic center. IUE and other UV telescopes have been used to study galactic centers in an effort to understand the processes occurring in the crowded, violent environments thought to prevail there.

See also Galaxy.


Resources

books

Bacon, Dennis Henry, and Percy Seymour. A Mechanical History of the Universe. London: Philip Wilson Publishing, Ltd., 2003.

Kaufmann, W. Discovering the Universe. 2nd ed. Freeman, 1991.

Mark, Hans, Maureen Salkin, and Ahmed Yousef, eds. Encyclopedia of Space Science & Technology. New York: John Wiley & Sons, 2001.


Jeffrey C. Hall

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
EUVE

—The Extreme Ultraviolet Explorer, launched in 1992. The EUVE telescope observes the short-wavelength end of the UV, from 70-760 Angstroms.

GHRS

—The Goddard High Resolution Spectrograph. An ultraviolet instrument that is part of the Hubble Space Telescope, GHRS extends the capabilities of IUE owing to its more modern instrumentation and the HST's large mirror.

IUE

—The International Ultraviolet Explorer. Launched in 1978, IUE operated for nearly two decades despite its original five-year design lifetime. IUE was one of the most successful and productive of space-based observatories, and has been used to observe nearly every kind of astronomical object, from planets and comets to stars, nebulae, and galaxies.

UV radiation

—Radiation with wavelengths between 100-4,000 Angstroms. UV radiation causes sunburn and, in sufficient doses, skin cancer. Fortunately for us, Earth's atmosphere prevents most UV radiation from reaching the ground, but this also means that the ultraviolet radiation from astronomical objects can only be studied by telescopes orbiting above the atmosphere.

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