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

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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.