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Ultraviolet

Contributed by: Ana In es´ G omez´ de Castro

Publication year: 2014

Study of astronomical objects by means of information obtained from the wavelength range of the , approximately 10–320 nanometers.

The resonance transitions of the most abundant atomic species in the are observed in this range up to 6 temperatures as high as 10, K. Molecular gas is also very sensitive to ultraviolet radiation, since the electronic ,+ transitions of the most abundant molecules, including H, 2 , CO, OH, CS, CO, 2 , and C, 2 , are in this range. The number of resonance transitions per 10-nm spectral range is shown in Fig. 1 for the two most abundant molecules, H, 2 and HD, and for the most abundant species in the . It is evident that the ultraviolet range far surpasses the optical range in the density of spectral tracers. Thus, ultraviolet observations are the most sensitive means of detecting and measuring the properties of diffuse matter in the universe over a very wide range of physical conditions, from the warm gas evaporating from extrasolar planets at a few thousand kelvins to the hot gas in the intergalactic medium and in supernova remnants that reaches temperatures of

300,000 K. Moreover, ultraviolet radiation from astronomical sources is a powerful photoionizing agent, fundamental for and the study of the chemical evolution in biogenetic environments, such as the protoplanetary and young planetary disks. See See also: ASTRONOMICAL SPECTROSCOPY .

Observatories

The ultraviolet spectrum is divided into the extreme-ultraviolet (EUV, 10–90-nm), far-ultraviolet (FUV,

90–200-nm), and near-ultraviolet (NUV, 200–320-nm) ranges. The shorter the wavelength of the radiation, the more it is absorbed, limiting the distances suitable for observation. EUV radiation is heavily absorbed even by the diffuse cloudlets in the interstellar medium, with densities of 10–100 particles per cubic centimeter. Ultraviolet radiation is fully absorbed by the ozone layer in the atmosphere of the Earth; hence, ultraviolet observations are carried out from space. For technological reasons, most of the ultraviolet information about astronomical sources has been obtained in the FUV ∕ NUV range (115–320 nm). See See also: ULTRAVIOLET RADIATION .

Ultraviolet astronomy began with instrumentation at high altitudes aboard sounding rockets for brief glimpses of the and . In 1972, the S201 experiment was deployed on the lunar surface by the mission

(NASA), chiefly to measure stars brighter than magnitude 10. Some experiments were run from crewed low-Earth platforms, such as the GLAZAR (1988) on board the space station, and the FAUST (Far Ultraviolet

Space Telescope) experiment run from the Spacelab-1 space station in 1984; both were equipped with ultraviolet imagers. Over the period 1985–1998, a number of missions were conducted or launched from the , AccessScience from McGraw-Hill Education Page 2 of 7 www.accessscience.com

WIDTH:DFig. 1 Number of resonance lines per 10 nm in the optical–ultraviolet range. Included are electronic transitions

of the two most abundant molecules, H, 2 and HD (the molecule made of an atom of hydrogen and an atom of deuterium), and resonance transitions of other abundant atoms and ions. Also shown are wavelength ranges of the Far Ultraviolet Spectroscopic Explorer (FUSE), the Hubble (HST), and ground-based . ( Courtesy of FUSE Project, Center for Astrophysical Sciences, The Johns Hopkins University, http: ∕∕ fuse.pha.jhu.edu ∕ educ ∕ bill 697 sci.html )

including the ASTRO missions containing the Hopkins Ultraviolet Telescope, the UltraViolet Imaging Telescope, and the Wisconsin Ultraviolet Photo-Polarimeter Experiment. In addition, ultraviolet spectroscopy at medium and very high resolution was accomplished with the Orbiting and Retrievable Far and Extreme Ultraviolet

Spectrometer and the Interstellar Medium Absorption Profile Spectrograph. The scientific data from these missions were acquired over the 7–10-day period that the shuttle orbited the Earth. See See also: SPACE FLIGHT ;

SPACE SHUTTLE .

By far the majority of ultraviolet astronomy has been carried out from orbiting space telescopes. The first major ultraviolet satellite observatories to be placed in space were the U.S. Orbiting Astronomical Observatories

(OAOs). OAO 2 operated from 1968 to 1972 and provided the first full survey of the many kinds of ultraviolet sources in the sky, whereas OAO 3 ( Copernicus ) operated from 1972 to 1980 and obtained high-resolution spectra of bright ultraviolet-emitting stars in order to probe the composition and physical state of intervening interstellar gas and to study the stellar winds of hot stars. A number of smaller satellites, including the European AccessScience from McGraw-Hill Education Page 3 of 7 www.accessscience.com

TD 1 and the Dutch ANS, also provided very important survey measurements of the ultraviolet brightnesses of astronomical sources.

The InternationalUltraviolet Explorer (IUE), which operated from 1978 to 1996, provided, for the first time, worldwide continuous access to the ultraviolet range to probe the full potential of ultraviolet astronomy. IUE was a collaborative project of NASA, the European Space Agency (ESA), and the United Kingdom Science and

Engineering Research Council (SERC). It consisted of a 45-cm-diameter (18-in.) telescope equipped with instrumentation for ultraviolet spectroscopy in the 120–320-nm spectral range. The telescope was in a high-Earth geosynchronous orbit to minimize the contribution from the Earth glow at ultraviolet wavelengths. It was operated in real time, as a ground-based observatory, with two sites: the U.S. site at Goddard Space Flight Center in Maryland and the European site at the Villafranca satellite tracking station in Madrid. The IUE obtained approximately 104,000 ultraviolet spectra of a wide range of astronomical objects, including comets and planets, cool and hot stars, exploding stars, external , and quasars.

The (HST) was launched into low-Earth orbit in 1990 and was serviced over the years by the shuttle program with missions STS 61 (1993), STS 82 (1997), STS 103 (1999), STS 109 (2002), and STS 125

(2009). The HST is a NASA-led mission with the collaboration of the ESA. The HST is a 2.4-m-aperture (94-in.) telescope equipped with instrumentation for , optical, and ultraviolet astronomy. Instrumentation flown for ultraviolet imaging in the HST has been the Wide-Field Planetary Camera 1 (WF ∕ PC-1, 1990–1993), to obtain images of astronomical objects over a wide field of view; the Faint Objects Camera (FOC, 1990–2002), to take advantage of the superb imaging quality of space observatories over small fields of view; and the Wide Field

Planetary Camera 2 (WFPC2, 1994–2010). Currently available instruments for ultraviolet imaging are the

Advanced Camera System (ACS), installed in 2002, and the Wide Field Planetary Camera 3 (WFPC3), installed in

2009. WFPC3 has only moderate access to the ultraviolet; it can observe only wavelengths greater than 200 nm.

The instrumentation flown in the past for spectroscopy was the Faint Objects Spectrograph (FOS, 1990–1997), for low-resolution spectroscopy and spectropolarimetry of weak ultraviolet sources, and the Goddard

High-Resolution Spectrograph (GHRS, 1990–1997), for high-resolution ultraviolet spectroscopy. Currently available instruments are the Space Telescope Imaging Spectrograph (STIS), installed in 1997, and the Cosmic

Origins Spectrograph (COS), installed in 2009. See See also: HUBBLE SPACE TELESCOPE .

Soon after the HST launch, it was realized that the primary mirror was suffering from spherical aberration. The

Corrective Optics Space Telescope Axial Replacement (COSTAR) system was implemented by the STS 61 mission. COSTAR was designed as an optical correction device for to be focused at the FOC, FOS, and

GHRS. It replaced, early in the mission, the High-Speed Photometer (HSP, 1990–1993), built for high-accuracy measurement of rapid photometric variations in the optical and ultraviolet ranges. Instrumentation installed later was built with its own correcting optics. The space shuttle program formally ended on August 31, 2011. No further instruments are planned for the HST . After the last refurbishing mission in 2009, the HST is expected to be fully operational until 2017. AccessScience from McGraw-Hill Education Page 4 of 7 www.accessscience.com

In 2003, a NASA-led international mission placed the Evolution Explorer (GALEX), a 50-cm-diameter

(20-in.) telescope, in a nearly circular orbit at an altitude of 695 km (432 mi) to map the ultraviolet sky. The ◦ telescope provides a very wide field of view of 1.25 diameter, ideally suited for its survey mission. GALEX has run several surveys, the largest being the All-Sky Imaging Survey (AIS). AIS covered 26,000 square degrees of the sky in two ultraviolet bands (134.4–178.6 nm and 177.1–283.1 nm), to limiting magnitudes of 20 and 21, respectively.

At shorter wavelengths in the FUV, the Far Ultraviolet Spectroscopic Explorer (FUSE, 1999–2007) was the first mission to explore the 90–120-nm range at high spectral resolution. The satellite was launched into an 800-km

(497-mi) low-Earth orbit. One of the primary goals of the mission was to measure the deuterium-to-hydrogen abundance ratio in the Galaxy and in the intergalactic medium.

The Far Ultraviolet Imaging Spectrograph (FIMS), also known as the Spectroscopy of Plasma Evolution from

Astrophysical Radiation (SPEAR) instrument, was launched in South Korea’s first space science satellite (STSAT-1) in late 2003. FIMS ∕ SPEAR was intended to conduct the first large-scale spectral mapping of diffuse cosmic emission in the far ultraviolet (90–175 nm).

In 1992, the extreme-ultraviolet window to the universe was opened with the launch of NASA’s Extreme

Ultraviolet Explorer (EUVE) satellite, which continued operation until 2001. The EUVE contained telescopes designed to produce images of the EUV sky and spectra of bright EUV sources in the wavelength range 7–76 nm.

The EUVE completed an all-sky survey of 801 astronomical targets. Because of strong absorption of EUV radiation by neutral hydrogen in the interstellar gas, it was expected that the EUVE would not be able to see very many galactic sources. However, the irregular distribution of that gas permitted the EUVEto probe to substantial 1 distances in some directions. Most of the sources of radiation detected with the EUVE over 8 ∕2 years of operation were stars with hot active outer atmospheres (or coronae) and hot stars.

The Cosmic Hot Interstellar Plasma Spectrometer (CHIPS, 2003–2008) was designed to carry out an all-sky study of the diffuse EUV background radiation at wavelengths from 9 to 26 nm to study the physical conditions of the

Local Bubble, the hot gas believed to fill the region of space in the immediate vicinity of the Sun.

Discoveries

The important discoveries of ultraviolet astronomy span all areas of modern astronomy and . Some of the notable discoveries in the area of astronomy include new information on the upper atmospheres of the planets, including planetary aurorae, and the discovery of the enormous hydrogen halos surrounding comets. The evaporating atmosphere of an extrasolar planet was first detected by its absorption of the

Lyman-alpha (121.5-nm) radiation produced by the HD209458, while the planet was in transit in front of the stellar disk; these observations were made with the STIS instrument on board the HST . See See also: AURORA ;

COMET ; EXTRASOLAR PLANETS ; TRANSIT (ASTRONOMY) . AccessScience from McGraw-Hill Education Page 5 of 7 www.accessscience.com

In studies of the interstellar medium, ultraviolet astronomy has provided fundamental information about the molecular hydrogen content of cold interstellar clouds along with the discovery of the hot phase of the interstellar medium, which is created by the supernova explosions of stars. The Local Bubble was thought to be

filled with such a hot gas, but the CHIPS, FUSE, and HST observations have shown that the hot gas is concentrated in the envelopes of the colder neutral clouds embedded in the Bubble, instead of fully pervading it.

The IUE, HST, and FUSE observatories have contributed to the understanding of the nature of the hot gaseous corona surrounding the Milky Way Galaxy and have set the grounds for the study of galactic winds. The spectrographs aboard the HST have revealed the existence of large numbers of hydrogen clouds in the intergalactic medium; these intergalactic clouds may contain much more normal (baryonic) matter than exists in the known luminous galaxies and stars. In addition, the HST spectrographs have detected the presence of a warm-hot (100,000–10,000,000 K) gaseous component in the intergalactic medium, which is expected to contain about 50% of the missing baryons in the universe. Galaxies in the universe are not distributed uniformly; rather they are located in the filaments of a gigantic cosmic web with large voids with no luminous matter within.

The warm-hot intergalactic medium is detected in the intracluster medium, within the cosmic web, and it is thought to have been shock-heated during the collapse of cosmological density perturbations that led to the formation of stars and galaxies. Finding the missing baryons is crucial to validate the standard cosmological model. The measures of the abundance of deuterium in the Milky Way Galaxy and beyond carried out with FUSE have provided important constraints on the conditions in the evolving universe when it was only several minutes old. See See also: BIG BANG THEORY ; COSMOLOGY ; GALAXY, EXTERNAL ; INTERSTELLAR MATTER ; MILKY WAY GALAXY ; UNIVERSE .

In stellar astronomy, ultraviolet measurements led to important insights about the processes of mass loss through stellar winds and have permitted comprehensive studies of the conditions in the outer chromospheric and coronal layers of cool stars. Ultraviolet astronomy has also provided important constraints on the star formation rate and its evolution up to redshift 2, about 80% of the life of the universe. The GALEX mission enabled the mapping of this evolution. It also enabled the detection and characterization of starbursts in galaxies, even those located far from the galactic disk as observed at optical electromagnetic wavelengths (320–750 nm). A composite of the optical and the ultraviolet image obtained with GALEX of the M83 is shown in Fig. 2 to illustrate the ability of ultraviolet wavelengths to pinpoint star-forming regions. Starbursts are systems with very high star formation rates per unit area. They are the preferred places for the formation of massive stars, the main source of thermal and mechanical heating in the interstellar medium, and the factory where the heavy elements form. Thus starbursts play an important role in the origin and evolution of galaxies. The connection of starbursts with galactic chimneys and their impact on galactic winds have been a major outcome from HST spectroscopic studies of the disk–halo interaction in spiral galaxies. Ultraviolet astronomy has also provided crucial information about the chemical enrichment of the universe. See See also: GALAXY FORMATION AND EVOLUTION ; NUCLEOSYNTHESIS ;

STARBURST GALAXY .

Ultraviolet observations of exotic astronomical objects, including exploding stars, active galactic nuclei, and quasars, have provided new insights about the physical processes affecting the behavior of matter in extreme AccessScience from McGraw-Hill Education Page 6 of 7 www.accessscience.com

WIDTH:CFig. 2 Ultraviolet image from NASA’ s Galaxy Evolution Explorer (GALEX), showing the galaxy M83, located 15 million light-years away. Ultraviolet light traces young populations of stars. In this image, young stars can be seen far beyond the main spiral disk of M83, up to 140,000 light-years from its center. ( Courtesy of NASA, JPL-Caltech, MPIA )

environments. Tracking the propagation of the radiation from supermassive black holes with the IUE enabled the study of properties of the circumnuclear region. These black holes have masses of about 100 million times the mass of the Sun and are surrounded by hot gas clouds moving in the deep gravitational potential of the black hole that reprocess the radiation produced in the ultraviolet by the accretion disk around the black hole. See See also:

BLACK HOLE .

Ultraviolet spectroscopy with the IUE, HST, and FUSE has unveiled some of the fundamental properties of interacting binary stars. They exhibit a wide variety of phenomena, such as accretion disks, wind-collimated jets, thermal disk instabilities, and both stable and thermonuclear shell burning in accreting white dwarfs, likely progenitors of type Ia supernovae, which are routinely used as candles for cosmological distance determinations.

See See also: BINARY STAR ; SUPERNOVA ; WHITE DWARF .

Ana In es´ G omez´ de Castro AccessScience from McGraw-Hill Education Page 7 of 7 www.accessscience.com

Bibliography

J. N. Bahcall and L. Spitzer, Jr., The space telescope, Sci. Amer. , 247(1):40–51, 1982

A. Boggess et al., The IUE spacecraft and instrumentation, Nature , 275:372–377, 1978

DOI: http://doi.org/10.1038/275372a0

A. I. G omez´ de Castro and W. Wamsteker (eds.), Fundamental Questions in Astrophysics: Guidelines for Future

UV Observatories , Springer, Dordrecht, the Netherlands, 2006 [also available as the dedicated volume 303 of the journal Astrophysics and Space Science ]

D. C. Martin et al., The Galaxy Evolution Explorer : A space ultraviolet survey mission, Astrophys. J. , 619:L1–L6,

2005 DOI: http://doi.org/10.1086/426387

H. W. Moos et al., Overview of the Far Ultraviolet Spectroscopic Explorer mission, Astrophys. J. , 538, L1–L6,

2000 DOI: http://doi.org/10.1086/312795

Additional Readings

H. Couper and N. Henbest, Encyclopedia of Space , Dorling Kindersley Publishing, New York, 2009

S. A. Stern et al., Ultraviolet discoveries at Asteroid (21) Lutetia by the Rosetta Alice ultraviolet spectrograph,

Astron. J. , 141(6):199, 2011 DOI: http://doi.org/10.1088/0004-6256/141/6/199

M. Vazquez and A. Hanslmeier, Ultraviolet Radiation in the Solar System , Springer, Dordrecht, Netherlands,

2010

MAST: Barbara A. Mikulski Archive for Space Telescopes

Network for UltraViolet Astronomy (NUVA)

Space Telescope Science Institute: Hubble Space Telescope