AccessScience from McGraw-Hill Education Page 1 of 7 www.accessscience.com Ultraviolet astronomy Contributed by: Ana In es´ G omez´ de Castro Publication year: 2014 Study of astronomical objects by means of information obtained from the ultraviolet wavelength range of the electromagnetic spectrum, approximately 10–320 nanometers. The resonance transitions of the most abundant atomic species in the universe 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 interstellar medium. 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 astrochemistry 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 Sun and stars. In 1972, the S201 experiment was deployed on the lunar surface by the Apollo 16 mission (NASA), chiefly to measure stars brighter than magnitude 10. Some experiments were run from crewed low-Earth platforms, such as the GLAZAR telescope (1988) on board the Mir 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 space shuttle, 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 Space Telescope (HST), and ground-based telescopes. ( 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 galaxies, and quasars. The Hubble Space Telescope (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 infrared, 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 light 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 Galaxy 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 Milky Way 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.