Ultraviolet Astronomy

Ultraviolet Astronomy, detection and study of ultraviolet radiation emanating from celestial objects. Research in ultraviolet astronomy covers virtually all aspects of astronomy. The study of ultraviolet data has advanced our knowledge of the cosmos, including the properties of interstellar and intergalactic matter, the properties of the outer layers of stars, the evolutionary processes in interacting binary stars, and the physics of quasars and active galaxies. For example, from the spectra obtained with the NASA/ESA International Ultraviolet Explorer satellite (IUE, which operated from 1978 to 1996, when it was switched off), astronomers learned that our Milky Way galaxy is shrouded in a hot halo of gas. IUE also measured the ultraviolet spectra of the supernova that appeared in the Large Magellanic Cloud in 1987 (SN1987A); these spectra were used to identify the supernova's progenitor star, the first time that observations existed of a star prior to its explosion as a supernova. The ultraviolet spectrum is also important because it is where most atoms and ions have their fundamental transitions to the ground state (the lowest energy level).

The Earth’s atmosphere prevents most of the ultraviolet radiation that comes from outer space from reaching the Earth’s surface. However, ultraviolet light with a wavelength between 410 nm and 300 nm, called the near-ultraviolet region, can penetrate the atmosphere to the Earth’s surface—although at the shorter wavelength end of this range, this is limited to the highest observatory sites. (1 nm, or nanometre, is a millionth of a millimetre. The mid-ultraviolet region extends between 300 nm and 200 nm, while the far ultraviolet is that from 200 nm to about 91 nm.) Radiation with wavelengths below 300 nm can be detected only above the Earth's atmosphere by orbiting telescopes and satellites, such as the NASA Orbiting Astronomical Observatory OAO-2 (1968), OAO-3 Copernicus (1972), the European Space Agency satellite TD-1 (1972), the Astronomical Netherlands Satellite (1974), IUE, the Hubble Space Telescope (1990), and the Hopkins Ultraviolet Telescope (flown on Space Shuttle missions in the 1990s). The extreme-ultraviolet (or EUV) range covers wavelengths shorter than 92 nm, the most energetic of which (below 50 nm) are, like X-rays, scattered and not reflected from conventional telescope mirrors. Consequently, telescopes that operate in this range must be built with a series of concentric, cylindrical mirrors, from which the UV photons are reflected at a grazing angle towards the telescope's focus.

The brightest UV objects outside the solar system are massive young stars (greater than 10 solar masses) and hot white dwarfs (of about 0.7 solar masses) whose surface temperatures exceed 20,000 K (the Sun is just below 6,000 K) making them appear blue-white in colour. Even higher temperatures (above 100,000 K) can be found in the accretion discs that surround compact objects (usually white dwarfs or neutron stars) in interacting binary-star systems, and in the supermassive objects at the centres of active galaxies. However, the UV spectra of quasars, the most luminous objects in the Universe, are red-shifted into the visible part of the spectrum, making them accessible to ground-based telescopes.

The UK's Wide Field Camera on board ROSAT (1990-1999) and NASA's Extreme Ultraviolet Explorer (EUVE, 1992-2001) probed much of the EUV domain from 91nm to about 10 nm, which is difficult to detect because of the continuous absorption of photons caused by the ionization of interstellar hydrogen and helium atoms. (At 91.2 nm, the Lyman limit, a photon has sufficient energy to completely ionize a hydrogen atom.) However, this is just the region where copious emissions are expected from objects at temperatures of a few hundred thousand Kelvin. Cosmic sources of EUV radiation were only discovered in 1975 during the Apollo-Soyuz mission, the brightest being HZ43, a hot white dwarf star. This demonstrated beyond doubt that interstellar hydrogen is not distributed uniformly through our galaxy, but is extremely patchy, with our Sun being immersed in a low-density “bubble” that allows us to see much further in the EUV. To the surprise of many astronomers, even galaxies have been observed with the EUVE, by looking in directions out of the absorbing plane of our own galaxy. EUVE made major contributions in the fields of active stellar coronae, hot white dwarfs, and cataclysmic variables (interacting binaries involving accreting white dwarfs).

A number of orbiting solar telescopes equipped with detectors able to observe in the UV spectrum have been launched since the mid-1960s. One was a solar observatory called Yohkoh, launched in 1991 by Japan, which has obtained high-resolution UV and X-ray images of the Sun, aided more recently by the ESA/NASA SOHO (Solar and Heliospheric Observatory), which has operated since December 2, 1995. The Sun’s surface (the photosphere) is too cool to emit in the UV spectrum, but its chromosphere and corona (immediately above the surface) are very much hotter and are thus copious sources of UV and X-radiation. The transition region above the photosphere is now being studied by NASA's TRACE (Transition Region and Coronal Explorer), launched from a Pegasus vehicle in April 1998.