Ultraviolet astronomy

(astronomy) Astronomical investigations utilizing observations carried out in the spectral region from approximately 350 to 90 nanometers.

Astronomical observations carried out in the region of the electromagnetic spectrum with wavelengths from approximately 10 to 350 nanometers. Ultraviolet radiation from astronomical sources contains important diagnostic information about the composition and physical conditions of these objects. This information includes atomic absorption and emission lines of all the most abundant elements in many states of ionization. The hydrogen molecule (H₂), the most abundant molecule in the universe, has its absorption and emission lines in the far-ultraviolet. See also Astronomical spectroscopy; Ultraviolet radiation.

Ultraviolet radiation with wavelengths less than 310 nm is strongly absorbed by molecules in the atmosphere of the Earth. Therefore, ultraviolet observations must be carried out by using instrumentation situated above the atmosphere. Ultraviolet astronomy began with instrumentation aboard sounding rockets. The first major ultraviolet satellite observatories to be placed in space were the United States Orbiting Astronomical Observatories (OAOs). The International Ultraviolet Explorer (IUE), launched into a geosynchronous orbit in 1978, realized the full potential of ultraviolet astronomy. See also Rocket astronomy; Satellite astronomy.

NASA's Extreme Ultraviolet Explorer (EUVE) satellite (1992–2001) contained telescopes designed to produce images of the extreme-ultraviolet sky and spectra of bright extreme-ultraviolet sources in the wavelength range from approximately 10 to 90 nm. Most of the sources of radiation detected with the EUVE were stars with hot active outer atmospheres (or coronae) and hot white dwarf stars. See also White dwarf star.

Although it had an inauspicious beginning, the Hubble Space Telescope, launched in 1990, has become the centerpiece of both ultraviolet and visible astronomy. The complement of instruments aboard the Hubble Space Telescope has included imaging cameras operating at ultraviolet, visual, and infrared wavelengths, and spectrographs operating at ultraviolet and visual wavelengths. See also Space Telescope, Hubble; Spectrograph.

The Far-ultraviolet Spectrograph Explorer (FUSE) satellite, launched in 1999, is designed to explore the universe in the 90–120-nm region of the spectrum at high spectral resolution. Galaxy Evolution Explorer (GALEX), launched in 2003, is a modest-sized ultraviolet imaging and spectroscopic survey mission that probes star formation over 80% of the age of the universe.

The important discoveries of ultraviolet astronomy span all areas of modern astronomy and astrophysics. Some of the notable discoveries in the area of solar system astronomy include new information on the upper atmospheres of the planets, including planetary aurorae and the discovery of the enormous hydrogen halos surrounding comets. 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. 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. The IUE, Hubble Space Telescope, and FUSE observatories have contributed to the understanding of the nature of the hot gaseous corona surrounding the Milky Way 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 environments. The spectrographs aboard the Hubble Space Telescope have revealed the existence of large numbers of hydrogen clouds in the intergalactic medium. These intergalactic clouds may contain as much normal (baryonic) matter as there exists in the known luminous galaxies and stars. See also Comet; Cosmology; Galaxy, external; Interstellar matter; Milky Way Galaxy; Planetary physics; Quasar; Satellite (astronomy); Star; Supernova.