THE INTRODUCTION of rockets in the 1950s and 1960s opened a new era in astronomy. Astronomers at sea level on Earth can observe the Universe through only two ‘windows’ in the electromagnetic spectrum: visible light and radio waves. Earth’s atmosphere absorbs all the other radiation (although some infrared can penetrate to the higher mountain peaks). But rockets could boost telescopes or, even better, satellites carrying telescopes, above the atmosphere. Astronomers can now observe stars, nebulae and galaxies over the whole range of wavelengths, including infrared, ultraviolet, X-rays and gamma-rays.
Until recently, however, astronomers have almost totally neglected one portion of the electromagnetic spectrum, that lying between X-rays and the ultraviolet. This region, the ‘extreme ultraviolet’, or EUV, covers wavelengths between 4.4 and 91.2 nanometres.
Our nearest and brightest source of EUV radiation is the Sun. Its rarefied outer atmosphere, or corona, consists of gas at a temperature of more than 1 million kelvin. This hot gas emits EUV radiation powerfully, especially at the shorter wavelengths. Solar flares, violent eruptions on the Sun’s surface caused by twisting of the solar magnetic field, are strong emitters at the longer EUV wavelengths.
For many years, astronomers have been observing the Sun’s EUV radiation with telescopes aboard satellites such as Skylab and the Solar Maximum Mission. In 1987, astronomers at Stanford University in California and NASA’s Marshall Spaceflight Center built a new type of EUV telescope, ‘tuned’ to a wavelength of 17 nanometres, and flew it on a rocket. It showed clearly the gases of the corona, at temperatures of between 1.0 and 1.3 million K, and also many prominences, which are loops and filaments of gas at a…
