Ultraviolet waves (in the wavelength of 10 nm to 390nm) are shorter than visible light but longer than X-rays. Some of the hottest and most energetic stars can be seen in ultraviolet light. However, it can be studied only from space, as ozone in the Earth’s atmosphere virtually blocks it in the stratosphere and protects life on Earth.
Massive young stars shine brightly in ultraviolet light, as they are hotter. Ultraviolet radiation is useful for studying the regions between stars, called the interstellar medium, where 99 per cent gas and 1 per cent dust are present. This is the region where new stars are born.
A pharmacist and doctor in Silesia, now part of Poland, Wilhelm Ritter discovered ultraviolet light in 1801. A year after William Herschel had discovered infrared light, Ritter looked for invisible light beyond the violet end of the visible light spectrum. He called the ultraviolet radiation chemical rays.
Nature reveals much about its elements in the ultraviolet. Theodore Lyman (1874-1954) was the first scientist to explore the ultraviolet region in the laboratory. He discovered the fundamental transitions of atomic hydrogen from one energy level to another. Hydrogen is the simplest element with only one electron. The hydrogen spectrum has several spectral lines corresponding to the absorption or emission of energy in the ultraviolet. Modern spectroscopy has identified the presence of hydrogen in space. As most of the ultraviolet radiation is absorbed by the Earth’s atmosphere, only high-rise balloons, rockets and satellites could reveal objects that radiate in the ultraviolet.
The ultraviolet region of the solar spectrum was first detected by a V-2 rocket used by the US Naval Research Laboratory in 1946. After a period of nine years, ultraviolet radiation outside the solar system was discovered by the same laboratory using an Aerobe rocket. It was only in 1962 that a satellite, Orbital Solar Observatory, was launched to study this radiation. This was followed by the Orbital Astronomical Observatory in 1968.
In 1966, a scientist of the US Naval Research Laboratory sent his ultraviolet camera on sounding rockets. In 1970 he detected molecular hydrogen in deep space. Two years later, his Far Electrograph Ultraviolet camera flew to the Moon aboard Appollo 16. The images showed new features of the Earth’s far outer atmosphere as well as galaxies.
The next major attempt was made, when the International Ultraviolet Explorer (IUE) was launched in 1978. It was a joint effort by the European Space Agency, NASA and the UK Science Research Council.
Ultraviolet astronomy has paid handsome dividends justifying the patience and money spent on the probes. It has been discovered that most of the light from hot stars, usually young, appears as ultraviolet. Their temperature and composition can be determined by analyzing the spectra. Ultraviolet radiation has also revealed stellar winds with velocities of 4,000 km/s, continuously depleting the mass of the stars.
The ultraviolet line spectrum reveals dust and gas in the space between stars and it is from the dust that stars emerge. As scientists noted the dimming effect of dust on starlight, there were indications of the presence of graphite and silicates. Though the Sun puts out only 4 per cent of its energy in ultraviolet, the radiation has given considerable data on solar activity and composition. It has been noted that while the optical spectrum of the Sun reveals dark absorption lines, the solar ultraviolet spectrum consist of only emission lines from the chromosphere and corona indicating the nature and location of their sources in the Sun’s different layers. The upper atmosphere of outer planets and the magnetospheres of Jupiter and Saturn are also studied in the ultraviolet region.
Farther away, ultraviolet radiation can throw light on the evolution of galaxies and supernova. For instance, the dark absorption lines in the spectrum of hydrogen gas in between quasars and the Earth will show whether the quasar is receding from us. The ultraviolet window will give us a panoramic view of the heavens where matter is continuously being cycled from the interstellar medium to stars and back through a series of mass loss and explosions. The physical and chemical state of the matter during its various phases would indicate the cosmic forces at work.