The International Ultraviolet Explorer (IUE), for instance, observed a supernova, which indicates the final stages of life of certain stars. The phenomenon was quite rare as it could perhaps be observed only once in 30 years in our galaxy. Though several occur in the Universe, they are too far away to be visible. Following the discovery of a supernova in the Messier spiral galaxy by an American amateur astronomer, IUE was turned on to observe it. The source was about 100 million light years from the Earth. The data showed tat the supernova’s ultraviolet emission had been absorbed by the interstellar gas in our galaxy. As the disrupted star expanded and cooled, the spectral features from it changed correspondingly. IUE was designed to indicate among others, more precisely how starlight is modified by interstellar dust and gas. The satellite could also examine faint stars, galaxies and quasars.
Recent studies have shown that the density of interstellar gas had been overestimated. Following a downward revision of the density, plans have been put forward to make a survey of the sky in the far and extreme ultraviolet range. Astronomers point out that such a survey can provide important data on the late stages of stellar evolution and reveal the density and temperature of the hot components of interstellar medium. The instruments are nowadays more sophisticated and it is hoped to find extragalactic sources as well. The high temperature of the interstellar medium was not expected previously. It is perhaps due to the heat imparted by repeated supernova explosions. In addition, new probes in the ultraviolet are expected to reveal more data on stars, hot white dwarfs, clusters, quasars and nuclei of galaxies.
Several satellites (Uhuru, Einstein, EXOSAT and ROSAT) have studied the sky in the ultraviolet region of 1 to 100 angstroms, while some others (Copernicus, IUE and Hubble) probed the cosmic scene in the 912 to 3,000 angstroms. But the intervening region of 100 to 912 angstroms was until recently considered unsuitable for satellite probes. This region, known as the Extreme Ultraviolet (EUV), was first explored in 1991 by a British instrument on ROSAT (Germany’s Roentgen Satellite). Over 270 research teams had sent proposals to study the region.
The first all-sky EUV survey has been completed by NASA’s EUV Explorer, launched in 1992. As normal telescopes could not be used, a special device that would not absorb the incoming photons was put on board. About 410 brightest sources of EUV have been detected. A wide variety of astronomical sources at considerable distances have been observed.
Some of the findings, including the distant hot B2 II star in Canis Majoris, 600 light years away, will require major revisions in our understanding of the atmosphere of hot stars and our ideas of the contents of the interstellar medium. Further, the EUV Explorer has detected stars like our Sun that posses a chromosphere and corona. The findings are expected to improve our knowledge of stellar corona. Moreover, the findings have come out with the first estimate of neutral helium in the interstellar medium.
The Far Ultraviolet Spectroscopic Explorer (FUSE), launched in 1999 has provided a new perspective on the far ultraviolet portion of the spectrum (about 90 to 120 nano-metres). The Fine Error Sensor on FUSE can see stars down to about 14th magnitude, which is about 5,000 to 10,000 times fainter than one can see on a typical clear night! The FUSE science team found a cosmic abundance of deuterium, a rare form of heavy hydrogen formed during the birth of the Universe. It is believed that the amount of deuterium is decreasing as matter gets cycled through stars. This would enable the astronomers to know how galaxies evolve.
Deuterium And The Big Bang
When the Universe was quite young, the simplest element, hydrogen, consisted of a positively charged nucleus, which had a single proton with a negatively charged electron in orbit around it. In some instances, the hydrogen atoms also had a second particle called a neutron in the nucleus with the proton. This type of hydrogen is called deuterium. In the early Universe, electrons were not bound to the nuclei and moved about freely. Gas with this property is known as plasma.
In the plasma, some of the hydrogen was converted into deuterium, some of which was in turn converted to helium. As the Universe expanded, the plasma cooled and the free electrons and nuclei combined to form complete atoms. The relative amounts of each element would indicate the conditions prevalent before complete atoms emerged. Stars are formed from the interstellar material, when chemical elements are synthesised. Later the stars return their products to the interstellar gas.
Theory predicts that some portion of the interstellar medium would get heated to very high temperatures and in the hot gas, atoms are ionized (the electrons that normally surround the atomic nuclei are stripped off the atoms).