Quasars or quasi-stellar objects are point-like sources and perhaps the most distant celestial objects that we can see. They are very bright and extremely energetic too. While pulsating stars or pulsars are rotating neutrons stars that spin very rapidly and accurately. Astronomers in India investigate quasars and pulsars even as the ground-based radio telescopes search for new ones. The work of Indian scientists in this field, based on their observations, is in step with the worldwide trends.
In the early 1960’s, astronomers found strange celestial objects which appeared like stars on photographic plates but emitted enormous radio waves. The astronomers concluded that they were entirely new kinds of objects and named then quasi-stellar radio sources or quasars for short.
Some two years after the discovery of quasars, astronomers were excited to note their peculiar feature. The strange objects were bolting away from the Earth at enormous speeds. Optical studies revealed the quasars as point-like sources whose spectra indicated red-shifts. Light waves put our by a receding celestial object appeared to shift towards the red end of the spectrum. In the optical spectrum of the quasar cataloged as 3C273, for instance, the hydrogen emission lines were shifted to the red, compared to the normal laboratory wavelengths.
Astronomers have concluded that quasars, which appear bluer than most stars, must be extraordinarily bight objects—10,000 times that of our Milky Way galaxy in brightness; yet in the course of a single day, their brightness can vary by 50 per cent. The rapid fluctuations of radiation have intrigued observes. Fluctuations were observed not only in the visible output of quasars but also in their infrared and X-ray radiation. In the optical and infrared portions, the fluctuations implied that the vast output of energy must be generated within a small area. Some sources are believed to have only the diameter of the solar system, even though the quantity of matter is more than a billion times the mass of the Sun! The generation of energy on this vast scale in such a small area continues to be a puzzle.
More tan 17,000 quasars are now known, though most are faint points of light. The most distant quasar has a red shift of 4.9 (the more distant quasars have higher red shifts). A double quasar has also been discovered (1993). This pair has a red shift of 2.3, corresponding to a look-back time of 83 per cent of the age of the Universe (assuming the age to be 20 billion years).
Quasars are essentially indistinguishable from radio galaxies. Usually quasars are much smaller, though more powerful, than radio galaxies. Often quasars have a strange feature. Their radio emission does not come from their visible areas but from two distant regions symmetrically placed on either side of their optical segment. The so-called double structure resembles radio galaxies. The most puzzling discovery was made by the Einstein Observatory (1978). It found that X-ray emission came from every known quasar. Before the Einstein probe, only three of the nearest quasars were found to be the source of X-rays. The most distant quasars seem to emit enormous energy in X-rays, as much as the entire output of a million, million Suns! Some astronomers have concluded that the quasars can account for the entire background X-radiation from outside our galaxy.
The quest for quasars has led astronomers to detect strange heavenly mirages, called gravitational lenses. Radiation from a quasar has been found deflected by an intervening galaxy at the line of sight. The gravity of the galaxy deflects the light (as predicted by Albert Einstein) and splits it, producing two or three images of the quasar. Several such gravitational lenses have been discovered since Jodrell Bank astronomer Dennis Walsh discovered the first in 1979.
And it has prompted astronomers to come out with new theories about the total matter in the Universe with implications relating to the expansion or collapse of the Universe. The images are hard to recognize, as they are less than one-thousandth of a degree apart. An international team of scientists led by the University of Manchester used several of the world’s radio telescopes to make ‘radio pictures’ of numerous distant quasars and found that one out of every 700 distant quasars showed that most seem to have some kind of a mysterious dark energy.
The observation, that X-rays from quasars are found closer to the center of the objects than radio waves are, has led to the speculation that there might be massive black holes in the center of many galaxies. It is believed that the nuclei of all galaxies went through an active phase in the past when they would have appeared as quasars. Stars falling into the black hole may give away enormous energy to the quasars. It is not known what exactly could be the source of that energy. One theory suggests that the highly energetic particles of energy are accelerated by the magnetic fields in massive rotating objects nearby. Another theory holds that collisions between rapidly moving stars produce vast energies. There are also micro-quasars, double stellar systems in our galaxy, discovered in the early 1990s.
The distant quasars also throw light on nearby phenomena. The numerous absorption lines seen in the spectra of quasars seem to originate from various sources. Some come from the quasar itself, while some others, from the intervening galaxies or intergalactic clouds. The data would tell us abut the chemical evolution of earlier epochs when galaxies such as ours were formed. Astronomers will continue to look for more distant quasars and thereby identify more distant galaxies as well.