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The Violent Universe
GURU: Today I will discourse upon the violence in astronomy.
DISCIPLE: Revered Sir! Will you be describing the violent phenomena in the Universe?
GURU: Yes, and I will also dwell upon the controversies amongst the astronomers about what these events imply–controversies which are no less violent than the phenomena themselves.
The one aspect of the star-studded night sky which impresses the casual observer most is its tranquillity. The peace and quiet of the heavens with their marked contrast to the hurly burly of life on the Earth have inspired poets, philosophers, and religious thinkers from time immemorial. Even the amateur astronomer viewing the night sky from his proverbial roof top telescope sees a picture which changes very slowly from night to night. The occasional visit of a comet, the fall of a meteorite, or in these modern times the passing of a man-made satellite are examples of events which introduce transitory variations on an apparently steady cosmic theme.
But appearances can be deceptive! What may appear peaceful and steady to the casual observer in fact hides many turbulent phenomena. The observational and theoretical advances in astronomy since World War II have revealed the existence of many types of violent events in the cosmos, events which by their sheer magnificence far surpass any spectacular happening on the Earth. This book is concerned with a description of such violent events in astronomy.
How is this violence in astronomy revealed to the astronomer? How can he estimate the physical power behind any particular violent event? What explanations can he offer for the cause of the event? Before we concern ourselves with such questions let us take a look first at a sample of the events themselves; events which point to some violent activity in astronomical objects.
The Crab Nebula
Figure 1.1 shows a photograph of an astronomical object in the constellation of Taurus. This object is not visible to the naked eye or even through an amateur's telescope. Its picture has been obtained by exposing a photographic plate for an extended period at one of the leading astronomical observatories.
The filamentary structure of the bright object gives it a crab-like appearance. To the astronomer, it also indicates some violent activity at the centre. This indication is borne out by a detailed examination of the nebula. For example, velocity measurements indicate that the bright matter in the object is moving away from the centre with speeds around 1000 km per second or more. The more recent investigations by radio and X-ray techniques show that the Crab Nebula is also a source of radio waves, X-rays, and gamma rays.
This remarkable object is believed to be the remnant left when a star exploded many centuries ago. To be precise, the explosion of the star was first seen on the Earth on 4 July 1054. How are we able to pinpoint the date so accurately? The reason for fixing on this date lies in the fortunate circumstance that the Chinese and the Japanese astronomers of the 11th century recorded this event in their chronicles, most probably for astrological purposes! According to these records the star was so bright in the initial stages that it could be seen in the day time. Even by 27 July 1054 the star was as bright as Venus. Subsequently it faded and by 17 April 1056 it was no longer visible to the naked eye.
It would be interesting to speculate why this object was not seen (or recorded) by observers in Europe and India, where it should have been visible. Did the intellectuals of medieval Europe, so dominated by the Christian dogma that God created the Universe in its entire perfection, fail to find a proper place in the scheme of things for the sudden appearance of a strange star? Could the astrologers of India have missed seeing the object, as July is a month of Monsoon rains?
Speculations apart, there are less direct indications that in two other parts of the world this remarkable object was seen. One indication is on the stone pictographs found in the caves of the Pueblo Indians of North America, showing a crescent Moon and a nearby bright object. The position of the object in relation to the Moon seems to agree with that of the exploding star in the Crab Nebula. These pictographs suggest that the Red Indian tribe in this area was sufficiently impressed by the event to have felt the need to record it on rocks. More recently Kenneth Brecher, Elinor Lieber, and Alfred Lieber have pointed out that the Arabs also noticed and recorded the event. A report by Ibn Buttan, a Christian physician of Baghdad who lived in Cairo until late 1052 or early 1053 and later spent a year in Constantinople, states the appearance of a spectacular star in Gemini some time between 12 April 1054 and 1 April 1055. (Because of the precession of equinoxes this position in Gemini corresponds to the present position of Crab in Taurus.)
An exploding star of the type which left the remnant in the form of the Crab Nebula is known as a supernova. Since 1054 two more supernovae have been seen to explode in our Galaxy; one by Tycho Brahe in 1572 and the other by Johannes Kepler in 1604. More supernova explosions have been observed in other galaxies. What is the cause of these explosions?
Does a supernova explosion blow apart the entire star? As we shall see later, there are reasons to believe that the central core of the star may survive the explosion. If so, in what way can we expect to see it? While theoreticians speculated about this question, an unexpected observational discovery in 1968 provided the most plausible answer.
Jocelyn Bell, a graduate student in the Cavendish Laboratory at Cambridge University, was making measurements of interplanetary scintillations, with the help of a large radio telescope. Apart from the expected pattern of radiation, she also detected another, rather unusual pattern. This pattern was remarkable for two reasons: it was highly regular and it was of the very short period (in seconds) of 1.337 279 5 ± .000 002 0. The fact that the period can be quoted to seven places of decimal indicates how regular the pattern of pulses was. The small time scale was remarkable since no astronomical objects were then known to show a pattern of radiation with such a rapid variation (see Fig. 1.2).
Jocelyn Bell, her supervisor Antony Hewish, and some of their colleagues at the Mullard Radioastronomy Observatory at the Cavendish Laboratory investigated this unusual pulse pattern. They ruled out the possibility that the pulses could have originated, in a planet going round a star. And with this conclusion went away the exciting possibility that these signals were being sent by an advanced civilization of 'little green men'! Instead, these radioastronomers came to the conclusion that the pulses originated in a compact astronomical source which was named a pulsar.
Although the first pulsar, now known as CP-1919 (CP stands for the Cambridge Catalogue of Pulsars), was detected by accident, its radiation characteristics were unusual enough to inspire radio astronomers all over the world to search for and find other pulsars. There was considerable excitement when a pulsar was detected in the Crab Nebula: for this discovery seemed to provide an answer to the question, raised earlier, about the remnant of a supernova explosion. To date, the number of pulsars exceeds 300 and the astronomers have succeeded in resolving the mystery of the regular short period pulses from these. strange objects.
Let us now turn to a young branch of astronomy, the branch which uses X-rays to detect cosmic sources of radiation. The direct observation of X-rays from outer space became feasible only after the dawn of the space age. For the Earth's atmosphere absorbs X-rays from outer space and thereby prevents their detection by ground-based instruments. When it became possible to launch artificial satellites well above the X-ray absorbing layers of the atmosphere, X-ray astronomy came into its own.
A major advance in X-ray astronomy was the launching of the satellite UHURU. This satellite was launched from the east coast of Kenya on 12 December 1970, on the seventh anniversary of its independence. Appropriately, the name UHURU of the satellite means 'freedom' in Swahili, the national language of Kenya.
The UHURU satellite carried an X-ray detector and, although earlier observations had revealed the existence of a few isolated X-ray sources, it was the first time that astronomers were able to get a long list of cosmic X-ray sources. The X-radiation came from many types of sources. Of these we will briefly look at one source which has generated considerable excitement.
Cygnus X-1 is a source in the constellation of Cygnus. The X-radiation from this source showed a periodicity of 5.6 days. That is, the radiation went through one cycle of maximum and minimum in this period. When astronomers examined their photographic plates for the region where Cygnus X-1 was detected they found, very close to its location, a large star of the type known as supergiant. From observations of this star using the visible spectrum it became clear that the star is not isolated but that it is a part of abinary system. The scenario is illustrated in Fig. 1.3.
Here we see two stars going round each other. Of these the star A is the supergiant star mentioned above. What is star B? The star B is not visible, but its presence can be inferred from the gravitational pull it exerts on its companion A. The X-ray emission of Cygnus X-1 seems to be coming from the vicinity of where star B should be located.
Cygnus X-1 is the most dramatic example so far of X-ray sources associated with binary stars. Why it is so dramatic we will see later.
Gamma Ray Bursts
An even younger branch of astronomy, younger than the X-ray astronomy described above, is the astronomy of gamma rays. Like X-rays, gamma rays also get absorbed by the Earth's atmosphere and hence have to be detected with the help of satellite-based detectors. The gamma ray detectors have not yet reached the same level of sensitivity, control, and adaptability as the X-ray detectors; and so gamma ray astronomy is still in its infancy.
Nevertheless the early observations of gamma ray astronomy hold out rich promise of things to come as the detector technology improves. Already astronomers have begun to detect what are known as gammaray bursts.
In a typical burst shown in Fig. 1.4, a large quantity of gamma rays is released in a short time of the order of a few seconds. The burst seems to be a once-and-for-all event; it is not repeated. What type of violent activity may be responsible for a gamma ray burst?
In Fig. 1.5 we see a somewhat larger system than just a star or two which we have so far considered. This is a globular cluster, a cluster of stars held together by their gravitational attraction. A cluster like the one shown here may contain as many as a hundred thousand stars.
Notice how the density of stars builds up towards the centre. This is characteristic of a gravitationally bound system, whether it be a single star or a cloud of gas, or a globular cluster. The number of stars increases towards the centre so much that, as seen in Fig. 1.5, it becomes difficult to make out individual star images.
The discovery of X-ray sources like those detected by the UHURU satellite has added a new dimension to the study of globular clusters, because it is found that some of the X-ray sources are located near the centres of globular clusters. For example, the cluster NGC 6624 (NGC stands for the New General Catalogue) houses the source whose UHURU catalogue number is 3U 1820-30. Is the process of X-ray emission due to the activity going on in the central region of the globular cluster? If so, what is the nature of this activity?
Nuclei of Galaxies
We now consider even bigger systems than globular clusters. Our Galaxy, shown schematically in Fig. 1.6, is a disc shaped object with a central bulge, the entire system containing some hundred thousand million stars. As in the case of globular clusters, the Galaxy also contains an increasingly high density of stars towards its centre. The dynamic activity in the crowded region of the galactic centre gives way to more steady and systematic motions as we move away from it. We are fortunate that our Solar System, the Sun with its planets and satellites, is located two thirds of the way out from the centre (see Fig. 1.6). It is doubtful if the Solar System would have survived intact in the central part of the Galaxy.
But, compared to the nuclear regions of many other galaxies, the centre of our Galaxy is a quiet place! In Figs. 1.7 and 1.8 we see two examples of galaxies whose nuclear regions show violent activity. The galaxy shown in Fig. 1.7, NGC 1068, is a galaxy belonging to a special class selected by C. Sevfert for their bright nuclear regions. Compared to the rest of the galaxy, the nuclear region of a Seyfert galaxy stands out in various ways apart from its brightness. There is explosive activity in the nuclear region as well as emission of X-rays. Are these two phenomena related?
Figure 1.8 shows another extraordinary galaxy, known as M 87 (M stands for the Messier Catalogue). There is a jet-like structure emanating from the centre. Whether it is a continuous jet or a succession of blobs of gas ejected by the centre of the galaxy is not yet known. Recent detailed studies of the nuclear regions of M 87 have led to some bizarre theoretical interpretations which we shall encounter in a later chapter.
Extragalactic Radio Sources
In 1946 J. S. Hey, S. J. Parsons, and J. W. Phillips discovered radio waves coming from the direction of the Cygnus constellation. The techniques of radio measurements in 1946 were not accurate enough to pinpoint the location of the source exactly. In 1951 F. G. Smith at Cambridge was able to achieve sufficient accuracy in locating this source to enable optical astronomers to institute a search for a source of visual light in the same place. The radio source was called Cygnus A.
Walter Baade at the Mt Wilson and Palomar (later renamed the Hale) Observatories did find an interesting object at the location of Cygnus A. Figure 1.9 shows the photograph of this object. This is the photograph of a radio galaxy. Baade in fact thought that the photograph shows two galaxies in collision.
It is interesting to recall a bet which Baade made with Rudolf Minkowski, another leading astronomer at the Mt Wilson and Palomar Observatories. The bet arose when, at the end of a seminar talk on Cygnus A, Minkowski made sceptical comments about the collision hypothesis, which had been proposed by Baade and Lyman Spitzer to account for the radio emission from Cygnus A. Baade was, however, confident enough to bet on his theory to the tune of one thousand dollars, but Minkowski talked him down to just a bottle of whisky! It was agreed by both sides that the evidence of emission lines in the spectrum of the gas in the source should be taken as a confirmation of the idea that colliding galaxies were involved. A few months later this evidence was obtained and Minkowski conceded the bet. However, Baade later complained that Minkowski himself finished the whisky that he had given in settlement of the bet!
Later events showed that Minkowski was right in consuming the whisky, for subsequent evidence justified his scepticism of the collision theory. Now it is realized that Cygnus A does not owe its radio emission to the collision of two galaxies. What goes on in Cygnus A is in fact characteristic of what goes on in the majority of radio sources located outside our Galaxy which have been discovered since 1951. The detailed evidence available in such cases points not to a collision, but to an explosion in the central region of the radio source; an explosion which throws out electrically charged particles in opposite directions, as shown in Fig. 1.10. These fast particles proceed a certain distance from the source and then radiate in the presence of the magnetic field in the region. Again we encounter evidence of violent activity. What is the process that leads to the emission of fast particles in a radio source? From where does the source derive its tremendous power?
Excerpted from Violent Phenomena in the Universe by Jayant V. Narlikar. Copyright © 1982 Jayant V Narlikar. Excerpted by permission of Dover Publications, Inc..
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