Space exploration
A guest star is born
Tom Margerison
It was in 1054, twelve years before the most remembered date in English history, that the Emperor of Khaifeng received his chief astronomer and calculator of his calendar to be told with all humility, "I have observed the appearance of a guest star." The star grew brighter and brighter until it surpassed Venus in brightness and could be seen in daylight as a reddish-white pinpoint of light in a blue sky. Within a month it was fading. A few weeks more and it had all but disappeared. Today the remains of that gargantuan explosion, a collection of wispy threads known as the Crab nebula, are a favourite subject of research among cosmologists concerned with the life and death of stars.
Now it seems that something rather similar, although less dramatic, is afoot in the Milky Way in the region of the constellation Orion. The discovery was made three weeks ago during a routine survey of the sky with an X-ray telescope mounted in the British satellite Ariel V. Such work can be done only in space because the X-rays are absorbed by the Earth's atmosphere. The earthbound astronomer, like a fish on the sea bottom, is limited in what he can know about the outside world by the restricted rays which penetrate to him.
One of Ariel V's two X-ray telescopes, built by the University of Leicester, looks out sideways from the spinning satellite, scans over the whole sky and surveys and logs every X-ray source it finds. The second, which looks along the axis of the satellite and was designed by the University of Birmingham and University College, London, can make more detailed observations of a particular patch of sky. When the first telescope picked up the X-rays from the region of Orion, it was noticed that each time the telescope scanned over the star it had become a bit brighter. "In a week it had grown to be 1,000 times brighter than when we first recorded it,"explained Professor K. A. Pounds, who is in charge of the experiment. By the weekend before last it was easily the most powerful source of X-rays in the whole sky, more than three times as intense as the next brightest, which is in the constellation Scorpio.
The problem was to know just what kind of body was producing such intense rays. X-rays, unlike light, cannot be focused by a lens nor reflected by a mirror, and the so-called telescope is not much more than a tube through which the X-ray detector looks. Professor Pound's telescope, which was designed for an overall survey of the X-ray-producing regions of the universe, can only pinpoint the position of the X-ray star to about one tenth of a degree; but by August 16, the brightness of the X-rays had become so great that he decided it was time to try to look for whatever was producing them with optical and radio telescopes, and he sent out telegrams to the world's observatories. He also arranged for the satellite to be turned to point the higher-resolution X-ray telescope at the star, to try to reduce the area of sky the astronomers would have to search, By Tuesday the 19th, before the results from the second X-ray were known, news came that a new star had been found, of twelfth magnitude, right in the position Pounds had predicted. "The message said that Bolly and Wolfson had found it," said Pounds. "We don't know who they are. They're probably graduate students who happened to have time on the forty-eight-inch telescope at Paloma just at the time our telegram arrived, and they decided to have a look."
When the Second X-ray telescope results were obtained, they agreed exactly with the position given by the Mount Paloma telescope. And last Thursday night came the news that the radio astronomers had picked up radio waves from the same star.
So, for the first time, X-ray, optical and radio astronomers can all observe the exciting happenings somewhere out in space, 1,000 light years away — not so very far by cosmological standards. At present the star is something like 30,000 times more luminous than the sun. The question is, what will happen next? Last week, it seemed that the X-rays had reached their maximum, possibly because they had come to the Eddington limit, which means that they had become so intense that the pressure of the escaping radiation prevents more matter falling into the star to produce more X-rays.
What, then, is the cause of this phenomenon? Already the theoreticians are at work. The most popular idea is that the X-ray star consists of two bodies, perhaps a million miles apart, one of which is in its old age and — with its nuclear fuel -used up — has collapsed into a super-dense state and become a neutron star, or even a 'black hole'. The super-dense partner of the pair is wrenching material from the other, and it is this material, falling headlong into it, which produces the X-rays. But the real problem is why this should happen suddenly in August 1975. Did the 'normal' star of the pair, for some reason expand a little, sufficiently for the super-dense one to get its clutches on some material? Or is this a cyclic event, perhaps happening once every revolution as the stars move round one another? And why has the star become so luminous?
It may be that the material stolen by the super-dense star, heated to the temperature (20 million degrees or so) needed to produce the X-rays, would send out sufficient light to explain the phenomenon completely. Or possibly the X-rays, striking the partner star, have increased its temperature so much that this is at least a contributory factor to brightness.
The real importance of the discovery is that, having found out which star is producing the X-rays, cosmologists have a unique opportunity to follow through the development of the story and perhaps to unravel some of the mysteries of these recurring sudden explosions in space.
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