by John Carey
After iron, the making of heavier elements in stars began to consume energy rather than releasing it. No star could earn a steady living that way. But in the explosion of a big star some of the enormous energy released went into building up dozens of chemical elements heavier than iron: gold, lead … all the way through the table of elements to uranium and beyond. Even so, heavy elements remained far less abundant in the cosmos than the lighter elements.
Many of the atoms so formed, and later incorporated into the Earth, were radioactive. Their nuclei were overcharged with energy and unable to survive indefinitely. But ‘not indefinitely’ could mean billions of years. From uranium, thorium, potassium and other radioactive elements, energy stored during the explosions of the ancestral stars slowly trickled out into the rocks of the Earth. It generated the heat that fired volcanoes, shifted the continents and built mountains. The great creakings called earthquakes, which accompanied these processes, were thus direct consequences – albeit greatly delayed and translocated – of those stellar explosions that made the stuff of the Earth available.
The idea that all living things – humans, gnats, slugs, trees – have their origin in Stardust, and the theory, referred to by Calder, that the universe is united in a constant process of creation and destruction, provide the key to Ted Hughes’s difficult poem ‘Fire-Eater’:
Those stars are the fleshed forebears
Of these dark hills, bowed like labourers,
And of my blood.
The death of a gnat is a star’s mouth: its skin,
Like Mary’s or Semele’s, thin
As the skin of fire:
A star fell on her, a sun devoured her.
My appetite is good
Now to manage both Orion and Dog
With a mouthful of earth, my staple.
Worm-sort, root-sort, going where it is profitable.
A star pierces the slug,
The tree is caught up in the constellations.
My skull burrows among antennae and fronds.
Sources: Nigel Calder The Key to the Universe: A Report on the New Physics, London, British Broadcasting Corporation, 1977. Ted Hughes, Lupercal, London, Faber and Faber, 1960.
Black Holes
The son of a Russian Jewish immigrant candy-store owner in New York City, Isaac Asimov (1920–92) began writing science fiction in his teens, and became the twentieth century’s most prolific as well as its most masterly, lucid and imaginative explainer of science to the common reader. This matchlessly clear and compact account of black holes was first printed in the Daily Telegraph in 1979.
Of all the odd creatures in the astronomical zoo, the ‘black hole’ is the oddest. To understand it, concentrate on gravity.
Every piece of matter produces a gravitational field. The larger the piece, the larger the field. What’s more, the field grows more intense the closer you move to its center. If a large object is squeezed into a smaller volume, its surface is nearer its center and the gravitational pull on that surface is stronger.
Anything on the surface of a large body is in the grip of its gravity, and in order to escape it must move rapidly. If it moves rapidly enough, then even though gravitational pull slows it down continually it can move sufficiently far away from the body so that the gravitational pull, weakened by distance, can never quite slow its motion to zero.
The minimum speed required for this is the ‘escape velocity.’ From the surface of the earth, the escape velocity is 7.0 miles per second. From Jupiter, which is larger, the escape velocity is 37.6 miles per second. From the sun, which is still larger, the escape velocity is 383.4 miles per second.
Imagine all the matter of the sun (which is a ball of hot gas 864,000 miles across) compressed tightly together. Imagine it compressed so tightly that its atoms smash and it becomes a ball of atomic nuclei and loose electrons, 30,000 miles across. The sun would then be a ‘white dwarf.’ Its surface would be nearer its center, the gravitational pull on that surface would be stronger, and escape velocity would now be 2,100 miles per second.
Compress the sun still more to the point where the electrons melt into the nuclei. There would then be nothing left but tiny neutrons, and they will move together till they touch. The sun would then be only 9 miles across, and it would be a ‘neutron star.’ Escape velocity would be 120,000 miles per second.
Few things material could get away from a neutron star, but light could, of course, since light moves at 186,282 miles per second.
Imagine the sun shrinking past the neutron-star stage, with the neutrons smashing and collapsing. By the time the sun is 3.6 miles across, escape velocity has passed the speed of light, and light can no longer escape. Since nothing can go faster than light, nothing can escape.
Into such a shrunken sun anything might fall, but nothing can come out. It would be like an endlessly deep hole in space. Since not even light can come out, it is utterly dark – it is a ‘black hole.’
In 1939, J. Robert Oppenheimer first worked out the nature of black holes in the light of the laws of modern physics, and ever since astronomers have wondered if black holes exist in fact as well as in theory.
How would they form? Stars would collapse under their own enormous gravity were it not for the enormous heat they develop, which keeps them expanded. The heat is formed by the fusion of hydrogen nuclei, however, and when the hydrogen is used up the star collapses.
A star like our sun will eventually collapse fairly quietly to a white dwarf. A more massive star will explode before it collapses, losing some of its mass in the process. If the portion that survives the explosion and collapses is more than 1.4 times the mass of the sun, it will surely collapse into a neutron star. If it is more than 3.2 times the mass of the sun, it must collapse into a black hole.
Since there are indeed massive stars, some of them have collapsed by now and formed black holes. But how can we detect one? Black holes are only a few miles across after all, give off no radiation, and are trillions of miles away.
There’s one way out. If matter falls into a black hole, it gives off X-rays in the process. If a black hole is collecting a great deal of matter, enough X-rays may be given off for us to detect them.
Suppose two massive stars are circling each other in close proximity. One explodes and collapses into a black hole. The two objects continue to circle each other, but as the second star approaches explosion it expands. As it expands, some of its matter spirals into the black hole, and there is an intense radiation of X-rays as a result.
In 1965, an X-ray source was discovered in the constellation Cygnus and was named ‘Cygnus X-1.’ Eventually, the source was pinpointed to the near neighborhood of a dim star, HD-226868, which is only dim because it is 10,000 light-years away. Actually, it is a huge star, 30 times the mass of our sun.
That star is one of a pair and the two are circling each other once every 5.6 days. The X-rays are coming from the other star, the companion of HD-226868. That companion is Cygnus X-1. From the motion of HD-226868, it is possible to calculate that Cygnus X-1 is 5 to 8 times the mass of our sun.
A star of that mass should be visible if it is an ordinary star, but no telescope can detect any star on the spot where X-rays are emerging. Cygnus X-1 must be a collapsed star that is too small to see. Since Cygnus X-1 is at least 5 times as massive as our sun, it is too massive to be a white dwarf; too massive, even, to be a neutron star.
It can be nothing other than a black hole; the first to be discovered.
Source: Isaac Asimov, The Roving Mind: A Panoramic View of Fringe Science, Technology, and the Society of the Future, London, Oxford University Press, 1987.
The Fall-Out Planet
The Gaia hypothesis was the brainchild of the scientist J. E. Lovelock, but its name was suggested by his friend, the novelist William Golding. Gaia was the Greek earth-goddess, also known as Ge, and the hypothesis states that the biosphere (i.e. the whole region of the earth’s surface, the sea and the air that is inhabited by living organisms)
is a self-regulating entity with the capacity to keep our planet healthy by controlling the chemical and physical environment. This extract is taken from Lovelock’s Gaia: A New Look at Life on Earth.
It seems almost certain that close in time and space to the origin of our solar system, there was a supernova event. A supernova is the explosion of a large star. Astronomers speculate that this fate may overtake a star in the following manner: as a star burns, mostly by fusion of its hydrogen and, later, helium atoms, the ashes of its fire in the form of other heavier elements such as silicon and iron accumulate at the centre. If this core of dead elements, no longer generating heat and pressure, should much exceed the mass of our own sun, the inexorable force of its own weight will be enough to cause its collapse in a matter of seconds to a body no larger than a few thousand cubic miles in volume, although still as heavy as a star. The birth of this extraordinary object, a neutron star, is a catastrophe of cosmic dimensions. Although the details of this and other similar catastrophic processes are still obscure, it is obvious that we have here, in the death throes of a large star, all the ingredients for a vast nuclear explosion. The stupendous amount of light, heat, and hard radiation produced by a supernova event equals at its peak the total output of all the other stars in the galaxy.
Explosions are seldom one hundred per cent efficient. When a star ends as a supernova, the nuclear explosive material, which includes uranium and plutonium together with large amounts of iron and other burnt-out elements, is distributed around and scattered in space just as is the dust cloud from a hydrogen bomb test. Perhaps the strangest fact of all about our planet is that it consists largely of lumps of fall-out from a star-sized hydrogen bomb. Even today, aeons later, there is still enough of the unstable explosive material remaining in the Earth’s crust to enable the reconstitution on a minute scale of the original event.
Binary, or double, star systems are quite common in our galaxy, and it may be that at one time our sun, that quiet and well-behaved body, had a large companion which rapidly consumed its store of hydrogen and ended as a supernova. Or it may be that the debris of a nearby supernova explosion mingled with the swirl of interstellar dust and gases from which the sun and its planets were condensing. In either case, our solar system must have been formed in close conjunction with a supernova event. There is no other credible explanation of the great quantity of exploding atoms still present on the Earth. The most primitive and old-fashioned Geiger counter will indicate that we stand on fall-out from a vast nuclear explosion. Within our bodies, no less than three million atoms rendered unstable in that event still erupt every minute, releasing a tiny fraction of the energy stored from that fierce fire of long ago.
The Earth’s present stock of uranium contains only 0.72 per cent of the dangerous isotope U235. From this figure it is easy to calculate that about four aeons ago the uranium in the Earth’s crust would have been nearly 15 per cent U235. Believe it or not, nuclear reactors have existed since long before man, and a fossil natural nuclear reactor was recently discovered in Gabon, in Africa. It was in action two aeons ago when U235 was only a few per cent. We can therefore be fairly certain that the geochemical concentration of uranium four aeons ago could have led to spectacular displays of natural nuclear reactions. In the current fashionable denigration of technology, it is easy to forget that nuclear fission is a natural process. If something as intricate as life can assemble by accident, we need not marvel at the fission reactor, a relatively simple contraption, doing likewise.
Thus life probably began under conditions of radioactivity far more intense than those which trouble the minds of certain present-day environmentalists. Moreover, there was neither free oxygen nor ozone in the air, so that the surface of the Earth would have been exposed to the fierce unfiltered ultra-violet radiation of the sun. The hazards of nuclear and of ultra-violet radiation are much in mind these days and some fear that they may destroy all life on Earth. Yet the very womb of life was flooded by the light of these fierce energies.
Source: J. E. Lovelock, Gaia: A New Look at Life on Earth, London, Oxford University Press, 1979.
Galactic Diary of an Edwardian Lady
The ‘big bang’ theory of the beginning of the universe was originally proposed by A. G. E. Lemaitre in 1927 and revised by George Gamow in 1946. According to the theory the universe began about 15 billion years ago in a hot, dense explosive phase. This would explain why, as the astronomer Edwin Hubble discovered in the late 1920s, the universe is expanding, with the galaxies receding from us and from one another. In 1965 two American radio engineers accidentally discovered microwave background radiation, seemingly coming from all directions in space, which is believed to be a remnant of the primordial fireball of the big bang. Edward Larrissy’s poem commemorates the fact that two best-selling books of recent years have been The Country Diary of An Edwardian Lady and Stephen Hawking’s A Brief History of Time, which is about big-bang theory.
In the beginning was a black bomb
That blew apart. A blinding smoke
Kept growing, growing
To a tropical fog, intolerably bright.
From this, white whorls of moonshine mist
Distilled, and then distilled
To petal-eddies on a dark pool.
And now they spin in clusters
Farther and farther apart
Like shining catkins, twisted into spools.
All forms, all time, all complexity,
From the first snowdrop to muffins and tea
Lay in that round black bomb
And will return there
When the hot afternoon is done.
Source: Edward Larrissy. Poem printed in the Independent, February, 1994.
The Light of Common Day
One of the greatest and most prolific writers of popular science and science fiction, Arthur C. Clarke was born in Minehead, Somerset, in 1917. As a child he made a map of the moon, using a home-made telescope, and started writing short stories while at Huish’s Grammar School, Taunton, under the influence of the English master, Captain B. E. (‘Mitty’) Mitford, who would assemble his budding writers once a week after school round a table on which was a large bag of toffees. Unable to afford a university education, Clarke worked as an auditor and, at the outbreak of the Second World War, went into the RAF as a radar instructor. He began publishing science-fiction stories towards the end of the war. His fiction anticipates various developments in space technology, and in a 1945 article he correctly predicted the development of satellite radio and TV. In the 1960s he collaborated with Stanley Kubrick on the film 2001: A Space Odyssey, based on a Clarke short story. The following essay, first published in 1963, was reprinted in his collection By Space Possessed (1993). Clarke says: ‘I am particularly proud of the concluding paragraphs.’
No man has ever seen the Sun, or ever will. What we call ‘sunlight’ is only a narrow span of the entire solar spectrum – the immensely broad band of vibrations which the Sun, our nearest star, pours into space. All the colours visible to the eye, from warm red to deepest violet, lie within a single octave of this band – for the waves of violet light have twice the frequency, or ‘pitch’ if we think in musical terms, of red. On either side of this narrow zone are ranged octave after octave of radiations to which we are totally blind.
The musical analogy is a useful one. Think of one octave on the piano – less than the span of the average hand. Imagine that you were deaf to all notes outside this range; how much, then, could you appreciate of a full orchestral score when everything from contrabassoon to piccolo is going full blast? Obviously you could get only the faintest idea of the composer’s intentions. In the same way, by eye alone we can obtain only a grossly restricted conception of the true ‘colour’ of the world around us.
However, let us not exaggerate our visual handicap. Though visible light is merely a single octave of the Sun’s radiation, this octave contains most of the power; the higher and lower frequencies are relatively feeble. It is,
of course, no coincidence that our eyes are adapted to the most intense band of sunlight; if that band had been somewhere else in the spectrum, as is the case with other stars, evolution would have given us eyes appropriately tuned.
Nevertheless, the Sun’s invisible rays are extremely important, and affect our lives in a manner undreamed of only a few years ago. Some of them, indeed, may control our destinies – and even, as we shall see in a moment, our very existence.
The visible spectrum is, quite arbitrarily, divided up into seven primary colours – the famous sequence, red, orange, yellow, green, blue, indigo, violet, if we start from the longest waves and work down to the shortest. Seven main colours in the one octave; but the complete band of solar radiations covers at least thirty octaves, or a total frequency range of ten thousand million to one. If we could see the whole of it, therefore, we might expect to discern more than two hundred colours as distinct from each other as orange is from yellow, or green is from blue.