Super-computers, heavy lift systems, magnetically cushioned super-trains, cheap electrical power transmission; these are the obvious prospects. Are there other important uses that have not yet been documented?
Almost certainly, there are. We simply have to think of them; and then, before scientists prove that our ideas are impossible, it would be nice to write and publish stories about them. It will not be enough, by the way, to simply refer to room-temperature superconductors. That was done long ago, by me among others ("A Certain Place In History," Galaxy, 1977).
TABLE 2.1
The boiling points of gases.
Gas
Boiling
point
Atm no
Mol wt
(C)
(K)
Radon
-61.8
211.4
86
222
Xenon
-107.1
166.1
54
131
Krypton
-152.3
120.9
36
84
Argon
-185.7
87.5
18
40
Chlorine
-34.6
238.6
17
71
Neon
-246.1
27.1
10
20
Fluorine
-188.1
85.1
9
38
Oxygen
-183.0
90.2
8
32
Nitrogen
-195.8
77.3
7
28
Hydrogen
-252.8
20.4
1
2
TABLE 2.2.
Temperatures at which materials
become superconducting
(in the absence of a magnetic field).
Material
Temp (K)
Titanium
0.39
Zinc
0.93
Uranium
1.10
Aluminum
1.20
Tin
3.74
Mercury
4.16
Lead
7.22
Niobium
8.90
Technetium
11.20
CHAPTER 3
Physics in the Large
3.1 Stars. Everything between atoms and stars, roughly speaking, belongs to chemistry. Although you and I are certainly subject to the laws of physics, we are chemical objects. Our metabolism and structure are controlled by the laws of chemistry. The same is largely true of planets. The shape of the Earth is defined by gravity, but most of the activities within it, or on its surface, or in its atmosphere, are decided by the laws of chemistry.
This is not true of stars. To understand how a star like the Sun can shine for billions of years, you need physics.
The modern view of stars, as giant globes of hot gas, began in 1609, when Galileo Galilei turned his home-made telescope upwards. Rather than a perfect sphere whose nature defied explanation, Galileo found that the Sun was a rotating object with lots of surface detail like sunspots and solar flares.
Over the next couple of hundred years, the size and the temperature of the sun were determined. It is a ball of gas, about a million miles across, with a surface at 6,000 degrees Celsius. What was not understood at all, even a hundred years ago, was the way that the sun stays hot.
Before 1800, that was not a worry. The universe was believed to be only a few thousand years old (Archbishop Ussher of Armagh, working through the genealogy of the Bible, in 1654 announced that the time of creation was 4,004 B.C., on October 26th. No messing about with uncertainty for him.)
In the eighteenth century, the scriptural time-scale prevented anyone worrying much about the age of the Sun. If it had started out very hot in 4000 B.C., it hadn't had time to cool down yet. If it were made entirely of burning coal, it would have lasted long enough. A chemical explanation was adequate.
Around 1800, the geologists started to ruin things. In particular, James Hutton proposed his theory of geological uniformitarianism (Hutton, 1795).
Uniformitarianism, in spite of its ugly name, is a beautiful and simple idea. According to Hutton, the processes that built the world in the past are exactly those at work today: the uplift of mountains, the tides, the weathering effects of rain and air and water flow, these shape the surface of the Earth. This is in sharp distinction to the idea that the world was created just as it is now, except for occasional great catastrophic changes like the Biblical Flood.
The great virtue of Hutton's theory is that it removes the need for assumptions. Anything that shaped the past can be assessed by looking at its effectiveness today.
The great disadvantage of the theory, from the point of view of anyone pondering what keeps the Sun hot, is the amount of time it takes for all this to happen. We can no longer accept a universe only a few thousand years old. Mountain ranges could not form, seabeds be raised, chalk deposits laid down, and solid rocks erode to powder, in so short a time. Millions of years, at a minimum, are needed.
A Sun made of coal will not do. Nothing chemical will do. In the 1850s, Hermann von Helmholtz and Lord Kelvin finally proposed a solution, drawn from physics, that could give geology more time. They suggested that the source of the Sun's heat was gravitational contraction. If the material of the Sun were slowly falling inward on itself, that would release energy. The amount of energy produced by the Sun's contraction could be precisely calculated.
Unfortunately, it was still not enough. While Lord Kelvin was proposing an age for the Sun of 20 million years, the ungrateful geologists, and still more so the biologists, were asking considerably more. Charles Darwin's Origin of Species came out in 1859, and evolution seemed to need much longer than mere tens of millions of years to do its work. The biologists wanted hundreds of millions at a minimum; they preferred a few billion.
No one could give it to them during the whole of the nineteenth century. Lord Kelvin, who no matter what he did could not come up with any age for the Sun greater than 100 million years and was in favor of a number far less, became an archenemy of the evolutionists. An "odious spectre" is what Darwin called him. But no one could refute his physical arguments. A scientific revolution was needed before an explanation was available for a multibillion-year age of the Sun.
That revolution began, as we saw, in the 1890s. The atom, previously thought indivisible, had an interior structure and could be broken into smaller pieces. By the 1920s it was realized that lightweight atoms could also combine, to form heavier atoms. In particular, four atoms of hydrogen could fuse together to form one atom of helium; and if that happened, huge amounts of energy could be produced.
Perhaps the first person to realize that nuclear fusion was the key to what makes the sun go on shining was Eddington. Certainly he was one of the first persons to develop the idea systematically, and equally certainly he believed that he was the first to think of it. There is a story of Eddington sitting out one balmy evening with a girlfriend. She said, "Aren't the stars pretty?" And he said, "Yes, and I'm the only person in the world who knows what makes them shine."
It's a nice story, but it's none too likely. Eddington was a lifelong bachelor, a Quaker, and a workaholic, too busy to have much time for idle philandering. Just as damning for the anecdote, Rudolf Kippenhahn, in his book 100 Billion Suns (Kippenhahn, 1979), tells exactly the same story—about Fritz Houtermans.
Even Eddington could not say how hydrogen fused to form helium. That insight came ten years later, with the work of Hans Bethe and Carl von Weizsäcker, who in 1938 discovered the "carbon cycle" for nuclear fusion.
However, Eddington didn't have to know how. He had all the information that he needed, because he knew how much energy would be released w
hen four hydrogen nuclei changed to one helium nucleus. That came from the mass of hydrogen, the mass of helium, and Einstein's most famous formula, E=mc2.
Eddington worked out how much hydrogen would have to be converted to provide the Sun's known energy output. The answer is around 600 million tons a second. That sounds like a large amount, but the Sun is a huge object. To keep the Sun shining as brightly as it shines today for five billion years would require that less than eight percent of the Sun's hydrogen be converted to helium.
Why pick five billion years? Because other evidence suggests an age for the Earth of about 4.6 billion years. Nuclear fusion is all we need in the Sun to provide the right time-scale for geology and biology on Earth. More than that, the Sun can go on shining just as brightly for another five billion years, without depleting its source of energy.
But how typical a star is the Sun? It certainly occupies a unique place in our lives. All the evidence, however, suggests that the Sun is a rather normal star. There are stars scores of times as massive, and stars tens of times as small. The Sun sits comfortably in the middle range, designated by astronomers as a G2 type dwarf star, in what is known as the main sequence because most of the stars we see can be fitted into that sequence.
The life history of a star depends more than anything else on its mass. That story also started with Eddington, who in 1924 discovered the mass-luminosity law. The more massive a star, the more brightly it shines. This law does not merely restate the obvious, that more massive stars are bigger and so radiate more simply because they are of larger area. If that were true, because the mass of a star grows with the cube of its radius, and its surface area like the square of its radius, we might expect to find that brightness goes roughly like mass to the two-thirds power (multiply the mass by eight, and expect the brightness to increase by a factor of four). In fact, the brightness goes up rather faster than the cube of the mass (multiply the mass by eight, and the brightness increases by a factor of more than a thousand).
The implications of this for the evolution of a star are profound. Dwarf stars can go on steadily burning for a hundred billion years. Massive stars squander their energy at a huge rate, running out of available materials for fusion in just millions of years.
(A word of warning: Don't put into your stories a star that's a thousand times the mass of the Sun, or one-thousandth. The upper limit on size is set by stability, because a contracting ball of gas more than about 90 solar masses will oscillate wildly, until parts of it are blown off into space; what's left will be 90 solar masses or less. At the lower end, below maybe one-twelfth of the Sun's mass, a star-like object cannot generate enough internal pressure to initiate nuclear fusion and should not be called a "star" at all.)
The interesting question is, what happens to massive stars when their central regions no longer have hydrogen to convert to helium? Detailed models, beginning with Fred Hoyle and William Fowler's work on stellar nucleosynthesis in the 1940s, have allowed that question to be answered.
Like a compulsive gambler running out of chips, stars coming to the end of their supply of hydrogen seek other energy sources. At first they find it through other nuclear fusion processes. Helium in the central core "burns" (not chemical burning, but the burning of nuclear fusion) to form carbon, carbon burns to make oxygen and neon and magnesium. These processes call for higher and higher temperatures before they are significant. Carbon burning starts at about 600 million degrees (as usual, we are talking degrees Celsius). Neon burning begins around a billion degrees. Such a temperature is available only in the cores of massive stars, so for a star less than nine solar masses that is the end of the road. Many such stars settle down to old age as cooling lumps of dense matter. Stars above nine solar masses can keep going, burning neon and then oxygen. Finally, above 3 billion degrees, silicon, which is produced in a process involving collisions of oxygen nuclei, begins to burn, and all the elements are produced up to and including iron. By the time that we reach iron, the different elements form spherical shells about the star's center, with the heaviest (iron) in the middle, surrounded by shells of successively lighter elements until we get to a hydrogen shell on the outside.
Now we come to a fact of great significance. No elements heavier than iron can be produced through this nuclear synthesis process in stars. Iron, element 26, is the place on the table of elements where nuclear binding energy is maximum. If you try to "burn" iron, fusing it to make heavier elements, you use energy, rather than producing it. Notice that this has nothing to do with the mass of the star. It is decided only by nuclear forces.
The massive star that began as mainly hydrogen has reached the end of the road. The final processes have proceeded faster and faster, and they are much less efficient at producing energy than the hydrogen-to-helium reaction. Hydrogen burning takes millions of years for a star of, say, a dozen solar masses. But carbon burning is all finished in a few thousand years, and the final stage of silicon burning lasts only a day or so.
What happens now? Does the star sink into quiet old age, like most small stars? Or does it find some new role?
And one more question. We can explain through stellar nucleosynthesis the creation of every element lighter than iron. But more than 60 elements heavier than iron are found on Earth. If they are not formed by nuclear fusion within stars, where did they come from?
3.2 Stellar endings. We have a star, of ten or more solar masses, running out of energy. The supply provided by the fusion at its center, of silicon into iron, is almost done. In the middle of the star is a sphere of iron "gas" about one and a half times the mass of the sun and at a temperature of a few billion degrees. It acts like a gas because all the iron nuclei and the electrons are buzzing around freely. However, the core density is millions of times that of the densest material found on Earth. Outside the central sphere, like layers of an onion, sit shells of silicon, oxygen and carbon, helium and neon and hydrogen, and smaller quantities of all the other elements lighter than iron.
When the source of fusion energy dries up, iron nuclei capture the free electrons in the iron gas. Protons and electrons combine. The energy that had kept the star inflated is sucked away. The core collapses to become a ball of neutrons.
The near-instantaneous gravitational collapse unleashes a huge amount of energy, enough to blow all the outer layers of the star clear away into space. What is left behind is a "neutron star"—a solid sphere of neutrons, spinning on its axis many times a second, only a few miles across but with a mass as much as the Sun's mass.
When such an object was observed, as a rapidly but regularly varying radio source, it seemed difficult to imagine anything in nature that could explain the signal. The team at Cambridge who discovered the first one in 1967 called it a pulsar. They wondered, even if they were reluctant to say so in public, if they had found signals from some alien civilization. When other pulsars were discovered and Thomas Gold proposed that the radio sources were provided by rotating neutron stars, astronomers realized that such a possibility had been pointed out long ago—in 1934, in a prophetic paper by Walter Baade and Fritz Zwicky. The most astonishing thing about the paper was that the neutron itself had been discovered only two years earlier, in 1932.
Could life ever exist on the surface of such a body, with its immense gravitational and magnetic field, and its extreme temperature and dizzying rotation? You might think not, but the novel Dragon's Egg (Forward, 1980) explores that wild possibility, as does Flux (Baxter, 1993).
And how much is a "huge" amount of energy? When a star collapses and blows up like this, in what is known as a supernova, it shines for a time as brightly as a whole galaxy. Its luminosity can temporarily increase by a factor of one hundred billion. If that number doesn't tell you much, try it this way: if a candle in New York were to "go supernova," you would be able to read a newspaper by its light in Washington, D.C.
The explosion of the supernova also creates pressures and temperatures big enough to generate all the elements heavier than iron
that could not be formed by standard nucleosynthesis in stars. So finally, after a long, complex process of stellar evolution, we have found a place where substances as "ordinary" as tin and lead, or as "precious" as silver, gold, and platinum, can be created.
For completeness, I will point out that there are actually two types of supernova, and that both can produce heavy elements. However, the second kind cannot happen to an isolated star. It occurs only in binaries, pairs of stars, close enough together that material from one of them can be stolen gravitationally by the other.
The star that does the stealing must be a small, dense star of the type known as a white dwarf, while its partner is usually a larger, diffuse, and swollen star known as a red giant. As more and more matter is stolen from the more massive partner, the white dwarf star shrinks in size, rather than growing. When its mass reaches 1.4 times the mass of the Sun (known as Chandrasekhar's limit) it collapses. The result is a huge explosion, with a neutron star left behind as a possible remnant. The outgoing shock wave creates heavy elements, and ejects them from the system along with the rest of the star's outer layers.
If you are thinking of using a supernova as part of a story, note that according to current theory the nearest binary star to us, Alpha Centauri A and B, is not a candidate. I am not discouraging the idea of using such a supernova, since I have just done it myself (Aftermath, 1998). The flux of radiation and high-energy particles from an Alpha Centauri supernova can do interesting things to Earth. But you'll need to do some ingenious talking if you want the idea to seem plausible.
Supernovas are rather like nuclear power stations. What they produce is important to us—it is the very stuff of which our own bodies and many of our most valued products are made. We prefer, however, not to have one in our own local neighborhood.
Borderlands of Science Page 7
