The region of space surrounding the Sun is not called the solar system simply because of the gravitational grip that the Sun exercises over its attendant planets. It is also because of the total dominance it has in governing the space environment from the inner planets out to a distance on the order of 100 to 200 astronomical units (AU). This outer boundary of the solar system, called the heliopause, is the place where the Sun's influence ceases and the interstellar medium—the stuff between the stars in our galaxy, the Milky Way—begins.
The Sun, our star, is something we tend to take for granted. We never question that it will rise each day to illuminate our daily routine. We never give a second glance to the ever-present source of beautiful light and warmth that makes for a perfect summer's day. This rather laid-back attitude is encouraged perhaps by the Sun's small apparent size; it subtends an angle of only half a degree on the sky. However, this apparent size hides its true scale. The Sun is an object of about 1,400,000 km (870,000 miles) in diameter with a mass some 330,000 times that of our own planet! If we did stop to reflect for a moment, it is a rather sobering thought that we are living only 150 million kilometers (93 million miles) away from a star! Fortunately for us, it is a rather stable star, its output being more or less constant over the last few billion years, and the astrophysicists tell us that it will stay that way for a few more billions years to come. This stability is derived from a long-term balance between the energy source within the Sun tending to blow it apart and the force of gravitation tending to hold it together.
I find it amazing that we did not understand the source of the Sun's energy until as recently as the 1930s, when physicists began to uncover the mysteries of nuclear fusion. As the name suggests, this is the process of fusing atoms together to form heavier atoms. The basic energy source that powers the Sun is the fusing together of hydrogen atoms to make helium atoms, and this involves the release of nuclear energy. The destructive capability of the hydrogen bomb is also frightening testimony to the power of nuclear fusion. Some years ago many drivers displayed a green bumper sticker saying, "Nuclear energy—no thanks!'' accompanied by a smiley Sun. Ironically, the antinuclear campaign's logo of the sun represented the largest source of nuclear energy in the neighbourhood!
How does nuclear fusion work? To begin, there are 92 different kinds of naturally occurring atoms, or elements. The Periodic Table lists these naturally occurring elements, starting at number 1, hydrogen, and ending with number 92, uranium. The currently accepted model of an atom is that it has a tiny, compact nucleus at its center composed of protons and neutrons, and this is surrounded by a cloud of orbiting electrons. The protons and neutrons are subatomic particles having about the same mass as each other, of the order of 0.000 000 000 000 000 000 000 000 001 of a kilogram, while the electrons are much smaller in mass (by a factor of around 2000). The protons each carry a positive electric charge and the electrons a negative one, while the neutrons are electrically neutral. This electric charge is the same as the static electricity that can sometimes build up on your clothing. You certainly get to know it's there if you touch a radiator, and the static charge gives you a mild electric shock as it dissipates to Earth. As the discussion above suggests, electric charges come in two varieties: positive and negative. We find that two like charges—two positives or two negatives—exert a force that repel each other, while a negative and a positive charge attract one another. The strength of this electric force between charges behaves like gravity in that it is governed by an inverse square law (see Chapter 1).
The numerical position of each element in the Periodic Table depends on the number of protons in the nucleus, so we have the simplest and lightest element hydrogen at number 1, consisting of a nucleus with one proton. At number 2 we have helium, with two protons (and two neutrons) in the nucleus, and so on up to uranium with 92 protons in the nucleus. To help visualize this, Figure 6.1 shows a hydrogen and a helium atom where the various particles are represented as billiard balls. Of course, the particles are not really like billiard balls, but instead have all sorts of weird properties that we need not discuss (if you are interested in knowing more, then I would suggest you find a popular book on quantum mechanics, which is the physics of the small world of subatomic particles). The nature of the subatomic world can be summed up by saying that we don't have any idea, for example, what an electron is! I find it remarkable that we can build a global consumer industry—the electronics business—on the basis of this fundamental ignorance. The saving grace, of course, is that our current theories are good at predicting how an electron behaves, so we don't really need to know what it is to build a television set or a computer games console.
We can complicate things a little by noting that atoms of a particular element can also exist in several forms, called isotopes, with different numbers of neutrons. For example, an atom with one proton and one neutron in the nucleus is an isotope of hydrogen called deuterium. One proton and two neutrons gives another isotope of hydrogen called tritium. These isotopes are also illustrated in Figure 6.1. The other puzzling thing about atoms with more than one proton in the nucleus is why the positively charged protons do not repel each other, and cause the nucleus to fly apart. The answer is that the physicists have discovered another force, the strong nuclear force, that binds the nucleus together. We have now come across three types of force so far in this book: gravity, electromagnetism (which includes the electric force), and now the strong nuclear force. So far, scientists have discovered only four fundamental forces in nature. The strength of the strong nuclear force exceeds that of the electric force, but it is
a very short-range force and so only acts, more or less, when the protons and neutrons come into contact with each other in a nucleus. The reason why there are only 92 naturally occurring elements is that when you get to number 93 (neptunium), it has 93 positively charged protons in its nucleus, and the repulsive electric force is just big enough to overcome the strong nuclear force, causing the nucleus to break up. At the time of this writing, scientists have created elements with up to 117 protons in the nucleus, but these heavy elements are unstable and don't hang around for long.
Now we can return to the process of nuclear fusion, which powers the Sun. The basic mechanism is to fuse together atoms or isotopes of hydrogen to form helium, and this results in the release of nuclear energy. The problem with fusion is overcoming the repulsive electric force that the protons have, and getting them close enough so that the strong nuclear force can grab hold of them and squeeze them together as a nucleus. Fortunately, the conditions in the core of the Sun are ideal for this to happen. The density is extremely high so that the protons are already very close together. The temperature is also extreme, on the order of 15 million degrees Celsius, which is such a large number as to make its meaning difficult to grasp. But the consequences for the protons is that, at these temperatures, they have high energies, and are rushing about at high speeds. This combination of the density and energy of the protons means that they can overcome their mutual electric repulsion, and get close enough for the strong nuclear force to bind them together. Thus we can form the nucleus of heavier atoms from light ones.
But where does the nuclear energy come from? In Chapter 1, we discussed what Einstein did for us. Another thing he gave us is an understanding that energy and mass are essentially different forms of the same thing. This he summarized in his famous equation E = mc2, which basically says that mass m can be converted into energy E and vice versa (where c = 300,000,000 meters per second is the speed of light). I know this book isn't supposed to have any equations in it, but this one is so well known that it has become a part of our culture. It pops up in the titles of books and television programs. The thing to note about a helium nucleus is that, remarkably, it weighs less than the two protons and two neutrons (Figure 6.1) that compose it. Some of the mass has been used up in the energy associated with the action of the strong nuclear force in binding the helium nucleus together. To be precise, the helium nucleus has a mass just 99.3% of the mass of its parts. When the protons and neutron fuse together to form a helium atom, 0.7% of their mass is converted to pure energy in a way described by Einstein's famous equation.
We can do a simple sum to calculate how much of the Sun's mass is being converted every second into energy by the process of nuclear fusion. If we go out into the garden and present an area of one square meter to the Sun, the solar power falling on that surface is roughly 1.4 kilowatts (neglecting any losses that may occur due to passage through the atmosphere). If we now multiply this power by the number of square meters on a sphere the size of the Earth's orbit around the Sun, we can calculate the total power radiated by the Sun. Using Einstein's equation, we can then estimate the amount of mass being converted to energy each second. This turns out to be a staggering 4^ million metric tonnes! This may sound like rather a lot of mass loss for the Sun to sustain, but the Sun can easily keep this up for many billions of years without it making much of a dent in the total mass. In fact, over the last 4^ billion years or so of the Sun's history, during which it has been shining at more or less a steady rate, about 100 Earth masses have been converted into pure radiate energy! Although this is a rather amazing statistic, nevertheless it represents only a tiny fraction of the mass available for nuclear energy production in the Sun.
Our very existence here on Earth is dependent on the Sun's stability, which is maintained by the Sun's massive gravitational field containing the awesome power of this nuclear furnace at its core.
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