Inside The

The Sun contains 99.8 percent of the Solar System's mass (most of the rest of it is in Jupiter), about 330,000 times the bulk of the Earth. Around 73 percent of the Sun is hydrogen, 25 percent is helium, and all the other elements added together comprise less than 2 percent of the solar mass.

The Sun agglomerated from a huge cloud of gas and dust, which was largely the debris left from previous expired stars and supernova explosions. In its core the temperature is over 10 million degrees Celsius (20 million degrees Fahrenheit), and the pressure is in excess of 200 billion times our atmospheric pressure. We say that the material within the Sun is a gas, and yet its density is 150 times that of water, 20-fold that of iron.

Under such conditions the repulsive forces between hydrogen nuclei may be overcome. (Hydrogen nuclei are simply bare protons—positively charged subatomic particles, the number of which within any nucleus controls the type of element it is.) Helium is produced as they coalesce. That is, the Sun is a natural fusion reactor. If we could do the same thing on Earth we would have a practically unlimited supply of energy, although one could not say that it would be free because many, many billions of dollars have already been spent in the as yet unsuccessful quest to produce controllable fusion. (Uncontrolled fusion is easy: it's called a hydrogen bomb.)

Since the time this fusion process began in the center of the Sun just over four and a half billion years ago, about half of the usable hydrogen fuel has been transmuted into helium. The word "usable" is significant here because, as the hydrogen at the middle is consumed, the shell where fusion is occurring moves outwards. But away from the center the temperatures and pressures eventually become too low to support hydrogen burning, and so fusion halts. This means that much of the hydrogen in the Sun will never be burnt. If the interior of the Sun were better mixed then it might have a longer lifetime, but things are as they are, and stellar interiors are heavily stratified.

Figure 5-1 shows a schematic cross section of the Sun. Energy generation through fusion occurs only in the core, which occupies about 25 percent of the overall radius. That energy is transported outwards through the radiative zone, the next 50 percent or more of the radius. The energy is carried through that zone by photons of light, rather than by conduction or convection. (Conduction is the process of hot atoms colliding with cooler ones and transporting heat away, in the same way as the handle of a long iron rod gets warm if the other end is left in a fire. Convection is

FIGURE 5-1. A cross section through the Sun showing its basic features at different levels.

the wholesale upward movement of hot atoms, like heated air rising above a stove.) Those photons deep within the Sun are not at the wavelengths of visible light; the temperatures there are so high that the photons are mainly in the gamma- and X-ray region of the spectrum.

The final 20 percent of the solar radius is known as the convection zone. In this layer the temperature gradient is sufficient for bubbles of hot sun-stuff to rise until close to the surface, giving the Sun its characteristic mottled appearance. (The effect is similar to making gravy or jam: the heating at the base of the pan makes the liquid want to rise, but not all of the liquid can rise at once, so it churns over in globules moving together.) After cooling, that material sinks again to the base of the convection zone, where it is heated again before beginning another round trip to the surface, as part of another cell. This convection results in the Sun's surface not being smooth, but covered with thousands of these globules, which are called granules. They are each the size of a continent, but short-lived, persisting for but a few minutes before dissipating and being replaced by some new rising globule.

What is usually referred to as the "surface" of the Sun is correctly termed the photosphere (that is, the region from which our eyes detect photons). This is not a solid surface, but a layer of ionized gas at a temperature of about 5,700 degrees Celsius (10,300 degrees Fahrenheit). The temperature of the photosphere determines the color we perceive: that's why the Sun appears yellow to us, whereas hotter stars appear blue or white, and cooler ones orange or red.

In Figure 1-1 we saw photographs of the Sun's surface, including some sunspots. These are cooler regions of the photosphere, typically at around 4,000 degrees Celsius (7,200 degrees

Fahrenheit). Their origin is not yet completely understood, although they are certainly related to convolutions of the intense solar magnetic field. In a sunspot the magnetic field is several thousand times as intense as elsewhere on the solar surface. One should not underestimate their size: many are 25,000 miles across, several times the diameter of the Earth.

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