The Sun is the sole source of light and heat for the maintenance of life on Earth. It is a star, a glowing ball of gas held together by its own gravity and powered by nuclear fusion at its center. In its physical and chemical properties the Sun is very similar to most other stars, regardless of when and where they formed. Indeed, our Sun appears to be a rather "typical" star, lying right in the middle of the observed ranges of stellar mass, radius, brightness, and composition. Far from detracting from the interest in the Sun, this very mediocrity is one of the main reasons that astronomers study it—they can apply knowledge of solar phenomena to so many other stars in the universe.
The Sun Data box lists some basic orbital and physical solar data. The Sun's radius, roughly 700,000 km, is most directly determined by measuring its angular size (0.5° ) and then employing elementary geometry. 
(Sec. 1.5). The Sun's mass, 2.0 
1030 kg, follows from application of Newton's laws of motion and gravity to the observed orbits of the planets. (More Precisely 2-3)The average solar density derived from these quantities, approximately 1400 kg/m3, is quite similar to that of the jovian planets and about one-quarter the average density of Earth.
The solar rotation period is found by timing sunspots (Sec. 2.5) and other surface features as they traverse the solar disk. These observations indicate that the Sun rotates in about a month, but it does not do so as a solid body. Instead, it spins differentially—faster at the equator and slower at the poles, like Jupiter and Saturn.
The Sun's surface temperature is measured by applying the radiation laws to the observed solar spectrum. (Sec. 3.4) The distribution of solar radiation has the approximate shape of a blackbody curve for an object at about 5800 K. The average solar temperature obtained in this way is known as the Sun's effective temperature.
Having a radius of more than 100 Earth radii, a mass of more than 300,000 Earth masses, and a surface temperature well above the melting point of any known material, the Sun is clearly a body very different from any other we have encountered so far.
The Sun has a surface of sorts—not a solid surface (the Sun contains no solid material) but rather that part of the brilliant gas ball we perceive with our eyes or view through a heavily filtered telescope. This "surface"—the part of the Sun that emits the radiation we see—is called the photosphere. Its radius (listed in our table as the radius of the Sun) is about 700,000 km. However, the thickness of the photosphere is probably no more than 500 km, less than 0.1 percent of the radius, which is why we perceive the Sun as having a well-defined, sharp edge (Figure 16.1).
Figure 16.1 This photograph of the Sun shows a sharp solar limb, although our star, like all stars, is made of a gradually thinning gas. The edge appears sharp because the solar photosphere is so thin.
The main regions of the Sun are illustrated in Figure 16.2 and summarized in Table 16.1. Just above the photosphere is the Sun's lower atmosphere, called the chromosphere, about 1500 km thick. From 1500 km to 10,000 km above the top of the photosphere lies a region called the transition zone, where the temperature rises dramatically. Above 10,000 km, and stretching far beyond, is a tenuous (thin), hot upper atmosphere, the solar corona. At still greater distances, the corona turns into the solar wind, which flows away from the Sun and permeates the entire solar system. (Sec. 6.5)
| TABLE 16.1 The Standard Solar Model | ||||||||||||||||||||||||||||||||||||||||||||||||
*These radii are based on the accurately determined radius of the photosphere. The other radii quoted are approximate, round numbers. |
Figure 16.2 The main regions of the Sun, not drawn to scale, with some physical dimensions labeled.
Extending down some 200,000 km below the photosphere is the convection zone, a region where the material of the Sun is in constant convective motion. Below the convection zone lies the radiation zone, where solar energy is transported toward the surface by radiation rather than by convection. The term solar interior is often used to mean both the radiation and convection zones. The central core, roughly 200,000 km in radius, is the site of powerful nuclear reactions that generate the Sun's enormous energy output.
The properties of size, mass, density, rotation rate, and temperature are familiar from our study of the planets. But the Sun has an additional property, perhaps the most important of all from the point of view of life on Earth—it radiates a great deal of energy into space, uniformly (we assume) in all directions. By holding a light-sensitive device—a solar cell, perhaps—perpendicular to the Sun's rays, we can measure how much solar energy is received per square meter of surface area every second. Imagine our detector as having a surface area of 1 m2 and as being placed at the top of Earth's atmosphere. The amount of solar energy reaching this surface each second is a quantity known as the solar constant. Its value is approximately 1400 watts per square meter (W/m2). Most of this energy reaches Earth's surface. Thus, for example, a sunbather's body having a total surface area of about 0.5 m2 receives solar energy at a rate of nearly 700 watts, roughly equivalent to the output of a typical electric room heater or about ten 75-W light bulbs.
Let us now ask about the total amount of energy radiated in all directions from the Sun, not just the small fraction intercepted by our detector or by Earth. Imagine a three-dimensional sphere that is centered on the Sun and is just large enough that its surface intersects Earth's center (Figure 16.3). The sphere's radius is 1 A.U., and its surface area is therefore 4
(1 A.U.)2, or approximately 2.8 1023 m2. Multiplying the rate at which solar energy falls on each square meter of the sphere (that is, the solar constant) by the total surface area of our imaginary sphere, we can determine the total rate at which energy leaves the Sun's surface. This quantity is known as the luminosity of the Sun. It turns out to be just under 4 1026 W.
Figure 16.3 We can draw an imaginary sphere around the Sun so that the sphere's edge passes through Earth's center. The radius of this imaginary sphere equals one astronomical unit. By multiplying the sphere's surface area by the solar constant, we can measure the Sun's luminosity, the amount of energy it emits each second.
The Sun is an enormously powerful source of energy. Every second, it produces an amount of energy equivalent to the detonation of about 100 billion 1-megaton nuclear bombs. Put another way, the solar luminosity is equivalent to 4 trillion trillion 100-W light bulbs shining simultaneously—about 1019 dollars' worth of energy radiated every second (at current U.S. rates)!
