Our Star - Weebly

10

Our Star

learning goals

10.1 A Closer Look at the Sun Why does the Sun shine? What is the Sun's structure?

10.2 Nuclear Fusion in the Sun How does nuclear fusion occur in the Sun? How does the energy from fusion get out of the Sun? How do we know what is happening inside the Sun?

10.3 The Sun?Earth Connection What causes solar activity? How does solar activity vary with time?

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Astronomy today encompasses the study of the entire universe, but the root of the word astronomy comes from the Greek word for "star." Although we have learned a lot about the universe up to this point in the book, only now do we turn our attention to the study of the stars, the namesakes of astronomy.

When we think of stars, we usually think of the beautiful points of light visible on a clear night. But the nearest and most easily studied star--our Sun--is visible only in the daytime. Of course, the Sun is important to us in many more ways than just as an object for astronomical study. The Sun is the source of virtually all light, heat, and energy reaching Earth, and life on Earth's surface could not survive without it.

In this chapter, we will study the Sun in some depth. We will see how the Sun generates the energy that supports life on Earth. Equally important, we will study our Sun as a star so that in subsequent chapters we can better understand stars throughout the universe.

essential preparation

1. Where do objects get their energy? [Section 4.3] 2. What is matter? [Section 5.1] 3. How does light tell us the temperatures of planets

and stars? [Section 5.2]

The Sun Tutorial, Lesson 1

10.1 A Closer Look at the Sun

We discussed the general features of the Sun in our tour of the solar system in Chapter 6 (see page 146). Now it's time to get better acquainted with our nearest star.

Ancient peoples recognized the vital role of the Sun in their lives. Some worshipped the Sun as a god. Others created mythologies to explain its daily rise and set. But no one who lived before the 20th century knew how the Sun provides us with light and heat.

Most ancient thinkers imagined the Sun to be some type of fire, perhaps a lump of burning coal or wood. It was a reasonable suggestion for the times, since science had not yet advanced to the point where the idea could be tested. Ancient people did not know the size or distance of the Sun, so they could not imagine how incredible its energy output really is. And they did not know how long Earth had existed, so they had no way of knowing that the Sun has provided light and heat for a very long time.

Scientists began to address the question of how the Sun shines around the middle of the 19th century, by which time the Sun's size and distance had been measured with reasonable accuracy. The ancient idea that the Sun was composed of burning coal or wood was quickly ruled out: Calculations showed that such burning could not possibly account for the Sun's huge energy output. Other ideas based on chemical processes were likewise ruled out.

In the late 19th century, astronomers came up with an idea that seemed more plausible, at least at first. They suggested that the Sun generates energy by slowly contracting in size, a process called gravitational contraction (or Kelvin-Helmholtz contraction, after the scientists who proposed it). Recall that a shrinking gas cloud heats up because some of the gravitational potential energy of gas particles far from the cloud center is converted into thermal energy as the gas moves inward (see Figure 4.12b). A gradually shrinking Sun would always have some gas moving inward, converting gravitational potential energy into thermal energy. This thermal

Chapter 10 Our Star 287

Figure 10.1 An acrobat stack is in gravitational equilibrium: The lowest person supports the most weight and feels the greatest pressure, and the overlying weight and underlying pressure decrease for those higher up.

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energy would keep the inside of the Sun hot. Because of its large mass, the Sun would need to contract only very slightly each year to maintain its temperature--so slightly that the contraction would have been unnoticeable to 19th-century astronomers. Calculations showed that gravitational contraction could have kept the Sun shining steadily for up to 25 million years. For a while, some astronomers thought that this idea had solved the ancient mystery of how the Sun shines. However, geologists pointed out a fatal flaw: Studies of rocks and fossils had already shown Earth to be far older than 25 million years, which meant that gravitational contraction could not account for the Sun's energy generation.

Why does the Sun shine?

With both chemical processes and gravitational contraction ruled out as the explanation for why the Sun shines, 19th-century scientists were stumped. There was no known way that an object the size of the Sun could generate so much energy for billions of years. A completely new type of explanation was needed, and it came with Einstein's publication of his special theory of relativity in 1905.

Einstein's theory included his famous equation E mc2, which tells us that mass itself contains an enormous amount of potential energy [Section 4.3]. Calculations demonstrated that the Sun's mass contained more than enough energy to account for billions of years of sunshine, if only there were some way for the Sun to convert the energy of mass into thermal energy. It took a few decades for scientists to work out the details, but by the end of the 1930s we had learned that the Sun converts mass into energy through the process of nuclear fusion.

How Fusion Started Nuclear fusion requires extremely high temper-

atures and densities (for reasons we will discuss in the next section). In

the Sun, these conditions are found deep in the core. But how did the

Sun become hot enough for fusion to begin in the first place?

Gravitational contraction released the energy that made the Sun's core hot enough for fusion.

The answer invokes the mechanism of gravitational contraction, which astronomers of the late 19th century mistakenly thought might

be responsible for the Sun's heat today. Recall that our Sun was born about 412 billion years ago from a collapsing cloud of interstellar gas

[Section 6.2]. The contraction of the cloud released gravitational

potential energy, raising the interior temperature and pressure. With-

out another source of energy to replace the energy output from its

surface, the Sun continued to contract until its central temperature

finally rose high enough to sustain nuclear fusion. Then the Sun was

finally able to replace the energy lost from the surface and stopped

contracting.

The Stable Sun The Sun continues to shine steadily today because it has achieved two kinds of balance that keep its size and energy output stable. The first kind of balance, called gravitational equilibrium (or hydrostatic equilibrium), is between the outward push of internal gas pressure and the inward pull of gravity. A stack of acrobats provides a simple example of gravitational equilibrium (Figure 10.1). The bottom person supports the weight of everybody above him, so his arms must push upward with enough pressure to support all this weight. At each higher level, the overlying weight is less, so it's a little easier for each additional person to hold up the rest of the stack.

Gravitational equilibrium works much the same in the Sun, except

the outward push against gravity comes from internal gas pressure rather

than an acrobat's arms. The Sun's internal pressure precisely balances

gravity at every point within it, thereby keeping the Sun stable in size

Everywhere inside the Sun, the outward push of pressure balances the inward pull of gravity.

(Figure 10.2). Because the weight of overlying layers is greater as we look deeper into the Sun, the pressure must increase with depth.

Deep in the Sun's core, the pressure makes the gas hot and dense enough

to sustain nuclear fusion. The energy released by fusion, in turn, heats

the gas and maintains the pressure that keeps the Sun in balance against

the inward pull of gravity.

pressure gravity

The outward push of pressure . . .

. . . precisely balances the inward pull of gravity.

Earth's atmosphere is also in gravitational equilibrium, with the weight of upper layers supported by the pressure in lower layers. Use this idea to explain why the air gets thinner at higher altitudes.

The second kind of balance is energy balance between the rate at which fusion releases energy in the Sun's core and the rate at which the Sun's surface radiates this energy into space. Energy balance is important because without it the balance between pressure and gravity would not remain steady. If fusion in the core did not replace the energy radiated from the surface, thereby keeping the total thermal energy content constant, then gravitational contraction would cause the Sun to shrink and force its core temperature to rise.

In summary, the answer to the question "Why does the Sun shine?" is that about 412 billion years ago gravitational contraction made the Sun hot enough to sustain nuclear fusion in its core. Ever since, energy liberated by fusion has maintained gravitational equilibrium and energy balance within the Sun, keeping it shining steadily and supplying the light and heat that sustain life on Earth.

Pressure is greatest deep in the Sun where the overlying weight is greatest.

Figure 10.2

Gravitational equilibrium in the Sun: At each point inside, the pressure pushing outward balances the weight of the overlying layers.

What is the Sun's structure?

The Sun is essentially a giant ball of hot gas or, more technically, plasma-- a gas in which atoms are ionized [Section 5.2] because of the high temperature. A plasma behaves much like an ordinary gas, except that its many positively charged ions and freely moving electrons enable it to create and respond to magnetic fields.

The differing temperatures and densities of the plasma at different depths give the Sun the layered structure shown in Figure 10.3. To help you make sense of the figure, let's imagine that you have a spaceship that can somehow withstand the immense heat and pressure of the Sun and take an imaginary journey from Earth to the center of the Sun. This journey will acquaint you with the basic properties of the Sun, which we'll discuss in greater detail in the rest of this chapter.

solar wind

photosphere

convection zone

chro

corona mosphere

core

Basic Properties of the Sun As you begin your journey from Earth, the Sun appears as a whitish ball of glowing gas. Just as astronomers have done in real life, you can use simple observations to determine basic properties of the Sun. Spectroscopy [Section 5.2] tells you that the Sun is made almost entirely of hydrogen and helium. From the Sun's angular size and distance, you can determine that its radius is just under 700,000 kilometers, or more than 100 times the radius of Earth. Even sunspots, which appear as dark splotches on the Sun's surface, can be

solar wind

sunspots

radiation zone

Figure 10.3 The basic structure of the Sun.

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Table 10.1

Basic Properties of the Sun

Radius (RSun)

Mass (MSun)

Luminosity (LSun) Composition (by percentage of mass) Rotation rate

Surface temperature

Core temperature

696,000 km (about 109 times the radius of Earth)

2 1030 kg (about 300,000 times the mass of Earth) 3.8 1026 watts

70% hydrogen, 28% helium, 2% heavier elements

25 days (equator) to 30 days (poles) 5800 K (average); 4000 K (sunspots) 15 million K

larger in diameter than Earth. You can measure the Sun's mass using Newton's version of Kepler's third law. It is about 2 1030 kilograms, which is some 300,000 times the mass of Earth and nearly 1000 times the mass of all the planets in our solar system put together. You can observe the Sun's rotation by tracking the motion of sunspots or by measuring Doppler shifts [Section 5.2] on opposite sides of the Sun. Unlike a spinning ball, the entire Sun does not rotate at the same rate: The solar equator completes one rotation in about 25 days, and the rotation period increases with latitude to about 30 days near the solar poles.

As a brief review, describe how astronomers use Newton's version of Kepler's third law to determine the mass of the Sun. What two properties of Earth's orbit do we need to know in order to apply this law? (Hint: See Section 4.4.)

The Sun releases an enormous amount of radiative energy into space. In science, we measure energy in units of joules [Section 4.3]. We define power as the rate at which energy is used or released. The standard unit of power is the watt, defined as 1 joule of energy per second; that is, 1 watt 1 joule/s. For example, a 100-watt light bulb requires 100 joules of energy for every second it is left turned on. The Sun's total power output, or luminosity, is an incredible 3.8 1026 watts. Table 10.1 summarizes the basic properties of the Sun.

commonMisconceptions

The Sun Is Not on Fire

We often say that the Sun is "burning," a term that conjures up images of a giant bonfire in the sky. However, the Sun does not burn in the same sense as a fire burns on Earth. Fires generate light through chemical changes that consume oxygen and produce a flame. The glow of the Sun has more in common with the glowing embers left over after the flames have burned out. Much like hot embers, the Sun's surface shines because it is hot enough to emit thermal radiation that includes visible light [Section 5.2].

Hot embers quickly stop glowing as they cool, but the Sun keeps shining because its surface is kept hot by the energy rising from its core. Because this energy is generated by nuclear fusion, we sometimes say that it is the result of "nuclear burning"--a term intended to suggest nuclear changes in much the same way that "chemical burning" suggests chemical changes. Nevertheless, while it is reasonable to say that the Sun undergoes nuclear burning in its core, it is not accurate to speak of any kind of burning on the Sun's surface, where light is produced primarily by thermal radiation.

The Sun's Atmosphere Even at a great distance from the Sun, you

and your spacecraft can feel slight effects from the solar wind--the

stream of charged particles continually blown outward in all directions

from the Sun. Recall that the solar wind helps shape the magnetospheres

of planets (see Figure 7.6) and blows back the material that forms the

plasma tails of comets [Section 9.2].

As you approach the Sun more closely, you begin to encounter the

low-density gas that represents what we usually think of as the Sun's

atmosphere. The outermost layer of this atmosphere, called the corona,

extends several million kilometers above the visible surface of the Sun.

The temperature of the corona is astonishingly high--about 1 million K--

explaining why this region emits most of the Sun's X rays. However, the

corona's density is so low that your spaceship feels relatively little heat,

despite the million-degree temperature [Section 4.3].

The Sun's upper atmosphere is much hotter than the visible surface, or photosphere, but its density is much lower.

Nearer the surface, the temperature suddenly drops to about 10,000 K in the chromosphere, the middle layer of the solar atmo-

sphere that radiates most of the Sun's ultraviolet radiation. Then you

plunge through the lowest layer of the atmosphere, the photosphere,

which is the visible surface of the Sun. Although the photosphere looks

like a well-defined surface from Earth, it consists of gas far less dense

than Earth's atmosphere. The temperature of the photosphere averages

just under 6000 K, and its surface seethes and churns like a pot of boil-

ing water. The photosphere is also where you'll find sunspots, regions of

intense magnetic fields that would cause your compass needle to swing

wildly about.

The Sun's Interior Up to this point in your journey, you may have seen Earth and the stars when you looked back. But blazing light engulfs you as you slip beneath the photosphere. You are inside the Sun, and incredible turbulence tosses your spacecraft about. If you can hold

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