26.1 The Sun
Reading Focus
Key Concepts
How does the sun produce energy?- Why does the sun remain stable over time?
- What is the structure of the sun?
- What are some features of the sun's atmosphere?
Vocabulary
core
radiation zone
convection zone
photosphere
chromosphere
corona
solar wind
sunspots
prominences
solar flare
Reading Strategy
Building Vocabulary Copy the table below. Then, as you read, write a definition of each vocabulary term in your own words. Add seven more rows to complete the vocabulary list.
Vocabulary Term |
Definition |
|---|---|
Core |
a. |
Radiation zone |
b. |
Convection zone |
c. |
Figure 1 The sun gives off tremendous amounts of energy. Plants on Earth use sunlight directly in photosynthesis.
You know that flowers like the ones in Figure 1 need sunlight. In fact, almost all life on Earth depends on sunlight. Without the sun's radiation, Earth would be a cold, dark, and lifeless planet. But what exactly is the sun? Scientists have learned that the sun is a glowing ball of gas. The sun is extremely hot, about 15 million K at its center and 5800 K at its surface. At such temperatures, most of the atoms in the sun have lost most of their electrons, forming a plasma of ionized gas. About 70 percent of the sun's mass is hydrogen and 28 percent is helium.
Energy From the Sun
The sun gives off tremendous amounts of energy in the form of electromagnetic radiation. But what is the source of this energy? Does the sun burn some type of fuel? If so, how long will it continue to burn before all its fuel is used up? Because the sun and Earth formed together, the answers to these questions can be used to estimate the age of Earth. As a result, scientists from many different fields began a debate that lasted more than 100 years.
In the late 1700s, the German philosopher Immanuel Kant wondered whether the sun produced its energy from an ordinary chemical reaction. He calculated that if it did, it could last only a few thousand years. In the mid-1800s, two physicists, Hermann von Helmholtz and William Thomson (better known as Lord Kelvin), proposed another idea. They hypothesized that gravity was causing the sun to shrink over time, converting its potential energy into thermal energy. According to their calculations, the sun was no more than 20 to 30 million years old.
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