1. The sun gives off tremendous amounts of energy
The sun gives off tremendous amounts of energy. Plants on Earth use sunlight directly in photosynthesis.

2. Energy From the Sun
produced in core by fusion of hydrogen nuclei into helium nuclei.
Fusion: nuclei combine into more massive nuclei, releasing enormous amounts of energy.
The sun gives off tremendous amounts of energy as electromagnetic radiation.

3. Energy From the Sun
Nuclear fusion within the sun’s central region converts mass into energy.
The green particles in the diagram are protons, and the purple particles are neutrons.
Hydrogen nuclei collide to form helium-3
Two helium-3 nuclei collide.
Helium-4 and hydrogen nuclei form.
Gamma ray

4. Forces in Balance
Sun has been a fairly stable energy source for billions of years
Inward and outward forces within the sun must be balanced so that temperature, pressure, and density at any depth remain fairly constant over time.
The sun is stable because an inward pull of gravity balances an outward push of thermal pressure from nuclear fusion
Ions move faster, exerting an outward thermal pressure
At the same time, gravity pulls the gas inward.
Astronomers estimate the sun will remain stable for another 5 billion years.

5. Astronomers infer the structure of the sun’s interior using:
The sun’s interior consists of the core, the radiation zone, and the convection zone.
Astronomers infer the structure of the sun’s interior using:
mathematical models
observations of vibrations (similar to earthquakes)

6. The Sun’s Interior
Core
The sun’s core is its central region, where nuclear fusion occurs.
The core has a diameter of about 400,000 kilometers, a little more than a quarter of the diameter of the entire sun.

7. The Sun’s Interior
Radiation Zone
The radiation zone is an area of highly compressed gas.
Energy is transferred by absorption and re-radiation of electromagnetic waves.
This region is so dense, energy can take more than 100,000 years to pass through it.

8. The Sun’s Interior
Convection Zone
Energy from the radiation zone passes into the convection zone, the outer layer of the sun’s interior.
Energy is transferred outward mainly by convection currents.
Hot gases in the convection zone
rise toward the sun’s atmosphere
while cooler gases sink downward.

9. The Sun’s Atmosphere
The sun’s atmosphere consists of three layers: the photosphere, the chromosphere, and the corona.
Photosphere
The innermost layer of the sun’s atmosphere, the visible surface of the sun, is the photosphere.
The photosphere is about 500 kilometers thick and has an average temperature of about 5800 K.
The photosphere is called the sun’s surface.
The photosphere’s surface has a bubbly appearance.
The bubbles, called granules, are the tops of convection currents.

10. In the chromosphere, the middle layer of the sun’s atmosphere:
Temperature is nearly 10,000 K
At these temperatures, hydrogen gas emits a reddish light
Normally visible only when the photosphere is blocked, such as during a total solar eclipse.

11. The Sun’s Atmosphere Corona
The chromosphere merges with the corona, the outermost layer of the sun’s atmosphere:
Gases in the corona are very hot, about 1 million Kelvin.
But the corona has such a low density that the total amount of thermal energy in it is relatively small.
The corona extends millions of kilometers above the chromosphere.
The corona gradually thins into the solar wind: a stream of electrically charged particles that flows outward from the sun through the solar system

12. The Sun’s Atmosphere
The corona can only be seen from Earth during a total solar eclipse or when viewed with a special telescope.

13. Features of the Sun’s Atmosphere
Features of the sun’s atmosphere include sunspots, prominences, and solar flares.
Sunspots
Sunspots are areas of gas in the photosphere that are cooler than the surrounding gases
Some are larger than Earth.
If sunspots could be seen apart from the rest of the sun, they would shine brightly.
Sunspots are often found in groups and are associated with intense magnetic fields on the sun.

14. Features of the Sun’s Atmosphere
Sunspots last from a few hours to a few months.
The number of sunspots varies in cycles, with about 11 years separating one sunspot peak from the next.
During peak periods, there may be a hundred sunspots on the sun.
Galileo Galilei discovered that the sun rotates by observing the motion of sunspots.
Sunspots near the sun’s equator move faster than sunspots near the sun’s poles.
This means that the sun rotates faster at its equator than near its poles.

15. Features of the Sun’s Atmosphere
Prominences
Prominences are huge loops of gas that erupt from sunspot regions
They extend upward from the photosphere into the chromosphere and sometimes into the corona.
Some prominences reach heights of more than 100,000 kilometers above the sun’s surface.
Prominences travel along the magnetic field lines that connect sunspots.

16. Features of the Sun’s Atmosphere
Solar Flares
The sun’s surface sometimes erupts dramatically, producing X-rays and hurling charged particles into space at speeds of 1000 km/s or more in a solar flare
Solar flares usually occur near sunspots and heat the corona to a temperature near 20 million K.
Solar flares increase the solar wind, so they can disrupt electric power transmission and radio, television, and telephone signals.

17. The sun has an interior and an atmosphere.
Convection zone (about 200,000 km thick)
Prominence
Radiation zone (about 300,000 km thick)
Chromosphere (about 1500 km thick)
Core (about 400,000 km in diameter)
Photosphere (visible surface)
Corona
(extending millions of km into space)
Solar flare
Sunspot (cool region)
Granulated surface
(caused by convection)

18. What is the source of the sun’s energy?
Assessment Questions
What is the source of the sun’s energy?
The friction between its molecules gives off thermal energy.
The conversion of potential energy into thermal energy as gravity pulls the sun into its own core.
Combustion of hydrogen gas on the sun’s surface.
Fusion of hydrogen nuclei into helium nuclei.
ANS: D

19. Assessment Questions
The sun has remained stable for billions of years because there is a balance between
gravity and the tendency of hydrogen and helium to rise.
gravity and thermal pressure.
thermal pressure and the repulsion between nuclei.
the amount of energy it absorbs and the amount of energy it produces.
ANS: B

20. Convection currents in the sun’s convection zone carry energy from
Assessment Questions
Convection currents in the sun’s convection zone carry energy from
the core to the photosphere.
the radiation zone to the photosphere.
the core to the radiation zone.
the photosphere to the corona.
ANS: B