1. The Sun

2. The Big Picture The Sun is our star About 4.5 billion years old
It is not merely the source of light that distinguishes day from night
It is also the main source of energy that powers weather, climate, and life on Earth
Humans wouldn’t exist without the Sun

3. Affects Earth
Supports life: photosynthesis, source of all food and fossil fuel
Drives seasons
Drives ocean currents
Drives weather
Drives climate

4. Overall Properties
A glowing ball of gas held together by its own gravity and powered by nuclear fusion

5. Rotation Can be measured by timing sunspots
The Sun rotates in about a month
It spends differentially
Faster at the equator
Slower at the poles
The equatorial rotation period at the equator is about 25 days
At a latitude of 60o there is a 31 day period
The rotation at the poles may be as long as 36 days

6. Temperature
The Sun’s surface temperature is measured by applying radiation laws
The average solar temperature obtained in this way is known as the Sun’s effective temperature
The surface temperature is well above the melting point of any known material

7. Structure The Sun does not have a solid surface
The Sun is made up of different regions
Core
Radiation zone
Convection zone
Photosphere
Chromosphere
Transition zone
Corona

8. Core Temperature 16 million K
The site of powerful nuclear fusion reactions
Generates the Sun’s enormous energy output
= 383 trillion kilowatts

9. Solar Interior Radiation Zone Convection Zone
Solar energy is transported toward the surface by radiation, not convection
Convection Zone
Below the photosphere
The material of the Sun is in constant convection motion
It takes a few hundred thousand years for photons to escape from the dense core and reach the surface
Temperature decreases from 8 million to 7,000 K

10. Photosphere Emits the radiation we see
Where sun spots are found (area of lower temperature and high magnetic activity)
The thickness is probably no more than 500 km
This is less than 0.1 percent of the radius

11. Chromosphere and Transition Zone
Just above the photosphere
Lower atmosphere
Transition Zone
1500 km to 10000
Where the temperature rises dramatically

12. Corona “crown” 10,000 km and above Thin hot upper atmosphere
At greater differences, the corona turns into solar wind:
Flows away from the Sun
Permeates the entire solar system

13. Solar Constant The Sun radiates a lot of energy into space
The amount of solar energy that reaches the Earth’s atmosphere is known as the solar constant
This value is approximately 1400 watts per square meter
Most of this energy reaches the Earth’s surface
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 lightbulbs

14. Energy Source Energy is radiated in all directions
Every second, it produces an amount of energy equivalent to the detonation of about 100 billion one-megaton nuclear bombs
Six seconds worth of solar energy output, suitably focused, would evaporate all of Earth’s oceans
Three minutes would melt our planet’s crust

15. Energy Transport
The energy produced by nuclear reactions in the core travels outward toward the surface in the form of radiation
Hot solar gas physically moves outward while cooler gas above it sinks, creating a characteristic pattern of convection cells
All through the convection zone, energy is transported to the surface by physical motion of the solar gas

16. Solar Spectrum
Astronomers can get an enormous amount of information about the Sun from an analysis of the absorption lines that arise in the photosphere and lower atmosphere