1. Activity of the Sun
X-ray image of the Sun.
T = K

2. Eruption (flare)
The arc follows magnetic field lines connecting two sunspots.

3. Solar wind
Flares generate streams of particles that reach Earth in 1-2 days. They produce Northern lights and can knock out the power grid.

4. The solar cycle The activity of the Sun has a maximum every 11 years.
It is related to a reversal of the Sun’s magnetic field.
The next maximum is expected about February 2013.

5. Fusion in the Sun
Light nuclei generate energy by fusing into a heavier nucleus.
The Sun fuses four H nuclei (protons) into helium.

6. Which interactions?
Fusion is a weak interaction process, since neutrinos are involved (Lect. 35).
The energy comes from the strong interaction.
The bottleneck is due to the electromagnetic interaction. Protons repel each other.
A temperature of 15 million degrees is required to have sufficient thermal energy to overcome the p-p repulsion.

7. Detailed fusion reaction in the Sun (optional)
The temperature at the center of the Sun is “only” 15 million degrees, about ten times less than in a fusion reactor or in a hydrogen bomb. The Sun gets away with such a “low” tem-perature because it produces energy at the extremely low rate of 6 W/kg . The human body produces heat at a rate of 1.2 W/kg, which is times higher. The Sun compen- sates by its huge mass.

8. Life cycle of the Sun
The Sun and the planets formed from a gas cloud 4 ½ billion years ago. Currently, the Sun is about half way through its stable period of fusing hydrogen into helium. But it is getting hotter and will evaporate all the water on Earth after 1 billion years. At the end of its stable period, the Sun will grow about 250 times bigger and become a red giant. Although its outer part cools, the large surface generates 2700 times the present radiation. Shortly thereafter fusion of helium into carbon begins. After changing its size a few more times the Sun eventually becomes an Earth-sized white dwarf consisting of carbon and oxygen. Then it slowly fizzles.