1. The Sun

2. The Sun – Our Star
Sole source of light and heat for the maintenance of life on Earth.
Held together by its own gravity and powered by nuclear fusion
Radius larger than 100 Earth Radii
Mass greater than 300,000 Earth masses
What we know about the sun can be applied to many other stars in the sky

3. The Sun - Overall Structure
Core
200,000 km
Site of nuclear fusion
Radiation Zone
300,000 km
Energy transported to surface by radiation
Convection Zone
Material of the sun is in constant convective motion
Photosphere
500 km
Thickness is why we perceive the sun as having a well-defined, sharp edge
Emits the radiation we see
Chromosphere
1500 km
Lower atmosphere
Transition zone
8500 km
Temperature rises dramatically
Solar Corona
Above 10,000 km
Thin, hot upper atmosphere
Solar Wind
Flows away from the sun and permeates the entire solar system

4. The Sun – Overall Structure

5. Luminosity Total energy radiated by the sun
Calculated from the fraction that reaches Earth (solar constant)
Equivalent to 10 billion 1-megaton nuclear bombs per second

6. The Solar Interior
Mathematical models, consistent with observation and physical principles, provide information about the Sun’s interior.
In equilibrium, inward gravitational force must be balanced by outward pressure.
Solar Temperature increases with decreasing radius
Core – 15 million K
Photosphere – 5800 K

7. Energy Transport
The radiation zone is relatively transparent; the cooler convection zone is opaque.

8. Evidence of Solar Convection
The visible top layer of the convection zone is granulated, with areas of upwelling material surrounded by areas of sinking material.

9. The Solar Atmosphere
Spectral analysis can tell us what elements are present, but only in the chromosphere and photosphere.
Hydrogen is, by far, the most abundant element

10. The Solar Atmosphere The cooler chromosphere is above the photosphere.
Difficult to see directly, as the photosphere is too bright, unless the Moon covers the photosphere and not the chromosphere during an eclipse

11. The Solar Atmosphere
Small solar storms erupt every few minutes in the chromosphere.

12. The Solar Atmosphere
Solar corona can be seen during eclipse if both the photosphere and the chromosphere are blocked.

13. The Solar Atmosphere
The corona is much hotter than the layers below it.
Roughly constant at 3 million K
It must have a heat source, probably electromagnetic interactions.

14. The Solar Wind
Electromagnetic Radiation and fast-moving particles escape from the sun all the time.
Radiation travels at the speed of light
Reaches Earth in 8 minutes
Particles travel slower
Reach Earth in a few days
The constant stream of escaping particles is the solar wind
The sun is constantly shedding mass

15. Sunspots
Sunspots appear dark because they are slightly cooler than their surroundings.

16. Sunspots Sunspots come and go, typically in a few days.
Sunspots are linked by pairs of magnetic field lines.

17. Sunspots
The rotation of the Sun drags magnetic field lines around with it, causing kinks.

18. Sunspots
The Sun has an 11-year sunspot cycle, during which sunspot numbers rise, fall, and then rise again.

19. Sunspots
This is really a 22-year cycle, because the spots switch polarities between the northern and southern hemispheres every 11 years.
Maunder minimum: few, if any, sunspots.

20. Active Regions
Areas around sunspots are active; large eruptions may occur in the photosphere.
Most frequent and violent around the time of sunspot maximum
Solar prominence is large sheet of ejected gas. Nearly 10 times the diameter of Earth

21. Active Regions
A Solar flare is a large explosion on the Sun’s surface, emitting a similar amount of energy to a prominence, but in seconds or minutes rather than days or weeks.
Temperatures can reach 100 million K

22. Active Regions
A coronal mass ejection emits charged particles that can affect the Earth.

23. The Heart of the Sun
Nuclear fusion requires that like-charged nuclei get close enough to each other to fuse.
This can happen only if the temperature is extremely high – over 10 million K.

24. The Heart of the Sun
The process that powers most stars is a three-step fusion process.
During a fusion process, total mass decreases.
4 Hydrogen nuclei combine to form one helium and energy

25. The Heart of the Sun
Neutrinos are emitted directly from the core of the Sun, and escape, interacting with virtually nothing. Being able to observe these neutrinos would give us a direct picture of what is happening in the core.
Unfortunately, they are no more likely to interact with Earth-based detectors than they are with the Sun; the only way to spot them is to have a huge detector volume and to be able to observe single interaction events.

26. The Heart of the Sun
Neutrino observatories