1. Measuring the Astronomical Unit
Mankind first found the scale of the solar system using the difference in the appearance of a transit of Venus from opposite sides of the Earth, whose size was known (requires accurate clocks as well).
In modern times we can just bounce radar off Venus and time the echo. We know “c” very precisely.

2. Measuring the Sun
Once you know the distance to the Sun (AU), you can convert its apparent size to a true size. Kepler/Newton can give you its mass (and thus density), and you can convert its observed brightness to an intrinsic luminosity once you know how far it is.
Distance = 1.5x108 km
Mass = 3.3x105 Earth masses = 2x1030 kg
Density = 1.41 gm/cc
Temperature = 5800K (at surface)
Luminosity = 3.8x1026 watts

3. The Sun at different wavelengths
Photosphere
Visible light
5800K
Chromosphere
Ultraviolet light K

4. Parts of the Sun Inside Surface
Energy is produced in the core of the Sun by nuclear fusion. Initially it flows away from the core as hot EM radiation. Eventually as the layers get cooler, they become too opaque. The energy in the outer 30% flows instead by convection. At the surface the energy is radiated into space.
There are several surface layers as well. Surprisingly, the atmosphere gets hotter on the outside – an influence of magnetic fields (which also produce many interesting features).
Inside
Surface

5. Convection
Convection is a mode of energy transport which crops up often. For example: heat flow in planetary interiors, in our atmosphere, and in cool stars. Hotter stuff is less dense, and therefore bouyant. It rises up until it can dump its energy (radiating it). Then it cools off, becomes less dense, and sinks back. There is a circulating pattern set up which carries heat outwards.

6. Granulation in the Photosphere (visible surface)
We see the tops of the convection cells at the visible surface of the Sun. The main pattern is called “granulation”; each granule is about the size of California. The pattern changes in a few minutes.

7. Sunspots
Sometimes the visible surface has a few dark spots on it. They aren’t really dark, they are just cooler than the surroundings (3500K). These are places where strong magnetic fields emerge from the solar interior. The fields are very important to the atmosphere of the Sun, and its interaction with us.

8. The Sun’s Magnetic Field
The ingredients for a magnetic dynamo are all present: a rotating, conducting, convecting interior. The Sun renews its fields constantly, on an 11-year cycle.

9. The Chromosphere (hot layer above photosphere)
The chromosphere is the red rim of the Sun which appears just over the photosphere in an eclipse. The red light is from a spectral line of hydrogen. Using a filter for just that line, we can see the whole chromosphere. It is much more structured than the photosphere, because at these heights the magnetic field rules the structure of the solar gases (they are fully ionized, or charged).

10. Chromospheric Structures
Spicules and fibrils: magnetic jets and loops

11. Views of the Corona
Above the chromosphere is the corona. Magnetic heating takes it to 2 million degrees, and the fields completely define its structure. These 3 images were all made on Aug. 11, The “white light” corona is just reflected sunlight; the real corona is an X-ray gas as seen in the 2 right images from space.

12. The real X-ray corona

13. Coronal loops

14. Prominences

15. Sometimes, the fields short out each other…
Since loops are constantly rising out of the Sun, sometimes opposite polarities will meet in the corona. The field reconnects, simplifying itself, and the excess energy is released as a huge explosion: flare.

16. Flares are best seen in the chromosphere
High energy particles stream down from the reconnection site, and heat the chromosphere. They can easily be seen in “H-alpha”.

17. Solar Shock Wave
The flare energies are vastly bigger than anything we know on Earth. The flare lasts ~ half an hour. Here the shock wave from the input of energy to the chromosphere can be seen spreading out. More commonly, there is a spray of material upward...

18. Coronal Mass Ejections
Huge blasts of high energy particles are followed by vast amounts of solar plasma travelling outward at hundreds of km/s. In a day or two they reach the orbit of the Earth…

19. The Sun-Earth Connection

20. Producing the “Aurorae”
Some of the high energy particles can get into the Earth’s magnetosphere (which otherwise protects us). They impact our upper atmosphere, and it glows.
The connections cause particles to stream preferentially down the magnetic poles, so normally you have to be at high latitudes to see them.

21. Beautiful Aurorae

22. The Magnetic Cycle of the Sun

23. The 22-year polarity reversal
The cycle is actually 22 years, with each 11 years having the north-south poles reversed. The Earth also reverses (more like 50,000 yr).

24. Why the Cycle Happens

25. Helioseismology