Hello Jeanne-Lyn,
We know so much about the Sun because of the large number of observatories both on the ground and in space that we have looking at the Sun, which are adding knowledge to what we have learned in the past from older observatories. By targeting different wavelengths, we can look at parts of the Sun at different temperatures and so build up a more complete picture. We observe the corona of the Sun in scattered white light where we first block off the disk of the Sun. This mimics an eclipse and shows the comlpex crown-like structure which gives the corona its name. The low corona which is at 1-3 million degrees can be observed with X-ray and Extreme Ultraviolet telescopes. The chromosphere is observed with red H-alpha light and emission from helium at 10,000 degrees. The surface is observed in normal optical light which shows the granulation and the sunspots. Finally, the interior of the Sun observed using helioseismology which uses the oscillations on the sun's surface to probe the underlying structure.
CHeers - David