We discussed how we know the basic paramters of the Sun: size, mass, density. It is composed of primarily hydrogen (.9 by number, .75 by mass); the rest is essentially helium. All other elements are present, but in amounts that range from 10^(-4) to 10^(-10) by number compared to hydrogen.

The energy of the Sun comes from fusing hydrogen to helium (at high density and temperatures exceeding 15 million K). It moves as radiation (high energy photons) from the core through the radiative zone, and the temperature falls. The outer section of the solar interior finds it easier to transfer the energy by convection: the rising of hot gas and sinking of cooler gas in convective eddies or cells. We see the tops of these at the surface of the Sun as granules. This surface is called the photosphere, and is where the Sun's energy escapes as visible light, with a temperature of 5800K. The photosphere appears sharp because of the distance of the Sun, but the whole Sun is gaseous.

Above the photosphere the temperature starts to rise. This is the chromosphere; it appears red because of a hot hydrogen spectral line (called "H-alpha"). The gas in the chromosphere is controlled by magnetic fields which have emerged from below; it has a tangled structure with many loops. Above that the temperature rises very quickly through a transition region, becoming a few million degrees in the solar corona. The characteristic light of the corona is X-rays because it is so hot (although we can see it in white light during an eclipse because it scatters photospheric light faintly). The reason that the outer atmosphere of the Sun gets so hot is that magnetic fields are constantly shorting out (reconnecting) and giving their energy to heat. This heat causes the outer corona to flow away in a fast solar wind.

The magnetic fields in the Sun arise from a combination of the convective motions and the rotation of the Sun. The process is called a magnetic dynamo, and operates in many other astronomical settings (inside the Earth, for example). Charged particles (and the gas or plasma in the Sun is generally ionized, meaning that electrons are free) are forced to move only along magnetic field lines, so the outer atmosphere of the Sun traces out the field. Through the solar wind, the blasts of high-energy particles from solar flares (when violent reconnection takes place), and the inhibition of interior energy by sunspots, the Sun somehow influences climate on the Earth (and can disrupt telecommunications). The sunspot cycle rises and falls every 11 years (and switches polarity so the full cycle is 22 years).