Reading: Chapter 18
For the next few days, we will talk about the Sun. We study the Sun in detail because it is the closest star to the Earth (which allows some important experiments to be performed) and it is the star for which we have the most detailed information. We will first present a qualititative description of the general features of the Sun and some empirically deduced properties of the Sun and secondly, we will discuss the physics of the Sun; that is, we will discuss how the Sun is put together and how it works.
The Sun is a hot, gaseous ball of self-luminous gas. It has neither a solid surface nor a solid interior and yet it appears to have an edge. Why? Let's see?
The Sun is a garden-variety G2 V (Main Sequence) star. It resides in the large grouping of stars referred to as the Milky Way galaxy. The Milky Way is a spiral galaxy (e.g., M51) composed of roughly 100 billion stars. The Sun lives in one of its spiral arms of the Milky Way, roughly 2/3 of the way out from the center of the galaxy, whizzing around the center of the galaxy at a speed of around 200 kilometers per seconds. The Sun is a little above average in mass for a Main Sequence star in our Galaxy (the average stellar mass is 30 % of the mass of the Sun), but overall, it is a fairly typical star.
The Sun is a variable star in that its luminosity and appearance varies on a time scale of roughly 11 years (actually 22 years or 44 years if you take account of how the polarity of the magnetic field of the Sun varies). The most obvious manifestation of this so-called Solar Activity Cycle is the variation in the number of Sunspots visible on the surface of the Sun.
Given that the Sun is a variable star, let's consider the properties of the Active and Quiet Sun in more detail.
