THE SUN: INTRODUCTION

Appearance of the Surface

The visible disk (i.e. seen in visible light from 400-700 nm) of the sun shows features such as sun spots, and limb darkening, which is the apparent darkening around the edge of the sun.

Sunspots appear dark only because they are cooler than the average temperature of the rest of the sun's "surface". Limb darkening is a similar effect. The most upper layers of the sun's surface are cooler than the lower layers. Limb darkening occurs because you are looking at the very top layers of the sun's atmosphere around the edges (it is a geometrical viewing effect).

Anatomy of the Sun

The structure of the sun from the center outward is

There are seven parts altogether if one includes the atmosphere is broken into 3 layers.

The basic properties of each of these are given in the lecture notes and book; I recommend you know the basic properties of each.

The Core

This is the center of the sun. It has a temperature of about 15 million K and a density of about 150 times that of liquid water. Water has a density of 1 gram per cubic centimeter (1 gram of mass compressed the volume of a sugar cube). The energy that powers the sun is released in the core. We will discuss this in a coming lecture.

The Radiative Zone

The core generates extremely high energy light (in the form of gamma rays) in its process of powering the sun. Once photons emerge from the core they begin their journey to the surface of the sun. In the radiative zone, the energy from the core is transported upward and outward by the photons themselves; this is called radiative transport and the region within the sun in which radiative transport is operating is called the radiaitve zone.

The photons emerging from the core are extremely high energy; they are gamma rays. As the photons transort the energy, they scatter off the ionized gas (plasma) in the deep interior sun. As the photons move further outward, the temperature of the gas is decreasing. Thus, as the photons scatter, they lose energy as they move up to these cooler layers. Eventually, the upper layers are around 6000 K, which is the temperature at which dense plasmas emit photons in the visible range of the electromagnetic spectrum (this is by the time the photons have made their way up to the base of the convective zone).

The Convective Zone

The convective zone is the region where the energy brought up from the core via radiative transport is now transported by convective transport, a process similar to boiling water in a pot on your stove. It is at the very top layer of the convective zone that photons escape to space; this top layer is the base of the photosphere. The hot and cool patterns of the convection are seen on the surface of the sun as granulation. Bright, hot zones are rising heat, and the relatively less bright zones are where cool gas is sinking back down (like boiling water). This is the process called convection.

Some granules are very large and are called supergranules. The heat is great just above these that it ionizes the gas in the chromosphere (see below) to form spicules. A spicule can be as tall as the diameter of the Earth in size.

The Atmosphere

The atmosphere comprises the photosphere, chromosphere, and corona, in order going outward from the center.

The temperature with height in the sun's atmosphere is as follows:

The corona can be seen when the sun is blocked out. The corona is the transition from the sun to the solar wind, which is a wind of charged particles being blown away from the sun.

High Energy Events in the Upper Levels of the Atmosphere

Sometimes energy erupts in a small area and forms a flare. Flares also create "thunder", causing sound waves to ripple across the sun's surface. Sometimes larger energy eruption occur and result in solar prominances. These usually are much 100s of time larger than the size of the Earth, and they are usually confined to magnetic loops. These magnetic loops confine the prominences to a horse shoe shape (the electrically charged particles trace out the shape of the magnetic loop as they move along the magentic field lines). Sometimes, however, thee magentic loops are broken; they are so energetic that they blow out from the sun and form holes in the corona. This is called coronal mass ejection (CME).

The Solar Wind

The Solar Wind is the streaming of elementary particles (protons, neutrons, and electrons) created in the corona and in CME. They move at 400-600 kilometers per second. This wind goes well beyond the solar system.