The Sun is our nearest star. Its radiant energies light up our daytime sky and make all life possible on this planet, even from as far away as 93 million miles. Like many things in nature, the Sun consists of many different parts that influence each other and exchange both energy and matter.

Deep within its core where gravitational pressures compress and heat its gases, atoms collide so furiously that some fuse together. At temperatures of 15 million degrees Centigrade, its abundant store of hydrogen turns to helium via thermonuclear fusion. Every second, 600 million tons of matter are converted to pure, radiant energy. Some of this energy goes into creating pressure that literally holds up the Sun against gravity. The rest leaks out of the dense core in the form of light, and deposits huge amounts of energy throughout the inside of the star. It takes many thousands of years for this light energy to make it to the surface because there is so much matter in the way.

Like some enormous onion, the Sun's interior is a collection of regions with unique combinations of temperature, density and the manner in which energy moves through them. In a region that astronomers call the radiative zone, light is the most efficient medium to transport energy from deeper inside the Sun. The gas moves very little, and rotates in unison with the rest of the Sun as though it were a solid substance. While the gas temperatures plummet to only a few hundred thousand degrees, light staggers to and fro until it gets about 1/3 of the way to the surface. The outer 1/3 of the Sun convects like a liquid in a boiling pot, because of a sharp change in the properties of the gas and its temperature. Astronomers can see these convection cells on the outer surface of the Sun, called the photosphere, which is the part that we see from the Earth. The small cells are called granules, but they are in fact nearly as large as the Earth and change their shapes in only a few minutes. Granules move about on top of larger 'super cells' which reach deep down into the interior of the Sun within the convective zone.

Stars much hotter than the Sun, such as Rigel or Deneb, have no convection zone at all, while stars much cooler than the Sun, such as Betelgeuse and Antares, have convective zones that reach almost to the cores of the stars. Because of the Sun's turbulent surface, many complex phenomena can occur on the surface as flows of charged particles cause magnetic fields, and these fields become entangled and amplified.

Teachers Section:

How do the different components of the Sun interact as a system?

Students acquire the skills they need to develop an understanding of a system. In the middle years, students identify the parts of systems and how one part connects and affects another. This leads to the analysis of parts, subsystems and interactions.

* (K-2) Most things are made of parts

* (3-5) In something that consists of many parts, the parts usually influence one another.

* (6-8) Thinking about things as systems means looking for how every part relates to others. The output from one part of a system (which include material, energy or information) can become the input to other parts. Such feedback can serve to control what goes on in the system as a whole.

* (9-12) Understanding how things work and designing solutions to problems of almost any kind can be facilitated by systems analysis. In defining a system, it is important to specify its boundaries and subsystems, indicate its relation to other systems, and identify what its input and its output are expected to be.

Science Nuggets:

2000...SOHO discovers a 16 month period in the gas speeds at the base of the convective zone.

1999...SOHO satellite observes explosions on the far side of the Sun by watching how the surface facing us vibrates.

1997...SOHO detects jet streams in the solar surface, and resolves the boundary between the radiative and convective zones using sound wave tomographic imaging.

1997...SOHO confirms that the interior of the Sun rotates with a common period, unlike it's convecting surface layers.

1995...Ulysses detects oscillations from

the Sun's interior in the interplanetary magnetic field.