STRUCTURE OF THE SUN AND STARS

THE STRUCTURE OF THE SUN CAN BE UNDERSTOOD FROM AN UNDERSTANDING OF A FEW CONCEPTS
I. Mechanical Equilibrium or Hydrostatic Equilibrium

If the Sun (or a star) is in hydrostatic equilibrium, the Sun (or star) is neither expanding nor contracting; it is sitting nicely in some equilibrium configuration. This means the forces that control the Sun (or star) are precisely balanced.
For the Sun (and stars):

The Sun is held together by gravity; the gravitational attraction between particles causes all of the particles in the Sun to attract each other making the Sun want to shrink overall.
The Sun maintains its current size because its interior is very hot which leads to a large internal gas pressure; the gas pressure overcomes the drive of gravity to shrink the Sun. Because of the large mass of the Sun, this requires extreme conditions. At the core of the Sun, the Sun maintains a temperature of 15 million K and has a density of 150 g per cubic centimeter, more than 13 times denser than a bar of lead.

II. Thermal Equilibrium

If the temperature structure of the Sun (or a star) is not changing with time, then the Sun (or star) is in Thermal Equilbrium. This will happen if energy production in the Sun exactly balances energy losses experienced by the Sun (or star).
For the Sun, because

We see the Sun ===> the Sun emits photons (particles of light) ===> the Sun is losing energy ===> the Sun should be getting cooler and therefore its internal gas pressure should be getting smaller ===> the Sun should go out of hydrostatic equilibrium and shrink.
The Sun is not shrinking (as far as we can tell, if it is shrinking, it is shrinking quite slowly) ===> the Sun maintains thermal equilibrium ===> the Sun generates an amount of energy that balances the energy losses due to radiation from its surface (its photosphere)!

We talk more about this later but for now, let me just say that the Sun generates this energy though nuclear fusion reactions that turn 4 hydrogen nuclei into a helium nucleus with the generation of energy and neutrinos. This happens in the inner 20 % of the Suns's radius, the Sun's core, where the interior temperature (15 million K) and density (150 grams per cubic centimeter) of the Sun are most extreme. As in an atom, the core is tiny in that it is less than 1 % of the Sun's volume. Although not as extreme as in an atom, a large amount of the Sun's mass resides in its tiny core; more than 1/3 of the mass of the Sun resides in its core!

When the Sun maintains hydrostatic and thermal equilibrium , it just sits there merrily chugging along. The regions in the HR diagram where we see clustering of stars are regions where the stars are roughly in hydrostatic and thermal equilibrium

III. Energy Source for the Sun and the Maintenance of Thermal Equilibrium
The important question of the energy source of the Sun is a longstanding one which was not answered satisfactorily until the twentieth century. Today we know that the Sun (and stars) generate energy in their cores (their central regions) through nuclear fusion as noted above. Finally, in order for us to understand how stars maintain thermal equilibrium and so to understand how the interiors of stars are put together and evolve, we must address how the ways in which the energy gets distributed throughout the Sun after it is produced in the Sun's core. So, What are the modes of this energy transport?
To finally close this topic for the Sun, we now discuss some of the details of how Nuclear Fusion works in the Sun (and stars).

PROBES OF THE SOLAR INTERIOR

I. Solar Neutrinos
An important test of our ideas (as discussed earlier) concerns the detection of the ghostlike particles known as neutrinos.

Solar Neutrino experiments were started in the 1960s by Brookhaven scientist, Ray Davis to verify that we understood how the Sun worked. No one thought that the experiment would that interesting; it would be difficult but the result would not be surprising. It came as a rude surprise when Davis's experiment detected fewer neutrinos than predicted by the best models of the Sun, throwing doubt onto whether we really did understand our Sun. Follow-up experiments also found ~1/3-1/2 of predicted neutrinos.

This conundrum persisted for ~35 years until the early 2000s when, first, the Super-K (Super Kamiokande) experiment showed neutrinos were chamaeleon-like in nature. Neutrinos, once produced, could change into forms undetectable by the early experiments. The SNO (Sudbury Neutrino Observatory) experiment, able to detect transmuted neutrinos, then came online and detected the predicted number of Solar neutrinos. The amusing result was that a simple observation of the Sun led us to a deeper understanding of how the Universe works on the sub-nuclear scale!
These important experiments showed that we have a good understanding of the Sun. As a major benefit, it also expanded our understanding of the properties of neutrinos, some of the fundamental particles which make up our Universe. For the more technically minded, the late John Bahcall's website www.sns.ias.edu/~jnb contains a very good discussion of Solar neutrinos and the Solar model.

II. Solar Oscillations

Solar oscillations are another sensitive probe of the interior of the Sun. Go to the website http://gong.nso.edu/info to find more information on the Global Oscillation Network Group, GONG.
The Sun is like a bell in that if you kick it, it rings. The source that excites the most of the observed Solar oscillations, is the convective layer and so, the Sun is like a bell that is continuously buffetted by many small kicks. The GONG experiment detects and studies the different ways in which the Sun rings. The exact manner in which the Sun rings is determined by the details of its interior structure (in particular, by the details of the convection layer). So, studying the way in which the Sun rings gives us another way to study what the Sun looks like beneath its surface. (The technique is similar to that used by geologists who use seismology to study the interior of the Earth. For the Sun, the science is called helioseismology.)

The Sun rings in many different ways depending on the details of the interior of the Sun, the sound waves bounce around and travel through the Sun in different ways and with different speeds. Some rattle around near the surface of the Sun, some are trapped in the convective layer of the Sun, and some stretch to the center of the Sun. (The motions generate sound waves -- The Boiling and Singing Sun.)
Scientists model these waves finding things which look like (the "cells" represent where the waves bounce off the surface of the Sun, and so on [see the above picture]). The following mpeg movie show Solar oscillations.
The current GONG results (depend on the sound wave speeds [the sound speed] and other aspects of the ringing) compare well to the predictions made by current Solar models. The GONG site's K-3 link is a little low-level, but is amusing.