[caption id="" align="alignright" width="363" caption="A team of researchers is asking how accounting for solar activity in the Sun's past could bring models of the Sun into agreement with observations."][more](http://apod.nasa.gov/apod/ap071203.html)[/caption]

First off, this isn't going to be some kind of anti-climate-change diatribe, in case you were worried by the title. No, what I want to talk about today is literally "The Problem with the Sun" according to the astrophysicists attempting to understand it. In a sentence, the Sun is just... well... all wrong (I know, that's totally explanatory). It's too hot in some places, too cold in others, too transparent or perhaps too opaque (depending on the color of light you consider), and it might not even be the age we think it is. I won't get into each of these problems in detail here (some have been discussed in a recent brief by Alireza Moharrer in Issue #20); I just want to highlight and give some background for recent work by a French team of solar astrophysicists on the Sun's history.

First, you need to know how we know anything about the Sun other than what we can see on the surface. We can construct an approximation of the interior structure of the Sun using only our basic physical understanding of how matter behaves under the influence of gravity and how it transports heat. From this construction, we know that in the deep interior of the Sun, the temperatures and densities are such that nuclear reactions turn lone protons and electrons into helium atoms. This conversion releases energy because, when it comes to protons, neutrons and helium atoms, 2+2 does not equal 4.

For most of its journey, this nuclear energy leaks slowly out towards the Solar surface by the diffusion of radiation, i.e. light. Unlike, say, the light streaming to your eyes from your computer screen, light generated in the Solar interior is surrounded by dense, opaque material. Its journey to freedom involves being repeatedly bounced around by the surrounding matter, but with a general preference toward moving to lower densities, i.e. outward. When I say this process is slow, I mean it; the energy contained in a photon generated in the center of the Sun won't reach the emptiness of interplanetary space for more than 30 million years! Which is to say, the energy reaching us from the Sun right now was originally generated before the first hominid species walked the earth.

After bouncing around from point to point inside the Sun, the diffusing light hits a nearly opaque wall. At this point, local patches of the Sun actually become unstable, and like a pot of water on the stove, the outer 30% of the Sun (by radius) begins to boil. Well, not exactly, but boiling is a pretty good approximation. The basic process at work, called convection, occurs because if a patch of gas in this part of the Sun was bumped upwards it would find itself less dense than its surroundings. Like a witch in water, it floats toward the surface, carrying energy with it. After travelling some distance upwards that blob of gas drops off its energy, and falls back down from whence it came to start the process all over again.

In the outer 30% of the Sun by radius, a region containing very little of the Sun's mass, solar matter is too opaque to permit this steady transfer by light to finish the job carrying energy out, and convection, a process akin to boiling, takes over, as hot blobs of matter carry energy upwards and colder overlying material falls back to be reheated. This convective process is not particularly well understood in the interiors of stars, but we see its effects on the Sun's surface, and there is a growing body of exciting modeling work on the subject, which makes awesome movies. One major outstanding issue in the modeling of the Sun, is how to correctly incorporate the Sun's magnetic field. Incorporating it's effects greatly complicates what were relatively simple physical equations. For this reason, magnetic models have only been constructed for relatively short-lived, or relatively localized models. That is, no model of the entire Sun, evolved from birth to the current day (or any appreciable fraction thereof) has yet been constructed. And it's not that the magnetic field is wholly unimportant. It is responsible for solar storms, a likely source of energy for the solar wind, and largely responsible for the fact that amazing movies like these exist. But on the scale of the whole sun, the energy contained in the magnetic field is actually quite small relative to the gravitational, radiative and gas heat energies, which has always justified assumptions that on that large scale, the magnetic field is relatively unimportant.

With a pencil and paper... ok with at least a laptop... astrophysicists can construct this mathematical model of the Sun I've described. But since we can only see the very surface of the Sun, the question remains, how do we know we've got any of that shrouded interior right? Well, the first question is, how right is "right"? Glossing over that larger scientific issue, we can confirm that our theoretical model gets things right to within a few percent using a technique called asteroseismology, or helioseismology when applied to the Sun. Much like a bell knocked by its clapper, when the interior of the Sun is jostled, even just a little bit, it rings at specific frequencies, or oscillatory modes. By identifying what amounts to the tonal range of the Sun, astrophysicists can learn something about it's interior structure. After all, a brass bell will ring differently than a steel one. With the Sun, different temperatures or chemical compositions will produce different tonal signatures. So by comparing the observed frequencies of the Sun with those of theoretical models with different characteristics, we can identify the best description of the Sun in our sky.

It just so happens that the interior of the Sun is constantly jostled by the outer convection zone I described previously. At the edge of this zone, the convective motions pummel the stable parts of the interior and set them ringing. While we can't hear the Sun ring like a bell (because, as we all know, in space, no one can hear you scream..), we can see patterns of ordered motion on the Sun's surface and patterns in its ejected wind. Just as a bell forms a distinct note or set of notes based on its shape, size, and internal structure (type of metal, etc.), a model of the Sun, constructed based on the physical principles laid out above, will support only a specific set of notes depending on its age, mass and internal structure. This fact allows astronomers to compare theoretical models against the real sun. Astronomers can, in fact, do this for many stars that show signs of ringing in the form of periodic variations in brightness, a burgeoning field of study known as astroseismology ("the study of star-quakes," if you like).

The recent work on the Sun's history was posted last week on the physics arXiv repository. The authors present evidence for the idea that the reason the Sun seems to be different from the current "Standard Solar Model" is that we have incorrectly reconstructed its history. By looking at some of the Sun's younger cousins in the galaxy, the authors argue that we have mistakenly discounted the importance of the solar wind and the Sun's magnetic field in its early development. If these effects were as important in our Sun's adolescence as it appears to be for it's otherwise nearly identical younger brethren, then our ideas about the Sun's internal structure may be all wrong.