Our understanding of the universe will not be complete until we can show how the current universe emerged from a beginning and where it is going. We combine observations with computer models to create stars, galaxies, and large-scale structures. But, we donít know which came first, the stars or the structures.
Stars are the chemical processing plants of the universe. Nuclear fusion transmutes the original material, H, He, and others, into all of the observed elements. Some processing occurs deep in stellar cores, others during the explosion of supernovae. Studying the Sun provides the only fixed point of stellar evolution. We know how bright, large, hot, and old the Sun is. Any model of stellar evolution must get these right to be calibrated.
An enormous amount of data is now available to study the Sun and stars. An accurate model of the Sun and its interior is needed to keep many other models consistent. Here are several areas that are actively studied in the Sun that will affect our knowledge of stellar evolution.
The neutrino fluxes from the Sun help to establish the actual changes that occur during the p-p burning in the Sunís core. This calibrates the rates for transformation of Hydrogen into Helium. Without such calibration the age of other stars is not accurate.
The composition of the Sunís surface is close to the original, but some nuclear processing has occurred as well as the solar wind removing some elements better than others. How does this small change in composition affect the future evolution of the Sun? How does the way the material interacts through the equation of state affect the evolution of the Sun?
The solar system has evolved along with the Sun. The Sun was dimmer and cooler when the Earth was formed. How were planets affected by the changes in the solar output over the past 5 Gyr?
All stars have times when energy is transported outward by convection. The Sunís convection zone is the only one that can be studied in detail. With current and upcoming helioseismic data we can see the plasma motions below the photosphere. Comparisons of these measurements with the ever-improving models will allow more accurate empirical models of convection to be applied to evolution calculations.
Surface abundances in evolved stars are not always the same as the initial material. Large convective ďdredge-upsĒ can move material that was in the core all the way to the surface. This also moves surface material, rich with nuclear fuel, into the core. Tracking the evolution of these stars requires a good understanding of how the material was moved through the star by convection.
Almost every star rotates. The speed of rotation changes dramatically as we move from the top of the main sequence (large stars, 150 km/s) to the bottom (small stars, 1 km/s.) How the rotation affects the nuclear evolution of a star is not well understood. How the change in rotation inside a star (differential rotation) affects the evolution is even less well understood.
It was noticed early that the Sun does not rotate as a solid body. The poles rotate once every 35 days, while the equator spins around once every 25 days. This differential rotation continues into the convection zone. Helioseismic measurements over a solar cycle show changes in the rotation pattern inside the Sun. Solar measurements will help to understand how rotation and evolution are linked.
Magnetic fields offer the opportunity to store energy in twists and whirls. Heat energy does not store easily (thatís why we need fireplaces and furnaces.) Convection cannot move material forever, the blobs hit the surface and stop, losing the momentum and energy back to the heat reservoir. Only energy converted into magnetic fields can be stored for a long time and then converted back to heat. How the magnetic fields of the Sun and other stars are created is an area of intense research. It probably comes from an interaction of convection and rotation.
The Sun is the only star that we can measure changes in the magnetic field across the face of the object. In some stars we can map the field changes by monitoring the changes with rotation, but the Sunís field is measured at many heights in the atmosphere and with a lot of detail. We also see the extension of the magnetic field into the chromosphere and corona of the Sun. Changes in the magnetic field produce many of the things we call solar activity. Our study of the Sunís corona and solar activity will also help us to understand the corona around other stars.
We have noticed that some climate cycles on the Earth are about the same as the time it takes to rotate around the Milky Way. If true, we have even more of a need to understand the motion of the Sun in the galaxy.
Nature waits patiently while small things grow into big effects.