![]() Its present position corresponds to its present structure, in which the convective zone only reaches about a third of the way down into the solar interior. This mixing continued as it moved downward in the HR Diagram, but as its core became hotter the contribution of absorption to opacity gradually diminished, and as the bottom of the convective zone moved toward the surface, the dot representing the Sun's characteristics began to move to the left in HR Diagram. When the Sun was first visible (as a protostar) it had just finished its ionization collapse, and was undoubtedly undergoing violent vertical mixing throughout its interior. In fact, as shown below those changes have probably made the Sun about 25% larger and 50% brighter than when it first formed, 4 1/2 billion years ago. To a good first approximation a star like the Sun remains the same throughout its Main Sequence lifetime, but it does change as it ages, just a little bit at first, but more and more the older it gets. For however long the fuel in the core lasts (as discussed in The Mass-Luminosity Diagram and the Lifetime of Main Sequence Stars), there is no obvious reason for the star to change, so year after year the dot representing the star's properties remains fixed in its Main Sequence position in the HR Diagram. When the rate of nuclear fusion is equal to the heat loss at the surface the star undergoes an adjustment of its structure, from one in which the heat loss is replaced throughout the star (by its overall contraction) to one in which the energy is produced only in the core and the rest of the star just passes the heat to the surface, and the contraction of the star comes to an end. Once nuclear fusion begins, however, the net loss of heat which has to be replaced by gravitational contraction is reduced, so the contraction slows as the rate of nuclear fusion increases. The compression of its gases raises the density, temperature, brightness, pressure and gravitational force in its interior and changes its external appearance, so that the dot representing its properties (in a Hertzsprung-Russell Diagram) moves downward or to the left over a period of time. Prior to reaching the Main Sequence the star's energy source is a gradual contraction. The Main Sequence (University of Utah), Main Sequence Stars (University of Oregon), and Stars (NASA’s Imagine the Universe) are three good places to go to learn more.The Main Sequence represents a long, stable stage of stellar life in which the thermonuclear fusion of hydrogen to helium in the core replaces the heat escaping at the surface. It took many decades of research to work out the details of stellar evolution – what nuclear reactions for what mass and composition of a star, how the size of a star reflects its internal structure and composition, how some stars can live on long after they should be white dwarfs, etc, etc, etc – and there are still many unanswered questions today (maybe you can help solve them?). So, broadly speaking, there are so many stars on the main sequence – compared to elsewhere in the H-R diagram – because stars spend much more of their lives burning hydrogen in their cores than they do producing energy in any other way! Stars are found elsewhere on the Hertzsprung Russell diagram, and their positions reflect what nuclear reactions are powering them, and where they are taking place (or not white dwarfs are cinders, slowly cooling). By the 1930s, however, the main outlines of the answers became clear … stars on the main sequence are powered by hydrogen fusion, which takes place in their cores, and the main sequence is just a sequence of mass (faint red stars are the least massive – starting at around one-tenth that of the Sun – and bright blue ones the most – about 20 times). Back in the 19th century, it would have been impossible to answer these questions, because quantum theory hadn’t been invented then, and no one knew about nuclear fusion, or even what powered the Sun. ![]()
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