Our closest stellar neighbor is, of course, our own Sun, officially named Sol. Since it is so close, we know the most about it, and it has provided a test case for astronomers and astrophysicists as they have sought to understand the structure and evolution of all stars. Because Sol is so close, we can observe many characteristics that are unobservable for other stars, phenomena that we can only infer must be associated with those we see as tiny points of light in the nighttime sky.
We combine accurate radar measurements of distances to the planets with the laws of dynamics and determine Sol's average distance to be 149,597,870 kilometers, or 92,955,807 miles. This distance is combined with accurate measurements of Sol's angular diameter to give a linear diameter of 1,392,530 kilometers or 865,278 miles. We could line up 109 Earths, side by side, across Sol's center. The laws of dynamics also permit us to determine the gravitational mass of Sol from the gravitational force it exerts on Earth and on the other planets. Its mass, the amount of material of which it is made, is 332,960 time the mass of our planet Earth. The solar constant, a measure of the rate at which we receive radiant energy from Sol, when combined with Sol's distance tells us its luminosity, its rate of energy emission. Sol radiates enough energy in one second to supply the current energy needs of the U.S. for about 9,000,000 years. If we combine Sol's diameter with its luminosity, we can deduce a temperature for its outer layers, its surface temperature. Finally, careful examination of its spectrum gives us Sol's chemical composition.
The age of our Sun and solar system, deduced from radioactive elements in meteors and in Earth herself, convince us that Sol is about 4 1/2 billion years old, so with nuclear fusion as a source of energy, the laws of physics, and a good computer, we can compute a model for our star that fits all the observations. There are surprisingly few parameters required for this computation. We need the mass, the luminosity, the chemical composition, and the temperature of Sun's "surface" for a unique model.
These parameters can be measured or inferred relatively easily, although with lower precision, from observations of the distant stars, so the computational techniques that have been honed with models for Sol are applicable to all of the stars, and consequently the theory of stellar evolution is one of the most successful theories in astrophysics.
Because Sol is so near, it is the only star whose surface can be seen clearly. All the other stars are true points of light, or so nearly true points of light that special techniques have to be used to determine that they have any dimension associated with them. We are not certain, for example, that any other stars have "sunspots," the most conspicuous of Sun's features visible from Earth. We certainly expect that they do, and, in fact, some observations of variability in relatively nearby stars that are similar to our Sun in luminosity and temperature are interpreted as due to "starspots," but such surface features are not actually observed.
Observations of Sol during total eclipses show features of the highest and most distant part of its outer layers, its "atmosphere." The most conspicuous part is the corona, a milky glow of light seen extending to several times Sol's radius. Closer to its surface is a thin red layer called the chromosphere. Neither of these can be observed directly in other stars, but observations by satellites designed to study Sol have detected X-ray emission from the chromosphere and corona. X-ray emission from some distant stars has also been detected by satellites that study distant X-ray sources, and these detections suggest chromospheres and coronas for many distant stars.
