The Sun
The Sun is a yellow dwarf star (G2V), about 4.6 billion years old, and the dominant gravitational force in the Solar System. It has a diameter of roughly 1.4 million kilometers and contains around 99.8% of the Solar System’s mass. Nuclear fusion in its core converts hydrogen into helium, producing energy that warms the planets. Above the core lie the radiative and convective zones, followed by the visible photosphere (~5,500 °C), the chromosphere, and the much hotter corona (~2 million °C).
Source: science.nasa.gov
APODs including "The Sun"
Hydrogen, Helium, and the Stars of M10
30/06/2001
Stars like the Sun use hydrogen for fuel, "burning" hydrogen into helium at their cores through nuclear fusion. But what happens when that hydrogen runs out? For a while, hydrogen burns in a shell surrounding the stellar core and the star expands to become a red giant. The bright reddish-orange stars in this beautiful two-color composite picture of the old globular star cluster M10 are examples of this phase of stellar evolution. Yet the bright blue stars apparent in M10 have evolved beyond the simple, hydrogen shell burning stage. These stars have become "horizontal branch" giants with core temperatures hot enough to burn helium into carbon. In this image, only the barely visible, faint, gray-looking stars are likely to still be burning hydrogen at their cores.