The Sun is a main sequence star, meaning that it is ‘middle aged’ at 4.6 billion years old. It has a surface temperature of approximately 5778 K (5505°C) and has a diameter of about 1,392,684 km. The Sun transforms 620 million metric tons of hydrogen into helium each second; once this supply has run out in about 5 billion years’ time, the Sun will expand into a red giant and then die, leaving behind its core and a planetary nebula. Once the planetary nebula has dispersed into space, a white dwarf will be left behind, which will also someday cool and die.
One method scientists use to study the interior of the Sun is helioseismology. This involves studying the propagation of the waves in its body, similar to how geologists learn about the interior of the Earth by monitoring seismic waves emitted by earthquakes. The waves in the Sun's body cause small oscillations of the surface that are observable.
There are three layers separating the surface of the Sun from its core. The inside layers of the Sun are the photosphere, the convective zone and the radiative zone. The photosphere is comprised of hydrogen at 5500°C, and is where sunspots occur. The convective zone contains currents which take heat to the photosphere, which is around 140,000 km thick. The energy from the core, which is where hydrogen is transformed into helium, goes through the radiative zone, about 380,000 km thick.
The solar atmosphere refers to the parts of the Sun above the photosphere. There are five principal zones of the Sun’s atmosphere: the temperature minimum; the chromosphere; the transition region; the corona; and the heliosphere. The heliosphere extends past the orbit of Pluto to the heliopause; here it forms a shock front boundary with the interstellar medium. The chromosphere, transition region, and corona are all much hotter than the surface of the Sun. Solar flares occur in the chromosphere.
NASA have developed a virtual tour of the Sun. You can click on features on the Sun to learn more about: sunspots; the Sun’s structure; the photosphere; solar active region; the corona; and the future of the Sun. you can also learn more about the current solar missions, ACRIMSAT and SORCE. There is also a video on the Sun’s role in climate change and some quick facts.