The Sun, the nearest star: general information
The Sun is the star nearest to the Earth, of spectral class G2. Its mass is ≈ 1.99 · 10³⁰ kg (≈ 333,000 times the Earth’s), its radius ≈ 696,000 km, mean density ≈ 1.4 g/cm³ and age ≈ 4.6 billion years. By mass it is ≈ 73 % hydrogen, ≈ 25 % helium and ≈ 2 % heavier elements. The surface temperature is ≈ 5800 K and the core ≈ 15 million K. Its luminosity (energy radiated each second) is L ≈ 3.8 · 10²⁶ W; the power falling on 1 m² facing the Sun at the top of the Earth’s atmosphere – the solar constant – is S ≈ 1361 W/m². Internal structure: the core (where energy is produced), the radiative zone, the convective zone and the photosphere, and outside it the chromosphere and the corona. The Sun rotates unevenly: ≈ 25 days at the equator and ≈ 35 days near the poles.
Never look straight at the Sun. A safe experiment: let a small hole in card (made by an adult) or gaps between leaves project an image of the Sun onto a white screen and observe its round shape (look at the screen, not at the Sun).