Nature’s favourite shape is a sphere or ball. Why? Because at every point on a sphere, the forces pointing in and out are the same, no matter where you look. If you were on a cube for example, the corners would look different to the middle of the edge. There are lots of spheres in nature – bubbles, stars, planets, even raindrops are ball shaped (not tear drops, that was only for cinemas!).
The Sun is actually not made of fire, it’s made of really REALLY hot gases – mostly hydrogen and helium. These gases are so hot they glow, and we call it “plasma”. It is a sphere because the weight of all this gas (pointed towards the middle) is balanced by all the energy that is made in the middle of the Sun through nuclear fusion (this energy points outwards). That said, the Sun and Earth both slightly bulge in the middle, because they spin.
The flickering you mention is what I am really interested in – understanding the Sun’s flickers is my research!!! It is to do with the massive magnetic field of the Sun….