The sun is like the earth: it is comparatively cool on the outside and quite hot on the inside. Only the circumstances are completely different. It is about 6,000 degrees Celsius on the surface of the sun, but the temperature inside is 15 million degrees.
In contrast to the earth, the sun is a huge fusion reactor. Under the enormous pressure inside, hydrogen atoms constantly fuse to form helium. 4 hydrogen atoms become one helium atom each. This releases energy.
Convert mass into energy
We know from Einstein: mass can be converted into energy, and that’s exactly what happens there. When the hydrogen atoms fuse to form helium, they lose about a hundredth of their mass, and that mass is converted into thermal energy.
And to give an idea: If one gram of hydrogen melts into helium, an energy of 180,000 kilowatt hours is released. On an electricity bill, this energy would cost 45,000 euros. But there are 6 billion tons of hydrogen that the sun converts into helium, every second. This creates the enormous temperature of 15 million degrees inside the sun, and of course this heat also migrates to the outside. But because the sun is so big, it takes a long time, namely 10 million years.
The coolest parts of the sun are hotter than the interior of the earth
For its part, the surface of the sun constantly radiates energy into space, including us, of course. It is therefore cooled from the outside, which is why it is “only” about 6,000 degrees on the surface. On the surface of the sun there are also the famous dark sunspots, which are a bit cooler, around 4,000 to 5,000 degrees. If you compare that to the earth, it is roughly the temperature at the center of the earth. This means that the hottest parts of the earth’s interior are still slightly cooler than the coolest points on the sun’s surface.