The marvellous balance of the Sun

Schematic diagram of the interior of the Sun. Image source: Kelvinsong.
This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

by Gabriel Ferrero

During the current, long stage of its evolution, the Sun shines because, very deep in its core, millions of tons of protons (nuclei of hydrogen atoms) fuse every second to form nuclei of helium. For each four protons fused, one new helium nucleus is formed. However, the mass of a helium nucleus is slightly lower than the sum of the masses of the four protons. That difference of mass becomes energy – more precisely gamma rays – following Einstein’s famous equation: E = mc2. Then, those gamma rays slowly travel across the interior of the Sun to its surface, degrading mainly into visible light along the way and finally escaping into space.

The huge rate of fusions per second generate an infernal environment. It is calculated that the core of the Sun should be at a temperature of around ten millions Kelvin. Such a tremendous temperature is related to huge pressure of around ten billion times the air pressure in car tires. If that pressure were not counterbalanced in some way, the Sun would explode.

But the Sun, like every star, is kept together by gravitational force. The core of the Sun – its heaviest region – attracts the surrounding region with a very large force. In the same way each interior shell attracts the outer shells and so the Sun remains united. Because of the large mass of the Sun, which is more than three hundred thousand times the mass of the Earth, its gravitational force is also enormous. If there were nothing to oppose its gravity, the Sun would collapse.

The Sun maintains its shape and brightness because of a very delicate and marvelous equilibrium between tremendous forces. At half the distance between the surface and the center of the Sun, the gravitational force pulling inward is like the weight of a thousand trillion trillion tons. And at that same point, the force of pressure pushing outwards is exactly, exactly the same.

It has been calculated that if, for some strange reason, the pressure should slightly decline, the Sun would suddenly shrink, it brightness would fall by some percentage points within minutes, and the average temperature of the Earth would drop off by several degrees in a couple of days. I let you imagine the catastrophic consequences of such an event.

However, this has not happened during the last four billion years! And nothing indicates that it would happen in the next three billion years neither. Nature has its laws, and second by second, pressure, gravity, temperature, density, chemical composition, viscosity, rotational speed, and many other variables, regulate themselves in every single point of the Sun’s interior. Because of this, the amount of light and heat reaching the surface of the Earth has remained practically constant for several billion years, allowing life to appear, grow and gloriously develop.

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