The Sun

On our stage, the role of main actor cannot but be conferred to the Sun, a star like many others in space, but very special for us because from the remains of its formation all the planets and the smaller bodies that rotate around it, and of which we are a part, have originated.
The Sun is so big that over 100 planets as big as Earth could be placed along its diameter. Its mass alone constitutes 99% of the total mass of the Solar System and it is capable of releasing, in the form of light and heat, an amount of energy equivalent to 1,000,000,000,000,000,000,000,000 100 W light bulbs, or 10,000 billion atom bombs, per second. The main motor resides in the Sun’s core where every second hundreds of millions of tonnes of hydrogen atoms, the most abundant chemical element in the universe, fuse together producing energy.
The Sun is a gigantic sphere of gas at a very high temperature and in perfect equilibrium that does not collapse on itself and does not get dispersed in space thanks to the state of balance between the gravitational and pressure forces which are of equal intensity but act in opposite directions.
Being made of gas, our star does not have a solid surface; we can think of the Sun as an enormous onion made of concentric layers of gas: what we see from Earth is the outline of the outer shell called the photosphere. The phenomena that take place in this region can be viewed even with small instruments as long as they have adequate filters: in fact, one must remember never to stare directly at the Sun because its light is so intense that it would cause permanent eye damage. On observing the photosphere, one notices that it is not compact, but made up of many small cells. This structure, called granulation, is caused by convective motion: columns of hot gas coming from the centre of the Sun reach the surface and then sink towards the interior. Also in the photosphere, groups of sunspots can be observed. These areas appear darker than the surrounding area because within them the gas is cooler than average. Even though they look small on the Sun’s surface, these structures are so big that they could contain five planets the size of the Earth. In contrast, faculae are areas that appear brighter because they are warmer than the surrounding gas. Above the photosphere lies the stellar atmosphere made up of two distinct areas known as the chromosphere and the corona, in which the gas is more rarefied. Even in these regions some phenomena can be observed, such as protuberances, gigantic columns of gas that rise almost perpendicularly to the Sun’s surface and which form beautiful arch-shaped structures. From the corona, the Sun extends into space releasing a stream of elementary particles called solar wind. Since the particles it is made up of, basically electrons and protons, are very energetic, the solar wind is harmful to all forms of life. Fortunately, the Earth has adequate defences: due to its magnetic field, that envelops the Earth like a protective shell, it is able to deviate these currents of particles, preventing them from reaching the surface. Some, however, manage to escape and to penetrate the upper parts of our atmosphere, colliding with the gas molecules and exciting them. The consequences of these interactions constitute one of the most beautiful natural phenomena: the spectacular polar auroras.
Moreover, it is the Sun that establishes the extreme boundaries of the Solar System, referred to as the heliopause. In fact, the solar wind creates a bubble in the interstellar medium, a very rarefied gas that pervades our galaxy. The interstellar medium is dotted by other bubbles similar to the Sun’s, proof that there are other stars that belong to as many solar systems that could be similar or very different from ours, just like the daisies in a meadow.

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