The question why the Sun is circle, only seems to be simple one. And confirmation of this is the fact that the answer to the “childish” question, scientists have only been able to find in the twentieth century. The complexity comes from the fact that, as the Sun represents a huge globe of hot gases, in the sky above us, instead of the round light we are accustomed to seeing, we should observe a blurry, shining spot with no pronounced boundaries. And we see a circle. And this circle is quite limited, leaving even the impression of a solid surface of the Sun. However, at the great temperatures not only of the Sun itself, but even millions of kilometers away from it, no substance can be found in a solid state. Then what is the essence of the circle we see?
Well, It’s all about the exotic way of how the stars shines – so unusual that even when science became clear of fusion reactions which energizes stars, the secret about daylight for some time remains unsolved.
The stars are “dressed” in a thin, absolutely opaque, and at the same time luminous “gas crust,” which is called a photosphere. The great secret of its composition is the presence of negative hydrogen ions, that is, hydrogen atoms that have captured an “extra” electron. Thanks to these atoms, the light generated in the bowels of the star does not go out. First, the photons of the visible part of the spectrum are swallowed by the ions, breaking off the second “extra” electron. In this process, the ionized atom becomes a very ordinary hydrogen atom.
Meanwhile, in the photosphere is carried out and the reverse process, in which when joining the free electrons to the neutral atoms of hydrogen are born photons of daylight. In this way, the Sun’s photosphere “re-translates” the light, and the quantity and energy of the absorbed photons is completely compensated by the amount and energy of the emitted photons.
The photosphere of stars is very, very thin; thousands of times smaller than the radius of the Star. The solar photosphere, for example, is only a few hundred kilometers thin while the diameter of our star is 1 392 000 kilometers (nearly 109 times the diameter of the Sun is greater than Earth’s).
just that thin “retranslation” light layer determines the sharpness and accurate outlines of the Star. And any one of them, at least in the visible spectrum, turns into a perfect circle.