Story of Gravity

from: Stéphanie Ruphy, LA RECHERCHE, espace blogs.

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What is the nature of universal gravity?
It is by far the least powerful of the four basic interactions today identified.
Happily for us, the effects of the electric force, which can be attractive or repulsive,
cancel out at our scale by virtue of the neutrality of matter. Imagine for a moment that two people situated one meter apart each possess one percent electrons in excess of protons. One could calculate that the electrical force of repulsion between these two negatively charged individuals would then be gigantic, a billion times a billion superior to their weights! As for the other types of fundamental interactions, their very short field of action confines their effect to the world of the nucleus of atoms.

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It is (gravity) which insures the cohesion of the xxx atoms which make up a star such as our sun. An immediate visual translation of this role is the spherical form of stars, and more generally of the satellite planets of a size superior to a critical value in the order of some one hundred kilometers. Objects of a more modest dimension, such as asteroids, present on the contrary more varied forms, often quite irregular. This difference is understandable through the nature of the forces which are principally responsible for the cohesion of matter. For smaller bodies, electrical forces dominate. These forces being of short range of effect, they are indifferent to the global shape of the object. While for larger bodies, gravity dominates and imposes a maximum of compactness which is translated to a spherical form.

Gravity dominates the entire life of a star. The latter is formed through the gravitational
implosion of a gas cloud: particules, at first dispersed, attract each other and get closer. This contraction liberates energy which gets converted to thermal energy and light emission. Gravitation is thus for a star the first direct source of light emission. But if it were the only one, a star such as our sun could only shine some thirty million years! Thanks to the thermal energy liberated, the temperature and pressure of the star increase as it contracts, up to the onset of nuclear reactions. Gravitation thus insures a sufficient confinement of gas at the heart of a star for the production of fusion of hydrogen into helium, the primary source of stellar light emission. The star thus finds itself in a state of hydrostatic equilibrium where the forces of internal pressure and gravitation cancel out.