Image nebula

How stars are born

Up to now we have spoken about our Solar System and the celestial bodies that make it up; now we are ready to leave the Solar System. An important fact to remember is that our Sun is one star in a collection of billions of stars in our Galaxy. Stars are enormous gaseous spheres that are formed when clouds, made up of gas (mainly hydrogen) and dust, contract. The interstellar clouds of gas, like the one shown in the figure, are very big. If we were to compare the Solar System to one of these clouds it would be as big as a dot. Their masses can be up to a million times larger than that of the Sun, and their temperatures are very low.

These clouds can contract spontaneously and slowly under the action of their own gravity or other external factors such as material expelled at high speeds from evolved stars or casual collision between two clouds during their movement within the galaxy. The subsequent collapse of a part of these clouds then follows.

When a cloud begins to contract, its internal gas particles become more compact and clashes among them are more frequent, these clashes cause the temperature to rise. A simple example can be used to explain this mechanism of friction, when we rub our hands together, to warm ourselves, the faster we rub them the warmer they become.

At a certain moment, the increased temperature of the cloud’s nucleus induces the hydrogen particles to transform into helium; energy is produced during this transformation. This energy pushes itself out and counteracts the cloud’s contraction, until it stops it altogether. In the meantime the cloud has changed shape and is now spherical, it has reached a level of equilibrium: and a star is formed.

The energy that is still being produced through the transformation of the particles in the nucleus now comes out from the star in the form of light and heat.

The evolution of a star can last millions or even billions of years according to the original mass of the newly-formed star and therefore to the energy production made by the nucleus, which gives the equilibrium between the pressure which causes the star’s expansion and the gravitational pull which causes the star’s compression. The whole existence of a star is therefore a matter of equilibrium between opposite forces.

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