Image supernova

The Supernovas

An expanding cloud of gas is shown in the figure, it represents all that remains after the explosion of a star with a great mass. The star is classified as a supernova because of this violent end. The areas where the lines are thicker represent the areas of cloud where the gas is denser.

The explosion occurs when the nucleus of a star with a large mass no longer reaches temperatures which are high enough to support the transformation of carbon atoms into iron. This nuclear transformation usually occurs when four carbon nuclei combine themselves into one iron nucleus. This particular nuclear reaction not only means that energy is not produced, but it also requires energy from the surrounding environment.

The outer layers, no longer balanced by the energy given off, suddenly collapse into the nucleus and cause the internal pressure and then the temperature to increase. In contrast with the case of white dwarfs,the total mass of the outer layers is now much greater and the subsequent rise in temperature of the nucleus is able to reach the levels necessary to suddenly produce an enormous quantity of energy which leads to a huge explosion during which the outer layers of the star are expelled at extremely high speed. This creates an immense expanding cloud.

A black hole, or a neutron star, is what is left behind in the middle of the supernova after the explosion. Its shape, however, is still spherical. Neutron stars cannot be seen in the figure because they are covered by the very thick central part of the gas cloud.

As the stars with a great mass are only a small fraction of the total, the explosion of a supernova is quite a rare event: it is estimated that every century on average three supernovas explode in our Galaxy.

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