Image red giant

Red giants

Red giants are amongst the brightest stars in the sky. They are called giants because of their great size, as the figure shows: on the left is an enormous sphere that represents a red giant and on the right the Sun is represented by a small circle. The radius of the biggest red giants are hundreds of times greater than the Sun’s, and their atmospheres can extend for millions of kilometres.

These stars are called red because their temperature causes the emission of electromagnetic waves with wavelengths which mostly correspond to the colour red.

The red giants are nothing more than the second stage of the evolution of a star: indeed, once almost all the hydrogen from its nucleus has been transformed into helium, there isn’t enough energy to counteract its contraction. The nucleus begins to become more compact and again its internal temperature rises until it induces the helium particles to transform into carbon. These transformations produce new energy which stops the contraction of the nucleus and which provides new fuel for the star. During this phase there is an excess of energy produced which is then dissipated thereby expanding the outermost layers of the star, greatly increasing its size and giving it the status of a red giant.

The Sun, too, will become a red giant in about 5 billion years: when this happens, its outer layers will expand until they reach the orbit of the planet Mars and the inner planets will be swallowed up, including Earth.

Once the red giant stage has been completed, the stars will evolve in completely different ways depending on their initial mass, that is the mass it possesses when it formed.

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