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The evolution of double stars and supernovae of Type Ia
Almost half the stars in the sky are double or multiple. If the two stars are close together then they can have dramatic effects on each other. The more massive of the two stars will evolve faster and when it becomes a red giant it may be so big that gravity draws its outer atmosphere across to the companion star. The transfer of material can lead to all kinds of interesting and exciting effects, depending on the properties of the two stars.
Stars that have lost their atmospheres to their companions are identical to the White Dwarfs in the centre of planetary nebulae. The less massive companion star, assisted by the extra mass it has gained, eventually becomes a red giant and starts to transfer material back onto its white dwarf companion. This can have the effect of increasing its mass beyond a critical limit of 1.4 times the mass of the Sun, known as the Chandrasekhar limit. When this happens the carbon-oxygen core can suddenly explode, converting half the mass by nuclear fusion into elements like chromium, manganese, iron, cobalt and nickel. This is called a type IA Supernovae. Because they are very bright and we think they always explode releasing about the same amount of energy, they are used as standard brightness light sources. The recent discovery that the expansion of the universe is accelerating, was made by observing these supernovae in galaxies 5,000 million light years away. Type Ia supernovae are also a major source of iron and other heavy elements.
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