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Our Sun is one of 100,000 million stars that make up our galaxy. The galaxy has the form of a spiral galaxy, similar in form to the Andromeda galaxy shown in the picture along side. The Sun lies in one of the spiral arms about 10,000 parsecs from the galaxy centre (1 parsec = 3.08 x 10
km, 1 light year = 9.46 x10
km).
The age of the Sun is thought to be 4.6x10
years, using the age of the oldest meteorites. At present all the energy released by the Sun comes from the conversion of Hydrogen into Helium in the nuclear core. The supply of energy in the core should last for another 5 x 10 years. Once all the hydrogen in the core is converted to Helium, the core will collapse under gravity and heat up. The Hydrogen in the surrounding envelope will ignite and the Sun will now have an inert Helium core and a nuclear burning envelope. The Helium produced in the envelope will settle in the core and will cause it to heat up and the envelope to expand. The Sun will become a red giant. The phase will last a few hundred million years and the Sun moves off the main sequence. Then the Helium burning stage will occur and the Helium of the core will be converted into Carbon (C).
The eventual fate of the Sun is to form a white dwarf. The density of white dwarfs is some 10kg m
. The final state of other stars depends crucially on the initial mass of the star.
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