2 Stars and galaxies
The Sun is just one of billions of stars in our Galaxy, the Milky Way. Like all stars, the Sun is a large ball of gas, mostly hydrogen, which is so massive that its centre has become a nuclear reactor. The weight of all the layers of the star pressing inwards creates huge temperatures and pressures at the centre. Under these conditions hydrogen undergoes nuclear fusion, creating helium. All of the energy that the Sun produces comes from this conversion of hydrogen gas into helium gas.
Our Sun is about 4.5 billion years old. 4.5 billion can also be written as 4500 000 000 or 4.5 × 109. The Sun is middle-aged: halfway through its lifetime as a ‘normal’ star, which we call a ‘main sequence’ star. More massive main sequence stars than the Sun are bigger, bluer and hotter (Figure 18). These stars burn through their resources of hydrogen relatively rapidly, after which they evolve into other types of stars and eventually ‘die’. Less massive stars are smaller, redder, cooler and burn hydrogen more slowly. Less massive stars therefore have longer lifetimes.
When the Sun runs out of its hydrogen supply in about another 4.5 billion years it will become a ‘red giant’. Its outer layers will expand, possibly reaching as far as the Earth’s orbit. Mercury and Venus will certainly be swallowed up. Eventually, these outer layers will be shed and just a dense core will be left where the Sun used to be. A remnant star like this is called a ‘white dwarf’.
Many stars will end their lives in the same way as the Sun, but others will have more dramatic deaths. More massive stars can explode as supernovas, and their leftover cores form extremely dense objects called neutron stars, or even black holes.
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