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The Evolution of Stars Like the Sun
Stars with masses between about 0.8 and 11 times the mass of the Sun have very similar histories although the speed with which they get through their lives depends on their mass. We will follow the life of our Sun.
Our Sun was born, along with its attendant planets, about 4,566 million years ago. We know this date quite accurately from measuring isotope ratios of various elements. For example, uranium-238 is a radioactive isotope of uranium with a half life of about 4.6 billion years. This means that after 4.6 billion years half of the original uranium-238 will have decayed into lead-206. By measuring the proportion of uranium-238 to lead-206 ina mineral sample, and assuming there was no lead-206 in the sample when it was first formed, it is possible to work out the age of the sample. (The number 206 in lead-206, refers to the total number of neutrons and protons in the lead atom. The usual notation is 206Pb.)
It is also possible to find unusual isotopes such as magnesium-25, in small samples of material less than a millimetre in diameter, between the grains in the meteorites called carbonaceous chondrites. These small fragments have remained undisturbed since the formation of the solar system. Magnesium-25 is produced by the decay of aluminium-26, which is no longer present because it is one of a number of short-lived radioactive isotopes with half lives of about a million years, which were created, either in a supernova that may have triggered the formation of the Solar System, or possibly by the young Sun itself.
During the last 4,566 million years, the Sun has been converting hydrogen into helium. It has already converted half the hydrogen in its core into helium and is 30% brighter than when it started its life. It will continue to get brighter as its radius increases and in 3,000 million years time, may have evaporated the Earth's oceans. In 5,000 million years hydrogen fusion will stop in the core, although it will continue in a shell around the core. With no central source of energy the force of gravity will compress the core and raise its temperature and density until it is high enough to start the fusion of helium to form carbon and oxygen. This new source of energy will once again provide the means to resist the inward force of gravity.
While the core is being compressed the outer layers of the Sun will expand. Although the outer layers will be much cooler than today, the Sun will be much bigger and we will call it a 'red giant'. There are many red giants in the sky. One of the closest is the star Betelgeuse, in the north-east corner of the constellation of Orion. Its diameter is greater than Jupiter's orbit around the Sun and its surface temperature is about 3,400 K. This is much cooler than the Sun but its greater surface area makes it 9,400 times more luminous.
When the Sun becomes a red giant, in about 7,000 million years time, it may have expanded as far as the Earth's orbit. The rocky surface of the Earth will liquify, obliterating all trace of life and may even evaporate, returning the material out of which we and it are made to interstellar space. Helium fusion will stop in the centre of the Sun but will continue in a shell, itself surrounded by a second hydrogen fusion shell. This will make the Sun even more luminous and unstable, causing it to vary in brightness, as well as bringing some of the carbon and oxygen from the core to the surface.
Soon after this, all nuclear reactions will cease in the Sun. Its surface will expand from a diameter of a few light hours until it is two to three light years across. It will then be called a 'planetary nebula'. The name was devised because, when first seen through telescopes, some of these nebulae appeared circular and faintly green, with a superficial resemblance to the planets Uranus and Neptune.
The gas in the planetary nebula will now be at such a low density that the inert core of the Sun will be visible for the first time. No bigger than the Earth, it will contain almost half the mass of the Sun and have a temperature of about 30,000 K. In a few tens of thousands of years the planetary nebula will fade away as the gas disperses into space, leaving the hot inert core, now called a 'white dwarf'. The white dwarf that was once our Sun, still attended by the outer planets, will spend the next perhaps nine thousand million years steadily cooling and fading in brightness. The oldest white dwarfs we currently see are almost as old as the universe and twice the age of the Earth. They still have temperatures of about 4,000K.
Planetary nebulae come in all sorts of shapes and sizes, showing great beauty and exhibiting extraordinary patterns and symmetries. The central stars, with temperatures up to 100,000K, supply a flood of ultraviolet light, which causes the planetary nebula to fluoresce. They disappear after a few tens of thousands of years as they expand and fade away, carrying with them as much as half the mass of their parent star. They have great significance for us because they contain, along with much hydrogen and helium, carbon, nitrogen and oxygen, which have been manufactured by the star. These elements are some of the important building blocks of life. Much of the carbon, nitrogen and oxygen on our Earth has come from these stars. If you are a romantic you can think of yourself as made of star dust: If you are less so you can think of yourself as made of the nuclear waste of stellar evolution!
Although we will see in the next section that many of the heavy elements, like chromium, manganese, iron, nickel and cobalt and even heavier ones, are made in supernovae explosions, there are some other important elements that are made in stars like the Sun when they become red giants. The mechanism takes place while there are two fusion shells and depends on some of the hydrogen from the surface being carried by convection into the core where it is processed to release neutrons. These combine with iron nuclei to make certain heavier elements such as strontium, yttrium, zirconium, barium, lanthanum and cerium. In this process there is a lower density of neutrons than in a supernova explosion, allowing a different selection of heavy elements to be made.
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