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The Sun – a very ordinary star

Posted in Astronomy, Science, Space on Tuesday, 30 August 2011

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This edited article about the Solar System originally appeared in Look and Learn issue number 1047 published on 3 April 1982.

Cutaway sun, picture, image, illustration

Cutaway picture of the sun showing the core, chromosphere, photosphere, sunspots, polar plumes and corona, by Harry Green

The Sun is a very ordinary star, similar to billions of others scattered across the Milky Way. An island of stars in space, the disc-shaped Milky Way is a galaxy so broad that light takes 100,000 years to cross it. This galaxy is much older than our Sun, which is a relatively young star, checking in at a mere 4.5 billion years.

The earliest stars that formed contained only hydrogen and helium – the raw materials of the galaxies. All other elements have been made inside stars, by a process known as “stellar nucleosynthesis”; at stars’ cores, simple elements like hydrogen and helium are literally fused into the heavier elements. This nuclear fusion is what drives the stars, their energy being released in line with Albert Einstein’s famous equation E=mc2 (in which E is energy, m is mass, and c is the speed of light). The elements produced include those we on Earth regard as commonplace, such as the carbon and oxygen so important to living things.
When old stars die they may explode and scatter the heavier elements across space, so that they are mixed, in tiny proportions, into the clouds of gas from which new stars form. From these new stars, an even smaller fraction of the heavy elements can settle out in the form of rocky planets like the Earth. In fact, everything on Earth except the hydrogen in water and the rare gas helium, has been made inside stars – we are, literally, stardust.

The Sun contains 99.9 per cent of all the material in the Solar System, and all but two per cent of this is hydrogen and helium. And within the innermost quarter of the Sun’s radius – only about 1.5 per cent of its total volume – half the mass is concentrated and 99 per cent of the energy is generated.

Nuclear fusion goes on only in the heart of the Sun. At the centre the temperature is about 15,000,000 degrees C, and a little bit higher just off-centre in the main nuclear fusion region. It then falls off dramatically to the visible surface which has a temperature of “only” 6,000 degrees C!

The fusion process involves the conversion of hydrogen into helium; eventually ail the hydrogen will be used up and nucleosynthesis will convert helium into carbon; but this will not happen for another five billion years.

At this point in its life the Sun will begin to expand into a “red giant” star, engulfing the inner planets, swallowing up the Earth.

Other Sun statistics make impressive reading: its diameter is 1,392,000 kilometres, so that the distance from the centre to the surface is 696,000 kilometres. The Sun’s mass is 330,000 times greater than that of the Earth, weighing 2 X 1027 tonnes (i.e. 2 followed by 27 noughts).

This huge mass provides ample nuclear fuel for the fusion reactions which take place in the interior, even though 600 million tonnes of hydrogen are converted into helium every second to make it glow as brightly as it does.

The energy produced deep in the Sun’s interior is in the form of short wavelength, high energy electromagnetic radiations. Within the inner 85 per cent of the radius, this energy radiates outwards very, very slowly, passed on from atom to atom – the radiation zone. For the rest of the way the atoms absorb the energy and rise upwards by convection, like water heated in a pan, in large gas masses – this is the convection zone.

Although the radiation from the interior travels at the speed of light, it is bounced around so much that it takes millions of years for the energy to get from the core to the surface, which is properly called the photosphere. It then takes just over eight minutes for the visible light to travel the further 150 million kilometres to Earth.

We see a surface of the Sun that is about 500 seconds old, but that surface was produced by nuclear reactions that took place about 30 million years ago!

The photosphere is enclosed by a low-density gas layer, the chromosphere, which is itself surrounded by the corona, a thinly spread layer of material boiled off from the Sun. Sometimes the surface layers are marked by dark spots of cooler gas – sunspots – which appear to come and go over an 11-year cycle.

Astronomers are still not sure exactly what causes sunspots, or why they should follow a regular rhythm. But observations from space have made it clear that these spots are just one manifestation of many activities affecting the Sun. When there are more spots, the Sun shows greater violent activity, producing huge “storms” and solar “flares” – outbursts of material from the surface which seem to be associated with changes in the Sun’s magnetic activity.

All of this activity produces a continuous “solar wind” of particles which streams across space, blowing past the inner planets like Earth. Here it interacts with the magnetic fields of our planet (the Van Allen belts) producing the magnificent “aurora” of the northern and southern lights.

Indeed, some features of our weather seem to follow the Sun’s cycle of activity, probably because of interactions with the solar wind.

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