The Sun
The Sun

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The Sun

2 Inside the Sun

To account for its brightness and activity, the Sun must contain a power source. However, the nature of that power source was a great puzzle in the nineteenth and early twentieth centuries. Fossil records and ideas about evolution were beginning to provide firm evidence that the Earth must be at least hundreds of millions of years old, rather than thousands of years as was previously thought, and the Sun must be at least as old as the Earth. The only fuels known at the time were coal, wood, oil, gas, and so on. It was fairly easy to calculate that, even if the Sun were made entirely of one of these fuels, and could get the necessary oxygen from its surroundings, it could burn for only a few thousand years at most while producing its current output of heat and light - not nearly long enough to sustain life on Earth over millions of years.

The problem of the Sun's fuel baffled many of the world's best scientists until nuclear reactions were discovered in the early twentieth century. Such reactions provided a totally new type of energy source. Rather than burning like coal or gas, nuclear reactions need no oxygen and produce vastly more heat and light for a given amount of fuel. Nuclear reactions give 'atomic' weapons their great destructive power but are harnessed more productively in electricity generation. The type of reactions that power the Sun - so-called fusion reactions involving hydrogen - are similar to those that occur in a hydrogen bomb, but in the Sun they proceed steadily rather than as an explosion.

Figure 13
Figure 13 Sir Arthur Eddington (1882-1944), astronomer and mathematician: one of the first astronomers to understand the internal constitution of stars.

The British astronomer Arthur Eddington (Figure 13) calculated that, if the Sun were made mainly of hydrogen undergoing nuclear reactions, it could last for millions of years while producing a more-or-less steady heat and light output. Furthermore, its outward appearance would closely resemble that of the actual Sun. We now know that hydrogen nuclear reactions will sustain the Sun for about ten thousand million years. The Sun is currently about half-way through the hydrogen-fuelled phase of its life.

Everything that is known about nuclear reactions is based on experiments performed in laboratories on Earth. Eddington's great triumph was being able to take that knowledge and work out what would happen if nuclear reactions happened on a far larger scale than was possible on Earth, and to relate his deductions to what was known about the Sun. This example illustrates an important feature of astronomy: everything that is known about the Universe beyond our Earth and Moon (apart from a few planets that space probes have visited) must be deduced by observing from a very great distance. Astronomers have two main strands to their quest to understand such distant objects. One strand involves the observations themselves: studying the appearance and movement of distant objects and analysing the radiation received from them. The other strand involves finding out how objects behave on Earth and using that knowledge to interpret and account for the observations.

Scientists have used what they know about nuclear reactions and about how very hot materials behave, together with detailed observations of the Sun, to piece together a model (that is, a mental picture) of what the Sun must be like deep inside. This is shown schematically in Figure 14 and described in the caption. The Sun does indeed consist largely of hydrogen and it is fluid throughout. The nuclear reactions occur only in the Sun's core - that is, deep in its centre -because the hydrogen fuel needs to be at a temperature of over 10 million °C before nuclear reactions can begin. Energy is carried away from the core by radiation that is repeatedly absorbed and re-emitted as it travels through the radiative zone. Closer to the surface, energy is transferred by a different process, known as convection, in which material, heated from below, expands and floats upwards, displacing cooler material that sinks downwards through the convection zone.

Figure 14
Figure 14 The solar interior. Temperature and density increase rapidly with depth inside the Sun, but only in the central core are the conditions right for nuclear reactions to occur. Beyond the core there are zones where energy is transported to the surface by processes involving radiation and convection (hot material rises while cooler material sinks to replace it).

As rising columns of hot material approach the top of the convection zone, the material above becomes thinner, increasing the chance of any emitted light escaping into space. This 500-kilometre thick region constitutes the photosphere, and the rising and falling columns of solar material account for the seething pattern of granules mentioned in Section 1.

Our understanding of the solar interior depends very much on our ability to understand the laws of physics that govern its behaviour. However, there are observations, based particularly on solar neutrinos and solar oscillations, that support and guide our theories. These are explored in the section of images below devoted to 'the solar interior', which you should look at in the next activity.

Activity 3 Describing the Sun

0 hours 30 minutes

At the start of this course, the Sun was referred to as a 'blindingly bright, yellow object'. From what you have read and studied so far, you now know rather more about the Sun than that simple description. Spend a few minutes looking through the text and the relevant pictures and captions below, including those concerning 'the solar interior'. Summarise what you have learned about the Sun's appearance and about its interior. Your summary should be in the form of a labelled sketch (maybe based on Figure 14). Try to make your summary as precise as possible: for example, include sizes and temperatures of the various parts of the Sun where you can.

Figure 15
Figure 15 A view of the solar interior

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This diagram shows the major features of the solar interior; the core, the radiative zone and the convective zone; along with external features such as the photosphere, the chromosphere and the corona. Also indicated are some of the waves that make it possible to learn about the interior by studying the motion of the solar surface.

Figure 16
Figure 16 A mode of solar oscillation

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This colour picture is a computer representation of one of nearly ten million different modes of solar oscillation. These modes are comparable to the vibrations of a ringing bell, or a plucked guitar string. Each part of a plucked string moves up and down in a certain period of time, and a snapshot of the string would capture some parts that were moving upwards while other parts were moving down. In a similar way, this picture shows regions of the Sun that are expanding and contracting within a period of about five minutes; regions caught in a state of contraction are shown in red tones and expanding regions in blue. While having observable surface effects, the vibrations also penetrate deeply into the solar interior and are therefore sensitive to the condition there. Many such modes of oscillation combine to create the observed movement of the solar surface. By measuring the strengths and frequencies of those modes arid using theoretical models, solar astronomers can infer much about the internal structure and dynamics of the Sun. This technique is called helioseismology because of its similarities to terrestrial seismology (the study of earthquakes and other terrestrial vibrations). Helioseismology is at the heart of the research programme of the Global Oscillation Network Group (GONG), which continuously monitors solar oscillations by operating an international network of instrument stations that always has at least one station in daylight.

Figure 17
Figure 17 Describing modes of solar oscillation

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This image shows three more modes of solar oscillation. Each mode is partly characterized by an arrangement of intersecting planes. The planes consist of points that are left unaffected by the movements that contribute to the mode they characterize. The arrangement of these planes can be specified by two numbers (designated l and m) that help define the mode concerned.

Figure 18
Figure 18 The solar interior

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This image shows differences in the speed of rotation of material inside the Sun. The false colours represent speed; red material is rotating the fastest, dark blue the slowest. The inner 70% or so of the Sun rotates at nearly the same rate. However there is marked differential rotation in the outer 30%, which corresponds to the solar convection zone where the energy is carried upwards by convection rather than by radiation. Note that the equator rotates much faster than the poles.


Figure 19 shows our summary description of the Sun. Yours may be presented in a different way but should include similar information.

Figure 19
Figure 19 A possible drawing for Activity 3.

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