Galaxies, stars and planets by The Open University

Author:The Open University
Language: eng
Format: epub
Publisher: The Open University
Published: 2016-03-01T00:00:00+00:00

5 The Sun and stars

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Safety warning

Never look directly at the Sun, either with the unaided eye or through spectacles, binoculars or a telescope. You risk permanently damaging your eyes if you do so.

The part of the Sun that you normally see is called the photosphere (meaning 'sphere of light'); this is best thought of as the 'surface' of the Sun, although it is very different from the surface of a planet such as Earth. The photosphere is not solid. Rather, it is a thin layer of hot gaseous material, about 500 kilometres deep, with an average temperature of about 5500 °C or 5800 K.

Figure 8a shows a visible light image of the Sun. Sunspots (Figure 8b) appear as small dark patches on the photosphere, which typically survive for a week or so, and sometimes for many weeks. The longer-lasting sunspots can be photographed repeatedly as they cross the face of the Sun so they can even be used to investigate the rate at which the Sun rotates. The number of sunspots changes with time, gradually increasing to a maximum every eleven years then decreasing to a minimum when no sunspots may be visible for some time - a cycle intimately linked to changes in the Sun's magnetic field. A close-up view of the visible surface of the Sun (Figure 8b) also reveals a seething pattern of granules seen all across the photosphere. Individual granules, resulting from upwelling hot gas due to convection (an important process in stellar energy transport) come and go in a few minutes, often to be replaced by other granules.

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