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The Sun

What is the Sun?

The Sun
Image: Courtesy of SOHO (ESA & NASA)
The Sun is a very hot gaseous body composed of almost 75% hydrogen and 25% helium. Less than 1% is made up of oxygen and several other elements. The diameter of the Sun is 1,400,000 km (840,000 miles) which is more than 100 times the diameter of the Earth. Its mass is more than 300,000 times that of the Earth with a surface temperature of about 5,500° C.

The source of energy in the Sun is the fusion of hydrogen nuclei (protons) into helium nuclei. In this process a small amount of mass is lost and transformed into energy. This nuclear reaction can only take place in the very hot (15,000,000°C) and dense centre of the Sun. The Sun loses half a million tons every second in this destruction of mass to give energy but will maintain its present output of energy for about 5,000 million years.

For this long period of time the Sun is called a main-sequence star but eventually the hydrogen in the centre will all have been converted into helium. The balance between the force of gravity pulling all the Sun's mass towards its centre, and the force due to the energy in the Sun which pushes matter outwards, will then be upset. The centre will contract and become even hotter while the outer part will expand and become cooler. The Sun will then be brighter, cooler and bigger - a red giant star. Ultimately all sources of energy production will come to an end and the Sun will collapse to become a very small hot object called a white dwarf.

The solar cycle

The Sun, as seen from the Earth, rotates about its axis once in just over 27 days and its activity rises and falls over an approximately 11 year cycle, producing variations in the Earth's magnetic field and changes in our upper atmosphere (the ionosphere) affecting the transmission of radio waves and therefore worldwide telecommunications. This cycle of activity was discovered by a German amateur astronomer Heinrich Schwabe as a result of observations carried out between 1826 and 1843; within ten more years a relationship had been established.

Butterfly diagram
Butterfly diagram for the years 1874-1976Image: Royal Observatory Greenwich
At the beginning of each cycle, sunspots occur in high latitudes on the Sun (about 40° from its equator) and in the course of about 11 years, occur in lower and lower latitudes and even on the equator itself. If the latitudes and durations of these sunspot groups are plotted against time, they produce the Butterfly diagram shown on the right.

The top chart shows the latitudes of sunspots as a function of time. They start each solar cycle at about 40° latitude and gradually move towards the equator. The lower plot shows the number of sunspots as a function of time. They peak about every 11 years or so.

The increase and subsequent decrease in sunspots (the areas of which are expressed in millionths of the Sun's visible hemisphere) are shown in the lower part of the figure. The shape of the graph is very similar to corresponding graphs of variations in the Earth's magnetic field (the geomagnetic index), showing the close relationship between activity on the Sun and terrestrial effects.

The period of rise from minimum phase (when sunspots may be absent for several weeks) to maximum phase (when 20 or more groups may be present at one time) takes on average four years, and the fall to the next minimum seven years. In the last 100 years the period of rise has ranged between 3.3 and 5.0 years and the period of fall between 5.7 and 8.3 years, so it is difficult to make predictions over any length of time.


These disturbed regions are seen as dark markings on the Sun's surface. Having a temperature of about 4,800°C, they appear dark by contrast with the brighter surrounding surface, the temperature of which is 6,000°C.
Detail of sunspots on the surface of the Sun.Image: Courtesy of SOHO (ESA & NASA)
The life of a sunspot can be as short as a few hours or as long as several months. Some are seen over several revolutions of the Sun about its axis and in such cases are actually observed for only about half their duration, because for 13 or 14 days of the 27 day revolution they are on the hemisphere facing away from the Earth.

Warning - do not look directly at the Sun
Sunspots can occur singly and in groups and they can be of very different sizes. Large sunspots are sometimes visible to the naked eye when seen through fog or when the Sun is dim and red at sunrise or sunset. At other times the disk is too bright to be looked at directly. Sunspots with areas of only one millionth represent the other end of the scale.

The photosphere, chromosphere & corona

The photosphere is the name given to the surface of the sun. The surface temperature of the sun is 5500 degrees centigrade, which is relatively cool when we consider that the core of the sun heats up to 15,000,000 degrees centigrade, as it burns to create energy. Sometimes the disk of the sun can be seen to get less bright at its edge. This is called Limb darkening.

It is also on the photosphere, where we see sunspots. These are areas, which are cooler (4000 degrees centigrade) than the surface and so appear to look darker. Associated with sunspots are faculae that are characteristic bright regions, which precede the appearance of sunspots.

The surface of the Sun can be seen, through a telescope (SEE WARNING), to have a granular appearance. These granules are the convection cells that carry the energy from below the apparent surface. 

Warning - do not look directly at the Sun

Solar flares

Usually associated with sunspots, these are observed as an increase in brightness of areas of hydrogen (known as flocculi) and can give rise to bursts of intense radiation in the ultra-violet region of the Sun's spectrum which cause sudden ionospheric disturbances and radio fadeouts, leading to disruption of telecommunications on the Earth's sunlit hemisphere. Flares also eject streams of electrically charged particles which affect the Earth's magnetic field and cause geomagnetic 'storms': disturbances affecting the compass needle. These 'storms' are sometimes accompanied in our latitudes by the aurora borealis, or Northern Lights.

Sun storm
This composite image presents the three most visible elements of space weather: a storm from the Sun, aurora as seen from space, and aurora as seen from the Earth.Image: Courtesy of SOHO (ESA & NASA)
Solar flares vary in size and intensity, the smallest lasting only a few minutes before the brightness begins to fade. These small flares produce negligible effects, but a large flare may last for several hours and produce partial or complete radio fadeouts for a corresponding period.


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