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The sun.

A nuclear furnace in space

Our Sun is a typical star and it is only because it is relatively close to us that it looks different from the others. Solar observation forms an important part of astronomy because, among other things, it gives us a close-up view of how a star behaves, and how it influences its surroundings.

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At over 1.4 million kilometres in diameter, the Sun contains 99.9% of the mass of the entire solar system: well over a million Earths could fit inside it.

The conversion of hydrogen into helium through nuclear reactions in the Sun's interior, where the temperatures and pressures are extremely high, generates power at an average rate of 383 billion trillion kilowatts. It would take Koeberg nuclear power station 7 billion years to generate the same amount of energy that is generated in the Sun every second. The core of the Sun is about 400 000 km in diameter and contains 60% of its mass in barely 2% of its volume. The temperature here is about 15 million degrees and the density of the gas is eight times that of gold.

But the energy released by the Sun is not always constant. It varies over the sunspot cycle by about 0.02%. The spots are associated with turbulent tangles of magnetic fields and boiling arc-shaped clouds of hot plasma.

Observing the Sun

An easy and safe way of observing the Sun is by projecting its image through a pinhole onto a shadowed screen. Take a cardboard box, remove one side, and tape a white sheet of paper on the inside of one end. Poke a small hole in the opposite end, cover it with aluminium foil and make a pin-hole in the foil. Stand with your back to the Sun and look at the sheet of paper, while pointing the box at the Sun. A large image of the solar disc will be projected inside the box. If the image is too faint, enlarge the hole; however, if the hole is too large, the image will be unclear.

A brighter, sharper image can be produced if a telescope (or half a binocular) is used to project the image onto a white sheet of paper (see Figure 1). Keep in mind that the concentrated heat can damage eyepieces, so allow the optics to cool down periodically. It's also a good idea to reduce the diameter of the lens using a piece of cardboard with a round hole in it, so that the eyepiece is not damaged by the intense light passing through it.

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Sunspots

Sunspots are strong concentrations of magnetic flux at the solar surface. Sunspots become visible when magnetic forces restrict the upward flow of ionized gas (plasma) in a certain area, causing it to cool down and darken. Sunspots consist of a dark central umbra (about 3 700[degrees]C) and a lighter surrounding penumbra (about 5 200[degrees]C). Careful examination of the solar disc should show these dark markings, usually grouped together. Individual spots last from a few hours to many weeks, and can be as large as the planet Jupiter. The Sun's rotation can be followed by watching the passage of sunspots across its disc.

The number of sunspots visible at a given time varies from none to over 100 in a cycle averaging 11 years. Peaks in numbers of sunspots have come as close together as 9 years and as far apart as 13.7. Solar astronomers traditionally refer to the cycle that started on March 1755 as "Solar Cycle 1". We are currently in Solar Cycle 24, which started in January 2008. Solar minimum occurred in December 2008 and solar maximum (when the sunspot count is highest) is expected to occur in May 2013.

Solar observers in Southern Africa can contribute significantly to the international effort of monitoring sunspot activity. All that is required is a small telescope (an aperture of 50-75 mm will give good results), a few minutes each day, and e-mail facilities.

Other solar features

To view faculae and granulation safely through a telescope, you will need a Alter that will dim the Sun's light by a factor of about a million. Only Alters designed for solar viewing should be used: other Alters, no matter how dark, generally transmit too much harmful infrared radiation. Mylar, an aluminised plastic, can often be bought very cheaply, and if properly mounted, works well. The filter must be located in front of the telescope. To view solar flares, prominences and elaborate surface detail, a special hydrogen-alpha (H[alpha]) Alter is needed.

In addition to visual observing, electronic devices can be used to monitor solar activity. A simple VLF radio receiver can be used to monitor the strength of signals received from distant transmitters. When a solar flare erupts, signal strength increases abruptly in an event known as a sudden ionospheric disturbance (SID). Such flares sometimes disrupt the Earth's magnetic field causing a magnetic storm that can be recorded with a magnetograph.

Solar telescopes in South Africa

At the SAAO's observing site near Sutherland, the University of Birmingham (UK) operates an automated telescope that monitors low-frequency solar oscillations. This is one of six automatic stations distributed globally, the Birmingham Solar-Oscillations Network known as BiSON, giving 24-hour coverage of the Sun. The 4-inch heliographic telescope at SAAO in Cape Town, erected in 1876, is used to view sunspots. At the Boyden Observatory near Bloemfontein, there is a 20-cm coelostat with instrumentation for making narrow-band H[alpha] observations.

Eclipses of the Sun

In a calendar year between four and seven eclipses (solar and lunar combined) can occur; at least two and at most Ave can be eclipses of the Sun.

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In 2012, two eclipses of the Sun occur, neither of which will be visible from Southern Africa. The annular eclipse on May 20 can be seen from China, Japan, the Pacific, and western United States of America. Its maximum duration is 5 minutes 46 seconds. The total solar eclipse on November 13 can be seen from far-northern Australia and the southern Pacific. Its maximum duration is 4 minutes 2 seconds. It will appear as a partial eclipse from other parts of Australia, New Zealand, and southern South America.

A solar eclipse occurs when the Moon blocks sunlight that normally falls on Earth and can only occur at the time of New Moon. Because the plane of the Moon's orbit is tilted slightly to the ecliptic, in most months the Sun-Moon-Earth alignment is not exact and the Moon's shadow misses the Earth. When the alignment is correct, the Sun is eclipsed.

When the observer is directly in the line Earth-Moon-Sun, the eclipse is either total or annular, depending on the distance to the Moon. When the Moon is at its farthest from Earth, it does not completely cover the solar disc, leaving a ring of sunlight visible. This is an annular eclipse (Latin annulus, 'ring'). For thousands of kilometres to either side of the path of totality, the eclipse appears partial (Figure 2).

During an annular-total eclipse, the eclipse starts as annular, then becomes total as the curvature of the Earth brings its surface closer to the Moon so that it intercepts the tip of the umbra, and then returns to annular before the end of the path.

Frequency of eclipses

From any given location, there will be, on average, a partial eclipse every couple of years, an annular eclipse once every 224 years, and a total solar eclipse once in 375 years. The next total solar eclipse in Southern Africa will be in 2030. Namibians will be able to view three total eclipses in 16 years, starting with the eclipse on 2360 March 18. Even more remarkable, in a 32 year period, Ave total eclipses can be seen from a region in southern Egypt. The island of Madeira is possibly the worst spot on Earth for eclipse watchers: the last total solar eclipse occurred in 292, and the next will be in 2620.

ASSA Solar Section

The ASSA Solar Section exists to promote solar observation in Southern Africa by helping novice observers, co-ordinating observation programs for the serious observer, providing an information service to the public and maintaining contact with a variety of international institutions. Persons interested in observing the Sun are invited to contact the ASSA Secretary, c/o SAAO, PO Box 9, Observatory, 7935. E-mail: [secretary@saao.ac.za] African lore

* The Sun is known as zuva in Shona, duvha in Venda, letsatsi in Sotho, and ilanga in Zulu and Xhosa. In praise poetry (izibongo), heroes and chiefs are often associated with the Sun's light and heat.

* Some Zulu believe that the Sun dies when it sets, being devoured by a race of pygmies, and a new Sun is born in the east every day.

* A solar eclipse, known as fifalo ya letsatsi to the Sotho and Tswana, ukufiphala kwelanga to the Zulu, meaning "darkening of the Sun", and called mutsha-kavhili or "two dawns" by the Venda, was considered an evil omen foretelling pestilence, famine, or the death of a chief.

* A Bushman tale tells how the Sun is really a rhinoceros. When it goes down in the west, it is killed and eaten by the people who live there, who then throw the shoulder blade of the animal to the east where it is reborn.

WARNING: Never look at the Sun without proper eye protection. Permanent blindness can result from the shortest look through binoculars or a telescope.
Quick facts about the Sun

Age (years)                               4.5 x [10.sup.9]
Mass (kg)                                 1.9891 x [10.sup.30]
Diameter (km)                             1.392 x [10.sup.6]
Escape velocity (km x [s.sup.-1])         617.7
Luminosity (watt)                         3.846 x [10.sup.26]
Mass conversion rate (kg x [s.sup.-1])    4.3 x [10.sup.9]
Spectral type                             G2 V
Absolute magnitude                        +4.83
Apparent visual magnitude                 -26.74
Distance from Earth (km)                  1.47-1.52 x [10.sup.8]
Core pressure (atmosphere)                2.452 x [10.sup.11]
Core temperature ([degrees]C)             1.571 x [10.sup.7]
Rotation period at equator (days)         26.8
Rotation period at poles (days)           36
Composition (% hydrogen)                  92.1
Composition (% helium)                    7.8
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Publication:Sky Guide Africa South
Geographic Code:6SOUT
Date:Jan 1, 2012
Words:1713
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