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THE SUN |
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SUN’S SURFACE The visible disk of the sun – what we think of as the Sun’s surface – is called the photosphere (from the Greek for "sphere of light"). After thousands of years working its way up from the core, the energy released by nuclear reactions inside the Sun finally bursts from the photosphere in a blaze of light. When Galileo first examined the Sun with a telescope almost 400 years ago, he was amazed to find its bright surface speckled with dark markings. These sunspots are caused by magnetic fields inside the Sun.
PHOTOSPHERE The photosphere is not solid like the Earth’s surface, but a seething sea of glowing gas 500 km thick that marks the tops of currents of hot, opaque gas rising from the interior. At the photosphere, the gas becomes transparent, allowing light to escape into space. Temperature range from 8,500°C at the bottom of the photosphere to 4,200°C at the top, with the average being about 5,500°C. By analysing light from the photosphere with a spectrograph, astronomers can tell that the Sun consists of hydrogen and helium.
SUNSPOTS Dark blotches, sunspots, periodically appear on the photosphere. They range from small spots known as pores, which are less than 1,000 km across, to clusters called sunspot groups that stretch for up to 100,000 km. Sunspots last from a few hours to many weeks.
SUNSPOT STRUCTURE Sunspots are shallow depressions in the photosphere where strong magnetic fields stop currents of hot gas from reaching the Sun’s surface. Sunspots are about 1,500°C cooler than the rest of the photosphere, and only look dark because of their brilliant surroundings.
SUNSPOT CYCLE The overall number of sunspots rises and falls over an 11-year cycle. The first spots of each new cycle are seen near the poles. They gradually increase in number, appearing closer and closer to the equator until the cycle reaches its peak. The cycle may be caused by the way different parts of the Sun’s surface rotate at different speeds, forcing bands of magnetic activity towards the equator.
MAGNETIC SUNSPOTS Sunspots occur in areas of violent magnetic activity called active regions. The magnetic fields inside the Sun are wound up and twisted by the different speeds at which the Sun’s surface rotates. Churning gas currents in the photosphere cause loops of magnetism to break through the surface and form sunspots. One end of each loop is a north magnetic pole, while the other end is a south magnetic pole.
SOLAR TELESCOPES A tower telescope is an optical telescope that tracks the Sun with a moving mirror (A heliostat) on top of a tower. The heliostat reflects light down a static, vertical shaft to measuring instruments at ground level. In a vacuum tower telescope, air is removed to stop the Sun’s heat from stirring up air currents that may distort the image.
EFFECT ON CLIMATE Some scientists think that solar events may influence the Earth’s climate, with periods of cooler weather linked to slow activity. One such period was 1645-1715, when the Sun was almost spots-free, and the sunspot cycle seemed to have stopped. Northern Europe went through a period of unusually cold weather now known as the Little Ice Age.
ROTATION SPEED The Sun’s is a globe of gas, so it does not all rotate at the same speed as a solid object would. The Sun’s equator makes one rotation approximately every 25 days, while areas near the poles turn once every 35 days. The way the Sun’s surface oscillates, or vibrates, suggests that the inner part of the Sun spins like a solid ball, with a rotation period of 27 days.
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