The Sun is a medium-sized star formed 4-6 billion years ago. It has used up around half of the hydrogen fuel in its core. In about 5 billion years it will expand to become a red giant, engulfing Mercury and Venus and possibly Earth.

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Nuclear reactions in the Sun’s core send waves of energy flowing to the surface, where it is given off as light, heat and other radiation.

The Sun’s diameter is 1,392,000 km (865,000 miles). More than 1 million Earths would fit inside the Sun

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The Sun is the source of heat and light for the whole Solar System. Its light takes 8.3 minutes to reach Earth.

Violent activity such as sunspots, flares and prominences are seen on the Sun’s surface.

Weird and wonderful

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The Sun has 11 –year cycles of sunspot activity due to changes in its magnetic field. The increased surface activity means that more changed solar particles reach Earth’s atmosphere, causing radio interference and aurora.

What is the Sun made of?

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The Sun is composed of 71 per cent hydrogen, 27 per cent helium, and 2 per cent heavier elements such as carbon, oxygen, nitrogen, neon and iron.

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The extreme heat and pressure in the Sun causes hydrogen atoms to fuse into helium, releasing energy. The particles that transport heat energy (called protons) follow a ‘random walk’ from the centre to the surface of the Sun by a combination of convection and radiation. The transfer of heat energy from the core to the surface can take up to 10 million years.

Inside the Sun

Around 5 million tones of the 700 million tones of hydrogen the Sun burns every second is converted into pure energy. This process also produces 695 million tones of helium per second. The core is surrounded by two inner layers, the radiative layer, and the convective layer. Above these is the visible surface, called the photosphere, and two further layers of atmosphere, the chromospheres and the corona.

The Sun’s radiation

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Most of the Sun’s radiation is in the ultraviolet, visible light and infrared wavelengths.

Sunlight is necessary for plant photosynthesis.

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Heat, in the form of infrared radiation, creates the mean global temperature necessary for life, and provides the energy for oceanic and atmospheric circulations.

Most of the Sun’s harmful ultra-violet radiation is blocked by the ozone layer, but it still causes sunburn, cancers and cataracts.

Surface features

Sunspots – Dark patches on the surface can be up to 100,000 km (62,000 miles) in diameter. Strong magnetic fields in these areas inhibit the flow of energy to the surface, so they are cooler than the surrounding area. Sunspots last for between one hour and one month.

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Spicules – Flame-like columns of gas that shoot up to 10,000 km (6,200 miles) from the surface.

Faculae – Temporary bright spots that appear on the Sun’s surface.

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Prominences – Flame-like loops supported by the Sun’s magnetic field that rise tens of thousands of kilometers. When seen against the Sun’s surface they appear dark and are called filaments.

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Solar flares – Explosive releases of energy eject clouds of atomic particles into space, triggering microwave and radio-wave radiation. These can cause electrical interference on Earth, affecting television screens and computers and creating surges in power lines.

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Prominences – The arcs of charged gas known as prominences are relatively cool compared to the surface of the Sun.

Solar wind

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The solar wind is a continuous flow of changed atomic particles that stream from the Sun’s surface at speeds of up to 1000 km/sec (600 mps). The fastest streams come from holes in the corona, the Sun’s outermost layer.

Sun Gods

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The Babylonian sun god was Shamash; for the Persians it was Mithras. The Egyptian god Ra was born in the sky each morning and died an old man each night. For ancient Romans, Phoebus Apollo rode a fiery chariot across the sky. The Aztec sun gods Tezcatlipoca and Huitzilopochti demanded human sacrifice, while the Japanese sun-goddess Ameratasu is reflected in Japan’s national symbol.