The mass of glowing gases at the heart of our Solar System has been radiating energy for 4.5 billion years. Gary Hayden looks at the Sun's life-sustaining power and its influence on human culture
"That lucky old Sun got nothin' to do, but roll around heaven all day."
The lyrics to Frankie Laine's 1949 chart-topping hit do little justice to our celestial neighbour. For the past four and a half billion years "that lucky old Sun" has worked 247, producing unimaginable quantities of nuclear energy deep within its core.
The Sun provides us with heat and light, and sustains life itself. Small wonder, then, that it has been worshipped throughout history. Many of the beliefs and legends that have developed as a result seek to explain natural phenomena, including the Sun's daily passage across the sky, its disappearance at night, the changing length of day from summer to winter, and solar eclipses.
The Ancient Egyptians worshipped Ra, the god of the Sun, who was said to ride across the sky each day in a solar barge, and return via the underworld each night. Before emerging to usher in a new dawn, he had to do battle with one of the denizens of the Underworld - a giant serpent named Apep, chief demon of the night.
The Sun god of Ancient Greece, Helios, drove a fiery chariot from east to west across the sky each day, and returned each night by sailing across the ocean in an enormous cup. In the 5th century bc, Helios's role as Sun god was gradually usurped by Apollo, son of Zeus.
The most extreme form of Sun worship was practised by the Aztecs, whose civilisation flourished in pre-Columbian Mexico. Their religion centred on the Sun god Huitzilopochtli - a young warrior who did battle with the darkness each morning, died each evening, and was reborn the following day.
Only human blood could provide Huitzilopochtli with sufficient nourishment to constantly renew his strength. So, to ensure the dawning of each new day, the Aztecs were required to offer up human sacrifices - thousands every year.
There is no record of Sun worship in China, but there are Chinese myths involving the Sun. According to one, there were 10 suns at the beginning of the world - far too many for life to flourish. So a hero, Hou Yi, shot down nine of them with his bow and arrows, thereby making the world inhabitable.
According to another Chinese legend, a solar eclipse occurs because a heavenly dragon (or sometimes a dog) takes a bite out of the Sun. This led to the tradition of banging pots and pans during a solar eclipse, to frighten the beast away.
Even our present-day Christmas celebrations are tied in with Sun worship.
In Ancient Rome, December 25 marked the feast of Sol Invictus, "the unconquered Sun". When Pope Julius I chose that date to celebrate the birth of Christ he was almost certainly motivated by a desire to hijack this still-popular pagan festival.
In the modern world, it is scientists rather than priests who probe the Sun's mysteries. But this doesn't detract from the Sun's majesty and wonder. We now know that the Sun is a star: a huge ball of glowing gases, hot enough and dense enough to trigger nuclear reactions at its core. It is a fairly typical star, similar to millions of others in our galaxy.
However, it has the distinction of being our star, around which orbit the Earth and other planets of our Solar System.
The Sun accounts for more than 99 per cent of the total mass of the Solar System. Situated at a distance of approximately 150,000 kilometres from Earth, the Sun's light takes about eight minutes to reach us - travelling at 300,000km per second.
The diameter of the Sun is about 1.4 million km, 109 times that of the Earth. So our planet could fit into a hollowed-out Sun 1.3 million times.
The Sun's mass is also impressive, weighing in at about 2,000,000,000,000,000,000,000,000,000 metric tons.
The Sun has been shining for about 4.5 billion years, and will continue to do so for another 5 billion years or more. The temperature of its outer visible layer, known as the photosphere, is around 6000C. But this pales in comparison to the temperature at its core, which is a staggering 15,000,000C.
The Sun is composed mainly of hydrogen. The high temperature and intense pressure at the core trigger nuclear reactions in which hydrogen nuclei are fused together to form helium. Every second, 700 million tons of hydrogen is consumed to fuel this reaction, and 5 million tons of matter is converted into heat energy which is released. In one second, the Sun emits more energy than mankind has used in the past 10,000 years.
The Sun's atmosphere is composed of a number of layers. The Sun is so bright that it is impossible to view these with the naked eye (and dangerous to attempt it), but scientists can observe all of the layers using special instruments.
The photosphere is where the visible light that reaches Earth originates.
It has a constantly changing mottled appearance, caused by continuous turbulent eruptions of hot gas. This outer layer is often pitted with dark regions, known as sunspots. Relatively cool regions of the solar surface, these are the result of complex interactions between the hot gas and the sun's magnetic field.
A thin layer called the chromosphere lies above the photosphere, and beyond this lies the corona. The corona is highly rarefied and very hot (about 2,000,000C). It extends millions of kilometres into space, and is visible to the naked eye as a ghostly halo during a total solar eclipse.
Prominences - immense clouds of glowing gas - sometimes erupt from the chromosphere and appear in the corona.
The Sun continuously emits a stream of charged particles, known as the solar wind. This extends beyond the corona into the outer regions of the solar system. The solar wind can affect the Earth, causing radio interference, power-line surges and the spectacular aurora borealis or Northern Lights.
The source and sustainer of life
Fuelled by the nuclear reactions at its core, the Sun generates unimaginable quantities of energy. On average, every square metre of the Earth's surface receives 350 watts of solar energy each year. Without the warmth and illumination of the Sun there would be no life on the surface of the Earth. The Sun's energy powers many of the complex processes and systems that enable plants and animals to survive. Plants use the Sun's energy to produce food. Sunlight enables them to convert water and carbon dioxide into sugar via photosynthesis. This allows plants to live and grow, and the plants in turn provide food for animals. Oxygen, which all animals breathe, is a by-product of photosynthesis. So the Sun provides us not only with nourishment, but also with breathable air.
The Sun drives the climate and weather systems that make life on Earth possible. The wind, rain and ocean currents are all dependent on solar energy. Sunlight heats the oceans and land, which in turn heat the air.
Warm air rises, creating an area of low pressure and causing air to rush in from surrounding areas. So the Sun powers the wind. Energy from the Sun causes water to evaporate from the world's oceans, seas and lakes. This is the first stage in the water cycle, which gives us rain. The sun warms the oceans around the tropics more than it warms the water around the poles.
This results in large amounts of warm and cool water being circulated around the globe, which has a dramatic and beneficial effect on the world's climate.
Sunlight affects the human body. Some of its effects are beneficial, but it can also be dangerous. Although the Sun may be the source and sustainer of life, its radiation must be treated with respect.
On the positive side, it is our main source of vitamin D, which strengthens bones and muscles, and boosts the immune system. It also stimulates the pineal gland - a pea-sized organ located in the brain. This gland produces chemicals called tryptamines, which have a beneficial effect on our mood.
On the negative side, exposure to the Sun's ultraviolet rays can lead to a variety of problems. Spending long hours in the Sun without eye protection increases the risk of developing eye-diseases - for example, exposure to high levels of sunlight makes you four times more likely to develop cataracts. Long exposure to the Sun damages the skin, changing its texture and weakening its elastic properties. This can lead to furrows and wrinkles, "liver spots" and pre-cancerous lesions. And exposure to the ultraviolet rays is widely recognised as a leading cause of skin cancer.
Certain groups of people are especially at risk, and need to be extra-cautious. Those with freckles, fair skin, and red or fair hair are all at increased risk, as are those with a large number of moles or a family history of skin cancer.
Fortunately, the health problems associated with exposure to the Sun are largely preventable. It's mostly a matter of seeking shade during the hottest part of the day (between 10am and 4pm), and using adequate protection when venturing out into the sunshine. In the summer months, Sun-worshippers everywhere would be wise to remember the three essential "S" rules - Slip on a shirt, Slap on a hat, and Slop on some sunscreen.