of residency on the red planet. This sunset shot is not only beautiful, but it also tells us something fundamental about the Martian sky. Repeated observations have revealed that twilight on Mars is a rather long affair, lasting for up to two hours before sunrise and two hours after sunset. The reason for this long slow progression to and from darkness is the fine dust that is whipped up off the surface of Mars and lifted to incredibly high altitudes. At this height the Sun’s rays are scattered by the dust from the sunlit side of Mars around to the dark side, producing the long, leisurely and beautiful journey between day and night. Here on Earth, some of the longest and most spectacular sunrises and sunsets are produced by a similar mechanism, when tiny dust grains are catapulted high into the atmosphere by powerful volcanoes, scattering light into extra-colourful moments on our planet.
Moving past Earth, which is 150 million kilometres out, we head to the heart of the Solar System. Mercury is the closest planet to the Sun, just forty-six million kilometres (twenty-nine million miles) away. It spins so slowly that sunrise to sunrise lasts for 176 Earth days. Beyond it there is nothing but the naked Sun, a colossal fiery sphere of tortured matter burning with a core temperature of about fifteen million degrees Celsius. The sheer scale of the Sun is difficult to conceive; at 40,000 kilometres (865,000 miles) across it is over 100 times the diameter of Earth, which means you could fill it with over a million Earths. Its mass is 2 x 1030 kilograms – 330,000 times that of our planet. If you add up the masses of all the planets, dwarf planets, moons and asteroids, you would find they contribute less than half a per cent of the total mass of the Solar System. The Sun is dominant – the rest is an afterthought.
Throughout human history, this majestic wonder has been a constant source of comfort, awe and worship, but our understanding of the Sun has developed slowly. For centuries, the finest minds in science struggled to understand how it created such a seemingly endless source of heat and energy. As recently as the nineteenth century science had little knowledge of what the Sun was made of, where it had come from, or the secret of its phenomenal power.
THE ENERGY OF THE SUN
In 1833, John Herschel, the most famous astronomer of his generation, travelled to the Cape of Good Hope in South Africa on an ambitious astronomical adventure to map the stars of the southern skies. This voyage was the end of an extraordinary odyssey for the Herschel family; he completed the work his father, William Herschel, had begun in the northern skies 50 years earlier.
In 1838, Herschel attempted to answer one of the most fundamental questions we can ask about the Sun – how much energy does it produce? It may seem an incredibly ambitious calculation, but Herschel knew that to measure this ‘solar constant’ he would need nothing more than a thermometer, a tin of water, an umbrella and the predictable blue skies of Cape Town.
When you want to measure the Sun’s radiation across billions of miles of space you need to start small. So Herschel began by asking how much energy the Sun delivers onto a small part of the Earth’s surface – in this case, onto a tin full of water. Herschel waited until December to conduct this experiment, when the Sun would be directly overhead, then placed his tin under the shade of the umbrella in the midday Sun. Once the water had heated up to ambient temperature he removed the shade to allow the Sun to shine directly onto the water. In direct sunlight, the water temperature begins to rise and by timing how long it takes the Sun to raise the water temperature by one degree Celsius, Herschel could calculate exactly how much energy the Sun delivered into the can of water.
The calculation was simple because Herschel already knew something called the specific heat capacity of water – in modern units it is the amount of energy required to raise the temperature of 1 kilogramme of water by one Kelvin. Kelvin is a temperature scale usually favoured in science: 1 Kelvin = 1 degree Celsius, and -273 K = 0 degrees Celsius. (For the record, the specific heat capacity of water is 4187 Joules per kilogramme per Kelvin.) From this calculation it’s a small step to scale the number up and work out how much energy is delivered to a square metre of the surface of the Earth in one second. It turns out that on a clear day, when the Sun is vertically overhead, that number is about a kilowatt. That equates to ten 100-watt bulbs being powered by the Sun’s energy for every metre squared of the Earth’s surface.
With this number Herschel could now take a leap of imagination and calculate the entire energy output of the Sun. He knew that the Earth is 150 million kilometres (93 million miles) away from the Sun, so he created an imaginary giant sphere around the Sun with a radius of 150 million kilometres. By adding up each of those kilowatts for every square metre of this entirely imaginary sphere, he was able to estimate the total energy output of the Sun per second. It’s a number that begins to reveal the sheer magnitude of our star. Every second the Sun produces 400 million million million million watts of power – that is a million times the power consumption of the United States every year – radiated in one second. It’s an ungraspable power, but a power that we have calculated using the very simplest of experiments and some water, a thermometer, a tin and an umbrella.
Every second the Sun produces 400 million million million million watts of power – that is a million times the power consumption of the United States every year – radiated in one second.
A STAR IS BORN
It’s a wonder of the Sun that it has managed to keep up this phenomenal rate of energy production for millennia. Stars like the Sun are incredibly long-lived and stable – our best estimate for the age of the Universe is 13.7 billion years, and the Sun has been around for nearly five billion years of that, making it about a third of the age of the Universe itself. So what possible power source could allow the Sun to shine with such intensity day after day for five billion years? The best way to find the answer is to go back to the beginning, to a time when this corner of the galaxy was without light, and the Sun had yet to begin.
The picture above shows the Milky Way. The dark areas with an absence of stars are called molecular clouds; clouds of molecular hydrogen and dust that are lying between us and the stars of the Milky Way galaxy. Taken by the Very Large Telescope (VLT) at Paranal Observatory, in Chile, this image is of Barnard 68, a molecular cloud well within our galaxy at a distance of about 410 light years. Take a close look, because you are looking at a future star, a cloud of dust and gas that in the next 100,000 years or so will collapse and begin its journey to becoming a new light in the heavens.
Barnard 68, like all molecular clouds, contains the raw material from which stars are made, vast stellar nurseries that are among the coldest, most isolated places in the galaxy. This particular cloud is around half a light year across, or twenty million million kilometres, and has a mass that is about twice that of our sun. Most importantly, it is incredibly cold; in the heart of this cloud the temperature is no more than 4 Kelvin, that’s -269 degrees Celsius. That matters because temperature is a measure of how fast things are moving, so in these clouds the clumps of hydrogen and dust are moving very slowly.
The stability of a cloud like Barnard 68 is in a fine balance. On one hand, the clumps of hygrogen and dust are moving around, which leads to an outward pressure that acts to expand the cloud. Counteracting this is the force of gravity – an attractive force between all the particles in the cloud that tries to collapse it inwards. In order for the cloud to become a star, gravity must gain the upper hand long enough to cause a dramatic collapse of the cloud. This can only happen if the particles are moving very slowly, i.e., if the temperature is low.
Over millennia gravity’s weak influence dominates and the molecular clouds begin to collapse, forcing the hydrogen and dust together in ever-denser clumps. We have a name for clumps of gas and dust collapsing under their own gravity: stars. As the clouds collapse further and further they begin to heat up and eventually in their cores they become hot enough for the hydrogen to begin to fuse into helium. The stars ignite, the clouds are no longer black and the life cycle of a new star has begun.
Five billion years ago a star was born that would come to be known as the Sun. Its birth reveals the secret of our star’s extraordinary resources of energy, because the Sun, like every other star, was set alight by the most powerful known force in the Universe.
THE FORCES BEHIND THE SUN
Nuclear fusion is the process by which all the chemical elements in the Universe, other than hydrogen, were