Our Star Is Born

Background

Nuclear fusion

The Sun's energy comes from nuclear fusion reactions deep in its core. At very high temperatures the nuclei of small atoms can be fused together to make larger ones. This is what happens in the heart of stars like our Sun. If the new, larger nucleus needs less energy to hold it together than the smaller one, a little mass is 'lost' and energy is released. The most common fusion reaction inside stars builds a helium nucleus from four hydrogen nuclei. In the Sun, 600 million tonnes of hydrogen are converted to 596 million tonnes of helium every second. The 'missing' 4 million tonnes of mass a second is converted to energy.

It was Albert Einstein who had the idea that mass is a form of energy. His celebrated equation

E = mc²

The Sun as an energy source

The Sun is the ultimate source of most of the Earth's energy resources. Without the Sun's energy that reaches Earth through space, there would be no plant or animal life on our planet. It is the Sun's energy, stored by plants and tiny organisms that lived on the Earth millions of years ago, that is released when we burn fossil fuels such as coal, oil and natural gas.

Sunlight is an example of a sustainable energy source: unlike fossil fuels it will not run out. The Sun's energy, in the form of sunlight, may be harnessed directly to heat water, or electricity may be produced using solar cells or using mirrors that focus the Sun's rays in a solar furnace. Fossil fuels, on the other hand, can only be burnt once; they are not recyclable. The gases produced from burning fossil fuels are partly responsible of the pollution of the Earth's atmosphere.

The energy of most alternative energy sources, such as wind and waves, also comes from the Sun. The warming of the Earth by the Sun generates winds whose kinetic energy can be converted into electrical energy by a windmill or wind turbine; and some of the wind's energy disturbs the surface of the sea and makes waves, which can also be used to generate electricity.

The solar cycle

In 1843 the German astronomer Heinrich Schwabe discovered that the number of sunspots visible on the face of the Sun varies periodically. At a maximum in the cycle, over a hundred sunspots may be present, but this number falls considerably as the cycle approaches its minimum. During the minimum there may be several weeks when no spots are seen. On average, the sunspot cycle spans 11 years, but there are considerable variations. Since records began, the length of individual cycles has varied from approximately 7 to 14 years.

In 1893, E W Maunder at the Royal Greenwich Observatory concluded from his study of old solar records that between 1645 and 1715 sunspots virtually disappeared. This period is known as the 'Maunder minimum'. It coincided with a marked cooling of the Earth's climate. The polar ice sheets and glaciers advanced further than at any time since the last ice age. On the frozen River Thames, celebrated winter 'Frost Fairs' were held. The period is also referred to as the 'Little Ice Age'.

At the end of each 11-year cycle, when few or no spots are visible, the polarity of the Sun's magnetic field is reversed. Thus a period of 22 years elapses before the Sun returns to its original magnetic pattern. Roughly midway between magnetic reversals, the Sun is at magnetic maximum: this is when the greatest number of sunspots are evident.