The Sun

The architecture of the Sun consists of a series of nested shells, one inside the other, similar to the layers of an onion.

At the very centre and stretching a quarter of the way to the surface is the core. The temperature is about 15 million degrees Celsius and the density of the compresses gasses is 12 times that of lead. A pinhead of this turbulent matter would be hot enough to ignite everything for 60 miles (100km) around. These conditions are extreme enough for nuclear fusion to occur. Every second, millions of tons of hydrogen are fused into helium. About 4.5 million tons of matter is converted into energy, which then radiates out from the core.

The radiative zone surrounds the core and extends four-fifths of the way to the surface. The gamma rays, X-rays and photons produced in the core fight their way outwards through the dense solar gases. It is a slow journey, and a single ray can take a million years to cross this zone. In the outermost fifth of the Sun, energy is moved in large bundles of gas, driven by the heat from within. The convective zone consists of layers of cells, each smaller that the one below. Each of the cells in the outer tier is about 600 miles (1,000km) across and is bordered by an area in which the gas sinks back into the Sun.

This pattern of rising and falling gas gives the Sun's surface, called the photosphere, a mottled and granulated appearance, bright where gas is rising and dark where it is sinking. The photosphere is no more than a few hundred miles thick and is usually marked by sunspots, which are often the size of the Earth in diameter and about 2,000 degrees Celsius cooler than the surrounding surface. The spots mark kinks in the Sun's magnetic field, where it has been contorted by the rotation of the gasses and by the turbulence in the convective zone.

Beyond the surface of the Sun, the Sun's atmosphere, or chromosphere, is a few thousand miles thick. Here the Sun's magnetic field arranges the gasses in long "hedges", like the iron filings around a magnet. Above them giant braided arches loop between sunspots - hanging there for months at a time, or for a few minutes - bursting in flares and prominences up to several hundred thousand miles out into space. More constant but less dramatic is the faint halo of white light called the corona. In places, particularly above the Sun's magnetic poles, gaps appear in the corona and the light glows less intensely. These holes are the sources of the solar wind, a great stream of atomic particles that have escaped the Sun's grip and slipped out into space, bathing the solar system and sweeping past the Earth.

It is estimated that the Sun is about half way through it's life. If this estimation is correct, in around 4 billion years from now the Sun will begin to die. The hydrogen in the core of the Sun is exhausted, leaving almost pure, very dense helium. Hydrogen burning will spread into the outer region of the star - which, as a vast red giant, will expand, engulfing the Earth and the inner planets. In about 5.5 billion years from now, the helium core will become hot and dense enough to burn. This will happen quickly and the star will flare up in a "helium flash". Helium burning gives the star a new lease of life, but when the helium in the core is exhausted, burning will continue in the outer shells. The complex processes of nuclear burning in the layered shells of an aged red giant lead to a phase in which the forces of expansion and contraction get out of step.

The star begins to oscillate, alternately shrinking and expanding, and possibly shedding some of it's outer layers into space. Once the helium in the core is consumed, the core will contract and heat up, the unused layers of hydrogen will expand into space to form "ring nebula" leaving a residual core that will become a small but very bright white dwarf star. Eventually the white dwarf, which is no more than a fossil star, will cool down to become a cold, dense black dwarf, no longer radiating energy and invisible to astronomers. It may take a trillion years to cool off completely.