Physical properties Nuclear fusion and light production Evolution

The Sun is a star in the outer part of the Milky Way Galaxy, formed from material produced inside a supernova. It is classified as a G2 V star with surface temperature about 5,800 Kelvin or 6 000°C and a core temperature of around 15 000 000°C. It is a fairly middle of the road star, which is about 5 billion years old.

The Sun is the star around which the Earth, planets, asteroids and comets of the Solar System revolve. It is the dominant body of the system, constituting more than 99% of the mass of the Solar System. It is more than 330 000 times heavier than the Earth and over 1 000 000 times larger.

The Sun is the source of an enormous amount of energy, and the only source of light and heat available to the Earth and other planets in the Solar System.

Physical properties
The temperature of the Sun's surface is so high that no solid or liquid can exist. It is predominantly made up of super heated gaseous plasma or gaseous ionised atoms, with a few small bits of molecules.  As a result, there is no fixed surface like on Earth, but a seething, boiling cauldron of exploding sunspots and prominences. The surface viewed from Earth is called the photosphere and is the layer from which most of the light produced by the Sun reaches us.

Under the force of its own gravity, the great mass of the Sun presses inward, producing a central pressure estimated to be around 10 000 times greater than that of the centre of the Earth. Under this immense pressure, the density of the Sun's core is about 100 times that of water or 100 g/cm³. At this temperature and pressure, the atoms are completely stripped of their electrons producing a plasma of nuclei and electrons. The nuclei collide with such force as to produce the nuclear reactions that are responsible for generating the heat and light produced by the Sun.

Although the Sun is a very stable source of energy, its output does vary by around 0.1 percent. It has an 11-year cycle of magnetic activity which produces regions of transient strong magnetic fields called sunspots. Solar prominences or great flares of luminous gas shooting thousands of kilometres into space are associated with these sunspots.

The results of these prominences can be seen on Earth as an increase in the solar wind, increased aurora activity and even interference with radio communications.  The sunspots themselves look dark against the brightness of the solar disc as they are actually a little cooler than the surrounding photosphere.

During a total eclipse, the outer atmosphere of the Sun called the chromosphere appears as a pink ring. Above the chromosphere is the corona, a dim, extended halo, which reaches far out into the Solar System as a flow of charged particles called the solar wind. Near the Earth the solar wind has a speed of 400 km/s.

Nuclear fusion and light production
The energy radiated by the Sun is produced in the core by nuclear fusion reactions combining  the nuclei of hydrogen atoms to form helium. The Sun is at least 90% hydrogen calculated by the number of atoms. At its present rate the Sun can burn for another 100 billion years.

The sun’s density is so high that the light particles (photons) produced in the core travel only a few millimetres before they collide with other nuclei and are scattered. The photons take a so-called ‘random walk’ outward until they escape from the Sun. Even at the speed of light this process takes 10 million years, so the light seen today was generated 10 million years ago. The final step from the Sun's surface to Earth, however, takes only eight minutes.

Evolution
The Sun has been shining for about 5 billion years. Considerable hydrogen has already been converted to helium in the core. The Sun becomes 10 percent brighter every billion years and it must now be at least 40 percent brighter than at the time of formation of the Solar System. The increase in solar brightness can be expected to continue as the hydrogen in the core is consumed and the region of nuclear burning moves outward from the core.

The evolution of the Sun should follow the same path as most stars. As the core hydrogen is used up, the nuclear burning will take place in a growing region surrounding the old exhausted core. The star will continue to grow brighter, and when the burning approaches the surface, the Sun will become a red giant. It will quickly grow as far as Venus or even the Earth, before exhausting most of its hydrogen fuel and collapsing to form a White Dwarf star. Fortunately, billions of years will pass before this happens!

Life of a low-mass star

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