Some Facts About the Sun
The Sun exerts a dominating influence on many aspects of the environment of mankind. It is to the Sun that humans have looked for thousands of years as an object of worship and as the means of determining the patterns of their lives.
It is only in the past few centuries that science has been able to tell us much about the real Sun - its size, its energy, its age and its future.
Despite the advances of science in our knowledge, the statistics of the Sun remain truly impressive compared with our everyday experience.
The Sun is the largest body in the solar system. Situated at the centre, its gravitation causes the eight planets to orbit around it. It is basically a large ball of mainly hydrogen and helium gas, although all the other elements may be found in small amounts. The surface and interior temperatures are too hot to have any liquid or solid material. One of the most interesting features of the Sun is the fact that it rotates more slowly at the poles than at the equator - this is called differential rotation. A summary of solar facts is given in the table below.
| Property | Value |
|---|---|
| Radius | 700,000 kilometres |
| Volume | 1.4 billion billion cubic kilometres |
| Surface Area | 6.1 billion billion square kilometres |
| Mass | 2000 billion billion billion kilograms = 330,000 x Earth mass |
| Distance from Earth | 149 million kilometres |
| Angular Diameter from Earth | 0.53 degrees of arc |
| Gravity at Surface | 290 metres/second/second |
| Escape Velocity at Surface | 618 kilometres/second |
| Average Density | 1400 kilograms/cubic metre |
| Total Radiated Power | 380,000 billion billion kilowatts |
| Power at Earth | 1370 Watts/square metre |
| Effective Temperature | 5770 degrees Kelvin |
| Solar Spectral Type | G2V |
| Apparent Rotation Period | 27.3 days (varies with latitude) |
| Age | 5000 million years (approx) |
The Sun has several different regions. At the centre is the core, which is where solar energy is produced via thermo-nuclear fusion. Above this is the radiative zone, where energy travels very slowly upwards. Then we reach the convective zone where heat is transported much faster to the surface, or photosphere. The inner atmosphere is termed the chromosphere, while the outer atmosphere is the corona. Material is continually boiling off from the Sun, and is carried outward by the solar wind. This varies in intensity with time. Fortunately for us, the solar output of heat and light is very constant (within 0.1%) over long periods of time. However, the Sun is not totally quiescent, but undergoes roughly cyclic outbursts of solar activity.
The most visible manifestation of solar activity is the appearance of dark sunspots on the photosphere. These, and all solar activity are believed due to large and changing magnetic fields threading the outer regions of the Sun, from the convective zone to the corona. Sometimes, magnetic fields change rapidly, releasing huge anounts of energy in solar flares and ejection of material from and through the corona (CMEs). These are only visible with special telescopes from the ground and from space. Solar activity tends to vary from a minimum to a maximum and back again in a solar cycle of about 11 years (although this may vary from 7 to 17 years in any particular cycle).
