The Sun

The Sun

The Sun is a star. It is a very typical star; it is neither the most luminous or the dimmest, the largest or the smallest, the hottest or the coolest. There is very little that is unusual about the Sun. In stellar terms, it is a yellow, G2 type star with a surface temperature just under 6000°C.

The Sun
 Mass 1.989 × 1030 kg 
 Mass (Earth = 1) 332,830 
 Equatorial Diameter 1,390,000 km 
 Equatorial Diameter (Earth = 1) 108.97 
 Mean Density 1.410 × 103 kg m-3 
 Mean Density (Earth = 1) 0.255 
 Rotational Period 25 to 36 days 
 Escape Velocity 618.02 km s-1 
 Escape Velocity (Earth = 1) 55.23 
 Luminosity 3.827 × 1026
 Apparent Visual Magnitude -26.8 
 Mean Surface Temperature 6,000°C 
 Age 4.5 × 109 years 
 Chemical Composition
All others

92.1 % 
7.8 % 
0.061 % 
0.030 % 
0.0084 % 
0.0076 % 
0.0037 % 
0.0031 % 
0.0024 % 
0.0015 % 
0.0015 % 


Mass is the amount of matter that an object contains. On the Earth mass can be measured by weight. The Sun contains thousands of times the mass of all the other objects in the Solar System put together. The Sun's mass is found by measuring its gravitational effect on the planets and other bodies.

Equatorial Diameter

The Sun's diameter is over 100 times that of the Earth. It is the largest object in the Solar System. The visible surface of the Sun is called the photosphere.


Density tells how concentrated matter in a body is. The Sun is a little more dense than water and a quarter the density of the Earth. The figure given is the Sun's average density. This density is similar to those of the Gas Giant planets. The Sun is gaseous throughout.

Rotational Period

This is how long the Sun takes to rotate once on its axis. The Sun is not a solid object as it is made of gas. Not all parts rotate together. The figures given here are from the pole to the equator. The polar regions rotate more slowly than the equatorial regions. The differential rotation causes the Sun's magnetic field to twist on itself. This produces sunspots.

The number of sunspots varies in an 11 year cycle which affects the Earth's climate and magnetic properties.

Escape Velocity

This is the speed that an object must attain in order to escape from the Sun's gravitational field. For the Earth, this speed is about 11 kilometres per second (7 miles per second).


This is the amount of energy that the Sun radiates, in Watts. Every second the Sun loses 6 million tonnes of matter that is converted into energy. The energy is produced by nuclear reactions in the core of the Sun. Hydrogen atoms combine to form Helium with a small loss of mass that is converted to energy in accordance with Einstein's famous equation,

E = mc2.

Apparent Magnitude

Apparent Magnitude tells how bright an object is as seen from the Earth. It was originally a scale set up for measuring the brightness of the stars. The brightest stars were called First Magnitude, the next brightest were called Second Magnitude, etc.

The brighter a star, the smaller its magnitude. Many stars are brighter than first magnitude. Some stars are so bright they have negative magnitudes. On this scale, Jupiter has a magnitude (at its brightest) of -2.6, Venus -4.4. The faintest stars visible to the naked eye are Sixth Magnitude. Pluto has a magnitude of +14, far too faint to be visible without a powerful telescope. On this scale the Sun is so bright its magnitude is the high negative value of -27.

Mean Surface Temperature

This is the temperature of the visible solar surface. There are many methods of measuring this temperature.

The first method involves studying the distribution of energy in the Sun's spectrum (Wein's Law)

The second method of measuring the temperature is by the application of Stefan's Law: the temperature is related to the luminosity.

A third method examines the solar spectrum for the condition of various elements. By seeing which elements are ionised and which are absorbing particular wavelengths of energy, the temperature can be found.

The centre of the Sun has a temperature in the millions of degrees.


The Sun's age is the age of the Solar System as a whole. A star's age is related to its luminosity (how quickly it is using up its energy) and its mass (how much material it contains).

Chemical Composition

The Sun's chemical composition is found from its spectrum. Like all stars, the Sun is made up mainly of Hydrogen.

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