diameter: 1,390,000 km.
mass: 1.989e30 kg
temperature: 5800 K (surface) 15,600,000 K (core)


The Sun is by far the largest object in the solar system. It contains more than 99.8% of the total mass of the Solar System (Jupiter contains most of the rest).

It is often said that the Sun is an “ordinary” star. That’s true in the sense that there are many others similar to it. But there are many more smaller stars than larger ones; the Sun is in the top 10% by mass. The median size of stars in our galaxy is probably less than half the mass of the Sun.

The Sun is personified in many mythologies: the Greeks called it Helios and the Romans called it Sol.

The Sun is, at present, about 70% hydrogen and 28% helium by mass everything else (“metals”) amounts to less than 2%. This changes slowly over time as the Sun converts hydrogen to helium in its core.

The outer layers of the Sun exhibit differential rotation: at the equator the surface rotates once every 25.4 days; near the poles it’s as much as 36 days. This odd behavior is due to the fact that the Sun is not a solid body like the Earth. Similar effects are seen in the gas planets. The differential rotation extends considerably down into the interior of the Sun but the core of the Sun rotates as a solid body.

Conditions at the Sun’s core (approximately the inner 25% of its radius) are extreme. The temperature is 15.6 million Kelvin and the pressure is 250 billion atmospheres. At the center of the core the Sun’s density is more than 150 times that of water.

The Sun’s power (about 386 billion billion mega Watts) is produced by nuclear fusion reactions. Each second about 700,000,000 tons of hydrogen are converted to about 695,000,000 tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the surface, it is primarily visible light. For the last 20% of the way to the surface the energy is carried more by convection than by radiation.

The Sun’s output is not entirely constant. Nor is the amount of sunspot activity. There was a period of very low sunspot activity in the latter half of the 17th century called the Maunder Minimum. It coincides with an abnormally cold period in northern Europe sometimes known as the Little Ice Age. Since the formation of the solar system the Sun’s output has increased by about 40%.

The Sun is about 4.5 billion years old. Since its birth it has used up about half of the hydrogen in its core. It will continue to radiate “peacefully” for another 5 billion years or so (although its luminosity will approximately double in that time). But eventually it will run out of hydrogen fuel. It will then be forced into radical changes which, though commonplace by stellar standards, will result in the total destruction of the Earth (and probably the creation of a planetary nebula).


There are eight planets and a large number of smaller objects orbiting the Sun. (Exactly which bodies should be classified as planets and which as “smaller objects” has been the source of some controversy, but in the end it is really only a matter of definition. Pluto is no longer officially a planet but we’ll keep it here for history’s sake.)


*The Sun is one of the millions of stars in the solar system. It is, however, larger than most (although not the biggest) and a very special star to us. Without the Sun there would be absolutely no life on Earth.
*The Sun is 870,000 miles (1.4 million kilometers) across. This is so big it is hard to imagine, but it would take more than one million Earths to fill the size of the Sun!
*The Sun is so big it takes up 99% of the matter in our solar system. The 1% left over is taken up by planets, asteroids, moons and other matter.

*The Sun is about 4.5 billion years old. It is thought to be halfway through its lifetime. Stars get bigger as they get older.
*As the Sun ages, it will get bigger. When this happens, it will consume some of the things close to it, and this includes Mercury, Venus and maybe even Earth and Mars. Luckily this is billions of years in the future.
*The Sun is the centre of the solar system.
*The Sun is 92.96 million miles (149.6 kilometers) away from Earth.
*The Sun is made of a ball of burning gases. These gases are 92.1% hydrogen and 7.8% helium.

*The Sun’s core is extremely hot! An unthinkable 13,600,000 degrees Celcius!
*The Sun has a very big magnetic field. It is the most powerful magnetic field in the whole solar system. This field is regenerating itself, but scientists are unsure how.