Leicester University Physics Department

The Sun

Solar flare

picture : Solar Flare taken from Bill Arnett's The Nine Planets

Solar System Index

The Sun

Mercury

Venus

Earth

The Moon

Mars

Jupiter

Saturn

Uranus

Neptune

Pluto

The Kuiper Belt

Introduction

Our Sun is one of many stars within the milky way, and all planets in the solar system orbit it. It provides heat and light so that life may exist on this world, and without it there could be no Solar System. The Earth itself would not exist. Fortunately however, it is expected to last for about another 5 billion years, so we've got little to worry about for the time being. When it finally does die, it will take the rest of the solar system with it, leaving behind a planetary nebula for anyone else in the galaxy to see.

The Scale of Things

The Sun is a huge fiery ball approximately 1.4 million km in diameter, over 100 times that of the Earth! The surface is typically about 5500 celsius, enough to vaporise most of even the hardest materials. The Sun's core is far hotter - an unimaginable 15 million celsius and 250 billion times the atmospheric pressure of the Earth. The incredible amounts of energy required to keep the sun going are produced by nuclear fusion in greater amounts than could ever be reproduced on the Earth.

Solar Eclipses

A solar eclipse occurs when the moon passes between the Earth and the sun. Total solar eclipses, like the one pictured, are rare, and it is often decades between each one at any place on the Earth. Partial eclipses are far more common to see : the dark disc of the moon covering only a small part of the bright disk of the sun. As well as being stunning to look at, solar physicists can use total eclipses as a rare opportunity to study the solar corona (atmosphere), since this part of the Sun can still be seen as a halo about the moon.

What is the Sun?

Diagram of the sun The Sun, or indeed any star, is a ball of gas held together by its own gravity. The light produced is due to nuclear burning in its core. The diagram shown here (courtesy of Los Alamos National Laboratory) shows much of the inner workings of the Sun, the bright centre being its hot core, through to the outer halo which is its corona.

The Solar Wind

As well as being the source for vast amounts of light, the Sun also produces what is known as the solar wind. This is a flow of gas and charged particles (hence a plasma) which stream outward from the Sun, interacting with magnetic fields and atmospheres of the planets to produce some stunning effects, such as the Aurora Borealis (northern lights). The Sun loses 2 million tonnes per second in this way!.

Solar Sailing

Light, like everything else in the universe, transfers energy to an object if it is reflected from it, just like a ball hitting a target will cause the target to move. Solar Sailing is based on this principle. The incredible amounts of light given out by the sun, if reflected off of a large sail, can cause a spacecraft to slowly accelerate. Solar Sails have not yet been used to power a spacecraft, since there are disadvantages. The spacecraft must be launched by normal means (a rocket), because the sails don't work in atmospheres. Also, the sails must be massive : at least one square kilometre (several sports fields) per kilogram of spacecraft. However, there are many plans to use solar sails in the future due to their relative cheapness, and maybe one day we will see them being used to carry small packages to martian colonies.