A galaxy is a large group of stars.

Our Sun is just one of millions of stars in our galaxy, the Milky Way.

The stars in a galaxy are often millions of times further away from each other than the planets in the solar system.

The Universe is made up of at least a billion galaxies.

Galaxies are often millions of times further apart than the stars in a galaxy.

The Universe is enormous, and is made up mainly of empty space.

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Stars do not stay the same forever. Over billions of years, they follow a sequence of changes until they eventually run out of fuel.

Stars, including the Sun, form when enough dust and gas from space is pulled together by gravitational attraction. Smaller masses may also form and be attracted by a larger mass to become planets.

Stars are very massive, so their force of gravity drawing together the matter from which they are made is very strong.

However, the high temperatures create forces which tend to make them expand.

During the main stable period of a star, these forces are balanced. This period may last for billions of years.

The Sun is at this stage of its life. The flow diagram below shows the stages it will go through later in its life.

                                                            stable period
                                                                     
                                                 fuel runs out and star expands

                                                              red giant
star contracts under its own gravity or a massive red giant may contract then explode

                          white dwarf                                                    supernova
                                                                                                           

                                                                                    throws out dust and gas into space

                                                                                                   neutron star

A white dwarf is a tiny dense star, which eventually fades away forever.

A neutron star is even more dense.

If enough matter is left behind after the explosion of the supernova, a large neutron star may be formed, which is so dense and its gravitational field so strong, that nothing can escape from it. Not even light or other forms of electromagnetic radiation.

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A Star's Energy

A star is powered by nuclear fusion reactions.

The nuclei of light elements, such as hydrogen and helium, fuse together to produce nuclei of heavier elements.

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Origin of Our Solar System

It is thought that our solar system was formed from material produced when earlier supernovae exploded.

Nuclei of the heaviest elements are found in the Sun.

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Red Shift 

In the 1920s, the astronomer Edwin Hubble noticed that light from distant galaxies had longer wavelengths than light from the Sun. This means it was shifted towards the red end of the visible spectrum.

He also noticed that galaxies further from the Earth showed a bigger red shift.

His explanations for these observations were that:

• Other galaxies are moving away from us very quickly
• The further away from us a galaxy is, the faster it is moving away

This suggests that the Universe is expanding and that it might have started billions of years ago with a huge explosion, which is often referred to as the ‘Big Bang’.

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Life Beyond Earth

If there is, or has been life on other planets or around other stars, we may be able to observe living organisms such as microbes, or their fossilised remains by actually visiting Mars or Europa (a moon of Jupiter), or by using robots to send back pictures or to collect samples which can be brought back to Earth.

We may be able to detect living organisms by the chemical changes they produce.

Living organisms on Earth have changed the atmosphere, for example, plants produce oxygen.

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SETI 

SETI stands for the Search for Extra-Terrestrial Intelligence.

This research has gone on for over forty years, using radio telescopes to try to find meaningful signals, in a narrow band of wavelengths (not just noise), that could suggest the existence of intelligent life.

So far it has not been successful.

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