AQA - GCSE - Unit 3 - Space

Early man tried to explain his known universe in religion and folk tales. Now we rely on science to explain it.

Our understanding of the Universe is an on-going process that involves observations, then calculations on them leading to development of a theory and a model that produces predictions.

These can then be checked by looking for evidence that the predictions are correct, supporting the theory that generated them and indicating our model is good. This has been the case since early times (eg. flat earth/round earth controversy). The theories we hold now could easily be displaced by better ones in the future as our ability to make observations and analyse them (because of the computer) improves.

From Key Stage 3 you should know that:

- the Universe is made up of matter and antimater (anything which has mass)

- the matter is not evenly distributed or static - it is 'clumpy' and moving.

- clusters of galaxies are separated by areas of space that seem to have nothing in them at all.

- galaxies are made up of billions of stars and there are hundreds of billions of galaxies. Galaxies of stars group together in clusters, and these clusters form even bigger clusters within the whole Universe (i.e. mass is not distributed evenly but in lumps!). The force that is pulling these masses together is called gravity.

- a star is a ball of hydrogen and helium pulled together by gravity - the density and pressure and temperature of the matter is so great that the process of nuclear fusion goes on within the star - emitting light and heat energy out into the universe. Also within the star heavier elements are formed as nuclei fuse together - stars 'manufacture' heavier elements.Some stars share systems and orbit around a point between them (binary star systems) The stars tend to group together in clusters within the galaxy

- planets orbit stars - some stars have no planets, but many have a lot of various size.

- natural satellites (moons) orbit planets.

- comets follow very elliptical orbits around stars.

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Our Earth is one of nine planets that orbit a rather unremarkable star called Sol. The Moon is a natural satellite that orbits our Earth. Other planets may have their own moons. Sol is one of millions of stars that make up the Milky Way (our galaxy) and may have planetary systems. .

Gravity is responsible for the pull of the moon on the Earth's oceans resulting in tides (Spring and neap tides etc. - see text book). Gravity is a force of attraction between all masses. The gravitational force of attraction between two masses is proportional to the product of the two masses involved and inversely proportional to the square of the distance between them (the bigger the masses, the bigger the pull - the further away they are, the less the pull).

On Earth the principal force of gravity it due to the attraction of objects to the Earth itself. This is so much bigger than all other pulls of gravity that they can be ignored and masses are pulled towards the centre of the Earth. In the Solar system it is the Sun that is the major massive body.

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In the beginning there was no Universe - no matter or antimatter at all!

At the 'Big Bang' matter and antimatter came into being and rapidly expanded outwards - as its very presence indicated the boundaries of the Universe the fact that this matter and antimatter spread outwards the size of the Universe rapidly increased.

At first the matter and antimatter was in a form we would not recognize - it was so full of energy, and therefore at such high teperatures, that the particles present were not of the form that matter is in in the Universe today. This makes very fascinating reading - but is beyond the scope of this syllabus and level! See this link.

Eventually (after a time lapse of about one second!) the protons, neutrons and electrons began to 'condense' out and matter as we know it began to form... but there were a lot of stages for the Universe to go through before the Universe we know came into being (and we don't have to worry about those!).

In this early stage the only element that existed was hydrogen (only one proton in the nucleus- so a single proton can be thought of as the nucleus of a hydrogen atom).The matter was much too highly energised to be pressed close together and also are all positive and so repel one another (like charges repel) - so bigger nuclei could not form at this stage. Fusion processes in stars later produce all naturally occurring elements. These elements are then distributed throughout the Universe by the explosion of a star (supernova) at the end of its life.

Astronomers believe that gravitational forces are responsible for the formation of
galaxies of stars, and for stars like the Sun having a long stable period. The gravitational force acts between masses. It is attractive and pulls mass together.

Stars 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. Gravitational forces pull inwards to balance the outwards radiation pressure (from the fusion process) to make a star stable.
A star goes through a life cycle. You only have to know the life cycle of stars of similar size to the Sun and stars much larger than the Sun.