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  • Created by: callum
  • Created on: 01-06-11 12:14

What is an Atom like?

Atoms are made up of Protons, Neutrons and Electrons.

particle      mass     charge

electron         -           1-

proton            1          1+

neutron          1         none 

The positive protons in an atom repel by electrostatic repulsion but are held together by a strong nuclear force.

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Evidence for the nuclear atom.

An experiment called the Rutherford-Geiger-Marsden experiment took place which led to evidence of an Atoms structure. 

Evidence for the nuclear atom.

- Most Alpha particles passed straight through the foil without any deflection, showing that atoms are mostly empty space.

- A few alpha particles were back scattered through angles greater than 90 degrees. They must have come close to a very tiny. very heavy positively charges nucleus.

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Nuclear Fusion.

In the centre of a star it is so hot that atoms loose all of their electrons. When hydrogen nuclei come very close to each other they fuse to make helium. Nuclear fusion makes vast amounts of energy, which stars emit as electromagnetic radiation. 

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Stars structure.


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Spectra and temperature.

Stars emit different amounts of radiation at different frequencies, depending on their temperature. 

Cool stars appear red, slightly hotter appear orange, then yellow, then white, and the hottest stars appear blue-white.

Astronomers can use a spectrometer to measure how much radiation is emitted at each frequency and get an accurate value for the temperature of the star from its peak frequency.( greater peak frequency means hotter temperature)

 Scientists can also work out the size of a star from its temperature and the total amount of radiation it emits per second called its luminosity.

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Different types of stars.

A Main sequence star is a star like our sun which fuses hydrogen to make Helium.

Red giants are stars that are past their hydrogen fusing stage and are fusing helium and heavier elements. The colour red means cool, but they are still very bright because of their giant sizes.

White dwarfs are very hot stars but they are not very bright due to their very small size.

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Absorption lines.

When light emitted from a star passes though the stars atmosphere some of the photons have exactly the right amount of energy to move electrons to higher energy levels. The atmosphere therefore Absorbs these frequencies ad the light does not reach the Earth. We observe black lines in the spectrum of light.

Every type of atom has its own unique electron energies, producing its own unique pattern of missing lines in an absorption spectrum.

Scientists can identify which types of elements are present in the star by comparing the lines in the absorption spectrum with emission spectra of elements i the laboratory.   

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What are gases like?

Stars are giant balls of hot Gases.

The particles of a gas move very quickly in random directions. When they hit the side of the container they exert a force as they change direction. This is what causes gas pressure.

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Pressure and volume.

If the Volume of the container decreases the particles will hit the sides of the container more often and the Pressure will go up.

Volume and pressure are said to be inversely proportional. Doubling the volume will halve the pressure and halving the volume will double the pressure.

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Temperature and Pressure.

Temperature corresponds to the energy of the gas particles. The hotter the gas, the faster the particles move, and the harder and more often they will hit the sides of the container. 

So increasing the temperature will increase the pressure.

If you continue to cool a gas the particles will move slower and slower. The lowest theoretical temperature is called absolute zero and is -273 degrees Celsius.  

The kelvin scale starts at absolute zero. The units are called kelvin (K)

Temperature in Kelvin = temperature in degrees Celsius - 270

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Temperature and volume.

If you decrease the temperature of a gas at a fixed pressure, The Volume of the gas would also decrease.

At a volume of absolute zero the volume would theoretically be zero.

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How are Stars formed?

Stars are formed when gravity compresses a cloud of gas. The volume goes down and both the temperature and pressure go up.

As the particles fall towards each other they go faster and faster and so the temperature of the gas rises. Eventually the temperature and pressure become so high that Fusion starts and a star is formed. 

The early stage of a star when it is still being compressed under the force of gravity is called a protostar.

Eventually the collision pressures resulting from fusion (called radiation pressure), balances the force of gravity and star becomes stable. It is then known as a main sequence star. 

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The life cycle of a star.

Throughout its life as a main sequence star hydrogen is being fused to form helium. Eventually hydrogen in the core will run out. The core of the star collapses inwards where new fusion reactions begin. Causing the outer layers to expand, and forming a Red giant.  Very large stars become Red supergiants. 

The core of a Red giant becomes hot enough for Helium nuclei to fuse to make heavier elements such as carbon, nitrogen or Oxygen. Eventually all fusion will stop . The core remaining for a Red giant will become a white dwarf, which will slowly cool and become less bright.

Red supergiants however undergo a huge explosion called a Supernova, where clouds of dust and gas are blown outwards. These may eventually form new protostars where the cycle begins again. The remaining very dense cores become either Neutron stars or black holes. 

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