The life cycle of a star.

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A LEVEL PHYSICS COURSEWORK ON THE LIFE CYCLE OF A STAR

In this physics coursework, I have been asked to carry out research of my selection and to develop it. I have selected to research the life cycle of a star, and I would conduct this by gathering the necessary information in a form of a report which explains this in detail. I have chosen to explore this particular topic firstly because I am extremely fascinated in space and the universe and secondly because I do not know much about the life cycle of a star and I deem this will help extend my knowledge.

Firstly when carrying out this research before describing the life cycle of a star I need to be familiar of what a star is, and how it is formed

What is a star, and how does it form?

Stars are basically huge balls of hydrogen gas. Hydrogen is by far the most common element in the Universe, and stars form in clusters when large clouds of hydrogen, which naturally forms a hydrogen 'molecule' (H+H=H2) with another atom, collapse.

The hydrogen clouds collapses very slowly, although they can be speeded up by the effects of a passing star, or the shockwave from a distant supernova explosion. As the cloud collapses, it speeds up its rotation, and pulls more material into the centre, where a denser ball of gas, the 'proto-star' forms. The proto-star collapses under its own weight, and the collisions between hydrogen molecules inside it generate heat. Eventually the star becomes hot enough for the hydrogen molecules to split apart, and form atoms of hydrogen. The star keeps on collapsing under its own weight, and getting even hotter in the core, until finally it is hot enough there (roughly 10 million degrees) for it to start generating energy, by nuclear fusion - combining hydrogen atoms to form a heavier element, helium. Energy is released from the core, and pushes its way out through the rest of the star, creating an outward pressure which stops the star's collapse. When the energy emerges from the star, it is in the form of light, and the star has begun to shine.

A Star is formed from a cloud of gas, mostly hydrogen, and the dust that is initially spread over a huge volume, but which is pulled together by its own collective gravity. This gravitational collapse of the cloud creates a body of large density, and the loss of gravitational potential energy in the process is very large indeed. The result is that the original particles acquire high kinetic energy, so that the collisions between them are very violent. Atoms lose their electrons. Not only has that, collisions taken place in which electrical repulsion of nuclei is no longer strong enough to keep them apart. They can become close enough together for the strong nuclear force to take effect, so that they merge. Fusion takes place, with hydrogen as the principal key material. This begins the process of conversion of mass to energy, and much of the released energy takes the form of photons which begins to stream from the new star.

Every star then exists in a state of slowly evolving stability. On the one hand there is the trend for the material to continue to collapse under gravity. On the other hand there is a tendency for the violent thermal activity and the emission of radiation resulting from fusion to blow the material apart. The more bigger star in general, the greater is the gravitational pressure and so the higher rate of energy is released by fusion, therefore bigger stars use up their supply of fusing nuclei more quickly than do smaller stars, such that bigger stars have shorter lives.

The enormous luminous energy of the stars comes from nuclear fusion processes in their centres. Depending upon the age and mass of a star, the energy may come from proton fusion, helium fusion, or the carbon cycle. For brief periods near the end of the luminous lifetime of stars, heavier elements up to iron may fuse, but since iron is at the peak of the binding energy curve, the fusion of elements more massive than iron would soak up energy rather than deliver it. This links to the below graph:
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Fusion in stars makes energy available to create radiation, consuming mass at an amazing rate. The sun, for example loses a mass of 4.5 million tonnes every second. Also, heavier nuclei are formed from smaller ones, so that the compression of a star changes. Concluding this, as the star dies the material dependant on its size is scattered in space.

The Hertzsprung - Russell Diagram

This simple graph shows ways in which to classify stars. Temperature is plotted on the x-axis. This is related to the colour as cooler stars are redder, hotter stars are ...

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