Parts of the cloud gradually contract and the gases become compressed, ‘clumps’ of denser gas are formed. The densest part being at its centre, here the gases become very hot, hot enough to trigger nuclear reactions.
At these temperatures atoms cannot retain their electrons and matter becomes a plasma of ionised atoms and unbound electrons. A nuclear reaction involves a change in the nucleus making it possible for one element to change into another.
Nuclear fusion happens in the centre of ‘clumps’ where lighter nuclei are fused together to form heavier nuclei. The nuclei need to approach each other at a high speed to overcome the repulsive forces between the positive nuclei.
So in the dense gas clouds, hydrogen gets turned into helium. The process releases a lot of energy causing the cloud to glow and the cloud has become a star.
Here are two examples of reactions which take place in the sun:
11 H + 21 H → 32 He + γ
21 H + 31 H → 42 He + 10 n
The nuclear reaction also generate hot wind which drives away some of the dust and gas, leaving behind a new star. This is often surrounded by planets which have condensed out of the remaining dust.
Heavyweight stars
What happens to a star next depends on its mass. The process of nuclear fusion occurs quicker in heavier stars because their centres are denser and therefore hotter. The temperatures get so high that further fusion can occur and make even heavier elements.
Layers of elements form within the star, with the heaviest elements near the centre where it is hottest.
When the core of a heavyweight star contains mainly iron it becomes unstable and explodes. Because when iron nuclei fuse together they absorb energy.
These explosions are called supernovae. As a result of this the elements in the star are dispersed as clouds of dust and gas, and the cycle begins again.
The Sun
The sun is a lightweight star: it is not as hot as most other stars and will last longer then heavyweight stars (until all the hydrogen is used up)
Once this happens the sun will expand into a red giant. As red giants become bigger they also become unstable and the outer gases drift off into space leaving behind a small dense core called a white dwarf.
How do we know this?
One of the most powerful analytical tools available is the spectroscopy. Based on the principle that under the right conditions a substance can be made to absorb or emit electromagnetic radiation.
If we analyse this electromagnetic radiation we can learn a lot about the substance such as what it is or its structure.
Absorption spectra
The glowing regions of stars emit light of all frequencies between the ultraviolet and infrared.
The surface (photosphere) glows like an object at about 6000K.
Outside the photosphere is the chromosphere which contains ions atoms and small molecules. These particles absorb some of the light. So when we analyse the light which reaches us we can see certain frequencies are missing (the ones which have been absorbed).
Even further out is the corona here the temp is so high that the atoms have lost many of their electrons.
Emission spectra
When the atoms, molecules and ions around the stars absorb electromagnetic radiation they are raised to higher energy states. The particles can lose their extra energy by emitting radiation. This can be detected on earth.
During a solar eclipse the glow of the suns photosphere id blocked out and we can only detect the light from the chromosphere.
Our Solar system
Our solar system condensed from huge gas clouds with gradually contracted under the force of gravity. As rings of dust and gas condensed around the run, the planets were formed. This material originated from a supernova so contained a range of elements. The non-volatile elements condensed near to the sun (the rocky planets). more volatile elements condensed away from the sun (the giant fluid planets).
Activity EL4.1
Activity EL4.2
Activity EL4.3
Activity EL4.4
Ideas 2.1
Ideas 2.2
Ideas 6.1
Ideas 2.3
Assignment 4
Assignment 5
