E5 Stellar Processes and Evolution

E5.1 Stars are formed when huge clouds of dust and gas are compressed. This cannot occur on its own as the gravitational attraction between them is not strong enough. Star formation is initiated by either a supernova explosion or by two dust clouds' collision. As gas clouds are pulled closer together, the gravitational potential energy converts to kinetic energy causing an increase in the temperature. Also as the particles get closer together the force between them increases causing a rise in the pressure of the dust cloud. The cloud therefore collapses into an increasingly hotter body.

The centre of the dense core rapidly contracts leading to high temperatures and pressure. This is called a protostar but cannot be seen as it is surrounded by a cloud of dust.

After 100000 years the radiation from the star blows away the dust and its mass stabilizes. It now forms a pre-main sequence star. The abundance of hydrogen in the universe suggests that the star is made mainly of hydrogen. When fusion reaction starts taking place, the radiation pressure eventually equals the gravitational pressure to form a body of stable mass. This is now a main sequence star like the sun.


E5.2 5.3Eventually when the hydrogen runs out, the gravitational pressure starts increasing causing the star to collapse. As this occurs the temperature and pressure of the star increases and it becomes possible to fuse helium into higher elements. However, these fusion reactions release an even greater amount of energy which causes the start to expand. When the star expands, the outer layers become cooler. This therefore forms a red giant star. The process of fusing higher and higher elements ensues till be reach iron which has the greatest binding energy per nucleon. After this fusing iron to form higher elements would be endothermic and the star cannot continue to shine. At this point, when the star has run out of nuclear fuel, it starts collapsing due to gravitational force. From here it may either turn into a neutron star or, if it is big enough, into a black hole.

E5.4
The initial mass of the dust cloud determines the size of the star that forms from it which helps us determine where different stars join the main sequence.

Hinemann HL Physics, Chris Hamper

E5.5