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In addition to the revision notes for Evolution of Stars on this page, you can also access the following Cosmology learning resources for Evolution of Stars
| Tutorial ID | Title | Tutorial | Video Tutorial | Revision Notes | Revision Questions | |
|---|---|---|---|---|---|---|
| 22.5 | Evolution of Stars |
In these revision notes for Evolution of Stars, we cover the following key points:
The evolution of stars similar to our Sun undergoes four stages: formation, maturation (in main sequence), red giant (and red supergiant), and white dwarf. Each of these stages has different features.
The formation (birth) of a star may occur inside any gas cloud. This process is very long; it includes a number of factors where the most relevant are gravitation (which tends to attract matter and compress it inside the just created sphere, and gas pressure (which increases with the increase in temperature of gas and tends to oppose the further compression of matter).
All stars in the main sequence involve the thermonuclear reaction of hydrogen fusion at their centre. This process is very slow and balanced; hence, this is the longest stage of a star life. The Sun is estimated to be in main sequence for 10 million years, where 4.5 billion years have already elapsed. The position of star in the main sequence graph after its formation is determined by its mass; the more massive stars lie in the upper part of main sequence graph in the H-R diagram.
In the red giant stage, the hydrogen (that acts as a nuclear fuel) in central part of a star begins to drain out; as a result, the gas pressure stars to decrease gradually. Therefore, the star starts shrinking as pressure - as an opposing effect to shrink - is smaller than before. Hence, gravitational force prevails over the resistive forces and the equilibrium breaks down. Matter stars a very fast process of compression known as gravitational collapse. On the other hand, the new values of pressure created after the collapse bring an increase in temperature and as a result, hydrogen starts to burn out again in a spherical layer that surrounds the star core. The energy produced in this process causes an expansion of external layers of the star.
Due to this process, the star magnifies in dimension in a very short time. When this occurs to our Sun, its outer layer will have reached the Earths orbit, melting therefore everything on its surface. This is because the surface temperature will reach 2000-3000 K. The Sun eventually comes out of the stable phase and turns into a red giant. As explained earlier, helium will convert to carbon, oxygen, etc. In the case of our Sun, this stage will last for about 2 billion years, until it will have burnt out the entire amount of helium.
When thermonuclear reaction cannot occur anymore the Sun will eventually turn into a white dwarf. It will continue illuminating the sky, losing energy without any source to replace it.
In stars that are more than 10 times heavier (and bigger) than the Sun, the first two stages are similar to those of the Sun. The evolution process starts to differ at red supergiant stage. At this stage, all helium resources are exhausted and as a result, the gravitational pressure has no more opponents and therefore, gravitational collapse starts again. When the core of the shrinking star turns into a neutron star, its outer layers moving towards the centre collide with the strong core and turn back at very high energy. This collision represents a supernova.
When radiation emitted by a neutron star reaches Earth, it comes in regular intervals determined by its period of rotation. In this case, we are observing a pulsar star. During this rotation, they radiate EM waves in a specific direction, which in many cases does not fit the axis of rotation.
A spatial black hole is created when the core of a collapsing star is heavier than three solar masses, the pressure produced by neutrons cannot withstand to gravitational force that "guzzles" everything around. Nothing is able to resist such a high-energy elan. A black hole does not radiate EM waves; it only absorbs, including everything from matter to radiation. Scientists can black holes due to the effects (mainly of gravitational nature) they cause in the surrounding space.
When moving in the interstellar space, planetary nebulae provide it with elements such as hydrogen, helium and other chemical element once contained in the parent star. It acts like a factory that produces heavy chemical elements by processing hydrogen and helium as raw material in more complicated processes than thermonuclear fusion.
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