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Monday, 26 March 2012

What is the Lifetime of a Star?

Stars come in varying shapes and sizes our Sun to VY Canis Majoris (the largest star we know of, 2100 times the radius of the Sun). However they can't stay stars forever so what are the processes that lead to their creation and their eventual demise?

During its lifetime a star goes through a multitude of large changes and there are many different ways for it to develop. This is called stellar evolution. There are three main stages that we have to look at. The creation of the star, what happens once it exhausts its supply of fuel, and what remains after this. So, how is a star formed?

A star starts as a large cloud of molecular gas and dust, called a nebula. Nebulae are also known as stellar nurseries as large amounts of starts are formed there. A lot of the most stunning and popularised pictures of astronomical objects that we would all recognise are of nebulae. Inside the nebulae some of the debris starts to be pulled together. The molecules break down and the gravitational potential energy they loose is lost as heat. As the process continues a larger and larger amount of heat is produced and as the molecules come closer to each other the pressure increases, eventually creating a protostar. A protostar is dissimilar to normal stars as they are just rotating extremely hot gas spheres. A lot of the development of stars relies on the mass of the original cloud that it starts with. There has to be more than a specific mass of gas before a star can start to form. James Jeans, a British physicist, created a formula that predicts the mass that a cloud would have to reach before it could start to contract:
Where n is the particle number density, m is the average mass of a gas particle in the cloud,
T is the temperature, and k is the Boltzmann's constant.
Though we may not use this formula much, it shows how basic the conditions have to be for a protostar to form, and explains, at a fundamental level, how abundant stellar activity is. Check out this video talking more about the formation and development of protostars.

The next stage of the process relies heavily on the mass of the protostar. When we measure the radius, or mass of stars, we use the radius and mass of the Sun:
 =  6.955 x 108m

If a star is massive enough, it will start hydrogen fusion but if the protostar is less then around 0.08 Solar masses (M) then the temperature never gets high enough. These are called brown dwarfs. If a brown dwarf is heavier than 0.0125 Mthen it fuses deuterium, an isotope of hydrogen with one neutron, rather than conventional hydrogen which has none. This process requires a lot less energy than hydrogen fusion, but does not produce nearly as much. These brown dwarfs as well as other sub-stellar objects (objects larger than a planet but smaller than a brown dwarf) do not shine very brightly and slowly cool and die.
On the other hand, in other more massive protostars, they reach 10 million ºK (degrees kelvin). At this temperature there is enough energy for hydrogen fusion to take place. When two hydrogen atoms clash, a sort of beta decay takes place. We have already talked about beta minus decay due to the weak interaction. This is beta plus decay, which is very similar, a sort of mirror image of beta minus decay. Due to the large amount of energy, One of the hydrogen atoms, a proton, decays through the weak force to become a neutron whilst releasing the anti-matter version of an electron (called a positron), an electron-neutrino and excess energy. This sort of decay is not common outside of stars as it requires a lot of energy.
Source: https://kjende.web.cern.ch/kjende/en/index.htm
Beta + decay
The passage of time is along the x-axis
When this happens, a atom of deuterium is produced. The positron produced in this decay annihilates with an electron. This produces two gamma ray photons and even more energy. After that, the deuterium fuses with another atom of hydrogen, to form an atom helium-3, another gamma ray photon, and a large amount of energy. This fusion can take place faster than the first stage, as it does not rely on the weak interaction. From that atom of helium there are three possible branches of fusion that could take place, these are called the pp I, pp II and pp III branches, pp staring for proton-proton. pp I is the simplest branch. Two of these helium-3 atoms fuse to create an atom of helium-4, an atom of hydrogen and excess energy. pp II and pp III are chain reactions, all resulting in the creation of heavier elements, though they are much less common:
pp II chain reaction (source:Wikipedia)

pp III chain reaction (source:Wikipedia)
At the end of all of these processes, approximately 0.7% of the original mass of the protons has been lost as energy. The energy released as gamma rays interact with electrons and protons and causes the heating of the interior of the star. This heat creates enough outward pressure to stop the star collapsing in on itself.

After a star runs out of hydrogen it starts to contract. It contracts until electron degeneracy, the inability for any electrons to be in the same quantum state, means that it cannot contract any further or it becomes hot enough for helium to start to fuse. However, what actual occurs relies on the mass of the star. A star which is less than 0.5 M☉ will not be able to fuse helium due to there not being enough pressure. These stars are called red dwarfs and the closest star to our solar system, Proxima Centauri, is one. After a very long time, they become white dwarfs. Stars with a mass of roughly 0.5 - 1 M become red giants. Red giants have very large radius, whilst having a relatively low surface temperature.  There are two main types of red giant, RGB stars, and AGB stars. RGB stars have a core of inactive helium, and hydrogen fusing into helium outside of the core. The other type, AGB, creates carbon through helium fusion in the triple-alpha process:
Triple-alpha process
In both types, the star is forced to expand hugely. When the Sun becomes a red giant in a few billion years, it will expand to at least 200 times its current radius. As the star is cooler than before, the light it produces is redder, resulting in its name.
If helium fusion starts to take place there will be a helium flash, where a huge amount of energy is realised. After all the helium has been fused, the star is left with a core of carbon and oxygen. The star changes size and temperature and there is a large amount of mass loss through stellar winds. The mass lost is usually in heavy element rich gas, particularly oxygen and carbon, which can sometimes clump together as planets.
The process is different for more massive stars. If a star is massive enough that helium fusion begins before electron degeneracy becomes prevalent, then it is likely that it will collapse into a type II supernova. A supernova is a pretty much an explosion caused by the quick collapse of one of these massive stars. Other massive stars may instead become red supergiants, extremely large red giants that produce a larger amount of heavier elements then normal red giants. Hot gasses are also expelled from these stars, and the atoms and molecules can form atoms around a second generation star.

After all of a star's fuel supply has been burned out, there are three different forms it can take, depending on its mass. If the star originally had a mass of 1 M☉ then it will form a white dwarf of mass 0.6 M. White dwarfs are very dense, stable stars, kept stable only due to the fact the electron degeneracy stops them from collapsing. The star does not create any more energy, but radiates its heat for billions of years. A white dwarf is so hot at its formation that it looses a lot of its energy in the form of neutrinos. Reasonably large white dwarfs, with a mass of a few solar masses, would be composed of oxygen, neon and magnesium, and medium sized white dwarfs, around the the sun's mass, will be composed of carbon and oxygen. Any smaller than this and the white dwarf will be mainly composed of helium. However, if a white dwarf's mass increases above something called the Chandrasekhar limit, then the star will collapse. This can lead to the ignition of carbon and oxygen or the creation of a neutron star, which we will talk about soon.
Another possibility for when a star runs out of fuel is that it will immediately collapse into a neutron star. This is through a process called electron capture. If the pressure is high enough a proton can absorb an electron and become a neutron. The star becomes a very dense ball of neutrons with a thin layer of other matter on top. These neutron stars are incredibly small whilst having a mass regular for a red giant and spin incredibly quickly, the fastest at over 600 revolutions per second. Some neutron stars emit detectable radiation when their axis is lined up with the earth, and these are called pulsars.
The last possibility is only only applies to very massive stars. If the star is massive enough, the star will be compressed to have a radius below the Schwarzchild radius. The Schwarzchild radius is the radius at which, if all the matter of an object of choice were to fit into a sphere with that radius, no light would be able to escape from its surface and is expressed by the formula:
c is the speed of light in a vacuum, G is the gravitational constant and m is the mass of the object
And what is an object from which no light can escape? A black hole! We have already talked about black holes briefly. They are objects of such high density (sometimes infinite, called a singularity) that not even light can escape from them and because of this all we know about them is that they exist. 


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Ned Summers.

Check out our last two posts:
What is the Theory of Everything? (Part 2): String Theory - The first look into a theory that could possibly become a theory of everything. 
What are Superconductors? - What are superconductors and how will they change the way we live our lives?

1 comment:

  1. i have learnt a lot thank you Janice

    ReplyDelete