Life
In order for nuclear fusion to take place, there must be tremendous amounts of pressure and heat. This pressure crushes together elements to create more massive elements and energy. Stars begin fusing hydrogen first because it is the least dense and the easiest to fuse. Four hydrogen nuclei fuse together to form one nucleus of helium. A by-product of this is further release of energy. Stars that are going through this hydrogen burning process are known to be on the main sequence. Stars spend most of their life (approx. 90%) on the main sequence. The main-sequence phase is around 7bil years.
Death
A star will eventually use up most of its hydrogen and be left with helium. At this time there is not enough pressure crushing down on the star to create a nuclear reaction with helium. Nuclear reactions cease inside the star, and because there is no longer any outward push from fusion, the star begins to collapse upon its self. Here is where the star leaves the main sequence. This collapse begins to create more and more pressure inside the star until it is sufficient to have the fusing process of helium begin in the core, while some of the remaining hydrogen burns just outside of it. The products of this helium burning are carbon and oxygen. The star swells, and depending on its size, either becomes a red giant or a red supergiant.
Small Stars:
After the hydrogen burning process is complete in stars with an initial mass of less than 8 solar masses, they become red giants. These red giants have a diameter of roughly 60 million miles. Helium is burning in the core producing oxygen and carbon, while a thin layer of hydrogen is burning around it where there is not sufficient pressure for helium burning. The red giant begins to brighten between 1,000 and 10,000 times.
The hydrogen-rich covering on the surface of the star begins to swell and becomes as large as the orbit of Earth or Mars. Because there is so little pressure now in the outside areas of the star, the surface temperature drops to about 5,000 - 6,500 degrees Fahrenheit. This temperature is actually very cool for a star. A strong solar wind begins to blow, and jettisons away most of this hydrogen covering. All that is left is a long-period variable star.
The centre of the star has now met its demise. During the formation of the solar wind the star ceases all nuclear reactions. The star is still very hot, up to several hundred thousand degrees Fahrenheit. Over a few hundred million years, the star cools and becomes a white dwarf. As the star cools more, it becomes dark and barely detectable. It is now known as a black dwarf.
The white/black dwarf is composed of carbon and oxygen. Surrounding this is a thin layer of helium, sometimes surrounded by hydrogen. The star is very compact. Although only about the size of earth, its mass can be from a little less than one half a solar mass to a little more than one solar mass.
Large Stars:
If a star starts off with a mass of less than 8 solar masses, then it will stop at the red giant stage. More massive stars continue to burn. The carbon and oxygen produced in the previous stage begin to fuse. Carbon begins to be crushed into neon and magnesium, while oxygen is being crushed into silicon and sulphur. Silicon and sulphur get crushed into an iron core.
This iron core now just sits in the centre of the star. The reason for this is that iron doesn't burn. Nuclear burning is only possible if an object is releasing energy. In order for iron to go through the fusing process, energy must be added. This leads to the collapse of the star. The addition of energy that the iron needs will only occur during the supernova explosion caused by the collapse of the star.
Because the iron is not fusing, it does not create any outward pressure do balance the effects of gravity. As the iron gets a mass of about 1.4 solar masses, gravity gets the upper hand and the core collapses from a size of about 5,000 miles to about 12 miles in less than a second. This sudden crush makes protons and electrons combine to form neutrons. The huge energy release is equivalent to 100 of our stars burning for more than 10 billion years. A small amount of energy is deposited in the lower layers of the shell surrounding the core, triggering the supernova explosion.
The energy deposited around the core creates a shock wave that runs outward toward the stars surface. As it is passing through, it heats up the shell sounding the core, starts nuclear burning, and throws off the shell faster than 10 million mph. This is when the iron fuses to create heavier particles. When the shock wave reaches the surface, it heats them very quickly and causes them to glow. In a day or 2, the star is brighter than a billion suns. In a couple of weeks, the explosion diminishes, although it may remain visible for months or years.
Finally the remains of the explosion contract so much that it becomes so dense and there is nothing that can escape. There is so much mass in it, and the gravity is incredibly high. It begins to suck in anything around it, and soon not even light can escape its clutches. Soon, it can never be seen at all…
- Explain the term protostar.
A protostar is when all the dust and gas has been gathered together. It is in a vague star shape but has not yet gained enough heat or energy to perform nuclear fusion, although it does have a lot of gravity and rotational force. The gravitational energy is converted into heat energy and it starts to become a main-sequence star.
- What decides if a star becomes a supernova or a black hole
Whether a star becomes a supernova or a black dwarf is decided by the size of the star. If it less than 8 solar masses then it expands into a red giant and eventually cools into a white dwarf and black dwarf. If it more than 8 solar masses then it becomes a supernova and black hole.
