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The Life Cycle of a Star
Nebula:
A nebula is a cloud in space made up of hydrogen and dust. Nebulae are the birth place of stars. There are different types of nebulae such as emission, reflection and dark nebulae.
Protostar:
These clouds of gas begin to draw in more and more gas and they enlarge, which is known as a protostar. Protostars are unstable because of the large amounts of reactions taking place in them. To become a star, a protostar must reach equilibrium, which is a balance between the matter being drawn in, and the heat and light being pushed away by the pressure of the gas. Once, fusion takes places with hydrogen being fused into helium, then the protostar can be classified as a star.
Main Sequence:
Once nuclear fusion has taken place, the star begins to radiate heat and light out into space. Because of this energy being lost, the star slowly contracts over billions of years. This is the longest life stage of a star and lasts for billions of years. This sequence is also where our sun is at the moment.
Gradually, the hydrogen in the star is fused into helium. As the star contracts, the temperature, pressure and density of the core increases.
Red Giant/ Red SuperGiant
As the star begins to near the end of its life, the hydrogen in the core runs out, and in order to maintain the equilibrium the star begins to burn helium. However, because the required temperature for helium to initate fusion is much higher then it is for hydrogen, the core begins to heat up. The shell, in an attempt to dissipate this heat, expands, and the star grows in size to a red giant. When our sun does this, the first three planets closest to the sun will be swallowed up by the sun. Fusion during this stage releases more energy than during the main sequence, making the star larger but more unstable with the core contracting much faster. A large star will form into a red super giant, a very large star into a red hypergiant, and a normal sized star into a red giant. Once all the helium has been used, the star resorts to the last kind of fusion, carbon fusion.
White Dwarf:
Small stars, the size or less than the size of our sun, shrink and form into a white dwarf. Because of the burn rate of these stars during the helium burn sequence, the small star creates a superwind, which is a lot stronger then a normal solar wind. This super wind, made up of electrons and photons, is strong enough to break off the shell of the star, forming a planetary nebula. This nebula continues to expand until it becomes part of the interstellar medium. The core of the star is left behind and is supported from further collapse be electrons. This white dwarf is about the size of earth.
Neutron Star:
A star from about the size of ours to about three times the size of ours will form a neutron star. These masses may not always be correct, since during different types of supernova events, a star with a larger mass than this may form a neutron star, but generally speaking these are the weights where a neutron star has a high chance of being created. When the outer layers of a star collapse inwards because of lack of fuel to support fusion, they rebound off the core and are expelled outwards. This is a supernova. During this massive explosion electrons and protons are forced out of the core, leaving a ball of neutrons. This ball continues to shrink to the size of about 20 kilometres in diameter, and is composed of neutrons. This star, despite being the size of a large city, is approximately 60000 times the Earth’s density. This would be like compressing a Boeing 747 to the size of a grain of sand. Sometimes a star initially collapses to a neutron star, then continues to collapse into a black hole such as in a type II-P supernova event.
Supernova/Hypernova:
A supernova is a massive explosion created at the end of a massive star's life. When a star can no longer maintain equilibrium, it collapses on itself and the outer layers rebound off the core, exploding outwards and creating a supernova. A supernova is created when a supermassive star is forced to use iron to fuel itself, in order to maintain equilibrium. However, the energy required to have iron fusion is more than is created by iron fusion, therefore the star begins to collapse. There are different types of supernovae, created by different stars of different masses and compositions. Type II/b and Type Ib/C supernovae create black holes, whilst type II/l and type Ib/c supernovae create neutron stars. These supernovae are all caused by iron core collapses. A hypernova is created by an iron core with more than 40 solar masses with near solar metallicity, which also creates a GRB (gamma ray burst), or a pair instability star. Pair instability is when pair production ( a particle and its anti-particle being produced), caused by reaction between gamma rays and atomic nuclei, reduces thermal pressure in the core of a star with 130 to 250 solar masses, with low to moderate metallicity, which causes a partial collapse. Runaway thermonuclear reactions (a reaction which is accelerated by heat, which in turn creates more heat) accelerate fusion to a point where the star completely explodes. During this explosion, no matter is left behind, leaving no remnant. Pair instability may also cause a GRB. A GRB is a flash of extreme energy gamma rays during a high energy explosion, in which a narrow beam of intense radiation is released. They are believed to be the brightest electromagnetic events in the universe.
Black Hole:
A black hole is an incredibly dense region of space, where the gravity is so great that not even light can escape. Created by large stars exploding in supernovae or even hypernovae, the theory of general relatively predicts, with decent evidence, that an object of significant mass could bend space itself. As light is pulled away by the intense gravity of a black hole, the only way we believe that we can detect a black hole is by detecting radiation, as predicted by quantum mechanics. As aforementioned, a black hole is caused by a star collapsing down to an incredibly dense point, called a singularity, during a supernova. However, certain types of star may create a black hole without a supernova, such as a star with solar mass 40-90 with low metallicity, or a star with more than 90 solar mass with low metallicity and a star that collapses due to photodisintegration, which creates a supermassive black hole. Photodisintegration is when high energy gamma rays knock a subatomic particle out of an atomic nucleus. This process is endothermic, meaning it is energy absorbing. Normally, within a massive star, more than 250 solar masses in size, photodisintegration’s energy absorbing effects do not affect the star. However, when it reaches carbon fusion, the heat and pressure reaches levels where photodisintegration can energy consuming process breaks the equilibrium, and the star begins to collapse. During this process, many heavy elements are produced. These are sprayed out into the universe by relativistic jets, which are extremely powerful jets of quark-gluon plasma, sprayed from the collapsing star and the created black hole. The creation of these jets are unknown, however because of the amount of energy required to launch one of these jets, most jets are believed to be launched from spinning black holes. These jets have been seen to be launched from the centres of several galaxies, which is evidence that black holes may be at the centre of most galaxies. The large emissions of radio waves from the centres of these galaxies, which are normally created by black holes, also lends itself to the this theory.
An important part of a black hole is the event horizon, mathematically predicted and tested with radiation telescopes. The event horizon is the theoretical boundary surrounding a black hole, from which no matter, not even light, can escape. An effect known as gravitational time dilation causes any object to observers seemingly take an infinite time to cross the event horizon. This has been predicted by the theory of general relativity, which states that the lower the gravitational potential, the slower the time. During this time, all actions happening on the object slow down causing the light being emitted to be fainter and redder, which is caused by gravitational redshift. If a person were to be crossing the event horizon, according to his watch he would take a finite time to cross the event horizon, but he would not be able to tell when he did because of lack of local evidence to support this. At the centre of a black hole is believed to be a gravitational singularity. In a static black hole, the singularity is a single point, believed to be in the first dimension. In a rotating black hole (Kerr black hole), the singularity is believed to be smeared out within the black hole. The singularity is all the mass of the black hole, and thus can be considered to infinite mass and infinite density. It is possible to avoid a Kerr singularity; however you will accelerate to a certain point, then maintain free fall. It appears to be possible to follow closed timelike curves (CTCs) which are an answer to Einstein’s Field Equations. CTCs are worldlines (the line of an object travelling through spacetime) in a Lorentzian manifold, a form of pseudo- Riemannian manifold, that is closed, therefore always returning to its starting point. Theoretically, this world line could be connected to earlier times, therefore travelling back in time to an earlier point. However, this theory opens up the possibility of a worldline that does not connect to an earlier point, therefore it defies causality, which states that a second event must always be traced back to an earlier event.
Nebula:
A nebula is a cloud in space made up of hydrogen and dust. Nebulae are the birth place of stars. There are different types of nebulae such as emission, reflection and dark nebulae.
Protostar:
These clouds of gas begin to draw in more and more gas and they enlarge, which is known as a protostar. Protostars are unstable because of the large amounts of reactions taking place in them. To become a star, a protostar must reach equilibrium, which is a balance between the matter being drawn in, and the heat and light being pushed away by the pressure of the gas. Once, fusion takes places with hydrogen being fused into helium, then the protostar can be classified as a star.
Main Sequence:
Once nuclear fusion has taken place, the star begins to radiate heat and light out into space. Because of this energy being lost, the star slowly contracts over billions of years. This is the longest life stage of a star and lasts for billions of years. This sequence is also where our sun is at the moment.
Gradually, the hydrogen in the star is fused into helium. As the star contracts, the temperature, pressure and density of the core increases.
Red Giant/ Red SuperGiant
As the star begins to near the end of its life, the hydrogen in the core runs out, and in order to maintain the equilibrium the star begins to burn helium. However, because the required temperature for helium to initate fusion is much higher then it is for hydrogen, the core begins to heat up. The shell, in an attempt to dissipate this heat, expands, and the star grows in size to a red giant. When our sun does this, the first three planets closest to the sun will be swallowed up by the sun. Fusion during this stage releases more energy than during the main sequence, making the star larger but more unstable with the core contracting much faster. A large star will form into a red super giant, a very large star into a red hypergiant, and a normal sized star into a red giant. Once all the helium has been used, the star resorts to the last kind of fusion, carbon fusion.
White Dwarf:
Small stars, the size or less than the size of our sun, shrink and form into a white dwarf. Because of the burn rate of these stars during the helium burn sequence, the small star creates a superwind, which is a lot stronger then a normal solar wind. This super wind, made up of electrons and photons, is strong enough to break off the shell of the star, forming a planetary nebula. This nebula continues to expand until it becomes part of the interstellar medium. The core of the star is left behind and is supported from further collapse be electrons. This white dwarf is about the size of earth.
Neutron Star:
A star from about the size of ours to about three times the size of ours will form a neutron star. These masses may not always be correct, since during different types of supernova events, a star with a larger mass than this may form a neutron star, but generally speaking these are the weights where a neutron star has a high chance of being created. When the outer layers of a star collapse inwards because of lack of fuel to support fusion, they rebound off the core and are expelled outwards. This is a supernova. During this massive explosion electrons and protons are forced out of the core, leaving a ball of neutrons. This ball continues to shrink to the size of about 20 kilometres in diameter, and is composed of neutrons. This star, despite being the size of a large city, is approximately 60000 times the Earth’s density. This would be like compressing a Boeing 747 to the size of a grain of sand. Sometimes a star initially collapses to a neutron star, then continues to collapse into a black hole such as in a type II-P supernova event.
Supernova/Hypernova:
A supernova is a massive explosion created at the end of a massive star's life. When a star can no longer maintain equilibrium, it collapses on itself and the outer layers rebound off the core, exploding outwards and creating a supernova. A supernova is created when a supermassive star is forced to use iron to fuel itself, in order to maintain equilibrium. However, the energy required to have iron fusion is more than is created by iron fusion, therefore the star begins to collapse. There are different types of supernovae, created by different stars of different masses and compositions. Type II/b and Type Ib/C supernovae create black holes, whilst type II/l and type Ib/c supernovae create neutron stars. These supernovae are all caused by iron core collapses. A hypernova is created by an iron core with more than 40 solar masses with near solar metallicity, which also creates a GRB (gamma ray burst), or a pair instability star. Pair instability is when pair production ( a particle and its anti-particle being produced), caused by reaction between gamma rays and atomic nuclei, reduces thermal pressure in the core of a star with 130 to 250 solar masses, with low to moderate metallicity, which causes a partial collapse. Runaway thermonuclear reactions (a reaction which is accelerated by heat, which in turn creates more heat) accelerate fusion to a point where the star completely explodes. During this explosion, no matter is left behind, leaving no remnant. Pair instability may also cause a GRB. A GRB is a flash of extreme energy gamma rays during a high energy explosion, in which a narrow beam of intense radiation is released. They are believed to be the brightest electromagnetic events in the universe.
Black Hole:
A black hole is an incredibly dense region of space, where the gravity is so great that not even light can escape. Created by large stars exploding in supernovae or even hypernovae, the theory of general relatively predicts, with decent evidence, that an object of significant mass could bend space itself. As light is pulled away by the intense gravity of a black hole, the only way we believe that we can detect a black hole is by detecting radiation, as predicted by quantum mechanics. As aforementioned, a black hole is caused by a star collapsing down to an incredibly dense point, called a singularity, during a supernova. However, certain types of star may create a black hole without a supernova, such as a star with solar mass 40-90 with low metallicity, or a star with more than 90 solar mass with low metallicity and a star that collapses due to photodisintegration, which creates a supermassive black hole. Photodisintegration is when high energy gamma rays knock a subatomic particle out of an atomic nucleus. This process is endothermic, meaning it is energy absorbing. Normally, within a massive star, more than 250 solar masses in size, photodisintegration’s energy absorbing effects do not affect the star. However, when it reaches carbon fusion, the heat and pressure reaches levels where photodisintegration can energy consuming process breaks the equilibrium, and the star begins to collapse. During this process, many heavy elements are produced. These are sprayed out into the universe by relativistic jets, which are extremely powerful jets of quark-gluon plasma, sprayed from the collapsing star and the created black hole. The creation of these jets are unknown, however because of the amount of energy required to launch one of these jets, most jets are believed to be launched from spinning black holes. These jets have been seen to be launched from the centres of several galaxies, which is evidence that black holes may be at the centre of most galaxies. The large emissions of radio waves from the centres of these galaxies, which are normally created by black holes, also lends itself to the this theory.
An important part of a black hole is the event horizon, mathematically predicted and tested with radiation telescopes. The event horizon is the theoretical boundary surrounding a black hole, from which no matter, not even light, can escape. An effect known as gravitational time dilation causes any object to observers seemingly take an infinite time to cross the event horizon. This has been predicted by the theory of general relativity, which states that the lower the gravitational potential, the slower the time. During this time, all actions happening on the object slow down causing the light being emitted to be fainter and redder, which is caused by gravitational redshift. If a person were to be crossing the event horizon, according to his watch he would take a finite time to cross the event horizon, but he would not be able to tell when he did because of lack of local evidence to support this. At the centre of a black hole is believed to be a gravitational singularity. In a static black hole, the singularity is a single point, believed to be in the first dimension. In a rotating black hole (Kerr black hole), the singularity is believed to be smeared out within the black hole. The singularity is all the mass of the black hole, and thus can be considered to infinite mass and infinite density. It is possible to avoid a Kerr singularity; however you will accelerate to a certain point, then maintain free fall. It appears to be possible to follow closed timelike curves (CTCs) which are an answer to Einstein’s Field Equations. CTCs are worldlines (the line of an object travelling through spacetime) in a Lorentzian manifold, a form of pseudo- Riemannian manifold, that is closed, therefore always returning to its starting point. Theoretically, this world line could be connected to earlier times, therefore travelling back in time to an earlier point. However, this theory opens up the possibility of a worldline that does not connect to an earlier point, therefore it defies causality, which states that a second event must always be traced back to an earlier event.