Stars like many other life forms have been linked to some origins which may be considered the birth of the stars. Like human beings, the stars are “born” and consequentially “die” at the end of their life which in this case may be considered their fate. Stars are formed in a cloud of dust and gas known as nebulae with nuclear reactions at the stars’ center or the core providing adequate energy to make them shine. The lifetime of the stars differs from one star to the other based on their sizes. As Dutch (2016) states, stars with larger body masses always have a shorter lifespan as compared to those with small masses since they burn much of their hydrogen at a fast rate than the smaller stars. The paper focuses on the stages of the life cycle of the stars and its effects on celestial bodies including the planets, moons and other stars.
Section I
Major Processes in Star’s Birth and Thermonuclear Fusion Initiation
No one has ever lived to see stars go through their life cycles, but technology has made it possible for astronomers to observe stars at various stages in their developmental cycles. According to Dutch, (2016) stars are believed that stars came into existence after the gases and dust between stars contracted due to gravity. Based on this notion, a cloud of dust and gas can exhibit two states in the interstellar space. They can either be fragile thereby remain in gaseous state or condense to form stars. The contraction process is believed to be initiated by violent explosions from other existing stars generating shock waves. The contents of the explosions enrich the gas in heavy chemical elements forming a cloud that condenses to form a star cluster (Dutch, 2016). The temperatures at these regions are freezing ranging between 10 and 20 K which is just above the absolute zero. At this low temperature, the dust and gasses are transformed into molecular atoms with the common ones being hydrogen and CO which bind together. The gas also clumps to high densities at this stage until it reaches a point where the star is formed.
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Once the clump is freed from the core parts of the cloud, it gains gravitational pull and identity thus becoming a protostar. The low gas falls into the center as the protostar forms releasing kinetic energy in the form of pressure, temperature, and heat. At first, the protostar has approximately 1% of its last mass but as the falling materials continue to grow, size of the star keeps on increasing. After about a thousand centuries, thermonuclear fusion begins at the core of the star producing a strong stellar wind that is responsible for ending the in fall of new materials. At this stage, the mass is fixed, and this forms a young star awaiting future evolution. Once a protostar becomes hydrogen-burning star; there is the formation of a strong stellar wind making the star to have a flow of gas out of the poles an experience that happens at the T-Tauri phase (Tinsley, 1980).
Thermonuclear fusion is the process of achieving nuclear fusion through high temperatures. Stars are nuclear fusion reactors with atoms engaging in tremendous collisions altering their atomic structures and releasing a large amount of energy regarding heat and light. At the protostar or T-Tauri phase, temperatures may reach 15, 000, 000 degrees initiating nuclear fusion in the cloud’s core (Tinsley, 1980). The effect of this is that the star starts to glow brightly, contrast, and finally become stable. Nuclear fusion involves the conversion of hydrogen into helium generating energy. Once the star begins to shine and becomes stable, it assumes a position on the main sequence where it stays.
Reactions in High Mass, Intermediate, and Low Mass Stars
The phrase high mass stars is used to refer to those stars with the greatest mass which is usually triple the mass of the sun. Due to their constitution, these stars are larger, brighter, hotter and have wide variety of colors ranging from white, blue-white, to blue. Due to their enormous sizes, these stars use up their hydrogen fuel rapidly thus depleting their source of energy and therefore have a short life. In the beginning, high mass stars go through the same processes and reactions as the low mass stars except that it all happens much faster in high mass stars than in low mass stars. As Nevis & Swain (2000) state in high mass stars hydrogen is burnt through the Carbon Nitrogen, and Oxygen cycle (CNO) with carbon acting as a catalyst in the fusion of nitrogen and hydrogen while oxygen absorbs the protons creating helium.
The high temperatures in the core of a high-mass star result in ignition of a helium fusion rather than helium flash. This occurs in both high and intermediate mass stars (Nevis & Swain., 2000). The two-high and intermediate mass stars burn helium into carbon in a few hundred thousand years depleting the helium in their cores. However, during the supergiant phase, the intermediate mass fail to attain the temperatures needed to burn oxygen or carbon due to degeneracy pressure in their cores. They thus blow away their upper atmosphere ending their lives as white dwarfs (Nevis & Swain., 2000).
Unlike the high or intermediate mass stars, the low mass stars spent billions of years fusing hydrogen into helium in their cores. In fact, as Nevis & Swain., (2000) state, the process of fusion differs from that of the intermediate and the high mass stars in that the low mass stars do this via the proton-proton chain. The low mass stars have a conventional zone in which the activities within this zone determine whether a star has similar activities as the sun or the sunspot cycle. The low mass stars experience a helium flash which contrasts with high and intermediate mass stars which have ignition of a helium fusion (Tinsley, 1980). Since the helium core is degenerative, the burning reaction of the helium is explosive allowing the fusion of large amounts of helium to carbon in a very short period. The helium flash is however not observable as the protons produced are trapped in the hydrogen layers. After the flash, there is a hot ideal gas allowing a slower helium reaction to occur at a stable rate in a period called the horizontal branch.
Section 2
The Fate of the Stars
When radiation pressure in a star falters, there is a gravitational pull of the star’s gas inwards. As this happens, the star heats up and gains energy, and the gas becomes more tightly compressed while the pressure increases. The impact of this is either the star collapses until the gravity is no longer effective forming a black hole or continue collapsing until a new force is developed to cease the force. On the other hand, the small star including red dwarfs continues to shine, gradually cooling as they deplete their fuel. Dutch (2016) states that their masses never collapse to start a new cycle of life since the masses are very small.
The stars with 10% and above that of the sun will face a different fate as compared to the small stars. For larger stars, as the hydrogen supply continues to run out, they begin to contract to acquire more energy. Eventually, the pressure and temperatures inside the star get high that the helium begins to fuse to make carbon. The star expands becoming as larger as 300 million kilometers in diameter. Many red giant stars are unstable, and they pulsate and vary their brightness instead of swelling to a given size and maintain it. They then shed matter into space either violently or steadily forming planetary nebulae which are a disk-like envelop (Dutch, 2016). Other giant stars are large enough to start other cycles of nuclear reactions after the depletion of their helium supply. Despite that, the energy will sooner or later be depleted. The final stage of the star will be the white dwarf which is very hot with a small surface area making them faint and lose their energy at a slower rate as compared to their initial size.
The sun like other stars will have its fate described as the red giant. It is estimated that in the next 10 billion years the sun will start to contract as the helium in its core gets depleted (Dutch, 2016). It will increase its hotness with the outer gasses expanding and eventually being destroyed due to the friction with the gas causes the spiral of the earth into the star. Eventually, the sun will eject the outer envelope leaving its core as a white dwarf (Nevis & Swain., 2000). The white dwarf will then be able to emit only a fraction of the energy it currently emits, and the earth will likely be frozen solid.
Despite the fact that each of the stars in the galaxy will have a fate termed as its end, each group of the stars will have a different fate based on many aspects. The fate of the stare is determined by two major factors. These are the star mass and nature of the star (single or binary) (Stahler, 1994). By the mass, if a star reaches the end of the asymptotic giant branch with less than 1.4 times the sun’s mass, its eventual end will be a white dwarf. On the other hand, if the mass is more than 1.4 that of the sun, it will collapse into a neutron star ending its life in a supernova explosion. At the Chandrasekhar limit- a limit above which the white dwarfs collapse under the influence of their weight, the inward gravitational force becomes stronger than the electron degeneracy pressure forcing the white dwarf to explode (Stahler, 1994).
Lower masses stars stop burning their nuclear when the core is converted to oxygen and carbon. Thus, it requires a lot of pressure and temperatures to reach the standard energy levels for the burning of these elements. It is this very same energy that holds the star’s outer layers against collapse. Due to this it implodes violently resulting to supernova (Stahler, 1994). The outcome of the explosion will further depend on the mass. Thus, there are two possible outcomes namely a black hole or a neutron star. Typically, if a star is above three solar mass limits, it creates a black hole with a strong gravitational field trapping anything that comes closer to it and remains there forever as the mass travels at speed higher than that of light (Tinsley, 1980).
Giant and Supergiant Stars
Giant stars or rather red giants are the most easily identifiable stars as they are larger than the sun. Giant stars form as a result of a star consuming its stock of hydrogen in its core resulting in no generation of an outward pressure to counter the inward pressure as a result of stopped nuclear fusion (Cain, 2016). Due to the depletion of the hydrogen, a shell of hydrogen is formed around the star’s core which ignites further prolonging the life of the star. The effect of the ignition of the shell is that the star begins to enlarge dramatically with time; it may become more than 100 times its size when at the sequence phase. At this age and size, it becomes a giant star with outer layers expanded and cool.
On the other hand, the supergiant stars are the largest stars in the galaxy. They are monsters with twelve times the mass of sun. Unlike other stars, the supergiant stars use hydrogen at an enormous rate depleting the hydrogen in their cores within a few million years (Cain, 2016). These stars live fast and die young by completely disintegrating. Due to their massive sizes, supergiant stars’ cores get much hotter than those of the giant stars fusing elements heavier that helium and hydrogen. At their disintegration, these stars go supernova (Cain, 2016).
Chemical Elements Produced and Types of Reactions Involved in Start Production
For many of their lives, stars fuse elemental hydrogen to helium with two of hydrogen atoms combining in a series of reactions to create helium. Small stars, through nuclear fusion, convert hydrogen into helium while those larger than the sun but less than eight times its mass undergoing further reactions that convert helium into oxygen and carbon through stellar evolution (Dutch, 2016). In even bigger stars, reactions go further to produce silicon and iron. All these elements are formed through the nuclear fusion reaction at the core of the star. The primary reaction mechanisms involved are the carbon-nitrogen-oxygen cycle and the proton-proton chain. Other elements which are much heavier cannot be formed through the above reactions and thus are formed through supernova explosion where neutron capture reactions occur. Heavier elements such as uranium and gold dare then produced.
The proton-proton- chain reaction is a fusion reaction dominating the stars smaller or the size of the sun. The reaction occurs only when the temperatures of the protons are high enough to overcome their Coulomb repulsion. On the other hand, the CNO cycle is dominant in a star with more than one-third of the sun’s mass (Stahler, 1994).
Effect of Temperatures on the Color of the Stars
Stars have different colors ranging from white, red, blue, to even gold. The temperature even in real life affects the color of the object. In stars, those with high temperatures will have their moving color closer to the bluish region of the spectrum. Those with low temperatures will have their color directed towards red. As the temperature of a star increases so does its color change from red to blue (Stahler, 1994). Stars approximate a black body radiator’s behavior. As the body gets hot, its color changes with further heating changing the dull color to reddish color. With continues heating, it could glow orange all the way to blue-hot color. The same principle applies to stars. Being made of solid dull matter, they follow the same color changes with temperature changes.
Determining Energy Source in Sun and Stars
Determining the exact source of energy for either the sun or the star is one of the major challenges that scientists have faced since no one can go close enough to these objects to determine the source of their energy. None the fewer scientists have developed theories to measure the amount of energy generated by these bodies. Understanding the source of energy is paramount. As Dutch (2016) states, the energy produced by the sun or stars is due to nuclear fusion in their cores. The energy comes in the form of light and heat. One of such techniques is the solar neutrino detectors like the kamiokande detector used in detecting neutrinos which provide a window for thermonuclear reactions in the core of the sun and stars.
Section 3
Degeneracy Pressure and its Importance to Neutron and White Dwarf Stars
Degeneracy pressure occurs during the compression of electrons into very small volumes. They gain a lot of momentum since their positions are well-known by the Heisenberg’s uncertainty principle (Stahler, 1994). The pressure is of great important to both neutron and dwarf stars as it not only supports but also results to helium flash. The dwarf and neutron stars are highly unstable and rely heavily on the degenerate pressure to counter the effect of gravitational force. In fact, the formation of dwarf stars heavily requires the input of degenerate pressure. Stars with less than 1.44 solar masses will only be strong enough to counter the effect of gravity to form dwarf stars if they have electron degeneracy pressure (Dutch, 2016). For a star with more than 1.44 solar mass, the neutron degeneracy supports the stellar body forming a neutron star.
Difference between Neutron and Electron Degeneracy
One of the major differences of these two degeneracies is that the degenerate electron pressure counteracts the force of gravity for stars with less than 1.44 solar mass while the degenerate neutron pressure counteracts the gravitational force of solar masses between 1.44 and 2 or even 3. In that aspect, the degenerate electron pressure occurs in solar masses between 1.44 and 3, there will be no effect, or rather it will fail, and the star will explode (Dutch, 2016). Additionally, when degenerate electron pressure is applied, the resultant star is a white dwarf while in degenerate neutron pressure results to a neutron star. In degenerate electron pressure, two electrons can occupy each energy level as long as they are spinning in the opposite direction. On the other hand, in degenerate neutron pressure, there is no way two neutrons can be located in the same energy state, and this can only happen under great force (Tinsley, 1980).
The Chandrasekhar Limit
The limit is the mass above which electron degeneracy pressure in the core of a star is insufficient to stabilize or balance the star’s gravity. It is the maximum mass of a stable white dwarf star. Therefore, white dwarf stars with greater masses than this limit will collapse disintegrating into different stellar remnants such as the black hole or neutron star however they avoid this by exploding being collapsing (Tinsley, 1980).
Weight of a Paperclip made of Neutron Matter
The weight of neutron matter is perceived to be the heaviest thus making a paperclip made from that matter to weight heavily. The surface of neutron star forms a crust of heavy nucleic material which is in fact not neutralized. If a paperclip were to be made of degenerate neutron material, then it would weight more than Mt. Everest. That is it would be more than 357 trillion pounds.
Type of Star that can End up as Black Hole and how to spot it
Not all stars can develop to be black holes. Rather, it is only those stars with large mases which can develop to become black holes. Reports indicate that stars whose mass is twenty times more than that of the sun can create black holes. Nevertheless, the massive of the core will determine if the star will for, a black hole or a neutron star (Tinsley, 1980). Most of the instruments used by astronauts rely on light, but currently, they detect black holes detecting its high-energy radiation emitted by the black hole’s swirling. Another way of detecting these black holes is by their gravitational influence.
Section 4
Types of Stars that may Support Life
Unlike most of the stars that are too hot to support any form of life, scientist indicates that the red dwarf stars may support planet earth’s life, but this has not yet been proven. There are those planets that are within the optimistic habitable zone. These planets and their related star include Proxima Centauri b with the supporting star being Proxima Centauri, Keppler with the supporting star being Kepler, Gliese with supporting star being Gliese. Despite the fact that these planets and the related stars have not been proven to support earth life, they have a higher possibility of supporting this life.
Formation of Planetary Systems
Planetary system formation coincides with the star formation process. Planetary systems come from protoplanetary disks forming around the stars. This system is made up of non-stellar objects that are gravitational bound. These objects revolve around the star system (Dutch, 2016). One of such planetary system is the solar system which includes the sun and its planetary system including earth. The formation of the solar system dates back to 4.6 billion years when a cloud of interstellar gas, ices, and dust with several generations of material collapsing to form the nebula. The sun and the rest of the solar system then formed from this nebula through the trigger of a supernova. Not all the material from the nebula collected to form the sun but rather some of these materials confined to a flat, spinning disc known as the protoplanetary disk. The material continued to collect around the disk with their gravitational forces increasing gradually creating kilometers-sized bodies. These bodies collided and either shattered or fused to form even larger body masses over time. These bodies formed the current giant planets.
Conclusion
The existing planetary system has been in existence for quite a long time, but their formation has revealed much than what is seen today. The existence of stars not only affects the life in other planets but also influenced their formation. Through different reactions, masses within the galaxy have collected to form the current planets, and stars that exist.
References
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Dutch, S. (2016). Life cycles of stars. The University of Green Bay . Retrieved on 4 March 2017, from https://www.uwgb.edu/dutchs/AstronNotes/STARS.HTM.
Nevins, W. M., & Swain, R. (2000). The thermonuclear fusion rate coefficient for p-11B reactions. Nuclear Fusion , 40 (4), 865.
Stahler, S. W. (1994). Early stellar evolution. Publications of the Astronomical Society of the Pacific , 337-343.
Tinsley, B. M. (1980). Evolution of the stars and gas in galaxies. Fundamentals of cosmic physics , 5 , 287-388.
