Stellar Evolution
Stellar evolution entails the process in which stars changes over time. The evolution explores the life history of a star in terms of how it condenses out of the interstellar matter to its death, which is sometimes catastrophic. This paper explores the stellar evolution in terms of its stages and the way of assessing its validity.
Overview
The stellar evolution is gravity-driven. The large mass of stars means that gravity is continually forcing them to contract. However, nuclear fusion stops the contraction, although temporarily, by offering the energy to produce high gas pressure that has the capability to counterbalance gravity. When the primary nuclear fuel is depleted, the star’s central pressure decreases (Stellar Evolution-The Death of Stars, n. d.). This paves the way for the core of the star to start collapsing. The collapsing occurs until either the nuclear fusion process starts or some other sources of pressure emerge.
Phases of Stellar Evolution
Evolution into the Main Sequence
The first stage of stellar evolution entails the contraction of the protostar (Swinburne University, n. d.). This is the stage in which protostar of the interstellar gases, such as hydrogen, helium a well as a few heavier elements, contract. This stage occurs in millions of years (Stellar Evolution: Phases of Stella Evolution, n. d.). In this phase, half of the gravitation potential energy emitted by the collapse of the protostar is discharged away. The remainder of the gravitational energy is used in the process of the formation of the new star. The majority of the contraction of the protostar is witnessed in isolated gas clouds (Stellar Evolution: Phases of Stella Evolution, n. d.). This is the region when a cloud contacts to develop one star. The major factor that determines how a star will evolve is its mass by the time it reaches the main sequence (Swinburne University, n. d.). As such, stars go through the main sequence phase differently depending on their masses.
The Main Sequence Phase
Stars spend about 90% of their lifetimes in the main sequence phase (Stellar Evolution-The Death of Stars, n. d.). One a new star is formed, it undergoes the main sequence phase in which it shines rapidly as it transforms its hydrogen reserves into helium. Once a star’s hydrogen reserves are changed into helium, it ends up having inhomogeneous chemical composition. This involves the star being rich in helium in its core, which is the parts in which nuclear reactions take place. However, the peripheral still remains with almost pure levels of hydrogen. The hydrogen located at the core of the star is utilized first. Once it is completely used, nuclear reactions changes shift to the successive concentric shells. Eventually, fusion happens only in the core’s thin outer layer (Stellar Evolution: Phases of Stella Evolution, n. d.). This is the only place that has adequate hydrogen levels and high temperature capable of sustaining the necessary chemical reactions. In the main sequence phase, a correlation exists between the star’s luminosity and its mass (Stellar Life Cycle, n. d.). As the luminosity of a main-sequence star increases, its masses get larger as well.
The next stage in the stellar evolution entails old stars dying. When the core’s helium content builds up, it contracts and produces gravitational energy (Stellar Evolution: Phases of Stella Evolution, n. d.). The gravitational energy increases the temperature of the core, enhancing the rate at which nuclear reactions occur. This results in the rate at which hydrogen is being consumed to increase steadily. The increased luminosity caused by the higher reaction rates forces the envelope to increase in size to pave the way for an enhanced supply of energy to the surface of the star in question. The expansion of outer layers of the star is followed by their condensation. At this point, the star comprises of a core that is not only dense but also helium-rich and which is surrounding by a huge fragile envelop comprising of significantly cool gas. At this point, the star has transformed into a red giant. As time goes by, the condensing stellar core reaches temperatures exceeding 100 million degrees kelvin (Stellar Evolution: Phases of Stella Evolution, n. d.). The burning of helium sets in, which halts the increase in the size of the envelope. The star returns to the red giant stage, decreasing in luminosity and size as it approaches the main sequence again. The sun is an example of how a star turns into a red giant, as demonstrated below.
(Stellar Life Cycle, n. d.)
As the star reproaches the main sequence, it probably becomes unstable. At the juncture, the star may shed off some of its mass. Also, it may turn into a supernova star or an exploding nova (Stellar Evolution: Phases of Stella Evolution, n. d.). When the evolution is nearing the end, increased contraction as well as higher temperatures ignites new thermonuclear reactions. Elements that weigh more than iron turn into supernova explosions (Stellar Evolution: Phases of Stella Evolution, n. d.). Besides, the late-stage star has a chemical composition that is highly inhomogeneous. Its resulting structure fractions into several concentric shells which comprise of different elements concentrated around the iron core as demonstrated below:
(Stellar Life Cycle, n. d.)
The outcome of the stellar evolution is reliant on the remaining mass of the old star. After the main sequence, a massive star evolves by forming a yellow giant first before developing into a red giant (Stellar Evolution-The Death of Stars, n. d).). Most of the stars do not form iron cores. If the final star does not have a mass that is 1.5 times higher than the sun’s mass, it ends up becoming a white dwarf (Stellar Evolution: Phases of Stella Evolution, n. d.). The white draft is formed when a star with low mass comes closer to the end of its life (Stellar Life Cycle, n. d.). The white dwarf star does not have enough mass to heat itself. As a result, the white draft continues shining using its previously stored energy. The star slowly becomes feeble for many years as it continues to radiate away the remainder of the heat energy until it transforms into a black dwarf, which is essentially a dead star. The white dwarf takes 10 million years to cool off to 20,000 degrees Kelvin and billions of years to translate to a black dwarf (Stellar Life Cycle, n. d.). However, not all stars end up becoming a black dwarf. Instead, stars that are too large end up becoming stable white dwarfs. These stars continue experiencing contractions until their temperatures reach approximately 5 billion o K (Stellar Evolution: Phases of Stella Evolution, n. d.). At this point, the evolution of the massive star is governed by the same process that dictates the development of smaller stars. This process entails the hydrogen fuel in the star’s core getting exhausted. As a result of its large mass, a massive star has high central temperature values. In the beginning, helium is burned to carbon in the process known as the triple-alpha process (Stellar Evolution-The Death of Stars, n. d). Through this process, the star is given a second chance in life. Because its core is not degenerate, the burning of helium does not start explosively.
Testing the Stellar Evolution
It is implausible for a person to follow a single star throughout its lifetime. This is because stars may exist in millions or billions of years (Stellar Evolution: Phases of Stella Evolution, n. d.). However, new stars are formed all the time. Thus, stars of various ages are found in the present epoch. This means that it is possible for the various stages of the stellar evolution to be determined in different stars already in existence. However, it is impossible to assess the age of a specific star by observing its characteristics. Instead, one must infer the age of the star in question from the evolutionary theory he or she is trying to validate.
To sum up, the stellar evolution explores the life cycle of a star from its formation to its end. Sometimes, a star may end up in death after all the nuclear fuel has been depleted or when it is unable to attain a stable configuration. Stars have finite energy reserves, which means that they cannot continue presenting the same level of luminosity for eternity. However, whether a star will end as a white or black dwarf is determined by its masses.
References
Stellar Evolution: Phases of Stella Evolution. (n. d.). Retrieved from https://www.infoplease.com/encyclopedia/science/space/astronomy/stellar-evolution/phases-of-stellar-evolution
Stellar Evolution-The Death of Stars. (n. d). PDF File.
Stellar Life Cycle. (n. d.). PDF File.
Swinburne University. (n. d.). Stellar evolution. Retrieved from http://astronomy.swin.edu.au/cosmos/S/stellar+evolution