Student Project

Sep 13, 2023
Hi, I doubt anyone'll want to go to the effort, but if theres anyone with some kind of qualification could you give me some feedback on this essay I've written as a school assignment? If you could let me know what's useful and what I could change that would be greatly appreciated. Also there were a few images but they kinda wouldn't attach nicely.
Thanks to anyone who can give me any suggestions!

Star Formation
All stars form from a cloud of dust and gas that has produced a dense region as a result of turbulence. In the occurrence of a region dense enough to collapse under its own gravity, a protostar can be formed (See Figure.1.) Protostars are formed because of the pressure, due to gravity, heating the core of what will become the star, all the while the core continues to draw in more gas and dust increasing its gravity (NASA, 2023.) Once the protostar has accreted (gained mass through the process gas or other small particles being drawn in via gravity, Ridpath, I., 2007) sufficient mass, 0.08 solar masses or greater, the pressure of the core reaches a temperature of ten-million kelvin. This is the temperature required to induce hydrogen fusion and classify the star as main-sequence the most common type of star, while protostars that do not reach this mass are termed brown dwarfs and are classified somewhere between planets and stars (Las Cumbres Observatory, n.d.)

Main Sequence Stars
Of the many billions of stars in the milky way around 90% are main-sequence stars, with their distinguishing feature being that they all fuse hydrogen to helium. Main-sequence stars, although not actually a sequence, are so named as they fall on a section of the Hertzsprung-Russel diagram (to be explained in Stellar Classification) in a section known as the main sequence (Tillman, N.; Biggs, B., 2022.) Photons formed in the core can take thousands of years to reach the photosphere (the visible surface of a star, Ridpath, I., 2007.) On average, the gamma photons formed by nuclear fusion within the sun twenty- to thirty-thousand years ago are only reaching the surface now. This is because the stars are formed from incredibly dense plasma, causing the photon to be absorbed and reemitted every centimetre of travel. The photon also loses energy in this process, and by the time it reaches the photosphere it has reduced to a wavelength of around 550 nm for the sun (See Figure.2), which is predominantly green in the visible light range (CSIRO, 2019.)

Red-giant stars range through sizes many hundreds of times that of the Sun. They form as the equilibrium of radiation to gravity in a main-sequence star is interrupted by the hydrogen running out (See Figure.3.) The gravity of the star causes it to shrink and become more dense, while the increase in pressure further heats the core allowing helium to begin fusing, while a shell of hydrogen continues to burn This increase in radiation effects the equilibrium and causes the star to swell to a much larger size with a cooler surface (as the surface area significantly increases) and redder colour (Nance, S., 2018.) The Sun, as a main-sequence star now is expected to become a red giant in roughly five-billion years and will stay in this phase of its life for one-billion years fusing helium into carbon and an array of other heavier elements. Larger stars than the sun, eight to forty times the mass, can also fuse the carbon in the red-supergiant phase (Tillman, N., 2023.) Another difference between red-giants and red-supergiants is the way in which they end their life, with red-giants fizzling out and red-supergiants going out with a bang (Scientific America, 2004.)

As stars burn through the last of their remaining fuel they begin to lose the equilibrium between radiation and gravity. Depending on the mass of the star (turning point is around eight solar masses) it can then go through a process where it expels a significant portion of its gas or it can collapse before rebounding and becoming a supernova. This rebound is caused by both shock waves originating from the collapsing core, and a great number of neutrinos that, although barely interacting with regular matter, collide with the collapsing gas releasing a large amount of energy (See Figure.4.) This process can generate more than one-hundred times the energy that a star like the sun generates throughout its lifetime, resulting in the collapsing star to stop collapsing and begin exploding, moving at around 30 000 km/s (CrashCourse, 2015.) This is just one type of supernova however, of the other three two are very similar. The difference between these types are the elements in their spectra, one has a moderate amount of helium, one has a large amount of hydrogen and helium, while the other has little of either. These three types (Type Ib, Ic, and II) are thought to result from stars of differing masses exploding in the method as discussed. The other type however, Type Ia, has little hydrogen but a large amount of carbon, these are thought to originate from white dwarf stars accreting too much matter (1.4 solar masses) to hold together (Kruesi, L., 2012.) White dwarf stars are created from low mass stars releasing their atmospheres.

Stellar Classification
Stars are organised into spectral classes which are represented by letters O B A F G K M called the Harvard System. These classifications are derived from an earlier system based on absorption lines from stellar spectroscopy which was later refined to this arrangement. The ordering O B A F G K M is given by the temperature in that O is a very hot star, > 25 000 Kelvin, while M is a very cool star > 3 000 Kelvin (Petersen, C., 2017.) These designations also give an overview of their colour O sats are blue, M stars are red, while F and G are whitish (the Sun is a G type star.) Further classifications L and T denote brown dwarf stars which are very dim and cool as they are too small to fuse hydrogen (AstroBackyard, 2020.) These classifications can be arranged into a graph known as a Hertzsprung-Russell Diagram, H-R Diagram (See Figure.5), which plots the luminosity, the amount of radiation a star emits (Ridpath, I., 2007), against temperature or Harvard classification. It can be seen on the diagram that a large portion of stars (90%) form a line known as the main-sequence. Other classifications of stars can also be seen on the H-R Diagram such as giants, supergiants, and white dwarf stars. A large amount of information can be found from this diagram including size and colour (Professor Dave Explains, 2018.)

Stellar Fuel
Stars require a significant source of energy to counter the collapse caused by the stars gravity, this energy comes from nuclear fusion within the hot, dense core of the star. There are three main types of fusion however, the most simple being the proton-proton chain (pp chain.) This process involves protons colliding at great speeds to overcome the electrostatic force repelling the like charges. Once the protons are close enough the strong nuclear force takes over and the two protons form a deuterium atom (a proton with a neutron, the neutron is produced by one proton undergoing beta plus decay.) Another proton then joins this atom producing a he-3 nucleus. This process occurs with many protons at a time within stars forming many he-3 nuclei in which two can combine to form a he-4 nucleus and two protons. Each step of this process, as seen below, produces gamma ray photons carrying large amounts of energy which support the star and prevent it from collapsing under its own gravity.
This process can also follow alternate cycles however they all produce the same result, he-4 from four protons and energy (Na, Y.-S., 2014.)

The CNO cycle is another of the three cycles in which atoms are fused within the cores of stars. The CNO (Carbon-Nitrogen-Oxygen) cycle is a catalytic cycle which produces a helium atom from two hydrogen atoms. The process is detailed in the equation below.
The CNO cycle has the same result as the PP chain however it is thought to be more typical in stars 1.3 times the mass of the sun, as these stars have temperatures around 17 000 000 K (bluish stars, See Figure.5.) The CNO cycle also has multiple sub-chains where a nucleus (such as N-13) gains a hydrogen nuclei (a proton) before it can decay. These chains always result in C-12 and He-4 being produced (, n.d.)

The triple alpha process differs from the PP chain and CNO cycle in that it utilises He-4 nuclei to produce C-12 nuclei. The process involves three He-4 nuclei (or alpha particles hence the name) merging almost instantaneously to produce one C-12 nuclei. The process is represented by Figure.6 below (
The first nuclear reaction in this process requires two He-4 nuclei to combine producing a Be-8 nuclei with half-life 10 seconds (Stanford, 2017.) This fusion also requires a net energy input of 0.09 MeV draining energy rather than producing it, however the second fusion counteracts this producing a 7.37 MeV gamma ray (Wikipedia Contributors, 2020.) This fusion of Be-8 and He-4 also yields a C-12 nuclei. As the half-life of Be-8 is so short though, the triple alpha process can only occur at high temperatures and densities such as those within the core of large, red-giant stars (Stanford, 2017.)

Nov 19, 2021
Good review.

"...causing the photon to be absorbed and reemitted every centimetre of travel"
Mean free path inside the Sun is closer to a millimeter not a centimeter.

" (Scientific America, 2004.)"
It's Scientific American

"the sun"
"Sun" is capitalized

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