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Chemistry of Stars
Episode 131st July 2022 • Chemistry Connections • Hopewell Valley Student Publication Network
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Chemistry Connections

Chemistry of the Sun

Episode #13  

Segment 1: Introduction to Fusion in the Sun

  • Welcome to chemistry connections, my name is Tyler Longo
  • And I am Sathya Kummarapurugu, and we are your hosts for episode #13. 
  • Today we will be discussing the processes that take place in the sun, through a chemistry-focused lens
  • Before a star is born there are clouds of dust in the area it will be formed. When clouds of dust begin to be pulled together by the force of gravity gaseous stars begin to be formed
  • As gravity drags these gas particles together making stars, the temperature in the core increases to very high temperatures. As the amount of thermal energy in the core increases the temperature also increases. As temperature increases the avg kinetic energy in the sun’s core increases, and according to the equation KE=0.5mv2 as Kinetic energy increases the velocity of the hydrogen atoms in the core increases. According to the collision theory, when a particle collides with another particle with enough activation energy, a bond may form, releasing energy. 
  • So since the temperature is so high, does that mean that the Hydrogen atoms within these stars begin to collide and form a bond?
  • Technically, since hydrogen particles are protons, two protons coming close together have a repulsive force between them by Columbus law since both protons are positively charged and particles with the same charge repel each other. Columb’s law states that objects of the same charge repel each other and objects of opposing charge attract each other. This attraction(electrostaticforce) and repulsion are directly proportional to the distance between the particles and charge magnitude. But the strong nuclear force overcomes the repulsive force and the two protons bind together
  • That’s really only the first step of the process in which hydrogen atoms become helium atoms through fusion. It’s actually a multistep process called a proton-proton chain reaction
  • So how it works is first, two hydrogen atoms collide together through the power of this force called the strong nuclear force. These are basically just protons. 
  • A proton is composed of an up, up, and down quark. These are one of the fundamental particles of the universe
  • So when the weak force is applied, this causes an up quark to become a down quark, thus changing the two of the particles from protons to neutrons
  • Yeah, There are also 4 other fundamental quarks: strange quarks, charm quarks, top quarks, and bottom quarks. 
  • What’s important is that up quarks have a charge of +2/3 and down quarks have a charge of -1/3, which means when two ups and one down come together to form a proton, it has a total charge of +1. Likewise, since a neutron has one up and two downs, it has a charge of 0
  • These quarks make up the fundamental particles of the universe such as neutrons and protons. Anyway, the protons are brought together by the Strong nuclear force and joined together by the strong nuclear force. The quarks are joined together by the gluons within each proton. When the strong force brings another proton towards the proton and then the protons collide with enough force, the protons stick together because the strong nuclear force joins a gluon to the quarks within the other proton causing the protons to bind and form a helium atom.
  • The sun conducts nuclear fusion within its core, and these interactions that occur between quarks are central to the fusion process.
  • When the protons fuse one Helium atom is created. The fusion releases a bunch of thermal energy. When bonds are formed energy is released and to break these bonds energy is required. Forming a bond through the strong nuclear force releases a lot of energy because the strong nuclear force is so strong at that microscopic scale. 
  • In the sun, about 74% of the mass is composed of hydrogen, and about 25% of the mass is composed of helium
  • That means we can use stoichiometry to figure out the mole ratio between these two elements within the sun. Remember, as we determined before, the isotope of helium found in the sun is helium 4. So using the numbers I said before, if we had a 100 gram sample of sun (lololol), then 74 grams would be hydrogen and 25 grams would be helium. 
  • But since helium is four grams per mole, this means 25 grams of helium equals about six moles of helium.
  • When you look at the ratio of 74 moles of hydrogen to 6 moles of helium, that comes out to being about 12 moles of hydrogen per mole of helium within the sun
  • Wait that actually makes so much sense!!!!
  • Yeah! Thinking back to the proton-proton chain reaction, six hydrogen-1 atoms were needed in the fusion reaction just to produce a helium atom (and also two extra hydrogens), so it makes sense for there to be many more moles of hydrogen than helium in the sun

Segment 2: The Chemistry Behind Pressure and Thermodynamics in the Sun

  • Let’s get back to the topic of pressure. PV=nrt is the ideal gas law where P is pressure, V is volume, n is the number of moles, r is the gas constant, and t is the temperature. According to this law, when temperature increases Pressure also increases. 
  • As the temperature increases, the core of a star exerts pressure outward. 
  • Yeah, and This pressure opposes the force of gravity. Since the force of the reaction and the force of gravity are equal and opposite the star is held in a delicate equilibrium and does not collapse in on itself.
  • Our sun has a surface temperature of around 5,778 K. According to the second law of thermodynamics, entropy always will naturally increase in the universe. Entropy is the amount that heat is dispersed or spread out. Since entropy must always increase in the universe the sun must disperse/transfer its concentration of thermal energy in some way. In space, stars transfer this heat through radiation, since convection or conduction is impossible in a vacuum, which increases entropy since the heat is more dispersed throughout space.
  • This is a great application of the ideal gas law since the intermolecular forces between hydrogens and heliums are sooo small. I mean, I guess there would be London dispersion forces, but these elements that we are dealing with have such small electron clouds that are barely even polarizable, so their behavior must be so close to that of an ideal gas and they are moving at such high speeds that I guess IMFs don’t play much of a role.
  • Although it is the lightest and most abundant element in the universe, Hydrogen is finite. The fusion in the sun is represented by the transmutation equation 11H + 11H→22He + heat. Also if you look closely, you’ll notice that this transmutation equation actually uses Isotopic notation. Isotopic notation is where the mass number (amount of protons and neutrons) is written on the top left of the element symbol and the atomic number (amount of protons) is written on the bottom left of the element symbol. Protons dictate what element an atom is. For example, if a particle has one proton it’s a hydrogen atom and if it has two it is a helium atom. 
  • I noticed that you listed heat as a product in that equation. That indicates that the reaction releases energy/heat, and it is exothermic, so the enthalpy, or change in heat energy, is negative. You also mentioned that hydrogen is finite, what does that mean??
  • That means hydrogen is the limiting reagent, which means it is the reactant that limits the amount of product produced. When the Hydrogen atoms run out the reaction stops. 
  • So at that point, if that reaction stops no more thermal energy can be created.
  •  According to PV=nrt as thermal energy decreases t decreases and P decreases. There will be no more outward pressure created and gravity becomes the dominant force in the star. The star collapses in on itself and explodes in a supernova, more entropy since the explosion spreads out the rest of the thermal energy in the sun.
  • So when the star runs out of hydrogen to use in the fusion reaction, the pressure decreases because thermal energy runs low and temperature decreases in the core and the star collapses, so it just disperses all of its energy because of entropy. Are there any other ways that a star can die?
  • Actually yeah: Black holes. D=m/v where m is mass and v is volume. As the star dies it expands and sheds some of its mass. The core, however, maintains most of its mass and when the star is MASSIVE ENOUGH (meaning it has a lot of mass) it has the ability to become a black hole. As the star collapses due to gravity, the volume of the star rapidly decreases while the mass of the core stays relatively constant. 
  • If the volume is rapidly decreasing, with mass staying the same, that must mean the density gets extremely high…
  • At about 2 x 1019 kg/m3, the star has enough density to become a black hole which terrified me as a child but fascinates me now.

Segment 3: Personal Connections

  • The topic of stars was mostly your idea, so I’m wondering, how did you come up with the topic??
  • Well, I never really understood how the sun worked so I wanted to explore something I didn’t know. I knew that fusion would have to go into quantum mechanics a bit so to force myself to learn it I decided to learn more about stars.
  • Yeah, that's a good point, I didn't know anything about the sun going into this project either, even though it is kind of a pretty big part of our lives. One detail that I kind of skipped over is that, in the proton-proton chain reaction in which protons are fused together, in the process of changing a proton to a neutron, one other product that is released is gamma rays. When leaving the sun, these get converted into lower-energy photons such as ultraviolet rays, which can cause sunburn. That’s an example of how this stuff relates to our experiences in real life.
  • I also wanted to know what fusion exactly was and how it worked. I also got to learn about quarks and how they are technically the fundamental building blocks of matter.
  • At the start of the project, I came across an article titled “proton contains more anti-down quarks than anti-up” and I thought it sounded extremely dumb, but now that we learned about quarks, things like this are able to make much more sense
  • Anyway, Thank you for listening to this episode of Chemistry Connections. For more student-run podcasts and digital content, make sure that you visit www.hvspn.com

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Warm Nights by @LakeyInspired 

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