Overview of Forces in Atoms
An atom is a basic unit of matter, comprising of a dense, very small, nucleus at it's centre, surrounded at a relatively huge distance, by negatively charged electron(s) in orbit (the orbit is described as the electron shell or cloud), which are attracted by the electromagnetic force to the nucleus - mediated by protons.
The Nucleus consists of electrically neutral neutrons, and positively charged protons (except hydrogen 1H, which doesn't have a neutron), which are bound together by the Strong Nuclear Force - mediated by gluons. Neutrons and protons are collectively known as nucleons. The strong nuclear force overcomes the repulsion between positively charged protons (if there is more than one), to make the nucleus formation possible. The number of protons determines the different chemical element, and the number of neutrons determines the isotope of the same element. The number of electrons is normally the same as the number of protons - making the atom neutrally charged; hence if there are less or more electrons than protons the atom is charged; negative if there are more electrons than protons, and positive if the opposite is the case. Nucleons are made of elementary particles known as quarks, of which there are six types (or flavours); in the proton example below, it is made up of one down quark, and two up quarks, which have the lowest mass of all the quarks. Quarks are also held together by the Strong Nuclear Force - mediated by gluons.
The example atom used above is the Helium atom 4He isotope, which has two protons (like all Helium isotopes), two neutrons (which makes it this specific isotope of helium), and two electrons. The illustration uses the Bohr Model of an atom, which has been superseded, however, pedagogically it is still appropriate to understand the fundamentals.
The Composition of the Sun
The Sun is made up of hot plasma (a partially ionised gas) interwoven with magnetic fields.
The Sun’s mass consists of about 71% Hydrogen (H), 27.1% Helium (He), and less than 2% is made up of heavier elements including Oxygen (O), Carbon (C), Neon (Ne), Iron, Beryllium-7&8 (8Be & 7Be), Boron-8 (8B), Lithium-7 (7Li) and others.
Below are the main neutral Hydrogen and Helium isotopes found in the Sun.
The Main Drivers of Nuclear Fusion
Gravitational Pressure - the compressive force
The Sun was produced by an initial gravitational collapse of a giant molecular cloud. Gravitational pressure acting on the gigantic mass, eventually compressed it enough - reducing its' size and increasing its' density - creating the conditions for fusion to occur in its' core. Due to the Sun’s mass (330,000 times that of the Earth) the gravitational force, acting within it, is incredibly high. The deeper into the Sun’s core (nearly 700,000 km from the surface to the core), the greater the column of mass above, and the greater the gravitational pressure: gravity has a gradient.
It is this same gradient in gravitational pressure, that means all Earth’s rocks are underfoot, with the denser elements (such as metals) accumulate at the core, and the atmosphere above containing the lighter elements, also spreading out in a gradient above (and not pressed down like a pancake on the rocks surface).
Internal Pressure - the expanding force
The compression by gravity, is balanced by an outward pressure gradient force in the opposite direction, which is a combination of these three factors:
Temperature is a measure of the average kinetic energy (the random motion energy) in the system. The higher the temperature, the higher the amount of kinetic energy (they are, therefore, directly related). The kinetic energy can be translational motion (back and forth - as shown in the image to the left, with the arrows indicating velocity and direction), rotational motion, or vibrational motion.
Confined particles, like those in the centre of the Sun, are liable to move around very fast. This high motion of particles tends to be a reaction to confinement - suggesting an underlying “restlessness” (Capra and Luisi, 2014) at the atomic scale. And so, even materials, such as rock, that may seem life-less and inert to us, are, at the level of the nuclei, highly dynamic, and in motion at high speeds.
Pressure is the amount of force per area (P = F/A). Hot gas expands for instance, creating a greater pressure on the surroundings. When particles bounce-off the edge of a boundary surface, they apply an outward force on the surface. Faster (hotter) particles apply more pressure. For example, a ballon is inflated as the particles inside are pushing harder against the inside of the ballon, than the particles outside the ballon.
3) Density (Volumetric Mass Density)
Is the measurement of the amount of mass per volume (ρ = m/V). Gases can be compressed to smaller volumes and therefore, have higher densities per volume. As the volume reduces (goes down), the pressure increases (goes up); they are, therefore, inversely related. This is because a decreased volume has a smaller surface area, and as it still has the same mass, and the same number of particles, there are now more particles (and mass) acting on a smaller surface area.