Stars
- Stars are the most basic unit of mass that is produced when gravity pulls mass together
- Stars are in a state of equilibrium. The force of fusion equally matches the force of gravity. When stars run out of fuel, fusion slows and stops, and gravity wins.
- Dark matter is not in the gravitation field of galaxies. Galaxies are suspended in the gravitational field of dark matter.
- Red dwarfs are the most popular types of stars, but are not visible to us. Blue stars are very rare, but hot and large enough for their light to reach us.
Nuclear Fusion
- Results from smashing hydrogen atoms in to each other under great force of gravity, producing heavier elements
- Two atomic nuclei combine to form one larger one, but this new element's nucleus is smaller in mass than the sum of the previous two independent atoms. The difference in mass is expelled as energy (light, radiation, etc)
- During fusion the enormous energy of the strong nuclear force is liberated, creating light
- Fusion produces electromagnetic radiation, consisting of a spectrum of energy, ranging from infrared to ultraviolet
Death
- When a star slightly larger than our Sun uses up all of its fuel, it turns in to white dwarf. If the star is part of a binary star pair, it can steal mass from its neighbor star, and go supernova.
- Type 1 A supernovae explode from identical amounts of mass, and as such give off a predictable amount of light energy. The intensity of this light energy can be measured and results in the distance away from us. Using this (standard candles), it was determined that the universe is expanding, but instead of gravity slowing them down, galaxies are accelerating away from each other. Dark energy causes this.
Equilibrium
Main sequence stars are in a state of equilibrium -- the inward force of gravity is opposed by the outward force due to the pressure of fusion.
Calculating Pressure
<math>P = nkT</math>
where P is the thermal pressure of the gas, n is the number of particles (number density) in each cubic centimeter, k is the Boltzmann constant, and T is temperature in Kelvin.
If the mass of each gas particle can be represented as m, and M is the total mass of the cloud, then <math>N_t = M/m</math> gives the total number of particles in the cloud.
Number density can also be found by dividing the total number of particles (<math>N_t</math>) by the volume (V) of the cloud. For example, using as sphere as our cloud, <math>V = 3/4 \pi r^3</math>, then
<math> n = \frac{N_t}{V} = \frac{M/m}{3/4 \pi r^3} = \frac{3M}{4m \pi r^3} </math>
Rearranging the formula for Pressure, solving for the number density, we get <math>n = P/kT</math>
The outward force of a spherical cloud due to pressure is thermal pressure of a gas times the area of the cloud. <math>F_p = pressure * area</math>
<math> F_p = nkT * A = \frac{3M}{4m \pi r^3} * kT * A = \frac{3M}{4m \pi r^3} * kT * \pi r^2 </math>
Reducing this, we get the outward pressure force of a spherical cloud of gas as
<math> F_p = \frac{3MkT}{4mr} </math>