Stars

• Stars are the most basic unit of mass that is produced when gravity pulls mass together
• 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.
• On a clear night we can see around 3000 stars, but there are over 100b stars in the Milky Way

Birth

• Clouds the size of our solar system coalesce for 100,000 years
• After 500k years, the center heats up, the core gets to greater than 8m degrees
• Fusion starts at 8m degrees

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)
• Einstein discovered matter is condensed energy, which can be releases by fusion
• 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

• Eventually the Hydrogen runs out, and fusion can stop
  • Our sun will fuse carbon -> oxygen, then stop.
  • Gravity wins, compresses the core, which heats up, causing outer layers to expand, creating a Red Giant
  • Core becomes unstable, blow away outer layers (planetary nebula) leaving a white dwarf
• If a white dwarf is part of a binary star pair, it can steal mass from its neighbor star, and go supernova.
• Iron is the heaviest element a star can fuse in its core. Since it can't be fused further, it absorbs energy.
  • Once Iron is formed, the core collapses, outer layers collapse down, causing a SUper Nova a few seconds after iron was formed
  • Neutron stars are left after supernovae

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>

Specific Stars

Sun

• 4.6b years old, 93M miles away, 1M Km in diameter
• Our sun is a second (or more) generation star. The iron in our blood was formed from a Supernova in the same space as our solar system, before our star was formed

Eta Carinae

• 5M times larger than the sun

Betelgeuse

• 300 times larger than E.C