Life Cycle of Stars

Terms and Definitions taken from lectures pertaining to "Life Cycle of Stars"

What is the relative Diameter of the Sun to Earth?

One Solar diameter is 109 Earth diameters.

What is the relative mass of the Sun to Earth?

One Solar mass is 330 Earth masses.

What is the elemental ratio of the Sun?

70% H, 28% He, and 2% trace elements.

What fusion process does the Sun use?

The Sun uses the Proton-Proton chain.

Explain the Proton-Proton chain.

• Protons repel each other due Coulomb's law.

• The particles in the Sun are moving so fast (due to the temperature) that they are able to overcome the Coulomb Force.

1. Two protons collide, creating Deuterium (1P, 1N). This also invokes Beta-Decay, releasing 1 neutrino, 1 positron, and some Gamma photons.

2. Another proton collides with the Deuterium and makes He-3, releasing a photon in the process.

3. The He-3 atom collides with another He-3 atom and makes He-4 (alpha particle), releasing two protons in the process.

What is the elemental ratio of the Interstellar Medium (ISM)?

The elemental ratio is 75% H and 25% He.

What is the average temperature of Molecular Clouds?

The average temperature ranges from 10k - 30k. This means the gas is cool enough to stick into clumps.

How are stars formed from molecular clouds?

1. A region of molecular clouds that is dense can begin to clump and begin to collapse.

2. If there is enough mass accumulated, the center can be compressed enough that the temperature will reach fusion “ignition”.

3. Once this happens the star then becomes a Main-Sequence Star.

• Multiple stars can be made from one molecular cloud.

What is the ratio of proton-to-neutron after the big-bang began to cool?

The ratio is 7-to-1.

How much larger are Red Giants than the star they formed from?

10x - 100x the diameter.

How much brighter are Red Giants than the star they formed from?

1000x brighter.

What percent of its life does a star spend in Red Giant form?

10% of its life.

Where does most of the light in a Galaxy come from?

Most of it comes from Red Giants, because of their extreme luminosity.

Why do Red Giants form?

Once the core has fused all the H into He, fusion stops. This reduces thermal pressure and the star no longer has hydrostatic equilibrium. This compresses the material outside the core and we get a “shell” of H being fused into He. This puffs out the outer layers, making it larger.

What happens when low-mass stellar red-giants finishing fusing the shell?

The He deposited into the core increases its mass which increases compression. This compression allows the rapid fusion of the He into C, N, O, and Ne. The core is super-heated as a result and this heat blows off the outer layers.

What happens when high-mass stellar red giants finish fusing the shell?

The core is massive enough to allow the fusion of heavier elements when the He deposition is finished. Fusion of elements less massive than iron release energy. This energy release warms layers outside it to fuse the elements to the next shell. This increases the mass of the core, further compressing. This compression, no longer has a counteracting force and compresses the core so much that electron degeneracy pressure is overpowered, allowing the protons and electrons to merge, making neutrons. This merge makes electron neutrinos that, due to the sheer quantity of them, are able to violently blow off the outer layers of the star in a super-nova.

What is the Chandrasekhar limit?

It is a mass that describes the point where neutron stars will be made. This is roughly 1.4 solar masses.

What is a type 1a Supernova?

It is when a binary system has a WD close enough and massive enough to sap mass from the main-sequence star. This material accretes and eventually falls to the surface of it, slightly compressing the WD. The material cannot compress anymore and the WD begins to rapidly fuse the material, causing the WD to fuse as well, which causes it to explode.

What is the diameter of the Milky Way Galaxy?

100,000 Light Years

How far is the sun from the center of the solar system?

28,000 light years.

What are the spiral arms of a galaxy?

Density waves, this explains why stars form in them.

Why do stars form in density waves?

The material is clumped together, allowing for mass accretion. This accretion allows for the formation of stars. The bigger ones die before they exit the density wave.