10.7 Stellar processes and Stellar Evolution

William Scaruffi

E.5.1. Describe Conditions that Initiate Fusion in a Star

• Deaths of Supernovas compress Hydrogen Clouds (Proto-Stars) enough for particles to come together. The more atoms that are pulled in by these clouds, the stronger gravitational force.

• Gravity on atoms gives atoms KE temperature increases

(High temperature an pressure)• Fusion Begins…

E.5.2 State the effect of a star’s mass on the end product of nuclear fusion

• High Temp and Pressure give H atoms enough energy to overcome electrostatic repulsion. Fusion Begins:

Small Star• Smaller than 4 Solar-Mass

Units• Fuses up to C• Reactions Stops, and outer

layers are blown off, exposing the core.

• Luminosity decreases (White Dwarf)

Big Star

• Larger than 4 Solar-Mass Units

• Fuse up to Fe• Chandrasekhar Limit:

e- + p+ form neutrons.

• Outer layers slam into core and BOOM: Supernova.

• All that it leaves is Neutrons (Hence a Neutron Star)

E.5.3 Outline the changes that take place in Nucleosynthesis when a star leaves the main

sequence and becomes a Red Giant.• When outward pressure reaches equilibrium with the

gravitational pull, the star becomes a MAIN SEQUENCE STAR (like our sun)

• H + H -> He; He + He -> C• When Hydrogen in the core runs out, fusion stops

Outward thermal pressure drops, core compressesCompressing core heats outside HydrogenOutside Hydrogen begins fusing and expands

E.5.4 Apply the Mass-Luminosity Relation

Mass-luminosity relationship:Only Main Sequence Stars.massive main sequence stars are more luminous than smaller stars. So it is then possible to predict where (on the graph) the different stars become part of the main sequence line, and therefore it is possible to predict their evolution.

The equation relating mass, m and luminosity, L is L m^3.5 ∝(L=W/m^2; m=Solar Mass Unit)

(e.g.)Question: Justify why the star Sirius is twice the size of our sun but it is about 10 times as bright.Answer:Luminosity of Our sun: 1^3.5 = 1Luminosity of Sirius: 2^3.5 = 11.31

E.5.5 Explain the use of Chandraskhar and Oppenheimer-Volkoff limits are used to predict

the fate of stars• Chandraskhar Limit

White Dwarf can be no more than 1.4 Solar Mass UnitsSo: White Dwarfs can’t result from stars that are more than 4 Solar Mass Units

• Oppneheimer-Volkoff LimitLess than 3 Solar Mass Units

Neutron Star will remain stableMore than 3 Solar Mass Units

Neutron Degeneracy is overwhelmedNeutron Star will collapse into a Black Hole

E.5.6 Compare the fate of Red Giants and Red Supergiants

E.5.7 Draw Evolutionary Paths on an HR Diagram

• Evolutionary Path: a star’s development (change in luminosity and temperature)

Large Star

Small Star

E.5.8 Outline Characteristics of a Pulsar

• Neutron Star that rotates really fast.• Since it’s a collapsed star, the magnetic flux is spread over a

smaller are, therefore Magnetic field is denser.• Strong Spinning Magnetic field causes EM radiation into a beam along poles.• Rotation Axis Beam’s Axis• Rays perceived to Pulse at a regular intervals from fixed perceiver.