When hydrogen is depleted, the star changes location on the HR Diagram.
Location Factors: Size of the star.
Helium fusion begins to form carbon once hydrogen is used up.
Low mass stars (up to 8 times the Sun's size) experience a “helium flash” as helium fuses.
Evolution of Low Mass Stars
Helium Fusion
Generates carbon, leading to the expansion of the star into a red giant.
Instability Phase
Carbon runs out causing instability and pulsations.
Planetary Nebula Formation
Outer layers are expelled, creating a cloud of gas and dust (misnamed as "planetary nebula" as it involves no planets).
End Stage
Only the core remains, becoming a White Dwarf Star.
No further nuclear fusion occurs; it only emits heat radiation.
Eventually cools down and becomes a Black Dwarf, roughly the size of a planet.
Evolution of High Mass Stars
Carbon Fusion
When helium is exhausted, the weight of the outer layers forces the carbon core to collapse, raising temperatures to fuse carbon into heavier elements (oxygen, neon, magnesium).
Further Nuclear Fusion
As additional nuclear fuels exhaust, the core collapses again to reach temperatures high enough for further fusion (silicon, sulfur, calcium, argon).
The process continues until iron is formed.
Iron Core Collapse in High-Mass Stars
Collapse Effect
The iron core collapses.
Resulting Explosion
Post-collapse, the star expands back to original size and undergoes a supernova explosion.
Outcomes
Smaller high-mass stars transform into neutron stars.
Larger high-mass stars may become black holes.
Neutron Stars
Formation
Created when a massive star runs out of fuel and collapses.
Characteristics
Rapidly rotating neutron stars are known as pulsars.
Emit pulses of radiation at regular intervals, akin to a lighthouse.
Possess extremely strong magnetic fields which direct jets of particles along two magnetic poles.
Discovery of Neutron Stars (Pulsars)
Jocelyn Bell's Contribution
In the 1960s, as a graduate student at Cambridge University, she utilized a radio telescope to study the twinkling of distant stars.
Discovered repeating radio waves occurring every second, initially labeling them as LGM for "Little Green Men."
Soon recognized these signals were from pulsars, which are neutron stars.