Stellar Evolution Flashcards
What do stars begin as?
Clouds of dust and gas
Where do clouds of dust and gas come from?
Previous stars blow themselves apart in supernovae
How do protostars form?
Denser clumps of cloud contract under the force of gravity, then fragments into regions called protostars
How do stars form from protostars?
Protostars contract and heat up. At a few million degrees, hydrogen nuclei fuse into helium, releasing a lot of energy creating radiation pressure that stops the gravitational collapse
How does a star move through the different categories of star?
Main Sequence → Red Giants → White Dwarfs → Fade away
What are the four stages of stars burning?
Core hydrogen burning
Shell hydrogen burning
Core helium burning
Shell helium burning
What is core hydrogen burning?
The pressure produced from hydrogen fusion in a Main Sequence star’s core balances the compressive force of gravity
In what kind of star does core hydrogen burning occur?
Main Sequence
How does a Main Sequence star become a Red Giant?
Nuclear fusion stops as all of the hydrogen is used up. The now-helium core contracts and heats up under the weight of the star. The outer layers expand and cool into a Red Giant
What is shell hydrogen burning?
Heat from the contracting helium core raises the temperature of this material enough for hydrogen to fuse
What is core helium burning?
The helium core contracts until it is hot and dense enough for helium to fuse into carbon and oxygen. This releases a huge amount of energy, pushing the outer layers of the star outwards
What is shell helium burning?
When the helium runs out, the carbon-oxygen core contracts and heats a shell around it so that helium can fuse in this region
At which stage of star burning do low-mass stars stop fusing?
Shell hydrogen burning
Why do low-mass stars stop fusing after shell hydrogen burning?
The carbon-oxygen core isn’t hot enough for any further fusion and so it contracts under its own weight
How does a low-mass star become a White Dwarf?
Once the core shrinks to Earth-size, electrons exert enough pressure to prevent collapsing. The helium shell becomes more unstable, pulsates and ejects its outer layer as a planetary nebula, leaving behind the dense core
