High Mass Stars Flashcards
How may high mass stars form?
•same way as low mass (gravitational collapse in molecular clouds)
•via competitive accretion
What is competitive accretion?
•all stars form as low mass
•stars continue accreting gas until it is all depleted
•some stars accrete more efficiently and grow to form high mass stars
How does hydrogen fusion differ in high and low mass stars?
•low mass- proton-proton chain
•high mass- CNO cycle
How long does it take high mass stars to reach main sequence?
Less than 150000 years
What can act as a catalyst for hydrogen fusion in high mass stars?
Carbon despite making up less than 2% of the material in stars
What is an intermediate mass star?
Stars with masses between 2 and 8 masses
How do intermediate mass stars initially behave like high mass?
Nuclear burning occurs through CNO cycle rather than through the p-p chain
What happens to stars <4m?
Eventually blow away upper atmosphere and end lives as carbon white dwarfs
What happens to stars between 4-8m?
Can burn carbon but nothing beyond so end as O/Ne/Mg dwarfs
What happens when hydrogen burning in shell around helium core occurs in high mass?
Outer layers expand producing a supergiant star
What happens when core temp reaches 100million K?
Helium burning starts in core
Explain helium burning in high mass?
•no helium flash as thermal pressure remains high
•only lasts a few hundred thousand years and produces inert carbon core
•helium shell burns between core and H shell
When does carbon burning start?
When core temp reaches 600 million K (in stars >4m)
Explain carbon burning
•carbon fused into magnesium
•only lasts few hundred years
•in a shell around inert core
•core shrinks till hot enough to fuse other elements
Does advanced nuclear burning provide more or less energy?
Less
What happens during final few days of advanced burning?
Iron begins to pile in silicon burning core
What happens when core fusion ceases?
Shell burning intensifies further inflating outer layers
What happens when core flares up?
Star contracts slightly but luminosity constant
•temp changes
Describe Betelgeuse
•red supergiant
•6 million years old
•between 12-17 solar masses
•650 solar radii
•120000 times suns luminosity
Why is iron unique?
Not possible to generate any kind of nuclear energy so core iron burning does not occur
How do elements heavier and lighter than iron release energy?
•lighter-fusion
•heavier-fission
What happens when gravity pushes electrons past quantum mechanical limit?
They combine with protons to form neutrons (Neutronization) and releasing neutrinos
What does the 6000km iron core collapse into?
A 10km ball of neutrons
What halts collapse of core?
Neutron degeneracy pressure
What happens to neutron core?
Remains as a neutron star that may rotate very fast to preserve angular momentum
What happens if remaining mass of core is large enough?
Gravity may overcome neutron degeneracy pressure to produce a black hole
What is a supernova?
Outer layers of a massive star being expelled at high velocities
Why can supernovas create elements heavier than iron?
They have so much energy
Explain how supernova nucléosynthesis occurs?
Through r-process
•nuclei bombarded by neutrons
•form unstable neutron rich nuclei
•unstable nuclei decay via beta decay to form stable nuclei with higher atomic number but same atomic weight
What are supernova explosions responsible for?
Creating nearly all of the heavy elements seen in nature
Where is most carbon produced?
Low mass stars
Why does fusion in stars produce more even numbered elements?
Mainly He capture reactions
Why are lithium, beryllium, and boron so rare?
•cannot be synthesised in stellar cores
•lithium and beryllium formed by nucleosynthesis in early universe
