High Mass Stars

Question 1

How may high mass stars form?

Answer 1

•same way as low mass (gravitational collapse in molecular clouds)
•via competitive accretion

Question 2

What is competitive accretion?

Answer 2

•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

Question 3

How does hydrogen fusion differ in high and low mass stars?

Answer 3

•low mass- proton-proton chain
•high mass- CNO cycle

Question 4

How long does it take high mass stars to reach main sequence?

Answer 4

Less than 150000 years

Question 5

What can act as a catalyst for hydrogen fusion in high mass stars?

Answer 5

Carbon despite making up less than 2% of the material in stars

Question 6

What is an intermediate mass star?

Answer 6

Stars with masses between 2 and 8 masses

Question 7

How do intermediate mass stars initially behave like high mass?

Answer 7

Nuclear burning occurs through CNO cycle rather than through the p-p chain

Question 8

What happens to stars <4m?

Answer 8

Eventually blow away upper atmosphere and end lives as carbon white dwarfs

Question 9

What happens to stars between 4-8m?

Answer 9

Can burn carbon but nothing beyond so end as O/Ne/Mg dwarfs

Question 10

What happens when hydrogen burning in shell around helium core occurs in high mass?

Answer 10

Outer layers expand producing a supergiant star

Question 11

What happens when core temp reaches 100million K?

Answer 11

Helium burning starts in core

Question 12

Explain helium burning in high mass?

Answer 12

•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

Question 13

When does carbon burning start?

Answer 13

When core temp reaches 600 million K (in stars >4m)

Question 14

Explain carbon burning

Answer 14

•carbon fused into magnesium
•only lasts few hundred years
•in a shell around inert core
•core shrinks till hot enough to fuse other elements

Question 15

Does advanced nuclear burning provide more or less energy?

Answer 15

Less

Question 16

What happens during final few days of advanced burning?

Answer 16

Iron begins to pile in silicon burning core

Question 17

What happens when core fusion ceases?

Answer 17

Shell burning intensifies further inflating outer layers

Question 18

What happens when core flares up?

Answer 18

Star contracts slightly but luminosity constant
•temp changes

Question 19

Describe Betelgeuse

Answer 19

•red supergiant
•6 million years old
•between 12-17 solar masses
•650 solar radii
•120000 times suns luminosity

Question 20

Why is iron unique?

Answer 20

Not possible to generate any kind of nuclear energy so core iron burning does not occur

Question 21

How do elements heavier and lighter than iron release energy?

Answer 21

•lighter-fusion
•heavier-fission

Question 22

What happens when gravity pushes electrons past quantum mechanical limit?

Answer 22

They combine with protons to form neutrons (Neutronization) and releasing neutrinos

Question 23

What does the 6000km iron core collapse into?

Answer 23

A 10km ball of neutrons

Question 24

What halts collapse of core?

Answer 24

Neutron degeneracy pressure

Question 25

What happens to neutron core?

Answer 25

Remains as a neutron star that may rotate very fast to preserve angular momentum

Question 26

What happens if remaining mass of core is large enough?

Answer 26

Gravity may overcome neutron degeneracy pressure to produce a black hole

Question 27

What is a supernova?

Answer 27

Outer layers of a massive star being expelled at high velocities

Question 28

Why can supernovas create elements heavier than iron?

Answer 28

They have so much energy

Question 29

Explain how supernova nucléosynthesis occurs?

Answer 29

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

Question 30

What are supernova explosions responsible for?

Answer 30

Creating nearly all of the heavy elements seen in nature

Question 31

Where is most carbon produced?

Answer 31

Low mass stars

Question 32

Why does fusion in stars produce more even numbered elements?

Answer 32

Mainly He capture reactions

Question 33

Why are lithium, beryllium, and boron so rare?

Answer 33

•cannot be synthesised in stellar cores
•lithium and beryllium formed by nucleosynthesis in early universe