Lecture 15 - Star Death

Question 1

what type of constellation is orion? why?

what is found in orion?

Answer 1

orion = winter constellation
- in the winter, it is UP at night so we easily see it in the dark
- in the summer, it is up during the day so we cannot see it because of the sun

has the orion nebula –> region of active star formation

Question 2

what is the significance of CO being in the gas cloud for star formation?

Answer 2

CO molecules are excited by collisions with H2 which converts KE –> PE, so if the molecule de-excites radiatively via line emission and the photon escapes, the cloud loses energy and cools down

Question 3

what are the 3 fates of stars? what does this depend on?

Answer 3

1. very low-mass stars ( < 2M_sun) –> burns out and becomes white dwarf
2. intermediate mass stars (1-8M_sun) –> puffs up to red giants, planetary nebulae, white dwarfs
3. high mass stars ( >8M_sun) –> supernovae

critically depends on its mass!

Question 4

how long does a star remain on the main sequence? what happens to it once it leaves?

Answer 4

a star remains on the main sequence as long as it can fuse H –> He in its core

becomes larger, redder, more luminous once it leaves the main sequence

Question 5

describe what happens when core H is used up

Answer 5

1. nuclear fusion stops, leaving an ash He core
2. core will collapse because there is no longer any fusion pressure to fight against gravity
3. as core collapses, H outside the core begins fusing He in a shell
4. outer layers puff out
5. luminosity increases –> broken thermostat

Question 6

describe H core fusion vs H shell fusion

Answer 6

H shell fusion is much more efficient and releases a lot of energy

Question 7

how does the core change vs the outer layers?

Answer 7

core shrinks, outer layers grow

Question 8

why is it said that red giants have a broken thermostat?

Answer 8

fusion rate increases in the shell but does not affect the contracting core

this causes an increased energy output which increases the thermal pressure inside the star, pushing the surface outward until luminosity increases to match the fusion rate

can’t regulate temp properly!

Question 9

what amplifies the gravitational pull when there is H shell fusion? and what is the result

Answer 9

He increases the mass of the core (heavier element) so the gravitational pull is stronger and it shrinks further

Then the H-burning shell shrinks and grows hotter and denser, increasing the amount of He in the core, further shrinking it

Question 10

does higher mass or lower mass stars become red giants more quickly?

Answer 10

higher mass stars become red giants more quickly

Question 11

once the H shell burns, what happens?

Answer 11

1. the core and shell continue to contract and heat up and the star grows larger and more luminous
2. it gets hot enough for He to fuse and make C

Question 12

why does He fusion require a high temp?

Answer 12

He has a larger charge causing more repulsion, so He requires higher temp to let the atoms slam into each other at higher speeds

Question 13

what is the reaction for He fusion?

Answer 13

3 He –> C + energy

TRIPLE ALPHA REACTION

Question 14

what happens in a low-mass star when the core temp rises enough for He fusion to begin?

Answer 14

He fusion begins very quickly, as soon as conditions are correct

Question 15

what is the main pressure in He core? what does this lead to?

Answer 15

degeneracy pressure is the main pressure in He core, fighting against gravity

doesn’t increase with temperature so the onset of He fusion rapidly heats the core and it contracts under gravity –> high fusion rate leading to He flash

Question 16

what is the result of He flash? (4 subsequent steps)

Answer 16

1. HUGE energy release in the core, increases temp so much that thermal pressure becomes dominant over degeneracy pressure
2. thermal pressure fights against gravity and the core expands
3. the H-fusing shell is pushed outward, lowering its temp and fusion rate
4. energy production decreases, luminosity decreases and outer layers contract

Question 17

describe the thermostat of Helium-Fusion stars

Answer 17

core thermostat is temporarily fixed so they neither shrink nor grow

fusion occurs in the core, so it regains the balance it had as a main sequence star except now it is He fusion in the core, H fusion in the shell

Question 18

where do helium-fusion stars move on HR?

Answer 18

moves down and left

Question 19

how do helium-fusion stars vary in luminosity and temp?

Answer 19

the rate of He fusion is the same in all low-mass stars, so they all have about the same luminosity

but if outer layers expelled more mass thru their stellar winds, they have smaller radii and higher surface temp so they move further left –> form HORIZONTAL BELT at 1 luminosity with varying temp

Question 20

what do helium-fusion stars form with carbon?

Answer 20

inert carbon core

Question 21

what occurs after helium-fusion star?

Answer 21

transitions to DOUBLE-SHELL BURNING

Question 22

what is double-shell burning?

how does this affect the properties of the star?

Answer 22

1. once core He fusion stops, He fuses into a C shell around the C core and H fuses into a He shell around the He layer
2. both shells and the inert core will contract, driving up the temp and fusion rates so that the star is larger and more luminous

Question 23

do double-shell burning stars reach equilibrium? what happens?

Answer 23

NEVER!!!

fusion rate periodically spikes upward with thermal pulses
- with each spike, convection pulls C from the core and transports it to the surface

Question 24

does C fusion occur after double-shell burning? why?

Answer 24

NO!!! C core cannot undergo fusion because the temp is not high enough

core never reaches high enough temp bc degeneracy pressure halts the collapse of the core before it gets hot enough

Question 25

what happens at the end of double-shell burning?

Answer 25

H and He are ejected into space as a planetary nebula

Question 26

what is a planetary nebula?

Answer 26

expanding, glowing shell of ionized gas

Question 27

what is the core left behind from a planetary nebula?

Answer 27

WHITE DWARF

Question 28

what do white dwarfs contain? describe the density of white dwarfs

Answer 28

contains mostly electron-degenerate matter –> low energy state electrons

very dense, essentially a solar mass crammed into the volume of earth

Question 29

what will happen to the Sun’s temp and size when it becomes a red giant?

Answer 29

temp: 1000x its current level (too hot for Earth)
radius: similar to radius of Earth’s orbit

Question 30

what is similar between the life as a low-mass star vs high-mass star (3) ? what different about these aspects?

Answer 30

1. in main sequence –> H core fusion
2. supergiant –> H shell fusion
3. supergiant –> He core fusion

these stages occur more quickly for low-mass stars

Question 31

why are high-mass stars necessary? why are they better than low-mass stars for this purpose?

Answer 31

to make elements!

high-mass stars get hot enough to make heavier elements

Question 32

what is the CNO cycle? how does it affect stars?

Answer 32

turns C into N and O

CNO act as catalysts for H fusion –> higher H fusion rate than proton-proton chain alone –> higher luminosity –> shorter lifetime

*higher H fusion rate means there are more photons bouncing around to cause a greater radiation pressure

Question 33

what is Helium capture?

Answer 33

high core temp allows He to turn C into O, Ne, Mg, etc.

Question 34

what do advanced reactions in high-mass stars cause?

Answer 34

at very high temp, make Si, S, Ca, Fe

Question 35

describe multiple shell fusion and its endpoint

Answer 35

each time the core depletes the elements it is fusing, it shrinks and heats up to allow other fusion reactions to occur –> like onion layers with lighter elements on the outside and heavier elements on the inside

only goes up to Fe

Question 36

why does multiple shell fusion only reach Fe?

Answer 36

fusion is only energetically favourable up to Fe because Fe has the lowest mass per nuclear particle –> would need a massive amount of irreversible energy to make things heavier than Fe

Question 37

how would the universe be different if H, rather than Fe, had the lowest mass per nuclear particle?

Answer 37

If H was the most stable, you could only fuse H to He but then it would go right back to H

no life on earth, few elements

Question 38

describe when and how a supernova forms

Answer 38

iron builds up in the core until degeneracy pressure can no longer resist gravity because there is no more fusion aka no more energy production

gravity takes over and the core suddenly collapses, making a supernova

Question 39

before a supernova, what is the main internal pressure?

Answer 39

degeneracy pressure prevents the core from being contracted but when iron builds up, gravity wins and it collapses

Question 40

what happens to degeneracy pressure after a supernova?

Answer 40

degeneracy pressure goes away bc all electrons combine with protons to make neutrons and neutrinos –> i.e. protons become neutral

Question 41

what is the result of supernovae?

what process occurs?

Answer 41

releases a large amount of gravitational energy which pushes off the outer layers, leaving behind a neutron star

the energy and neutrons released allow for elements heavier than iron to form via EXPLOSIVE NUCLEOSYNTHESIS

Question 42

what type of imaging can we use to look at supernovae?

Answer 42

X-ray detects the energetics of explosions, like energetic gases

Question 43

how do we use exploding stars to find that the universe is expanding at an increasing rate?

Answer 43

1. each star that explodes the same way has the same luminosity
2. can use luminosity to find apparent distance, then distance
3. use distance to find redshift to determine how much growth occurred btwn time of explosion and now

Question 44

with a mass of 1 solar mass, what is the eventual fate of our own sun?

Answer 44

will run out of He to fuse, ending up as a white dwarf and planetary nebula

Question 45

what causes failed stars/brown dwarfs? what can failed stars do? what happens to the properties of them?

is jupiter a brown dwarf? why?

Answer 45

caused by degeneracy pressure stopping the contraction of objects <0.08M_sun before core temp is hot enough for H fusion

they can fuse deuterium and if massive, lithium

they emit infrared light due to heat remaining from contraction, then luminosity will decline as it loses thermal energy

jupiter is NOT a brown dwarf –> mass is too small for fusion