Lecture 15 - Star Death Flashcards
what type of constellation is orion? why?
what is found in orion?
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
what is the significance of CO being in the gas cloud for star formation?
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
what are the 3 fates of stars? what does this depend on?
- very low-mass stars ( < 2M_sun) –> burns out and becomes white dwarf
- intermediate mass stars (1-8M_sun) –> puffs up to red giants, planetary nebulae, white dwarfs
- high mass stars ( >8M_sun) –> supernovae
critically depends on its mass!
how long does a star remain on the main sequence? what happens to it once it leaves?
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
describe what happens when core H is used up
- nuclear fusion stops, leaving an ash He core
- core will collapse because there is no longer any fusion pressure to fight against gravity
- as core collapses, H outside the core begins fusing He in a shell
- outer layers puff out
- luminosity increases –> broken thermostat
describe H core fusion vs H shell fusion
H shell fusion is much more efficient and releases a lot of energy
how does the core change vs the outer layers?
core shrinks, outer layers grow
why is it said that red giants have a broken thermostat?
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!
what amplifies the gravitational pull when there is H shell fusion? and what is the result
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
does higher mass or lower mass stars become red giants more quickly?
higher mass stars become red giants more quickly
once the H shell burns, what happens?
- the core and shell continue to contract and heat up and the star grows larger and more luminous
- it gets hot enough for He to fuse and make C
why does He fusion require a high temp?
He has a larger charge causing more repulsion, so He requires higher temp to let the atoms slam into each other at higher speeds
what is the reaction for He fusion?
3 He –> C + energy
TRIPLE ALPHA REACTION
what happens in a low-mass star when the core temp rises enough for He fusion to begin?
He fusion begins very quickly, as soon as conditions are correct
what is the main pressure in He core? what does this lead to?
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
what is the result of He flash? (4 subsequent steps)
- HUGE energy release in the core, increases temp so much that thermal pressure becomes dominant over degeneracy pressure
- thermal pressure fights against gravity and the core expands
- the H-fusing shell is pushed outward, lowering its temp and fusion rate
- energy production decreases, luminosity decreases and outer layers contract
describe the thermostat of Helium-Fusion stars
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
where do helium-fusion stars move on HR?
moves down and left
how do helium-fusion stars vary in luminosity and temp?
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
what do helium-fusion stars form with carbon?
inert carbon core
what occurs after helium-fusion star?
transitions to DOUBLE-SHELL BURNING
what is double-shell burning?
how does this affect the properties of the star?
- 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
- both shells and the inert core will contract, driving up the temp and fusion rates so that the star is larger and more luminous
do double-shell burning stars reach equilibrium? what happens?
NEVER!!!
fusion rate periodically spikes upward with thermal pulses
- with each spike, convection pulls C from the core and transports it to the surface
does C fusion occur after double-shell burning? why?
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
what happens at the end of double-shell burning?
H and He are ejected into space as a planetary nebula
what is a planetary nebula?
expanding, glowing shell of ionized gas
what is the core left behind from a planetary nebula?
WHITE DWARF
what do white dwarfs contain? describe the density of white dwarfs
contains mostly electron-degenerate matter –> low energy state electrons
very dense, essentially a solar mass crammed into the volume of earth
what will happen to the Sun’s temp and size when it becomes a red giant?
temp: 1000x its current level (too hot for Earth)
radius: similar to radius of Earth’s orbit
what is similar between the life as a low-mass star vs high-mass star (3) ? what different about these aspects?
- in main sequence –> H core fusion
- supergiant –> H shell fusion
- supergiant –> He core fusion
these stages occur more quickly for low-mass stars
why are high-mass stars necessary? why are they better than low-mass stars for this purpose?
to make elements!
high-mass stars get hot enough to make heavier elements
what is the CNO cycle? how does it affect stars?
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
what is Helium capture?
high core temp allows He to turn C into O, Ne, Mg, etc.
what do advanced reactions in high-mass stars cause?
at very high temp, make Si, S, Ca, Fe
describe multiple shell fusion and its endpoint
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
why does multiple shell fusion only reach Fe?
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
how would the universe be different if H, rather than Fe, had the lowest mass per nuclear particle?
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
describe when and how a supernova forms
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
before a supernova, what is the main internal pressure?
degeneracy pressure prevents the core from being contracted but when iron builds up, gravity wins and it collapses
what happens to degeneracy pressure after a supernova?
degeneracy pressure goes away bc all electrons combine with protons to make neutrons and neutrinos –> i.e. protons become neutral
what is the result of supernovae?
what process occurs?
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
what type of imaging can we use to look at supernovae?
X-ray detects the energetics of explosions, like energetic gases
how do we use exploding stars to find that the universe is expanding at an increasing rate?
- each star that explodes the same way has the same luminosity
- can use luminosity to find apparent distance, then distance
- use distance to find redshift to determine how much growth occurred btwn time of explosion and now
with a mass of 1 solar mass, what is the eventual fate of our own sun?
will run out of He to fuse, ending up as a white dwarf and planetary nebula
what causes failed stars/brown dwarfs? what can failed stars do? what happens to the properties of them?
is jupiter a brown dwarf? why?
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
