The Beginning & Life of Stars Flashcards
(101 cards)
1
Q
how many
A
new stars form per year in our galaxy
2
Q
what determines what life path the star will take
A
mass
3
Q
Stars are divided into 3 basic groups:
A
Low mass , intermediate mass, high mass
4
Q
Low mass
A
0.08MSun≤M < 2MSun
5
Q
Intermediate mass
A
2MSun
6
Q
HIGH MASs
A
8MSun
7
Q
a diagram useful for following stellar evolution
A
H-R diagram
8
Q
The ‘empty’ space is filled with interstellar medium (ISM).
A
Made up of gases & dust: 70% H, 28% He & 2% heavier elements
Half of the heavy elements are interstellar dust
9
Q
Interstellar medium differs in temperature
& density
at different places.
A
(Hot & Low density) vs (Cold & High density)
Most have in-between temperature & density
10
Q
stars are born in the coldest (10…30K)& highest density (~300 molec./cm3) types of interstellar clouds.
A
Consequently, molecules (mainly H2 ) can form
11
Q
First generation of stars at the beginning of the Universe (after the Big Bang) were born in clouds that never cooled below 100 K (no C, hence no CO to radiate thermal energy!)
A
only stars with masses ≥30MSun could have been born
12
Q
interstellar dust
A
The interstellar clouds where stars are born are usually called molecular clouds.
Non-uniform! → high-density regions can be present (hundreds of times denser than the average)
Molecular hydrogen (H2) = the most abundant element, but cannot be observed directly: too cold to produce emission lines
Instead, other molecules are monitored: CO is most abundant among other elements, and produces radio emission lines
More than 120 other molecules have also been identified in
molecular clouds by their radio emission signature, e.g. H2O,
ammonia, ethyl alcohol, etc.
13
Q
Ionization nebulae
A
UV light from short-lived high-mass O & B stars excites & ionizes the gas around them
The violet-blue light of the massive stars is scattered & absorbed by nearby
dust clouds.
Gas re-emits with strong emission at the red H α line
-> These nebulae tend to appear reddish → indicate active star formation.
14
Q
Reflection nebulae
A
Dusty gas clouds reflect & scatter the light on their dust grains.
Why do reflection nebulae look bluer than the nearby stars? For the
same reason our sky is blue, and sunsets are red -> Violet-blue light is preferentially scattered by gas molecules and small dust particles.
The brightness of the reflection nebula is determined by the size and density of the reflecting grains, and by the color and brightness of the
neighboring star(s).
15
Q
Stars form when gravity causes a molecular cloud to contract aaaaand
A
and the contraction continues until the central object becomes hot enough to sustain nuclear fusion in its core.
Competition between gravity & thermal pressure determines whether a star can form
16
Q
Gravity overcomes thermal pressure only in clouds of
A
high-density
17
Q
Observations suggest that gravity can form stars more easily if some other force triggers the cloud compression what is this
A
Collision between 2 molecular clouds
Collision of debris/shockwave from exploding star with molecular cloud
18
Q
The minimum mass that a clump of
gas must have to collapse under its
gravity is called the
A
Jeans mass
MJeans
19
Q
mjeans formula
A
mjeans is proportional to T^2/sqrt(p)
20
Q
Once gravity overcomes thermal pressure
A
gravitational
contraction shrinks the molecular cloud.
21
Q
Gravitational potential energy converted into thermal energy — > continue this process in start formation
A
Thermal energy is quickly lost through photon emissions (IR & radio waves) by colliding molecules.
Cloud’s temperature↑ if it cannot get rid of that thermal energy as quickly as it is being generated.
Pressure will also ↑-> the process can be brought to a halt!
22
Q
Molecular clouds are turbulent & lumpy, err care to explain
A
Small, dense clumps can shrink on their own during contraction
Gravity strength ↑ as the cloud shrinks in size
23
Q
Accelerating nature of this process splits a large molecular cloud into many fragments, then?
A
Each becomes a star system
24
Q
Large molecular clouds do not normally form a single extremely massive star but
A
many individual stars
25
Not fully understood how a small cloud (a few MSun) forms what
an isolated star → Thus, most stars are born in clusters.
26
(gravity > Thermal pressure) requires
minimum of ~100MSun .
27
Star-forming clouds often hold much more mass
(~1,000MSun)
28
Star-forming clouds often hold much more mass
| (~1,000MSun) which may not be used all to form stars probably due to:
Turbulent gas motion: Fast moving gas clumps
Dissipation -> The solar wind from newborn star blows material away
Magnetic fields threading the clouds
29
Protostellar accretion disk
Gas cloud has some initial, small overall rotation.
Rotates faster as it contracts (angular momentum
conservation)
Inner part orbits faster than outer-> friction & heat
generated
Collisions between gas particles in cloud gradually
reduce random motions and up+down motions->Friction decays orbits of individual gas particles, which thus slowly fall onto the protostar
Protostellar accretion disk
Collisions flatten the cloud into a disk->The result is a rotating protostar with a rotating accretion disk of gas & dust.
Sometimes the disk coalesces into planetary systems (we do not
know exactly how or how often this happens!)
Accretion = the process in which material falls onto another body.
The accretion disk transfers mass & angular momentum to the protostar->Protostar mass gradually↑
30
Accretion =
the process in which material falls onto another body.
| The accretion disk transfers mass & angular momentum to the protostar->Protostar mass gradually↑
31
Birth of a protostar
Photons cannot escape as protostar density↑
More likely to run into a molecule & get absorbed
Convert radiated energy back into thermal energy
InternalT &p↑
When core is dense enough to trap all radiation,
T &p rise dramatically.
Pressure pushes back against gravity-> contraction slows down
Dense core becomes now a protostar
Surface temperature of protostar remains surprisingly constant at ~3,000 K while its core temp. slowly rises.
Convection carries thermal energy to surface in early
stages and keeps protostar’s surface temperature constant
Otherwise it wouldn’t contract!
Outer gas layers have little pressure to support them when dense core forms -> they start to “rain” down onto protostar -> it must be constantly fed with material to keep growing and contracting further
32
Protostellar jets
Internal convection + Rapid rotation of protostar-> strong magnetic field
Magnetic fields threading
the clouds
Restrict
linear motion of charged
particles-> ↑ friction on
other (neutral) particles
moving ⊥ to the field lines
This can slow or halt the gravitational collapse of a molecular cloud
Magnetic fields also generate a strong protostellar wind, carrying additional angular momentum to interstellar space.
ALL slow down the rotation of the protostar
33
Protostellar jets (continued)
Many protostars also fire streams of gas into space.
Magnetic field lines (threading the protostellar disk) twisted into a ropelike configuration->channel jets of charged particles along
the rotation axis->
2 protostellar jets shoot in opposite directions of rotation axis
Winds & jets clear gas cocoon around protostar
Herbig-Haro objects
Protostar core temp. is only ~ 1m K when
it beings to blow away surrounding gas.
Half of energy is radiated away; half
remains inside
interior heats up &
surface temperature↑.
Radiative diffusion takes over as primary
energy transport process
Core heating up still comes only from gravitational contraction, NOT
fusion->To ignite fusion it must continue to add mass & contract!
34
How does nuclear fusion begin in a newborn star?
Nuclear fusion ignites when significant mass
accretes & core temperature >10mK
--->Gravitational contraction stops when core energy
generation equals energy radiated from surface
(i.e. hydrostatic equilibrium is finally achieved)->
(then, and only then)
…
A new main sequence star is born!
35
Length of time from protostar formation to birth of
| main-sequence star depends on
the star’s mass.
36
O or B stars formation time
< 1m years
37
our sun (G stars) formation time
~30-50m years
38
M stars:
> 100m years
39
Massive stars may live & die long before the
smallest stars even start to fuse H!
40
Some protostars end up close together & orbit around each other
Binary star systems:
Pairs with larger angular momentum -> large orbits
Close Binary Systems have orbital separations
< 0.1 AU & orbital periods of only a few days
41
Life track of a 1Msun star Assembly of protostar
a protostar assembles from a collapsing cloud fragment. It is concealed beneath a shroud of dusty gas
42
Life track of a 1Msun star Convection Contraction
the protostar shrinks and heats as gravitational potential energy is converted into thermal energy
43
Life track of a 1Msun star Radiative Contraction
surface tempoerate rises when radiation becomes the dominant mode of energy flow within the protostar
44
Life track of a 1Msun star Self Sustaining Fusion
the fusion rate increases until it balances the energy radiated from the star's surface
45
Life track of a 1M star
```
Stage 1:
Assembly of protostar
Stage 2:
Convective contraction
Stage 3:
Radiative contraction
Stage 4:
Self-sustaining fusion
```
```
SAME stages, but
progress through them
at different rates for
protostars of different
masses
```
46
going zouk in 2018
Degeneracy pressure
2 particles cannot occupy the same space
& momentum .
Particles play a game of musical chairs
Particles become very energetic in very dense materials
Ground states are completely filled-> the other
e‒ are forced into higher
and higher energy states-> moving at progressively faster speeds-> approach light speed !
Two types: Electron & Neutron Degeneracy Pressures->
ANY degeneracy pressure built up within a star is independent temperature, but increases as volume decreases.
47
Red dwarfs:
Mass M between 0.075…0.5MSun with 0.01%...0.1% of Lsun
VERY long lived: ~0.1…10 tyears!
Dimmest red dwarf star: 8.3% of Sun’s mass.
48
The smallest mass of a newborn star:
0.08MSun = 80MJupiter
49
Brown dwarf:
Protostar with M<0.08 MSun → has closely
packed electrons (e– ) ->
Insufficient mass-> e– degeneracy pressure (independent of temperature!) stops gravitational contraction-> Core NOT hot enough
to ignite H fusion!
If it has a mass M >13MJupiter it fuses protons with deuterium (D)→for Tcore> 0.5mK
If it has enough mass, M>63
MJupiter (>0.06MSun), it can also ‘burn’ Li!
Main cause for a brown dwarf’s long term “failure” as a star: its insufficient mass to continue core compression & ignite fusion
50
Is a brown dwarf a planet?
Brown dwarf = “Failed star” that slowly radiates its internal thermal energy away-> radiates in red & IR
until cools to planet-like temperatures (<1000 K)
51
Key differences between brown dwarfs and other
| various astronomical objects:
Sun-like star: Partial convection & nuclear fusion (H ‘burning’) (Note: red dwarfs have full convection)
Brown dwarfs: Full convection & core fusion (‘burning’) of
D (and Li, if massive enough)
Jupiter-like planet: Partial convection & NO type of fusion
Brown dwarfs form in the same way as stars, by condensation in an interstellar gas cloud. Planets, by contrast, accrete from material in a circumstellar disk.
52
Key features of brown dwarfs:
Always with mass M<80MJupiter= 0.08MSun
Core is mainly H
Surface temperatures typically below 3,000 K
The more massive it is, the higher its (surface) temp.
Those with M >13MJupiter , fuse protons with D (internal temp. T> 0.5mK)
Those with 65MJupiter
53
A brown dwarf may be defined in one of two main ways: by its mass or its origin
by its mass or its origin
54
It is hypothesized that there are stars that start out as ordinary Hfusing red dwarfs and then
get whittled away to brown dwarf size.
55
A brown dwarf may be defined in one of two main ways: by its mass or its origin
Brown dwarfs form in the same way as stars, accretion at the centre of a circumstellar accretion disk. Planets, by contrast, accrete from material inside a circumstellar disk.
The more massive a brown dwarf, the higher its surface temperature
56
Brown dwarfs do not glow, even dully, for very long, ELABORATE
even those at
the high end of the mass range ( >60MJupiter) use up their meagre supply of fuel in ~10m years-> they no longer undergo nuclear
fusion-> they gradually cool down and fade from dim dark red to black.
Surprisingly, some brown dwarfs also emit X-rays.
57
Maximum mass of a star is, eh and I want some elaboration ah
~150 MSun
Not well-defined (can be ~100MSun but always <200Msun) due to radiation pressure≡ photons exert tiny pressure when striking matter
Very massive stars are very BRIGHT: a few million times the solar luminosity
radiation pressure > thermal pressure and counterbalances gravity
58
Largest masses of newborn stars - Eddington limit
Eddington limit (or Eddington luminosity) = the
maximum luminosity for which a body (such as a
star) achieves hydrostatic equilibrium, i.e. a
balance between the force of radiation acting
outward and the gravitational force acting inward.
M =120MSun is the theoretical Eddington limit, when gravity is barely strong enough to hold in the star ‘s radiation pressure and gas.
Energy generated furiously in stars with M >150
MSun = beyond Eddington limit-->gravity cannot resist radiation pressure->
Such stars get rid of extra mass by blowing away their outer layers → it’s unclear how they manage to form in the first place!-> probably by merger of several protostars
59
Very large stars can be observed, although they are much rarer than small stars (Sun-like or red dwarfs)
explain the heavyweights MRLTAge
Very large stars can be observed, although they are much rarer than small stars (Sun-like or red dwarfs)
M = tens of Msun , up to ~max.150MSun
R = can be up to hundreds of RSun
L = on the order of mLsun → can radiate as much energy in tens of
seconds or minutes as the Sun does in a months or years!
T = can be well above 10,000 K, even as high as >30,000K
Age = such stars cannot be old, their age is typically on the order of m years or shorter
60
Heavyweight stars dude, due to their immense radiation what happens
they typically eject huge amounts of
material as stellar winds → unclear how they could form in the first place from collapse of a molecular cloud, most probably by merger of a few protostars
No planets would exist around these stars, since planets take longer to form than such a star takes to live and die (and it consumes most/all of
the cloud!)
61
how will the heavyweights end their life
It will end its life in a brilliant supernova or (IF Mstar>~40MSun) hypernova in a short time of ~ m years or even much sooner
play a crucial role in:
Recycling cosmic matter, and
Creating heavy chemical elements
62
hypernova
a stellar explosion with an energy of over 100 supernovae
63
stars remain in a balanced state until their
H are used up.
Thermal pressure balances gravity
Fusion energy balances radiative energy flow from surface
64
Once fusion stops, the star’s fate depends on its
birth mass
65
Intermediate-mass stars have lives similar to
highmass stars but they end like low-mass stars
66
talk about high mass convection
No convection zone near surface. Convection in upper core moves energy out due to the very high energy production in the core.
67
talk about 0.5~1.5Msun mass convecion
deeper convection in cooler outer layer, radiation diffusion deep in the star
68
talk about very low mass convecion
convection zone extends to the core
69
Low-mass stars are similar to
to our Sun.
Generate energy & shine steadily like the Sun
Interior structures are also similar, but
Cooler interiors ->Deeper convection zone
Minor differences in the way energy travels to the surface.
Energy escape through radiative diffusion &/or convection
Convection zone depth depends on internal temperature, and hence on its mass
70
Life stages of very low-mass stars
Convection determines the activity on a star’s surface.
Very low-mass stars of spectral type
M are very active:
Deep convection
Fast rotation -> tangle, twist & knot their mgn. lines -> Flares are produced when these field lines snap and reconfigure
```
Very long lives (trillions of years!!) of low-mass stars are
generally uneventful(in terms of star structure/core evolution):
```
71
Red dwarfs do NOT become red giants in their post-main sequence phases.
Fusion consumes H -> particle number
↓ & core shrinks->fusion rate↑->luminosity↑ gradually (like our Sun)
However, they remain small and grow hotter to become blue
dwarfs (not observed yet, will happen after 6t years!)
Eventually, after they run out of nuclear fuel they slowly fade away as He white dwarfs.
Collapse countered by e- degeneracy pressure
72
Life stages of Sun-like stars: Subgiant stage
```
Larger main sequence
stars ‘quickly’ run into
troubles: Fusion stops
when core H is depleted.
Once again out of balance----> no longer can resist
crush of gravity
The star will undergo
dramatic changes!
```
73
subgiant stage crush of gravity shrinks the core rapidly.
Plenty of fresh H surrounds the He core
Gravity shrinks both inert He core & surrounding H shell
H shell becomes hot enough to ignite fusion
H shell burning proceeds at a higher rate than core fusion -> increase (↑) in energy output ->build-up of thermal pressure
74
Life stages of Sun-like stars: Subgiant & Red giant stages
Star grows in size & luminosity to become a subgiant.
Outer layers expand outward to push surface outward until its luminosity
matches the elevated fusion rate
Yet nothing else can be done to inflate the inert core now at the heart of the star
Star size↑-> A ↑↑-> energy dissipation ↑↑-> Surf.
Temp.↓
Weaker gravity at surface-> large masses escape in stellar wind
The star is caught in a vicious circle → Red giant stage
Newly produced He keeps adding to the mass of the core->
Mcore↑-> The core shrinks further under its own weight->
Its T & density both↑->
Fusion rate in the hydrogen-burning shell ↑↑ !-> Even more He ’ash’ is added to the core, and so on and on...
75
Life stages of Sun-like stars: Subgiant & Red giant stages
The longer a star undergoes initial H-shell fusion, the
larger and more luminous it becomes → this is why there is a continuous line of stars right up to the most luminous red giants = stars on the verge of He flash
The increased radius of a red giant-> weaker gravitational pull-> huge amounts of stellar winds but at much slower speeds Life stages of Sun-like stars: Subgiant & Red giant stages
Even more He ’ash’ is added to the core, and so on and on...
Consequently, the core & the
shell continue to shrink in size &
heat up (while the star as a whole
continues to grow larger & more
luminous → becomes a red(super)giant)
, until
…He flash occurs: inert
core & shell collapse and heat up to 100mK.
He fusion begins (He → C)→“tripleα reaction”
Star creates most of the C that
made organic molecules (& life)
In very low-mass stars (<0.45 Msun) at the end of their life, He fusion may NOT start: Core collapse is countered by e– degeneracy pressure→He white dwarf
76
Life stages of Sun-like stars: He flash CONTINUED
He fusion is ignited; core held up by e – degeneracy pressure.
Core heats up rapidly without expansion:
Degeneracy pressure does NOT ↑ with temperature T→ cannot expand to cool down!→T ↑↑→soaring He →C fusion rate→He flash
:
Enormous energy released into core very quickly:
Huge amounts of He (~0.4MSun!) are fused into C within a few minutes
Degenerate core is intensely heated→"vaporizes", its nuclei escape it
Core reverts back into a (extremely dense) normal gas, and in seconds its
powerful thermal expansion dominates again & pushes back gravity
H-burning shell expands→ its p ↓→Tshell ↓→fusion rate of H ↓↓
Outer layers contract
Surface temp.↑
Star color turns back into yellow
77
Life stages of Sun-like stars: Helium burning
Total energy production drops even though the star has both He & H fusion→ Luminosity ↓→Outer layers contract from their peak size→ Surface temp.↑ →Star color turns back into yellow from red red.
Life stages of Sun-like stars: Helium burning
All low-mass stars fuse He into
C at the same rate→
Similar luminosity but outer layers
of different masses
the exact final luminosity depends on mass
expelled through stellar winds during
red giant phase
As C slowly accumulates at the core
of the star, fusion of 12C +4He→ 16O
proceeds, hence O % ↑ with the carbon concentration.
H fusion still takes place in a shell
around the He-burning core
78
A dying Sun-like star
Star goes out of balance again when core He is exhausted.
C core shrinkage & collapse stopped by degeneracy pressure
Can never become hot enough for C fusion (600
m K)
Collapse triggers a He-burning shell around the C core.
H shell burns atop He shell--->double-shell burning star
Shells contract together with
inert core-->L, R, T, fusion rates
& stellar winds ↑↑↑--> The star
(Sun) expands to an even
greater size & luminosity than
in its first red giant stage! = Red Supergiant
79
He burning inside a dying star never reaches equilibrium; (talk to me more about a dying sun like star)
instead it proceeds in a series of thermal pulses.
Fusion rate spikes upward every few thousand years
Surface enriched with C drenched up from core by strong convection during each thermal pulse--> known as carbon stars
Expanded dying star has a very weak grip on its outer layers-->matter ejected outward: cool, slow stellar winds, whose particles
cluster into dust grains when T drops below 1000…2000 K.
The pulsation creates shock waves that propagate through the stellar
atmosphere and enhance the dissipation of material from the star and the creation of the planetary nebula
80
Planetary nebula
Outer layers ejected through stellar winds/other process.
Huge gas shell expanding away from the inert C&O core
intense X-ray & UV radiation from
exposed core ionises expanding gas.
Gas shell glows brightly as a planetary
nebula --> slowly fades out as C core cools
Also cools by neutrino emission
Nebula will disappear within 1 m years, leaving behind a white dwarf.
The white dwarf eventually also
disappears from view as it becomes too
cold to emit any visible light (more
details: next lesson!)
81
smaller-than-Sun star
M = 0.4Msun
82
Life cycle of a small mass star summary
Convective motions mix the He-rich product throughout the entire interior. At the end of their main-sequence lifetime, these small stars are
uniformly He-rich
Main sequence: core burns H in to He.
Red subgiant & giant: inert He core with H-burning shell.
He burning star: core fuses He into C. H-burning shell still present!
Double-shell burning red (super)giant: inert C core, H & He-burning shells.
Planetary nebula: extremely heavy mass loss (dissipates outer layers).
White dwarf: inert bare C core that no longer generates energy in it =
final stage -----> slowly cools & fades out
83
Big Bang made what atom in what percentages
75% H, 25% He
84
we have seen how He, some Li, C & O are generated by nuclear fusion in
low mass stars
85
we have seen how He, some Li, C & O are generated by nuclear fusion in low mass stars
Other heavy elements are made in:
High-mass stars: only their cores can reach high enough temperatures
Supernova explosions→ extremely high temperatures occur for a very short period of time before and at the moment of explosion
BH-BH, NS-NS or BH-NS mergers can also create very heavy elements as tremendous temperatures (1011K) are obtained for a very short
(kilonova)
86
hydrogen fusion in Low-mass star (e.g. Sun)
Fusion via proton-proton (p-p) chain
| The p-p chain provides about 98.4% of the Sun's energy output
87
The early life stages of high-mass stars are similar to lowmass stars, but proceed much more rapidly.
Fuse increasingly heavier elements until all sources are used up
Implosion by gravity results in self-destruction ------>Supernova
88
high mass stars fusion how sia
Fusion via CNO cycle chain
The remaining 1.6% of the Sun’s energy production is generated by another cycle of nuclear reactions, called the CNO cycle or CNO bicycle: the carbon-nitrogen-oxygen cycle.
Nuclei of carbon, nitrogen, oxygen and fluorine are produced in this cycle, but these are only transient intermediates.
In stars with M>1.5MSun , the CNO cycle is the dominant means of hydrogen fusion (hydrogen ‘burning’)
89
cno cycle
In a high-mass star the strong gravity compresses the H core to a higher temp. than in a low-mass star
Much hotter core allows protons to slam into C, O & N
C, N, O act as catalysts in the nuclear fusion reaction.
Amount of C, O & N in core (< 2%) is sufficient for catalysis
Much faster fusion process with catalysts
luminosities of high-mass stars are much higher and their lives much shorter
Effectively still 4 1H nuclei →1 4He nucleus
```
The CNO cycle begins at 15
m K & becomes more
dominant at higher temperatures.
Enormous amounts of energy are generated in the core--->Significant radiation pressure drive strong, fast-moving stellar
winds at the photosphere
```
90
Life evolution of Giants & Supergiants
For intermediate-mass stars
(2…8MSun), e‒ degeneracy pressure halts the collapse of C core & prevents reaching high fusion temperatures to burn C or O.
Upper layers are eventually blown away
Becomes a white dwarf
a high-mass star responds much like a low-mass star but much faster as its core H runs out.
H-burning shell develops & the star’s outer layers expands
Core shrinks/collapses
& He fusion ignites gradually as T↑
No He flash for mass >2MSun as thermal pressure is stronger, preventing degeneracy pressure from being a factor
A high-mass star fuses He into C very rapidly.
----- > Inert C core after a few 100,000s years
C core collapses, forming a He-burning shell
Outer layers swell further
91
Advanced nuclear burning
| For high-mass stars (M>8MSun) gravitational contraction of C core continues until it reaches 600mK
Electron degeneracy pressure never has a chance to play a role
C starts to fuse into heavier elements
Gravitational equilibrium is restored temporarily
Once C is depleted (~100s years), core again collapses & heats until it can fuse a still-heavier element.
Simplest sequence of fusion stages occurs through
successive helium-capture reactions (He fuses with C & O to create heavier elements, then fuses with them as well generate even heavier elements, with which again it fuses, and so on)
92
Advanced nuclear burning (continued)
At even higher temp.s, heavy nuclei fuse to each other
Some heavy-element reactions release free neutrons, which may fuse
with heavy nuclei to make still rarer elements
The star is forging a variety of elements!
Each time the core depletes the elements it is fusing ------> It shrinks and heats up until it becomes hot enough for
other fusion reactions to start → The duration of each new step ↓ drastically!
New type of shell burning
ignites each time the core shrinks.
In the star’s final days, its central region looks like the inside of an onion: A layer-over-layer stratified structure of shells, each fusing differnt elements
Fe piles up in the core as a result of Si fusion
```
The star’s life track zigzags across the top of the
H-R diagram.
In most massive stars the
changes happen so quickly
that the outer layers don’t
have time to respond
```
93
The iron (Fe) problem
Energy (released in nuclear
processes as mass variation
Δm per nuclear particle) ↓ from H to Fe.
Trend reversed beyond Fe.
Those elements can only generate
nuclear energy through fission
Fe has the lowest released energy per nuclear particle!
Cannot release energy by either fusion or fission
No further energy is generated once core turns to Fe
Fe piles up until even e– degeneracy pressure can no longer support the core
ultimate nuclear waste catastrophe!
= SUPERNOVA !
94
Supernova
Degeneracy pressure briefly supports the inert Fe core.
Once gravity pushes the electrons (e–) past the quantum limit, e– combine with protons to form neutrons.
Huge amounts of neutrinos released
e– degeneracy pressure suddenly vanishes!
Fe core of ~1MSun & a size larger than Earth
collapses into a neutron ball 10’s km across.
The gravitational collapse of the core releases an
enormous amount of energy:
In less than a second, core temperature rises to over 100b K (!!!) as the iron atoms are crushed together
Drives outer layers off in a titanic explosion (type II supernova)
Elements heavier than Fe are produced by rare fusion reactions shortly before and during a supernova explosion
95
if a core collapse is stopped by neutron degeneracy pressure
neutron star
96
if a core collapse is stopped by neutron degeneracy pressure -- > neutron star
if the remaining mass is still large then what
Gravity > neutron degeneracy pressure --- > core continues to collapse ----> black hole (BH)
97
supernova in our galaxy
```
The only extragalactic
supernova near enough to be
visible burst into view in 1987.
A supernova pops off every
100 years per galaxy.
400 years since the last visible
supernova in our galaxy→one is long overdue!
A potential candidate is Eta
Carinae.
Highly unstable!
```
98
Most stars are NOT
single.
| >50% occur in binary or multiple systems
99
Most binary stars live & die as if
they were isolated
exceptions occur in close binary systems.
Example is the ‘demon star’ Algol (Perseus constellation)
Close, eclipsing binary star
A main sequence star (3.7MSun) & a subgiant (0.8MSun)
The gravity of each star attracts the near side more strongly than
the far side
Stars are stretched into elongated shapes, and
Stars become and remain tidally locked (always show the same face to one another)
Both stars born at the same time
Less massive star is in a more advanced stage of life!
This apparent contradiction to stellar evolution model is
known as the Algol Paradox.
100
Algol-type binaries contain a
```
faint orange-red F/K giant or
subgiant star, called the
secondary (younger) , and a
luminous blue B/A main
sequence companion, called
the primary (older)
```
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Explanation of the Algol Paradox:
Mass exchange occurs when
the giant grows so large that its tidally distorted outer layers succumb to gravitational attraction of the smaller companion
The 0.8MSun Algol subgiant was more massive, expanded into a red giant and transferred much of its matter onto the companion
This will reverse a few million years from now, when the currently massgaining 3.7MSun star will expand into a red giant itself and transfer mass
back to its companion
