The Beginning & Life of Stars

Question 1

how many

Answer 1

new stars form per year in our galaxy

Question 2

what determines what life path the star will take

Answer 2

mass

Question 3

Stars are divided into 3 basic groups:

Answer 3

Low mass , intermediate mass, high mass

Question 4

Low mass

Answer 4

0.08MSun≤M < 2MSun

Question 5

Intermediate mass

Answer 5

2MSun

Question 6

HIGH MASs

Answer 6

8MSun

Question 7

a diagram useful for following stellar evolution

Answer 7

H-R diagram

Question 8

The ‘empty’ space is filled with interstellar medium (ISM).

Answer 8

 Made up of gases & dust: 70% H, 28% He & 2% heavier elements
 Half of the heavy elements are interstellar dust

Question 9

Interstellar medium differs in temperature
& density
at different places.

Answer 9

(Hot & Low density) vs (Cold & High density)

 Most have in-between temperature & density

Question 10

stars are born in the coldest (10…30K)& highest density (~300 molec./cm3) types of interstellar clouds.

Answer 10

 Consequently, molecules (mainly H2 ) can form

Question 11

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!)

Answer 11

only stars with masses ≥30MSun could have been born

Question 12

interstellar dust

Answer 12

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.

Question 13

Ionization nebulae

Answer 13

 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.

Question 14

Reflection nebulae

Answer 14

 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).

Question 15

Stars form when gravity causes a molecular cloud to contract aaaaand

Answer 15

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

Question 16

Gravity overcomes thermal pressure only in clouds of

Answer 16

high-density

Question 17

Observations suggest that gravity can form stars more easily if some other force triggers the cloud compression what is this

Answer 17

 Collision between 2 molecular clouds

 Collision of debris/shockwave from exploding star with molecular cloud

Question 18

The minimum mass that a clump of
gas must have to collapse under its
gravity is called the

Answer 18

Jeans mass

MJeans

Question 19

mjeans formula

Answer 19

mjeans is proportional to T^2/sqrt(p)

Question 20

Once gravity overcomes thermal pressure

Answer 20

gravitational

contraction shrinks the molecular cloud.

Question 21

Gravitational potential energy converted into thermal energy — > continue this process in start formation

Answer 21

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!

Question 22

Molecular clouds are turbulent & lumpy, err care to explain

Answer 22

 Small, dense clumps can shrink on their own during contraction
 Gravity strength ↑ as the cloud shrinks in size

Question 23

Accelerating nature of this process splits a large molecular cloud into many fragments, then?

Answer 23

Each becomes a star system

Question 24

Large molecular clouds do not normally form a single extremely massive star but

Answer 24

many individual stars

Question 25

Not fully understood how a small cloud (a few MSun) forms what

Answer 25

an isolated star → Thus, most stars are born in clusters.

Question 26

(gravity > Thermal pressure) requires

Answer 26

minimum of ~100MSun .

Question 27

Star-forming clouds often hold much more mass

Answer 27

(~1,000MSun)

Question 28

Star-forming clouds often hold much more mass

(~1,000MSun) which may not be used all to form stars probably due to:

Answer 28

Turbulent gas motion: Fast moving gas clumps
Dissipation -> The solar wind from newborn star blows material away
Magnetic fields threading the clouds

Question 29

Protostellar accretion disk

Answer 29

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↑

Question 30

Accretion =

Answer 30

the process in which material falls onto another body.

 The accretion disk transfers mass & angular momentum to the protostar->Protostar mass gradually↑

Question 31

Birth of a protostar

Answer 31

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

Question 32

Protostellar jets

Answer 32

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

Question 33

Protostellar jets (continued)

Answer 33

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!

Question 34

How does nuclear fusion begin in a newborn star?

Answer 34

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!

Question 35

Length of time from protostar formation to birth of

main-sequence star depends on

Answer 35

the star’s mass.

Question 36

O or B stars formation time

Answer 36

< 1m years

Question 37

our sun (G stars) formation time

Answer 37

~30-50m years

Question 38

M stars:

Answer 38

> 100m years

Question 39

Massive stars may live & die long before the

Answer 39

smallest stars even start to fuse H!

Question 40

Some protostars end up close together & orbit around each other

Answer 40

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

Question 41

Life track of a 1Msun star Assembly of protostar

Answer 41

a protostar assembles from a collapsing cloud fragment. It is concealed beneath a shroud of dusty gas

Question 42

Life track of a 1Msun star Convection Contraction

Answer 42

the protostar shrinks and heats as gravitational potential energy is converted into thermal energy

Question 43

Life track of a 1Msun star Radiative Contraction

Answer 43

surface tempoerate rises when radiation becomes the dominant mode of energy flow within the protostar

Question 44

Life track of a 1Msun star Self Sustaining Fusion

Answer 44

the fusion rate increases until it balances the energy radiated from the star’s surface

Question 45

Life track of a 1M star

Answer 45

 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

Question 46

going zouk in 2018

Answer 46

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.

Question 47

Red dwarfs:

Answer 47

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.

Question 48

The smallest mass of a newborn star:

Answer 48

0.08MSun = 80MJupiter

Question 49

Brown dwarf:

Answer 49

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

Question 50

Is a brown dwarf a planet?

Answer 50

Brown dwarf = “Failed star” that slowly radiates its internal thermal energy away->  radiates in red & IR
until cools to planet-like temperatures (<1000 K)

Question 51

Key differences between brown dwarfs and other

various astronomical objects:

Answer 51

 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.

Question 52

Key features of brown dwarfs:

Answer 52

 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

Question 53

A brown dwarf may be defined in one of two main ways: by its mass or its origin

Answer 53

by its mass or its origin

Question 54

It is hypothesized that there are stars that start out as ordinary Hfusing red dwarfs and then

Answer 54

get whittled away to brown dwarf size.

Question 55

A brown dwarf may be defined in one of two main ways: by its mass or its origin

Answer 55

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

Question 56

Brown dwarfs do not glow, even dully, for very long, ELABORATE

Answer 56

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.

Question 57

Maximum mass of a star is, eh and I want some elaboration ah

Answer 57

~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

Question 58

Largest masses of newborn stars - Eddington limit

Answer 58

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

Question 59

Very large stars can be observed, although they are much rarer than small stars (Sun-like or red dwarfs)
explain the heavyweights MRLTAge

Answer 59

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

Question 60

Heavyweight stars dude, due to their immense radiation what happens

Answer 60

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!)

Question 61

how will the heavyweights end their life

Answer 61

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

Question 62

hypernova

Answer 62

a stellar explosion with an energy of over 100 supernovae

Question 63

stars remain in a balanced state until their

Answer 63

H are used up.
 Thermal pressure balances gravity
 Fusion energy balances radiative energy flow from surface

Question 64

Once fusion stops, the star’s fate depends on its

Answer 64

birth mass

Question 65

Intermediate-mass stars have lives similar to

Answer 65

highmass stars but they end like low-mass stars

Question 66

talk about high mass convection

Answer 66

No convection zone near surface. Convection in upper core moves energy out due to the very high energy production in the core.

Question 67

talk about 0.5~1.5Msun mass convecion

Answer 67

deeper convection in cooler outer layer, radiation diffusion deep in the star

Question 68

talk about very low mass convecion

Answer 68

convection zone extends to the core

Question 69

Low-mass stars are similar to

Answer 69

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

Question 70

Life stages of very low-mass stars

Answer 70

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):

Question 71

Red dwarfs do NOT become red giants in their post-main sequence phases.

Answer 71

 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

Question 72

Life stages of Sun-like stars: Subgiant stage

Answer 72

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!

Question 73

subgiant stage crush of gravity shrinks the core rapidly.

Answer 73

 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

Question 74

Life stages of Sun-like stars: Subgiant & Red giant stages

Answer 74

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…

Question 75

Life stages of Sun-like stars: Subgiant & Red giant stages

Answer 75

 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

Question 76

Life stages of Sun-like stars: He flash CONTINUED

Answer 76

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

Question 77

Life stages of Sun-like stars: Helium burning

Answer 77

 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

Question 78

A dying Sun-like star

Answer 78

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

Question 79

He burning inside a dying star never reaches equilibrium; (talk to me more about a dying sun like star)

Answer 79

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

Question 80

Planetary nebula

Answer 80

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!)

Question 81

smaller-than-Sun star

Answer 81

M = 0.4Msun

Question 82

Life cycle of a small mass star summary

Answer 82

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

Question 83

Big Bang made what atom in what percentages

Answer 83

75% H, 25% He

Question 84

we have seen how He, some Li, C & O are generated by nuclear fusion in

Answer 84

low mass stars

Question 85

we have seen how He, some Li, C & O are generated by nuclear fusion in low mass stars

Answer 85

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)

Question 86

hydrogen fusion in  Low-mass star (e.g. Sun)

Answer 86

Fusion via proton-proton (p-p) chain

 The p-p chain provides about 98.4% of the Sun’s energy output

Question 87

The early life stages of high-mass stars are similar to lowmass stars, but proceed much more rapidly.

Answer 87

 Fuse increasingly heavier elements until all sources are used up
 Implosion by gravity results in self-destruction ——>Supernova

Question 88

high mass stars fusion how sia

Answer 88

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’)

Question 89

cno cycle

Answer 89

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

Question 90

Life evolution of Giants & Supergiants

Answer 90

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

Question 91

Advanced nuclear burning

For high-mass stars (M>8MSun) gravitational contraction of C core continues until it reaches 600mK

Answer 91

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)

Question 92

Advanced nuclear burning (continued)

Answer 92

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

Question 93

The iron (Fe) problem

Answer 93

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 !

Question 94

Supernova

Answer 94

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

Question 95

if a core collapse is stopped by neutron degeneracy pressure

Answer 95

neutron star

Question 96

if a core collapse is stopped by neutron degeneracy pressure – > neutron star
if the remaining mass is still large then what

Answer 96

Gravity > neutron degeneracy pressure — > core continues to collapse —-> black hole (BH)

Question 97

supernova in our galaxy

Answer 97

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!

Question 98

Most stars are NOT

Answer 98

single.

 >50% occur in binary or multiple systems

Question 99

Most binary stars live & die as if

Answer 99

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.

Question 100

Algol-type binaries contain a

Answer 100

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)

Question 101

Explanation of the Algol Paradox:

Answer 101

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