Transcript Star Types - University of Massachusetts Amherst

Stellar Evolution
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The birth, Life and Death of stars
Assigned Reading: Chapter 12 + Chapter 13
The main sequence begins as soon as the
star is supported by hydrogen fusion
The main sequence
exists because stars
balance their weight
with energy outflows,
produced by nuclear
fusion in their core
A main-sequence star can hold its structure for a very
long time. Why?
Time = c2 M / L = c2 M / M3.5 = 1 / M2.5
Thermal
Pressure
Gravitational
Contraction
Main Sequence Stars
•Main
Sequence stars are all fusing H to He in
their cores.
•Life time of a star is determined by its mass.
•Nature makes more low-mass stars than highmass stars. Low-mass stars also live longer. That
is why there are a lot more low-mass stars.
KEY QUESTION:
What happens after the main sequence (when
hydrogen in the core runs out)?
Low-End of Main Sequence
Most common stars, but very hard to see
This one is CHRX 73 A+B, a 0.3 Mo red dwarf plus a 15 MJ brown dwarf
High-End of Main Sequence
Very luminous byt very rare
Stars.
Very hard to measure the mass
Also, very hard to find stars with
M>100 Mo.
Large mass ejection
This one is Eta Carinae: two
Stars, one of 60 Mo and the
The other of 70 Mo.
When core hydrogen fusion ceases, a star
leaves the main sequence and becomes a giant
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The thermal pressure in the core can no longer support the
weight of the outer layers.
The enormous weight from the outer layers compresses
hydrogen in the layers just outside the core enough to initiate
shell hydrogen fusion.
This fusion takes place at very high temperatures and the new
thermal pressure causes the outer layers to expand into a giant
star.
Both the cooling/collapsing inert He core and the H-burning
shell contributes to energy output. Star overproduces energy: it
expands, surface cools, and becomes a luminous giant
Anatomy of a Star that is
leaving the Main Sequence
Hydrogen
fuel
Helium
“ash”
Hydrogen
burning core
shell
ABSOLUTELY NOT
IN SCALE:
In a 5 Mo star,
if core has size of a
quarter, envelope has
size of a baseball
diamond.
Yet, core contains
12% of mass
Up the red giant branch
Eventually, hydrogen will burn only in the outer parts of the
mostly-helium core. The star will swell to enormous size and
luminosity, and its temperature will drop, becoming a red
giant.
Sun in ~5 Gyr
Sun today
How does the Helium core push back?
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As matter compresses, it
becomes denser (and heats up!)
Eventually, the electrons are
forced to be too close together.
A quantum mechanical law called
the Pauli Exclusion Principle
restricts electrons from being in
the same state (i.e., keeps them
from being too close together).
The resulting outward pressure
which keeps the electrons apart
is called electron degeneracy
pressure – this is what supports
the core
Stars with M > 3 Mo never
develop degenerate He core
Indistinguishable particles
are not allowed to stay in
the same quantum state.
Helium fusion begins at the
center of a giant
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While the exterior layers expand, the helium core continues to
contract and eventually becomes hot enough (100 million Kelvin)
for helium to begin to fuse into carbon (if M > 0.5 Mo)
 Carbon ash is deposited in core and eventually a heliumburning shell develops. This shell is itself surrounded by a
shell of hydrogen undergoing nuclear fusion.
He fuses through a number of reactions, generally
referred to as the “3-a” reactions
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He + He + He = C + energy
… and produces an element “crucial” to our existence:
 CARBON
For a star with M<Msun, the carbon core
never gets hot enough to ignite nuclear
fusion (star needs 600,000,000 K to do so).
After helium fusion gets going…
The Sun will expand and cool again, becoming a red (super)
giant. Earth, cooked to a cinder during the red giant phase, will
be engulfed and vaporized within the Sun. At the end of this
stage, the Sun’s core will consist mostly of carbon, with a little
oxygen.
For low mass stars
Planetary Nebula
At the center of the nebula
there is the dying star.
Destiny of stars with roughly M
< 8Mo
M <0.4 Mo He WD
M < 4 Mo, C WD
M < 8 Mo, C + O + Si WD
Nuclear burning in massive stars (>4 Mo)
The lead-up to
disaster in
massive stars
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Iron cores do not
immediately collapse due
to electron degeneracy
pressure.
If the density continues to
rise, eventually the
electrons are forced to
combine with the protons
– resulting in neutrons.
What comes next … is
core collapse.
Massive Star Explosions: Supernovae
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The gravitational collapse of the core releases an
enormous amount of energy.
All the shells ignite, and the stars literally explodes
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100 times the total amount of energy produced by the Sun
in its lifetime is released in a matter of seconds.
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For a few days, the star is ~as luminous as a whole galaxy!!!
Then luminosity decays in following months:
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It can fully disintegrates, nothing is left of it (Type Ia)
Or a neutron star or black hole (core cadaver) is left (Type II)
E.g. A Type Ia SN dims by a factor of 100 in about 170 days
Chart of light intensity versus time is called “Light Curve”
(see fig13-13, page 300).
Supernova 1987a before/after
Supernova Remnant Cassiopeia A
Stellar Evolution in a Nutshell
M < 8 MSun
M > 8 MSun
Mcore < 3MSun
Mass controls the
evolution of a star!
Mcore > 3MSun
End Products of Stars
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M < 0.08 Msun  Brown dwarf (fusion never starts)
0.08 Msun < M < 8 Msun  White dwarf
Helium White Dwarf: 0.08 Msun < M < 0.4 Msun
 Carbon White Dwarf: 0.4 Msun < M < 4 Msun
 Oxygen-Neon White Dwarf: 4 Msun < M < 8 Msun
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M > 8 Msun  Supernova II+ neutron star or a black
hole
If Supernova Ia (explosion of the degenerate core),
the whole star disintegrates, nothing is left of it
All of the Heavy Elements are Made During Supernovae
The Key Point in the Production of
Elements in the Universe
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Hydrogen and Helium are initially created in the
Big Bang
Stars process Hydrogen and Helium into heavier
elements (elements lighter than iron) during the
nnuclear-burning phase of their lives.
Elements heavier than iron are generated only in
the deaths of high mass stars (supernovae).
We were all once fuel for a stellar furnace.
Parts of us were formed in a supernova.
The evolution of stars
Points to remember:
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How is the helium core of a star supported?
(electron degenerate pressure)
What causes the expansion of a star to become a red
giant? (shell burning, energy over-production)
What is a supernova? (the consequence of an
unstoppable gravitational collapse)
When does a massive star explode as a supernova?
(once it cannot support itself anymore)
Where does the energy come from in a star like the Sun? Why?
Nuclear fusion.
What elements can such a star produce?
Carbon and Oxygen.
Why cannot the star produce heavier element?
not enough mass to reach the temperature.
Why more massive stars have higher central temperatures?
high pressure to balance the gravity.
What is the heaviest element that can be fused into in a star? Why?
Iron, which is the most bound nucleus.
Observing Stellar Evolution
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How can we see stellar evolution in action?
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Stellar Clusters, a group of coeval stars, I.e. all
born at the same time, but with different masses
(hence different life time)
How can one estimate the age of a stellar
cluster?
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By looking at the HR diagram of the cluster,
namely at the evolutionary phase of stars of the
same age but with different mass
Our First Measurement of Age
Star Clusters
Open cluster: 103 stars, up to 30 pc
in size, found in disk of galaxy.
All have mostly young stars
Globular cluster: up to 106
stars and 150 pc in size, in
disk and halo of galaxy. All
have old stars
Why are clusters useful to astronomers?
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All stars in a cluster are at about same distance
from Earth.
All stars in a cluster are of about the same age.
Clusters therefore are natural laboratory in
which mass, rather than age, of stars is only
significant variable.
The Hertzsprung-Russell Diagram
More mass,
more fuel,
very fast burning.
Shorter
Lifetime
of Star
Less mass,
less fuel,
slow, steady burning.
Longer
How do we know the age of a star?
The H-R Diagram of a Cluster
We can date
a cluster by
observing its
population of
stars.
Turn-Off point:
Age indicator
The oldest clusters
known have been
measured to be
~14 billion years old.
All these stars in the
cluster have burned
themselves out!
Variable Stars
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Chepeids
RR-Lyrae
Variability due to Instability
Variability is PERIODIC
Instability caused by presence of ionized He
More luminous variable stars have large Period
Variability is EXTREMELY USEFUL,
because it is an absolute distance indicator
Cepheid Variable Stars
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Cepheid variable stars have
variable brightness that is
very regular.
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The period of the variation
can be from days to weeks
Pulsation due to instability:
He ionization layer acts a
energy sponge
it seems to be a reliable
indication of the star’s
luminosity!
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Cepheids: the Period-Luminosity Relation
Henrietta Leavitt
Henrietta Leavitt
(1868-1921).
Luminosity=4D2B
Standard Candles
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If we know an object’s
true luminosity, we can
measure its distance by
Luminosity
measuring its apparent Brightness 
2
4  distance
brightness.
An object that has a known luminosity
is called a standard candle.
What is burning in stars?
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Gasoline
Nuclear fission
Nuclear fusion
Natural gas
Survey Questions
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4.
How is the helium core of a star
supported?
What causes the expansion of a star to
become a red giant?
What is a supernova?
When does a massive star explode as a
supernova?.
The death of stars and
stellar remnants
•He white dwarfs
M<0.4 Mo
•C White dwarfs (planetary nebulae)
0.4<M<4 Mo
•Carbon-Neon-Silicon White Dwarfs
4<M<8 Mo
•Two types of Supernovae
M>8 Mo
•Type Ia, the exploding stars disintegrates
•Type II (core collapse), the star leaves remnants:
•Neutron stars (basically, a neutron white dwarf,
I.e. degenerate gas of neutrons)
•Black holes
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Stellar evolution can be deeply altered
in a binary system: mass transfer
An originally
massive star can
loose mass
and become less
massive (longer
life)
A nearly dead star can
be rejuvenated by
accretion of fresh fuel
A Type of Stellar Remnant:
Planetary Nebulae
At the center of the nebula there is
the dying star.
This is a white dwarf, where small
and hot: it photo-ionizes the nebula
The nebula formed out of the mass
loss during the red super-giant
phase.
Destiny of stars with roughly M <
8Mo
M <0.4 Mo He WD
M < 4 Mo, C WD
M < 8 Mo, C + O + Si WD
NOVAE: an example of binary stars
Novae are nuclear explosions on the surface of white dwarf and neutron stars
Brightness changes by a factor of 4000!
Two basic types of supernovae
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Type Ia – from the
thermonuclear detonation of a
white dwarf with M ~ 1.4 Msun
after accreting matter from its
companion.
(1.4 Msun is called Chandrasekhar limit)
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Type II – from core collapse of a
massive star  neutron star or
black hole.
Type Ia: White Dwarf Supernova
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If a White Dwarf accretes enough matter
from a companion star, it will eventually
nova.
If, after the nova, it does not shed all the
mass it gained, it will continue to accrete
mass until it novas again.
If this process continues (accretion, nova,
accretion, nova, etc.) such that the WD
continues to gain mass, once it has a
mass of 1.4Msun, the core will collapse,
carbon fusion will occur simultaneously
throughout the core, and the WD will
supernova.
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How might it be possible for a White
Dwarf to flare back to life?
Remnant from a Type Ia supernova:
A lot of irons!
Another distance indicator: White Dwarf
(Type Ia) Supernovae in distant galaxies.
L=4D2 B
•A 25 Mo star burns:
•H in 7 million years
•O in 6 month
•Si in 1 day
•Then… Booom
•Core collapse in
•~0.001-0.01 sec
Remnant from a Type II supernova
Crab Nebula
The supernova
explosion that
created the Crab was
seen on about July 4,
1054 AD.
Another Type of Stellar Remnant:
Neutron stars
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A neutron star --- a
giant nucleus --- is
formed from the
collapse of a massive
star.
Supported by neutron
degeneracy pressure.
Only about 10 km in
radius.
A teaspoon full would
contain 108 tons!
Very hot and with
very strong magnetic
field
Jocelyn Bell
Neutron stars
discovered as
pulsar
Pulsar, as a light house
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A fast rotating,
magnetized
neutron star.
Emits both strong
radiation (radio)
and jets pf highenergy particles.
Jets not very well
understood; their
existence is due to
the rotation and to
the presence of
magnetic fields
The Limit of Neutron Degeneracy
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The upper limit on the mass of stars
supported by neutron degeneracy
pressure is about 3.0 MSun (predicted
by Lev Landau)
If the remaining core contains more
mass, neutron degeneracy pressure
is insufficient to stop the collapse.
In fact, nothing can stop the collapse,
and the star becomes a black hole.
Black holes
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When the ball of neutrons collapses, it keep its mass,
but shrinks to smaller and smaller sizes.
No amount of pressure can stop the collapse, because
in those extreme situations, pressure itself contributes
more to gravity than it does of opposing it.
It forms a singularity – a region in space with the mass
of the parent material, but with virtually null volume
and hence potentially infinite gravitational field.
In a singularity gravity is so strong, i.e. the space is so tightly
curved, that nothing can escape, even light!
The most interesting aspects of a black hole are not what it’s
made of, but what effect is has on the space and time around it.
Review Questions
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What are type-Ia supernovae?
What do a type-II supernova leave
behind?
Why does a neutron star spin fast?
What is a pulsar?