Transcript life cycle of stars

Evolutionary Path of a Solar-Mass Star
Planetary nebula
Asymptotic giant branch
Horizontal branch
Main sequence
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Astronomy 100 Fall 2003
Helium
flash
The Life of a 1 Solar Mass Star:
0.4 MSun < M < 4 MSun
Example of how low mass stars will evolve on the
HR Diagram–
http://rainman.astro.uiuc.edu/ddr/stellar/archive/suntr
ackson.mpg
Nov 5, 2003
Astronomy 100 Fall 2003
A Low Mass Stellar Demise
Solar-mass mainsequence star
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Helium-burning
red giant
Astronomy 100 Fall 2003
White dwarf and
planetary nebula
Evolution of a Solar-Mass Star
Red giant
Shell hydrogen burning
1010 yr
Main sequence
Core hydrogen burning
Tcore ~ 16 million K
109 yr
Helium flash
Our Sun has about 5
billion more years left on
the main sequence.
Shell helium burning
Nov 5, 2003
Astronomy 100 Fall 2003 Planetary Nebula and White Dwarf
White Dwarfs and Planetary Nebulae
• Outer layers of the red
giant star are blown
away by radiation from
the hot new white
dwarf– loses from 20 to
more than 50% of its
mass
T > 200,000 K
• As they expand, they
are lit from within by
the white dwarf
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Astronomy 100 Fall 2003
NGC 2440
Electron Degeneracy
e
e
p
p
e
p
e
p
e
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p
e
p
e
e
p
e
p
e
p
Matter in the core of
a normal star
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e
e
p
e
p
p
p
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p
p
Electron-degenerate
matter in a white dwarf
1 ton per cubic cm
Astronomy 100 Fall 2003
Degeneracy Pressure
Electrons are forced
into higher energy
levels than normal – all
of the lower levels are
taken
Effect manifests itself
as pressure
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NASA
Relative Size of White Dwarf
12,000 km
White dwarf– but will weigh
about 0.7 Solar Masses
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Astronomy 100 Fall 2003
Binary Systems?
• In a close binary pair of stars with slightly
different mass, the first higher mass low-mass
stars evolves into a white dwarf.
• Then later on the other stars evolves into a red
giant.
• What happens?
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Astronomy 100 Fall 2003
What Happens in Binary Systems?
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Astronomy 100 Fall 2003
Novae
Accreted
hydrogen
envelope
If enough material piles
up onto the surface of a
white dwarf, can undergo
explosive nuclear fusion
100 m
White dwarf (carbon-oxygen)
White dwarf blows off this
envelope and brightens
by 100x – 1000x over a
period of days – weeks
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Astronomy 100 Fall 2003
Nova Cygni 1992
Novae
Process often repeats
Novae are very common,
about 20 in our galaxy a
year.
BUT, it is possible that the
whole star can explode–
causing a Type Ia
Supernova– too much
material exceeds the
electron degeneracy (1.4
solar masses)
Nov 5, 2003
Astronomy 100 Fall 2003
Stellar Evolution for Intermediate Stars:
4 MSun < M < 8 MSun
.
Example of how 8 stars 1 through 8 solar masses
will evolve on the HR Diagram–
http://rainman.astro.uiuc.edu/ddr/stellar/archive/onet
oeighttrackson.mpg
Nov 5, 2003
Evolutionary Path for Intermediate Stars
Carbon
ignition
Mass loss
Blue supergiant
Helium
flash
Protostar
Main sequence
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Astronomy 100 Fall 2003
And when the Hydrogen Runs out?
• The more massive stars have
convective cores and radiative
envelopes, but still very similar
to low-mass in the first few
stages.
• First the hydrogen is burned in
the core– still not hot enough to
burn helium
• Then the core starts to shrink a
little– hydrogen shell burning
(around the inert helium core)
starts.
• This stops the collapse, and
actually the outer envelope
expands quickly becoming a Red
Supergiant.…but then…
http://www-astronomy.mps.ohiostate.edu/~pogge/Ast162/Unit2/LowerMS.gif
Evolution of an Intermediate-Mass (> 4 MSun) Star
5 x 106 yr
Main sequence
Core hydrogen burning
Tcore ~ 40 million K
Red supergiant
Shell hydrogen burning
106 yr
C Burning Core
105 yr
105 yr
103 yr
Red supergiant
Core carbon burning
Tcore > 600 million K
Blue supergiant
Core helium burning
Tcore ~ 200 million K
Red supergiant
Shell helium burning
White Dwarf
Stellar Demise of a Massive Star
10 MSun mainsequence star
Nov 5, 2003
Helium-burning
red supergiant
Other supergiant
phases
Astronomy 100 Fall 2003
Core-collapse
supernova
Stellar Evolution for Massive Stars:
M > 8 MSun
.
Example of how a 15 solar mass star will evolve on
the HR Diagram–
http://rainman.astro.uiuc.edu/ddr/stellar/archive/high
massdeath.mpg
Evolutionary Path of High-Mass Stars
Supernova
Blue supergiant
Carbo
n
ignitionHelium
flash
Protostar
Main sequence
High Mass Stars
• These are very similar to the intermediate mass stars, but
as they have more mass, they can “burn” heavier and
heavier atoms in the fusion process.
• Until they create Iron– after that it takes energy to produce
heavier atoms
• Nothing left!
Stage
Temperature
(million K)
Duration
H fusion
40
7 million yr
He fusion
200
500,000 yr
C fusion
600
600 yr
Ne fusion
1,200
1 yr
O fusion
1,500
6 months
Si fusion
2,700
1 day
Game
Over!
Supernova Explosions in
Recorded History
1054 AD
• Europe: no record
• China: “guest star”
• Anasazi people
Chaco Canyon, NM:
painting
Modern view of this region
of the sky:
Crab Nebula—remains of
a supernova explosion
Supernova Explosions in
Recorded History
November 11, 1572
Tycho Brahe
A “new star”
(“nova stella”)
Modern view (X-rays):
remains of a supernova
explosion
November 11, 1572
Tycho Brahe
On the 11th day of November in the evening after sunset ... I
noticed that a new and unusual star, surpassing the other stars
in brilliancy, was shining ... and since I had, from boyhood,
known all the stars of the heavens perfectly, it was quite
evident to me that there had never been any star in that place
of the sky ...
I was so astonished of this sight ... A miracle indeed, one that
has never been previously seen before our time, in any age
since the beginning of the world.
Types of Supernovae
• Type I
• Type II
• Type III, IV, V…. .sorta
Types of Supernova
Type I
• Observationally, astronomers originally
classed supernovae into two “types”, I and
II.
• Type I had no Hydrogen emission lines in
their spectra whereas Type II exhibited
Hydrogen emission lines. Later it was
realised that there were in fact three quite
distinct Type I supernovae, now labelled
Type Ia, Type Ib and Type Ic.
Type Ia
• Type Ia supernovae (SNIa) are thought to be
the result of the explosion of a carbonoxygen white dwarf in a binary system
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Astronomy 100 Fall 2003
Type Ib and Ic
• Types Ib and Ic supernovae are categories
of stellar explosions that are caused by the
core collapse of massive stars. These stars
have shed (or been stripped of) their outer
envelope of hydrogen, and, when compared
to the spectrum of Type Ia supernovae, they
lack the absorption line of silicon.
Type Ib + Ic
• Compared to Type Ib, Type Ic supernovae
are hypothesized to have lost more of their
initial envelope, including most of their
helium. The two types are usually referred
to as stripped core-collapse supernovae.
Type II supernovae
• A Type II supernova (plural: supernovae or
supernovas) results from the rapid collapse
and violent explosion of a massive star. A
star must have at least 8 times, and no more
than 40–50 times, the mass of the Sun
(M☉) to undergo this type of explosion.
Type II
• Unlike the Sun, massive stars possess the mass needed to
fuse elements that have an atomic mass greater than
hydrogen and helium, albeit at increasingly higher
temperatures and pressures, causing increasingly shorter
stellar life spans.
• The degeneracy pressure of electrons and the energy
generated by these fusion reactions are sufficient to
counter the force of gravity and prevent the star from
collapsing, maintaining stellar equilibrium.
Type II
• The star fuses increasingly higher mass elements,
starting with hydrogen and then helium,
progressing up through the periodic table until a
core of iron and nickel is produced.
• Fusion of iron or nickel produces no net energy
output, so no further fusion can take place, leaving
the nickel-iron core inert. Due to the lack of
energy output creating outward pressure,
equilibrium is broken and the core is compressed
by the overlying mass of the star.
Type II Supernova
• When the compacted mass of the inert core
exceeds the Chandrasekhar limit of about 1.4 M☉,
electron degeneracy is no longer sufficient to
counter the gravitational compression. A
cataclysmic implosion of the core takes place
within seconds. Without the support of the nowimploded inner core, the outer core collapses
inwards under gravity and reaches a velocity of up
to 23% of the speed of light and the sudden
compression increases the temperature of the inner
core to up to 100 billion kelvin.
Supernovae and nucleosynthesis
of elements > Fe
Death of low-mass star: White Dwarf
• White dwarfs are
the remaining cores
once fusion stops
• Electron
degeneracy pressure
supports them
against gravity
• Cool and grow
dimmer over time
A white dwarf can accrete mass
from its companion
Tycho’s supernova of 1572
Expanding at 6 million mph
Kepler’s supernova of 1609
Two kinds of supernovae
Type I: White dwarf supernova
White dwarf near 1.4 Msun accretes matter from red
giant companion, causing supernova explosion
Type II: Massive star supernova
Massive star builds up 1.4 Msun core and collapses into
a neutron star, gravitational PE released in explosion
light curve shows how luminosity changes with time
A neutron star:
A few km in
diameter,
supported
against gravity
by degeneracy
pressure of
neutrons
Discovery of Neutron Stars
• Using a radio telescope in 1967, Jocelyn Bell
discovered very rapid pulses of radio emission
coming from a single point on the sky
• The pulses were coming from a spinning neutron
star—a pulsar
Pulsar at center
of Crab Nebula
pulses 30 times
per second
Pulsars
Thought Question
Could there be neutron stars that appear as pulsars to
other civilizations but not to us?
A. Yes
B. No
Thought Question
Could there be neutron stars that appear as pulsars to
other civilizations but not to us?
A. Yes
B. No
What happens if the neutron star has
more mass than can be supported by
neutron degeneracy pressure?
1. It will collapse further and become a black
hole
2. It will spin even faster, and fling material
out into space
3. Neutron degeneracy pressure can never be
overcome by gravity
• Neutron degeneracy pressure can no longer
support a neutron star against gravity if its mass is
> about 3 Msun
18.3 Black Holes: Gravity’s Ultimate
Victory
A black hole is
an object whose
gravity is so
powerful that not
even light can
escape it.
Escape Velocity
Initial Kinetic
Energy
=
Final Gravitational
Potential Energy
1 2 GmM
mv 
2
r
Where m is your mass,
the mass of the object that you are trying to escape from, and
r is your distance from that object

Magic Numbers
•
•
•
•
1.4 M☉
8 M☉
25 M☉
40+ M☉
Nov 5, 2003
Astronomy 100 Fall 2003