Download Planetary Nebulae – White dwarfs

Life of a Low-Mass Star AST 101
Introduction to Astronomy:
Stars & Galaxies
Planetary Nebulae – White dwarfs
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END STATE:
PLANETARY NEBULA
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WHITE DWARF
WHAS IS A WHITE DWARF?
Exposed core of a low-mass star that has died
Mostly made of Carbon and Oxygen
No fusion to maintain heat and pressure
to balance gravity pull
Electron degeneracy pressure balances
inward crush of its own gravity
Very high density and hence gravity
Maximum mass=1.4 Msun (Chandrasekar limit)
Funky properties of white dwarf material!
Size Of A White Dwarf
Size of
Earth
1 Kg chocolate cake!
2 Kg chocolate cake!
0.4 Msun white dwarf!
0.8 Msun white dwarf!
Clicker Question
•  Hubble Space
Telescope spies
12-13 billion year
old white dwarfs
–  Formed less than
1 billion years
after the creation
of the universe
© HST and H. Richer
(University of British
Columbia)
Which is correct order for some
stages of life in a low-mass star?
A.  protostar, main-sequence star, red giant,
planetary nebula, white dwarf
B.  protostar, main-sequence star, red giant,
supernova, neutron star
C.  main-sequence star, white dwarf, red giant,
planetary nebula, protostar
D.  protostar, main-sequence star, planetary
nebula, red giant
E.  protostar, red giant, main-sequence star,
planetary nebula, white dwarf
Clicker Question
Which is correct order for some
stages of life in a low-mass star?
A.  protostar, main-sequence star, red giant,
planetary nebula, white dwarf
B.  protostar, main-sequence star, red giant,
supernova, neutron star
C.  main-sequence star, white dwarf, red giant,
planetary nebula, protostar
D.  protostar, main-sequence star, planetary
nebula, red giant
E.  protostar, red giant, main-sequence star,
planetary nebula, white dwarf
Sirius A & B
Main Sequence & White Dwarf
Chandra image, X-ray light
Time scales for Evolution
of Sun-like Star
H core burning
Main Sequence
1010 yr
10 billion years
Inactive He core, H shell burning Red Giant
108 yr
100 million years
He core burning (unstable), ”
Helium Flash
Hours
He core burning (stable), ”
Horizontal Branch 107 yr
10 million years
C core, He + H shells burning
Bright Red Giant
104 yr
10 thousand years
Envelope ejected
Planetary Nebula 105 yr
100 thousand years
Cooling C/O core
White Dwarf
Cold C/O core
Black Dwarf
∞
Clicker Question
The Big Bang produced only hydrogen and
helium. Suppose the universe contained
only low mass stars. Would elements
heavier than Carbon and Oxygen exist?
A.  Yes
B.  No
Hubble image, visible light
Clicker Question
The Big Bang produced only hydrogen and
helium. Suppose the universe contained
only low mass stars. Would elements
heavier than Carbon and Oxygen exist?
A.  Yes
B.  No
General Principles Are the Same:
Battle Between Pressure and Gravity
•  Main sequence
lifetimes are much
shorter
•  Early stages after
main sequence
–  Similar to a low mass
star, but happen much
faster
•  No helium flash
Lives of Intermediate/High-Mass
Stars
•  Low mass: < 2 times the Sun
•  Intermediate mass: 2-8 times the Sun
•  High mass: > 8 times the Sun
Intermediate-Mass Stars
•  May burn up to carbon but do not have enough mass
to get temperatures high enough to go any higher up
the periodic table
•  Degeneracy pressure stops the core from collapsing
and heating enough: particles are squashed together
as much as possible
•  End their lives with planetary nebulae, white dwarfs,
similarly to low-mass stars.
High-Mass Stars (M >8 MSUN)
•  Sequence of expansion/contraction
repeats as higher and higher
elements begin to fuse
•  Each heavier element requires
higher core temperatures to fuse
•  Most elements are formed via Helium Capture
–  A helium (2 protons) nucleus is absorbed, energy is
released
•  The elements are created going up the periodic
table in steps of 2
•  Core structure
keeps on building
successive shell
-  Like an onion
•  Lighter elements
on the outside,
heavier ones on
the inside
Other Reactions
Carbon (6), Oxygen (8), Neon (10)
Magnesium (12)….
“WE ARE ALL STAR STUFF!”
- Carl Sagan
“We are all star-stuff”
•  All heavy elements are created and dispersed
through the galaxy by stars
•  Without high mass stars, no heavy elements
•  Our atoms were once parts of stars that died
more than 4.6 billion years ago, whose
remains were swept up into the solar system
when the Sun formed
HIGH mass stars keep creating
elements up the period table UNTIL….
IRON (Fe, 26 protons )
•  Iron does not
release energy
through fusion or
fission
–  Remember: All
energy created by
the loss of mass
from the fusion or
the fission
(E=mc2)
There Is No Way Iron Can Produce Any
Energy to Push Back Against the Crush
of Gravity in the Star’s Core
The star is DOOMED!!!
Clicker Question
Clicker Question
What is the heaviest element that can be
created through fusion?
A. 
B. 
C. 
D. 
Carbon
Silicon
Iron
Uranium
What is the heaviest element that can be
created through fusion?
A. 
B. 
C. 
D. 
No significant changes
in luminosity
Star travels back and forth
on the HR diagram
In the most massive stars,
changes happen so quickly
that the outer layers do not
have time to respond
Outer layers subject
to strong winds
Carbon
Silicon
Iron
Uranium
Massive
red giant
or supergiant:
Fierce hot
winds and
pulsed ejecta
Hubble
Question: why do we see the glowing gas surrounding
the star to grow in time?
Wildest of all !
ETA CARINAE
Supermassive
star (150 MSUN )
late in life,
giant outburst
160 yr ago
Note: the star emitted a
pulse of radiation some
time ago.
Violent bipolar
ejecta + disk
at equator
Red Giant
with
intense
brightening
`Light Echo
from pulse
Star V838
Monocerotis
HST-ACS
•  The core of a high
mass star accumulates
iron as the layers
above it fuse
•  Without any outward
pressure, the core
once again starts to
contract.
•  Electron degeneracy
pressure supports the
core for awhile until the
mass of iron gets too
heavy (how heavy?)
•  When mass is too large
(>1.4Msun), core collapses and
iron atoms get compressed into
pure neutrons
•  protons + electrons ! neutrons
+ neutrinos
–  This takes less than 0.01 seconds
•  Electron degeneracy pressure GONE!
–  Core collapses completely
• Eventually neutron degeneracy pressure stops the
collapse abruptly
• Infalling atmosphere impacts on the core.
• Time for a demo…
Supernova!
•  The lightweight atmosphere impacts on
the heavy core and is “bounced” off in
a huge explosion
•  Plus huge energy release from
neutrinos!
The star s former surface zooms outward with a velocity of 10,000 km/s!
Big and small balls Demo
• 
What do you think will happen?
A.  The little ball will bounce up together with the
others
B.  The little ball will bounce higher than the others,
but no higher than when the little ball is dropped
alone
C.  The little ball will bounce much higher than the
other balls