Transcript Slide 1

Overview of this set:
The Main Sequence: Brief review
Stellar evolution off the Main Sequence
Cepheid variable stars: Measuring distance
Because It gets very dense: T is rising!
T-> 100,000,000K-> 3 a Very Fast
3a
Degenerate gas P not dependent on T if it did
the core would expand and slow reaction
Evidence for Stellar Evolution:
Variable Stars
Some stars show intrinsic
brightness variations not caused
by eclipsing in binary systems.
Most important example:
d Cephei
Light curve of d Cephei
Cepheid Variables:
The Period-Luminosity Relation
The variability period of
a Cepheid variable is
correlated with its
luminosity.
The more luminous it
is, the more slowly it
pulsates.
=> Measuring a
Cepheid’s period, we
can determine its
absolute magnitude!
Cepheid Distance Measurements
Comparing absolute and apparent magnitudes of Cepheids,
we can measure their distances (using the 1/d2 law)!
The Cepheid distance
measurements were
the first distance
determinations that
worked out to
distances beyond our
Milky Way!
Cepheids are up to
~ 40,000 times more
luminous than our sun
=> can be identified in
other galaxies.
Pulsating Variables: The Instability Strip
For specific
combinations of radius
and temperature, stars
can maintain periodic
oscillations.
Those combinations
correspond to locations
in the Instability Strip
Cepheids pulsate
with radius changes
of ~ 5 – 10 %.
Pulsating Variables: The Valve Mechanism
Partial He ionization zone is opaque and
absorbs more energy than necessary to
balance the weight from higher layers.
=> Expansion
Upon expansion,
partial He ionization
zone becomes more
transparent, absorbs
less energy => weight
from higher layers
pushes it back inward.
=> Contraction.
Upon compression, partial He ionization zone
becomes more opaque again, absorbs more
energy than needed for equilibrium => Expansion
Ie:ionization: He+ +photon -> H++ =opacity of radiation
R is getting smaller
T is going up
Opacity holds back
radiation
Star expanding
High T drops opacity
Radiation floods out and
We get
Absolute
Magnitude
By knowing the
Distance of
Some
Cepheids
So get P gives M look at
m use distance modulus to find
Distance…..Cepheids are visible in other galaxies! SEE HW Examples
Period Changes in Variable Stars
Periods of some Variables are not constant over time
because of stellar evolution.
 Another
piece of evidence for stellar evolution.
The End of a Star’s Life
When all the nuclear fuel in a star is used up,
gravity will win over pressure and the star will die.
High-mass stars will die first, in a gigantic
explosion, called a supernova.
Less massive
stars will die
in a less
dramatic
event, called a
nova
Red Dwarfs
Stars with less
than ~ 0.4
solar masses
are completely
convective.
 Hydrogen
and helium remain well mixed
throughout the entire star.
 No
phase of shell “burning” with expansion to giant.
Star not hot enough to ignite He burning.
Sunlike Stars
Sunlike stars
(~ 0.4 – 4
solar masses)
develop a
helium core.
 Expansion
to red giant during H burning shell phase
 Ignition
of He burning in the He core
 Formation
of a degenerate C,O core
Mass Loss From Stars
Stars like our sun are constantly losing mass in a
stellar wind ( solar wind).
The more massive the star, the stronger its stellar wind.
Farinfrared
WR 124
The Final Breaths of Sun-Like Stars:
Planetary Nebulae
Remnants of stars with ~ 1 – a few Msun
Radii: R ~ 0.2 - 3 light years

Expanding at ~10 – 20 km/s ( Doppler shifts)
Less than 10,000 years old
Have nothing to do with planets!
The Helix Nebula
The Formation of Planetary Nebulae
Two-stage process:
The Ring Nebula
in Lyra
Slow wind from a red giant blows
away cool, outer layers of the star
Fast wind from hot, inner
layers of the star overtakes
the slow wind and excites it
=> Planetary Nebula
The Dumbbell Nebula in Hydrogen and
Oxygen Line Emission
Planetary Nebulae
Often asymmetric, possibly due to
• Stellar rotation
• Magnetic fields
• Dust disks around the stars
•binaries
The Butterfly
Nebula
marker
?
Photons carry momentum =E/c
10.
7. All H-> He core collapse
GP energy to KE to heat H shell
Forms H shell ignites * moves to the
right now a SUBGIANT. Note energy
output increased but outer layers cooling
8. H shell energy intensifies, (Triple
alpha starts here) Helium flash, He to C
fast, outer expansion to Red Giant.
Radius increase s but outer T nearly
constant. I.e. star expands cools outer
layer but energy coming out
Compensates since it is rising. The
greater the temp gradient dT/dr) the
faster the energy flow.
9.After He flash Energy production slows
( core no longer degenerate) star goes to
Horizontal branch. Outer layer weakly
held, Mass loss starts
10. All He cor -> C , oore contracts
He shell heats up *-> asymptotic Giant
Oscillations of energy can now occur.
11. Outer core held loosely _>PN form
Deep Sky:
The Planetary Nebula (show)
Glowing gaseous shrouds shed by dying sun-like stars trying to stabilize as
they run out of nuclear fuel.. Typically 1,000 times the size of our solar
system These Ten have names like Owl, the Cat's Eye, the Ghost of Jupiter,
Ring. This glorious final phase in the life of a star lasts only about 10,000
yrs.
Note: The
Star Remnant
At center of
The Planetary
Nebulae.
Called “Planetary”
Because the
Resemble planets
In a telescope
GOOD-BYE
PLANETS!
PN Formation is still a mystery and can be complex as this image shows
Physical conditions… from lines which are sensitive to temperature and density
Appears as ring since
Line of sight in shell to us
Has more material at edges
Of shell.
Like looking through a
Balloon!
See this at CSI observatory
The Remnants of Sun-Like Stars:
White Dwarfs
Sunlike stars build
up a Carbon-Oxygen
(C,O) core, which
does not ignite
Carbon fusion.
He-burning shell
keeps dumping C
and O onto the core.
C,O core collapses
and the matter
becomes
degenerate.
 Formation
of a
White Dwarf
White Dwarfs
Degenerate stellar remnant (C,O core)
Extremely dense:
1 teaspoon of WD material: mass ≈ 16 tons!!!
Chunk of WD material the size of a beach ball
would outweigh an ocean liner!
White Dwarfs:
Mass ~ Msun
Temp. ~ 25,000 K
Luminosity ~ 0.01 Lsun
The Chandrasekhar Limit
The more massive a white dwarf, the smaller it is.
 Pressure becomes larger, until electron degeneracy
pressure can no longer hold up against gravity.
WDs with more than ~ 1.4 solar
masses can not exist!
White Dwarfs (2)
Low luminosity; high temperature => White
dwarfs are found in the lower left corner of the
Hertzsprung-Russell diagram.
HW #14
read Chapter 10
Questions
Problems 1,2,4,6,9,12