Transcript Lecture 11

ASTR 1200
Announcements
I hope to have the exams returned next Tuesday,
but no promises….
Class will be in the planetarium next Tuesday
(October 14)
Website
http://casa.colorado.edu/~wcash/APS1200/APS1200.html
Full, Artistic H-R
As mass of
MS star increases,
both R and T
increase
increasing
size
sAT4
T constant
on any vertical
line
Newly Formed Star
-5
Giants
Rigel
Capella
0
M
Sirius
Protostar
Procyon
Sun
+5
Main Sequence
Then sits while
burning H
a Cen B
White Dwarfs
+10
Sirius B
Prox Cen
+15
O
B
A
F
G
Spectral Type
K
Large,
Low T.
Settles down
to MS
M
MS Lifetime
What determines amount of time a star stays on Main Sequence?
Just like a kerosene heater: Amount of fuel and rate of burn.
More Mass = More Fuel
More Luminosity = Greater Burn Rate
We can scale from the Sun: M = 1M
L = 1L
Sun lasts 1010 years
M
MSLife  10
L
10
M in solar masses
L in solar luminosities
Some Lifetimes
Sun
Sirius
Prox Cen
Rigel
Mass
Luminosity
Lifetime in Billion Years
1
2
.4
8
1
10
.001
10,000
10
2
4000
.008
Dinky little stars like Prox Cen will last trillions of years
Huge stars like Rigel are gone in a few million
There aren’t many large stars out there, because they don’t last.
10,000 O stars of the 100,000,000,000 Milky Way stars
Chemical Energy for Sun?
Chemical Energy Generates 2eV per atom in forming molecule (burning)
2eV = 3x10-19 Joules
Number of Atoms in Sun:
M
2 x1030 kg
N

 1057
 27
m p 1.6 x10 kg
Available Energy
E  3x10 19 x1057  3x1038 J
Time it can run:
E 3x1038 J
11
t 

7
x
10
s  20,000 years
26
L 4 x10 W
Gravitational Energy?
Available Gravity Energy:
Time it can run:

GM 2 6 x1011 x 2 x1030
E

R
7 x108

2
24 x1049
41


3
x
10
J
8
7 x10
E 3x10 41
15
7
t 

10
s

3
x
10
years
26
L 3x10
Sun can only run 30million years on gravity.
It does this during formation
Best understanding of Sun until Einstein.
Nuclear Energy for Sun?
Nuclear Energy Generates 2MeV per atom in forming molecule (burning)
2MeV = 3x10-13 Joules
Number of Atoms in Sun:
E  3x10 13 x1057  3x10 44 J
Available Energy
Time it can run:
M
2 x1030 kg
N

 1057
 27
m p 1.6 x10 kg
E 3x1044 J
17
t 

7
x
10
s  20,000,000,000 years
26
L 4 x10 W
Sun can run 20 Billion years on nuclear energy
Which is what it does.
What Happens When Hydrogen Starts to
Deplete?
Percentage of Helium in Core Rises
Nuclear Reaction Rate Slows
Energy Leaks Away and Pressure Drops
Pressure from above squeezes core,
making it smaller and hotter.
Nuclear Reaction Rate Increases
Star Gets Brighter!
The less fuel a star has left, the faster it burns it….
Effect on Star
Core Shrinks and Gets Hotter
-5
Star gets brighter and bluer?
Giants
Rigel
Capella
0
M
Sirius
Procyon
Sun Main Sequence
+5
a Cen B
+10
White Dwarfs
Sirius B
Prox Cen
+15
O
B
A
F
G
K
Spectral Type
Actually Goes Up and To Cooler Side
Why??
M
Outer Envelope of Star Expands
Force of photon flux from below lifts outer parts of star.
Core gets smaller and hotter, but
Surface gets larger and cooler.
Path on H-R Diagram
Over Time a Star becomes
a red giant.
What’s Happening in Core?
H
H
H->He
H->He
He
In the center, H is depleted, but He too cool to burn.
“Shell Burning” describes source of nuclear power around dead center
He Compressed In Middle
H
H->He
He
Temperature and Density
Rise in Center
Star gets more luminous
Electron Degeneracy
•Helium Density Rises
•Center of Sun has density of 10g/cc (H2O = 1g/cc)
•When density in center of star reaches ~30,000g/cc
•a new phenomenon kicks in
•Electron Degeneracy
•A purely quantum mechanical phenomenon
A Metaphor for Degeneracy
Electron Degeneracy Pressure
Because of electron degeneracy pressure, core stops shrinking.
Gains Mass, Temperature Rises, Stays Same Size
Has all the characteristics of a bomb.
Burnable Material
Confined Space
What ignites it?
Helium Flash
When degenerate He core reaches about 108K, the nuclei
can burn by the Triple Alpha Reaction. (He+He+He->C)
They start to burn and release energy. Pressure rises and temperature
rise, but volume does not increase. P and T rise some more.
Finally, P gets so great it lifts the degeneracy and thermal pressure
equilibrium is re-established.
This is the “He Flash”
Could blow a star apart, but it doesn’t.
He Flash
Star changes L and R
in just 10,000 years!
After the Flash
H
H
H->He
H->He
He->C
He->C
C
Helium Burns in the Center
Starts to Development Dead C Core
Red Giants
Earth Orbit
Nucleus
Fate of The Earth
We will be swallowed by the Sun
In 5 billion years it will start to swell.
T will rise on Earth
Oceans will boil and then evaporate into space
Sun will cover the sky. Giant ruddy ball.
Then it will engulf us.
Mountains will melt.
Planet should survive.
Will look like a polished bowling ball.
Red Supergiant
H
H->He
He->C
C
Meanwhile, down in the core,
the C is becoming degenerate.
Luminosity is becoming so great it
blows the H envelope into space.
Planetary Nebulae
Gas blown out into space and
illuminated by central star.
The star is the degenerate C core.
The Ring Nebula
Gorgeous Planetary Nebulae
from Hubble Space Telescope
Notice Rings.
Star has “episodes”
Hourglass Shape
Star throws material out in its ecliptic plane.
That’s the equatorial plane of the star.
Creates a dense disk around star.
Hourglass 2
Next Explosion is
Constrained and Expands
as Hour-Glass
What’s Left?
Eventually all the envelope is blown away.
The planetary nebula dissipates.
All we have is the hot, degenerate core.
Settles down to become a White Dwarf