Download Lecture 22 - Seattle Central

ASTR 100
Lecture 22:
Life of a high-mass star
Keep reading about star death,
Next week, start reading about Our Galaxy:
Ch. 12, 13, 14 in “Essential”
This/next week’s material:
Going to track stars over their entire lifetime, fill out
the rest of the H-R diagram: Cradle to grave
Black holes and Einstein’s version of gravity
Summary:
We know about how stars are formed
We know larger stars live fast and die young
Talk about what happens to high mass stars from
formation to “death”
Last weird branch of the HR diagram + neutron stars,
pulsars, black holes, core-collapse supernova
Chemical enrichment of the Universe
Star formation….
“The Initial Mass Function”:
For every O star, there are ~200 K and M stars
Updated picture of collapse
of a star-forming cloud:
In a star-forming cloud, what’s
pulling inwards? What’s
pushing outwards?
Protostellar/protoplanetary
disk forms.
Jet powered by magnetic
fields transports lots of
material.
Gravity in.
Thermal pressure out.
Cloud to main sequence,
we understand:
Powered by gravitational
contraction, burn brighter
than fusion, but just for a
few 104-5 yrs
Some comparative star anatomy:
On the main sequence, stars merrily fuse Hydrogen to
Helium for however long
…Then what
Today, finish talking
about the remnants
from “low mass” stars
High mass stars:
(3-100 Solar masses)
What’s happening on the main sequence? More
and more Helium, less and less Hydrogen
Runs out and Hydrogen
fusion stops….
Radiation pressure stops,
Gravitational contraction…
Subgiant: No fusion, but gravitational contraction
increases luminosity of star, making star expand….
Non-burning, Helium core
gets smaller, rest of star
gets bigger
Hydrogen around Helium
core also gets crushed and
eventually starts fusing:
Shell-burning
Solar wind increases
Solar mass stars have enough
pressure for:
Helium to Carbon in core,
Hydrogen to Helium in the shell
This story agrees with observations from
cluster HR diagrams and simulations in Solar mass stars.
Much harder to see observationally in high-mass stars,
too fast, too few. A lot of this story is theory/simulation.
Composite image of X-ray and visible light
For solar-mass stars, we stopped at Carbon. Not
enough gravitational pressure to go further.
E = mc2
A high mass star (Supergiant) has enough mass and
gravitational pressure to burn past carbon:
Multiple shells of fusing material
Differentiated. But it stops at iron.
All sorts of crazy stuff going on in the cores.
Flowchart of stellar
nucleosynthesis pathways:
Memorize by Monday’s quiz!
Just kidding.
The ballet of contraction/shell burning occupies the
entire Supergiant branch of the HR diagram
Towards Iron this isn’t really a lot of power….
Law of diminishing returns. The Silicon shell, only 5 days.
High mass stars have enough gravitational pressure
to get out to Iron. Crazy. Now what?
What’s keeping the star from crushing?
The iron isn’t fusing (no radiation pressure), but it’s solid.
“Electron degeneracy pressure” = electrons staying in orbit
Solid isn’t so solid (the nucleus is 106x too large in that pic):
In fact, we’ve seen “Electron degeneracy
pressure” before.
It’s what keeps the Earth and White Dwarfs from
collapsing …and fruit bowls on kitchen tables
This is this fact from chemistry with
“electron shells”, no two electrons
can be in the same place, so they
stack up like bricks.
Actually. You can crush an atom.
These were some of the
steps we never talked
about in making
hydrogen.
p + e + (v) = N
A proton, electron, (and
anti-neutrino) can get
smooshed together to
make a neutron
No more electron, no
more electron
degeneracy pressure.
(Neutrons can decay into a proton, electron,
and anti-neutrino too….)
The iron core gets crushed so that it’s no longer iron,
the electrons and protons combine into neutrons, the
volume of the core reduces by a factor of 1018
Outer core falls in
at about 25% of
the speed of
light, the core
temp rises to
~1011K
Option 1) The core (formerly iron) is now a stack of
neutrons none of which can occupy the same state.
This “neutron degeneracy pressure” fends off the falling
outer layers
Option 1) Summary of core-collapse (Type II) Supernova
Option 2) neutron degeneracy pressure isn’t enough to
resist the gravitational inflow….
A hole in spacetime….
Option 1)
Computer
simulations
verify speed
and power of
core-collapse
Supernovae
Option 1) Planetary nebulae on steroids….
Supernova remnants: A large fraction of the star’s mass,
chemically enriched with elements up to and BEYOND
iron, back into space.
SN1006 about 60ly across.
Lots of historical
records of “guest stars”
In stellar mass death, we had Planetary nebula
(cloud) and dense core remnant (White dwarf)
Core remnant of just neutrons: A Neutron “star”
~1 Solar mass, ~20km, spin ~1000x per second
“Pulsars” are Neutron stars whose radiation we can
see.
Like lighthouses….
Pulsars are the directions we gave to our Solar system on
the Voyager/Pioneer records/plaques
Where does the Uranium come from?
E = mc2
mc2 = E
Lots of energy from the gravitational collapse
http://www.youtube.com/watch?v=Xaj407ofjNE
The Life and Times of a high-mass star (pics not to scale)
1)
2)
3)
4)
5)
6)
7)
Protostar ~104yrs
Main sequence star ~106yrs
Red Supergiant (shell burning) ~105yrs
Helium Burning ~106yrs
Multiple shell burning ~104yrs
Supernova a few months
BH/pulsar/neutron star …forever
Story of stars is sort of the story of
pressure vs. gravity….
Star-forming clouds:
Thermal pressure vs. gravity
Main sequence stars:
Radiation pressure vs. gravity
White dwarfs
(and all solid matter):
Electron degeneracy pressure
vs. gravity
Neutron stars/pulsars:
Neutron degeneracy pressure
vs. gravity
Black holes:
…. Gravity wins.
Covered entire HR diagram now!!!
Key terms:
neutron “star”, pulsars, electron degeneracy pressure, neutron
degeneracy pressure, core-collapse (Type II) supernova
Key Ideas:
What are the main stages in a high mass star’s life?
What happens in the core of a high mass star at the end of its life?
Why does fusion stop at Iron in high mass stars?
Where do elements heavier than Iron come from?
What are the two possibilities when the electron degeneracy pressure in a
high mass star’s Iron core fails?
How do we know about core-collapse supernova?
What are the two parts of a core-collapse supernova remnant and what
are their properties?