Transcript Stellar Deaths - Mid

Stellar Deaths
Novae ans Super Novae 16
Hydrostatic Equilibrium
• Internal heat and
pressure from fusion
pushes outward
• Gravity pulling mass
inward
• Two forces balance!
• When all hydrogen
transformed to
helium, Sun will
begin to collapse!
What Powers the Sun?
Nuclear Fusion:
What is that?
Need high temperatures. Why?
Energy is produced in this process. How?
What Powers the Sun?
Nuclear Fusion: An event where the nuclei of two atoms join together.
Need high temperatures. Why? To overcome electric repulsion.
Energy is produced. (A small amount of mass = a lot of energy)
E = mc2. Sum of mass and energy always conserved in reactions.
Mass is just “frozen” energy!
Why High Temperatures?
• To overcome electric
repulsion
• High temp => fast
atomic motion
• Nuclear forces - very
short range (10-15m)
• 100 times EM force
• 1039 times gravity
• Fusion video
Astronomy Picture of the Day
CPS Question
• Sunspots are associated with “loops”
created in the Sun's magnetic field as a
result of _____ .
– A) prominences
– B) flares
– C) differential rotation
– D) the solar wind
Question
When a star runs out of hydrogen, what
happens next?
Evolution of a Low-Mass Star
(< 8 Msun , focus on 1 Msun case)
- Helium ash collects in core.
- Too cool for He burning. Why?
-
- Core contracts. Heats up. H burning shell
- Higher temp. => Brighter! Star expands!
- "Red Giant". Diameter ~ 1 AU!
- Does fusion rate at this stage increase or
decrease? Why?
Red Giant
Evolution of a Low-Mass Star
(< 8 Msun , focus on 1 Msun case)
- Helium ash collects in core.
- Too cool for He burning. Larger electric
repulsion.
-
- He Core contracts. Heats up. H burning
shell
- Higher temp. => Brighter! Star expands!
- "Red Giant". Diameter ~ 1 AU!
- Rate increases. Phase lasts ~ 1 billion
years
Red Giant
Creation of Heavier Elements
- Core shrinks and heats up to
108 K, => Helium fuses into
Carbon.
- All He -> C.
- Core shrinks and heats up.
- Outer parts burn faster
Each phase shorter than the
last.
-
Red Supergiant
Death of a Low Mass Star
• What factor(s) eventually determine
when this process stops?
"Planetary Nebulae"
- Low mass star (< 8 Msun) cannot achieve 600
Million K temp. needed for Carbon fusion
-
- As core collapses Contraction stopped by the Pauli
exclusion principle: two objects cannot occupy the
same space.
-
- As the core reaches its end stages of collapse the
outer shell burns He even quicker, making the Star
becomes unstable. Ejects outer layers in pulses.
"Planetary Nebula" (Historical name, nothing to do
with planets.)
- Once all the fuel has burned off, what’s left is a
Carbon core called a “White Dwarf”
-
Stellar Lifetimes
• Is the lifetime of a high mass star shorter
or longer than that of a lower mass star?
Why?
Evolution of Stars > 8 MSun
Higher mass stars burn out
faster and fuse heavier
elements.
Example: 20 MSun star lives
"only" ~10 million years.
Heaviest element made in
core of any star is iron.
Products of outer layers
become fuel for inner layers
Eventual state of > 8 MSun star
Stellar Explosions
Novae
White dwarf in
binary system
WD steals mass from companion. Eventually passes , a burst of fusion.
Brightens by 10'000's! Cycle may repeat every few decades => recurrent
novae.
Nova Cygni with Hubble
May 1993
Jan 1994
1000 AU
Is all of the accreted matter expelled into space during a nova?
A Carbon-Detonation or “Type I” Supernova
Despite novae, mass continues
to build up on WD.
At 1.4 MSun (the "Chandrasekhar limit"), gravity overwhelms the Pauli
exclusion pressure supporting the WD => contraction and heating.
Carbon fusion everywhere at once.
Tremendous energy makes star explode. No core remnant.
Death of a Very High-Mass Star
M > 8 MSun
Iron core at T ~ 1010 K radiation
photodisintegrates iron nuclei into protons
and neutrons.
Core collapses in < 1 sec.
Neutrons “rebound”. Shock ejects outer
layers => Core-collapse or Type II
Supernova
Ejection speeds 1000's to 10,000's of km/sec!
Remnant is a “neutron star” or “black hole”.
(Supernova Demo)
Supernova 1987A in the Large
Magellanic Cloud
In 1000 years, the exploded debris might look something like this:
2 pc
Crab Nebula: debris
from a stellar
explosion observed
in 1054 AD.
Or in 10,000 years:
50 pc
Vela Nebula: debris
from a stellar
explosion in about
9000 BC.
Remember, carbon-detonation (Type I) and core-collapse (Type II)
supernovae have very different origins
Testing our Theories
• How can we test our theories of stellar
evolution when the lifetimes of stars are
so long?
Star Clusters
Two kinds:
1) Open Clusters
-Example: The Pleiades
-10's to 100's of stars
-Young (10's to 100's of millions of years)
2) Globular Clusters
- few x 10 5 or 10 6 stars
- Billions of years old
Why are star clusters useful for stellar evolution studies?
Clusters are useful for stellar evolution studies because all of the
stars:
1) formed at about same time
2) are at about the same distance
3) have same chemical composition
The ONLY variable property among stars in a cluster is mass!
Making the Heaviest Elements
• Since iron is the heaviest element that
can be made by stellar fusion, where do
the heavier elements come from?
Making the Elements
H and some He were made in Big Bang. Rest made in
stars, and distributed by supernovae.
Heaviest elements made in supernovae.
Solar System formed from such "enriched" gas 4.6
billion years ago.