This Week: Chapter 7, How Stars Live And: Chapter 8: How ...
Transcript of This Week: Chapter 7, How Stars Live And: Chapter 8: How ...
A Star’s Fate depends on its Mass
! Small Stars: live “forever”
! Medium Stars: become red giants, planetary nebulae, then white dwarfs (which quietly “retire”).
! But White dwarfs can explode if their mass gets larger than 1.4 MSUN (“Chandrasekhar limit”)! This explosion, called a Type Ia Supernova, would only happen in a binary star system.
This Week: Chapter 7, How Stars Live And: Chapter 8: How Stars Die
The Death of Stars
Massive stars start on the Main Sequence, quickly fusing their H. They eventually expand, and turn into SuperGiants
After a supergiant runs out of fuel it will cause a huge explosion called a supernova.
It could become neutron star or a black hole
Pre-summary: Death of Massive Stars (M>8 MSun)
Death of Stars: Massive Stars" Massive stars burn up their Hydrogen fuel quickly. " They then expand dramatically. " They can become 1000 times larger than the Sun! " They are called: red supergiants.
" Betelgeuse is a red supergiant in the constellation Orion
Evolution of Massive Stars
When a massive star runs out of Hydrogen fuel, its core begins to contract and heat up.
Next, it fuses Helium into Carbon & Oxygen in its core. (Outside the core, a shell of Hydrogen can fuse to Helium.)
Outside the core: no fusion
Death of a SuperGiant
A SuperGiant is hugeMost of the star is inert H & He
But at its core, fusion is creating many new elements....
...In a desperate attempt to provide energy.
Carbon runs out, then Neon fuses...then
Oxygen, then Silicon
Cooler outer layers fuse elements now gone from the core.
The center of the star looks like an onion!
A Massive Star’s Core
Death of Massive Stars" Massive stars fuse heavier and heavier elements:
Neon, Magnesium, … and even Silicon.
" However this process stops with Iron. " Fusing Iron will not produce additional energy.
" When the core of the star is full of Iron, the star has no source of energy….it is in big trouble
" Gravity takes over and causes the Iron core to collapse.
Core-Collapse: Supernova!
" The Iron in the core of the massive star begins to disintegrate
" " Now there is nothing to hold back gravity, so the core
collapses. " The electrons collide with the protons to form neutrons
" This also produces ghostly particles called neutrinos. " The temperature of the core reaches up to 100 billion
kelvins! " The star explodes catastrophically!
" This is called a supernova (Type II)
Core-Collapse SupernovaeWhen the core collapses, the innermost matter becomes neutrons.
The result is an incredibly dense object called a neutron star
The in-falling matter bounces off this dense core.
Also, copious neutrinos push the matter outward.
How Do Supernovae Explode?How do we know?
Supernova explosions are not well understood.
None have gone off in our galaxy since 1604.
Using computers, we can make a model of the explosion.
These models must be tested by observations of a real supernova.
Anim. & Demo.
Supernovae in History
Chaco Canyon Petroglyph
" Historical records show that many people witnessed a supernova in 1054.
" It was observed in Arabia, Ireland, and N. America " Observers in China, Japan, & Korea noted a “Guest Star”
The star was so bright it could be seen during the daytime!
It faded after a few weeks
Today in that part of the sky we find ...
" The Crab Nebula is expanding! " So it must have been smaller in the past. " Long ago, it was all just a point. " When? " 1054 AD !
10 light years
https://www.youtube.com/watch?&v=Qm1VscNlMK8
NASA video on Crab Nebula
In 1572, Tycho observed a “Nova Stella”, or New Star
This where we get the words:
--Nova--Supernova
The new star faded after a few weeks.
Tycho’s Star Chart
Tycho’s Supernova
In 1604, Kepler also observed a supernova.
Today in that place we see a large cloud, called a supernova remnant.
Remant of Kepler’s Supernova (X-ray image)
Observing Supernovae" A supernova happens every century or so in a galaxy
" But, there hasn’t been a supernova in our Galaxy for 400 years….
" We’re overdue!
" get ready....
Supernova in 1987" On Feb. 24, 1987, a star in the Large Magellanic Cloud exploded
as a Type II Supernova " It was seen in Australia, Chile and Zimbabwe. " At just the same time, a burst of neutrinos was detected in Japan. " This confirms theories of supernova explosions.
Kamiokande Neutrino Detector
HubbleImage ofSN1987a
Supernovae become incredibly bright for a short period of time, then fade away.
1 billion times the luminosity of the Sun!
Making Heavy Elements
During its life, a massive star can create many heavy
elements:
Mg, Na, Al, Si, etc.
These elements make up rocks and
the Earth.
They were all formed in stars!
Some of the energy released by a supernova transforms into mass & new elements are created.Freaky Fact: This where heavy elements come from.
! Gold (Au)! Silver (Ag)! Lead (Pb)! Uranium (U)
They are blasted into space by supernovae and eventually form into stars and planets.
Supernovae