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)

The Lifetimes of Stars

See Table 12-2 (p. 386 )

Massive

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

Antares, a SUPERGIANT in Scorpius

SUPERGIANT

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

A massive star consumes fuel at an increasing rate, before it blows up.

Table 13-1 p. 414

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.

Computer simulation of Supernova Explosion (I)

Core

Computer simulation of Supernova Explosion (II)

Core

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

A Supernova Remnant

" 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

Tycho’s SupernovaObserved in Xrays

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)

A nearby supernova remnant in Vela

The Veil Nebula,a Supernova Remnant(in UV light)

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 1987A

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!

Supernova in galaxy M51, 2005

And another in 2011 !

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

Stellar Recycling

The atoms in us were created in

stars, which later died.

They ejected their outer layers into the

ISM.

These atoms were then incorporated

into Earth.

Someday they may be returned to the

ISM