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Supernovae:
The Death of Massive Stars
Low- vs High-Mass
The Dependence on Mass
- everything happens faster!
Counter-Intuitive Behaviour!
When the most massive stars run out of fuel, you
would expect gravity to win.
(They are too massive for Chandrasekhar to save
them!) They should – and indeed do – collapse
inwards.
But the immediate consequence is a spectacular
outward explosion – a supernova!
Why??
A Colossal Explosion!
Material comes off at very high velocities (measured
by Doppler shifts), and involves a large fraction of
the mass of the star.
It can be ejected at speeds of ~10,000 km/sec (but it
slows down when it encounters the Interstellar
Medium [ISM])
A fantastically vigorous explosion! The dying star
becomes as bright as a whole galaxy of many
billions of stars.
Supernovae!
These super-bright stars
were first recognized as
a special class by Fritz
Zwicky, in the 1930s.
(He also foresaw neutron
stars and dark matter.)
He was quite a character.
Observations of Supernovae
A typical big galaxy has about one bright
supernova a century
The Milky Way is a little ‘overdue’ – the
most recent couple were seen in the time
of Tycho (1572) and Kepler (1604).
There was a spectacular one in 1054 AD.
SN 1054 AD -- A Millennium Later!
(the Crab Nebula)
An Animated Version
https://vimeo.com/71117055
Our Nearest Galaxy Neighbours
The LMC
a small nearby galaxy (only 150,000 l.y. away!)
Supernova
1987A
The most
recent ‘nearby’
supernova
In a More Remote Galaxy
(about 55 million light years away)
…and here
a star dies
in a very
distant
galaxy
The Lead-Up to the Explosion

The fuel runs out, reactions stop, the temperature and
pressure fall, and the core contracts.

Potential energy is converted to thermal energy: the core
gets even hotter than before.

A new fuel supply (the ‘ashes’ of the previous cycle) can
now be fused. This continues for fuels beyond Carbon
(unlike the sun-like stars that are supported by electron
degeneracy and never get hot enough).

Each successive fuel is of ‘lower quality’ (binding energy
curve!) and less abundant  reduced potential lifetime

Eventually the available fuel no longer provides energy
(i.e. we are at the peak of binding energy curve). The star
is doomed to collapse.
The Binding Energy Curve Again
Along the Way: Stratification
(‘Onion-Skin’ Structure)
Two Important Reminders
1.
2.
The stratification is NOT because heavy elements
settle to the centre. It is merely a consequence of the
fact that the progressively heavier elements are
created near the centre, where the temperature
progressively becomes high enough to do so!
The outer envelope of the star is still the pristine
material from which it was made (mainly Hydrogen
and Helium). This means that before the explosion,
the star’s spectrum does not reveal any of the
enormous and frantic compositional changes which are
occurring deep within it.
One Important Implication…
We cannot tell, from its outward
appearance, that a particular
star is on the verge of using
up its last bit of fuel in the
core and is about to explode!
There is no obvious “fuse” we
can watch, gradually counting
down the seconds.
The Internal Changes Occur Deep Within
a Hugely Expanded Outer Envelope
Consider the Temperatures!
Consider
Representative
Reactions,
as the Star
Evolves
You can see why the temperature needs to be extremely high to force
these nuclei together! The inter-nuclear repulsion is very strong.
Potential Lifetimes
(here, for a star of 8 solar masses)

Hydrogen
10 million years

Helium
1 million years

Carbon
300 years

Oxygen
200 days

Silicon
2 days (!!!)
One Result:
Enrichment of the ISM
Remember that low-mass stars (like the Sun) gently
‘puff off’a planetary nebula shell that is still largely
H and He.
By contrast, the massive stars throw out a great deal
of newly-generated heavy elements, from the deep
interior!
That’s where your atoms were created! – the iron in
your haemoglobin, the calcium in your bones, the
sodium and potassium in your blood,…
Joni Mitchell
Says It All
(Woodstock)
I came upon a child of God, walking down the road
I asked him, where are you going?
And this he told me
He said I'm going down to Yasgur's Farm,
Just join in a rock and roll band.
Get back to the land and set my soul free.
(He said) we are stardust, we are golden,
And we got to get ourselves back to the garden.
Can We Understand the Details?
What elements will be produced, and in
what proportions?
How do those calculations agree with the
observed amounts in the universe as a
whole? (not just in a special localized
enmvironment, like the Earth...)
The ‘Cosmic Abundance’
of the Various Elements
First, how do we do
the calculation?
An aquarium pool containing 1 whale and 1 person
is 50:50 by number (whales vs people) but
99.9% whales by mass
The Overall
Cosmic
Abundance
For every 90 Hydrogen atoms (a total mass of 90), there are 9 Heliums
(with a total mass of 9x4 = 36); and so on for the other elements.
Thus Hydrogen is about 70% of the total mass, the rest (~25% or so)
being mostly Helium. This justifies my earlier statements that the
universe is ~2/3-3/4 Hydrogen, with most of the rest being Helium
and just a few percent of ‘everything else.’
One Surprise!
There’s a
Remarkable
“Odd-Even”
Effect
Two Important Questions
1.
2.
Why do we see the striking odd-even pattern
of abundances? (Why is there more Carbon
and Oxygen than Nitrogen, for example?)
Where do the ‘super-heavy’ elements (like
Uranium and Gold) come from? (The binding
energy curve suggests that nuclear fusion can
routinely make heavy elements up to Iron, but
no farther.)
The Binding Energy Curve Again
1. The Odd-Even Effect
Explained
In all stars, the first round
of thermonuclear fusion
converts Hydrogen to
Helium. By doing so,
it pairs up the protons.
Subsequent reactions dominantly involve species
with even numbers of protons.
2. Where Do the Super-Heavy
Elements Come From?
The binding energy curve tells
us that the heaviest elements
(those beyond iron) can’t be
simply built up by progressive
phases of thermonuclear fusion
in the core of the star.
To answer the question of their origin, we need to consider the actual
mechanism of a supernova. Only then will we know how the very
heaviest elements are created (in trace quantities) – and how they
get out!