Download a Supernova!

The Supernova Event
Imagine a Free-Fall on Earth
1. The force of gravity is roughly constant as you fall, because
your distance from the center of the Earth changes very little.
So you accelerate steadily.
2. Eventually, air resistance becomes important and sets a
limiting ‘terminal velocity.’( A parachute helps to make that a
slow, safe speed.)
https://www.youtube.com/watch?v=vvbN-cWe0A0
In a Collapsing Star
When gravity wins, all the atoms of the star fall
freely towards the centre. There is no‘wind
resistance’; it is a complete free-fall.
Moreover, the whole star is collapsing inward. As a
result, all the atoms get much closer together
and the force of gravity increases. The inward
collapse accelerates fantastically.
How Long Will a Fall Take?
The time to impact depends on
 the gravitational force acting (remembering that it
may grow as the collapse continues); and
 how far you have to fall.
1.
2.
Felix’s stratospheric jump: with no air resistance, he
would have fallen 24.2 km in a little more than a minute
and hit the ground at twice the speed of sound. Ouch!
If you completely stopped the Earth in its orbit, it would
fall straight into the Sun in about three months.
How About a Massive Star?
The core of a big star, with as much mass as the whole sun in a
dense ball about the size of the Earth, would collapse completely
in about 1 second!
This produces:



a star’s worth of material rushing inward, and accelerating as
it goes
attracted by a fantastically strong and growing gravitational
force
densities that approach a trillion times that of water.
What happens then?
A Now-Familiar Story
The vigorous collapse
 releases gravitational potential energy
 causes extreme heating (but not enough
to halt the collapse and support the star)
 resulting in lots of very energetic radiation
(gamma rays).
The Gamma Radiation Can
Tear Complex Nuclei Apart
A lot of hard work is undone!
Disruption
The gamma rays rip apart many (but not all!) of
the heavier nuclei that have been so gradually
fused together by thermonuclear reactions over
millions of years.
The disruption of these nuclei leads to a‘sea’of
inward-falling protons and neutrons (plus
electrons, of course! -- they have been present
ever since the star formed: one per proton!)
Can the Electrons
Stop the Collapse?
They are being compressed ever more
closely together.
Do they become degenerate, and provide
extra sustaining pressure, saving the star?
Electron Degeneracy
Cannot Save Us!
The cores of these massive stars exceed the
Chandrasekhar limit.
Instead, in the crush, the
electrons are ‘squashed
together’ with the protons,
thereby converting a lot of
the free-falling protons to
additional neutrons.
This ‘neutronization’ process also creates neutrinos!
A Neutron Star is Born!
The central part of the star – perhaps equivalent to a
few times the mass of the sun – is more or less
instantly turned into a ball of infalling neutrons.
Suddenly, the material attains the critical density (that is,
the neutrons become ‘degenerate’) and the collapse
comes to an abrupt halt.
Deep in its interior, the core of the huge star has
‘hatched’ a tiny neutron star (about the size of a city,
perhaps 10 km in diameter).
There is a
Rebound
Stellar material that started a little farther out falls in fractions of
a second later, and rebounds off the now-dense core.
This rebound is aided quite significantly by a momentary ‘overcontraction’of the neutron star core, which itself re-expands
slightly and gives an extra outward ‘kick’ to the infalling
material.
As a consequence, the envelope of the star (and thus a large
fraction of the original stellar material) is blasted into space
at high speed: a Supernova!
A Neutrino ‘Wind’
In addition, the outflow
of fantastic numbers of
created neutrinos helps
to blow off the outer
shell of material.
(Not all of the elusive neutrinos get through the
dense outer layers! Those that are captured
help to ‘push’ the expanding shell of material
outwards.)
The Consequence –
Enrichment of the ISM
Not all the nuclei are torn apart, so we can understand why
the supernova shell (the expanding material) helps to
‘enrich’ the interstellar medium (ISM) by carrying out
some of the heavy elements – calcium, magnesium,
silicon, iron, etc – that were produced earlier, in the
star’s interior.
A Lingering Question
Earlier, we also claimed that the ultra-heavy elements (past
iron) are produced in small amounts by supernovae, and
flung out into interstellar space.
How does that happen?
(Remember that the binding energy curve seems to tell us
that nothing heavier than iron can be produced at all by
these stars.)
Neutrons Again
In the dense centre of the star, nuclei are torn apart, and the
star ‘neutronizes’ completely. But somewhat farther out,
other things happen.
In particular, some of the heavy nuclei survive, but are now
subjected to a bombardment by large numbers of newlycreated neutrons.
Since these neutrons are uncharged, they have no trouble
penetrating a heavy nucleus: there is no repulsion!
What happens if we bombard a heavy nucleus (say, Iron) with
neutrons?
Heavier Elements Can Be Built Up
By capturing neutrons, Iron can be transformed to heavier
isotopes. Natural radioactive decay processes can
convert these isotopes to another element (like Cobalt).
The same can occur again (yielding Nickel) – and so on.
Right Up to the
Very Heaviest Elements
“r” (rapid) and “s” (slow) process
The cosmically observed amounts agree with our
theoretical calculations.
The Alchemist’s
Dream Realized!
This process produces
the heavy elements well
beyond iron (like
platinum, uranium, etc…)
– and also gold, the
ultimate goal of the
medieval alchemists.
This Comes at a Price!
It requires an input of energy to build up
the very heaviest elements. The
supernova provides it!
Really it comes from the fantastic release of
gravitational potential energy as the mass
of an entire star collapses inward.
What We See
When it happens
(a supernova in a remote galaxy)
Long afterwards
(a supernova remnant in the Milky Way)
What a Supernova Produces


A very bright source of light…
A shell of heavy-element-enriched hot gas ,
expanding at speeds of some tens of thousands
of km/sec.

Neutrinos in unimaginable numbers…

A compact neutron star (often).
Amazing Numbers!
As noted, the visible light output can briefly match that of a
whole galaxy of 100 billion stars.
But there is 100x as much energy again in the form of
rapidly-moving material (kinetic energy)… and 100 times
as much again in the form of neutrinos.
That is, the energy in the form of neutrinos is 10,000 times
as much as the visible light it produces!!
In Short
Supernovae are critical for us because they
produce heavy elements and strew them
into the interstellar medium for later
generations of stars, planets – and people!
…but what they do most effectively is
flood the universe with fantastic numbers
of neutrinos!
The ‘Light Curve’of a Supernova
A supernova becomes as bright as a whole galaxy
(containing billions of stars). It fades away over
successive weeks or months.
Why So Bright?
A supernova becomes bright for two reasons:


It has a huge, hot, expanding surface – a
large ‘radiating area;’ and
The shell of gas contains certain newlyformed radioactive elements that decay,
producing light.
Reconsider SN 1987A
Note how the decay
of radioactive Cobalt
plays a role.
Computers allow us
to model all this
behaviour!
How About the Neutrinos?
Reconsider Supernova
1987a. We saw a very
dramatic increase in
visible brightness.
We claim that this was
accompanied by a huge
flood of neutrinos.
Neutrinos Detected!
The Kamioka detector in Japan detected
eleven neutrinos from Supernova 1987A,
just as the light arrived! Kamioka also
could tell the direction from which they
came. [Sadly, the Sudbury Neutrino
Observatory was not yet operational.]
Are these disappointing numbers? NO! Remember that the
source is 150,000 light years away, and that neutrinos are
elusive and very hard to capture. In fact, the detection
completely bears out our astrophysical understanding.
Visible at Immense Distances
Note the faint supernova just to the right of the very
remote fuzzy galaxy… billions of light years away!
We can measure its brightness and monitor how it slowly
fades away, to confirm its nature.
One Special Application:
Very Distant Supernovae
We use such remote supernovae to determine the
distances to remote galaxies (by how faint they
appear)
In this way, we can unravel the size, structure and
evolution of the universe.
This sort of work won the Nobel Prize in 2011, for
reasons we will come back to later.