The Death of Sun-Like Stars:
White Dwarfs
As The Core Contracts,
the Envelope Expands
The stars ‘leave’ the main sequence (no real
moton in space, of course)
They become red giants
They are said to ’climb the giant branch’
Climb that Branch
(this refers to changes in the appearance of the
star, not its spatial location, of course)
Reconsider the Masses
…and Note
…that stars of various masses become red
giants of rather similar appearance.
The Shrinking Core Gets Hotter
…and finally reaches 108 degrees. This ignites
“triple-alpha burning”, producing C
Complicated Reactions
As with the p-p cycle, the fusion takes place
in a series of steps, but the net result is that
helium nuclei are converted to carbon
(principally), with a net release of energy.
A Layered Structure Develops
- but this is not because heavy elements
settle to the centre!
Red Giants are Not as Stable as
Main Sequence Stars
Main sequence stars have long stable lives.
Once a red giant, things are not so placid!
The onset of He burning is very vigorous; the
outer properties of the star can change
erratically.
So Red Giants Change with Time
We will not explore the details!
Suffice it to say that astrophysicists
understand the behaviour…
The Future Luminosity of the Sun
The Future Size of the Sun
Eventually, He Fuel Will Run Out!
What Then?
The same argument as before would seem to apply:
 the loss of energy generation in the core leads to…
a loss of pressure support, and thus…
 gravitational contraction and heating, until…
 a new (less efficient) fuel supply kicks in, converting C
to even heavier elements. but with reduced life
expectancy
A Further Expectation
Suppose we merge Carbon nuclei to form
something heavier (say, C12 + C12  Mg24).
The ‘poor quality’ of this new fuel (remember the
binding energy curve!) suggests that it won’t last
very long.
And so the cycle should continue, with
progressively poorer fuels being used in turn…
But No!
After the conversion of Helium to Carbon,
the Sun undergoes no more significant
nuclear reactions at all!
How? Doesn’t this mean that gravity will
win, and the sun will dwindle down to a tiny
dense object? With no nuclear reactions,
what can prevent such a collapse?
Meet Wolfgang Pauli
…and Chandrasekhar
..and Eddington
(here with Einstein)
Collectively, They Helped Develop an
Amazing New Understanding
…with unfamiliar
‘new physics’!
First, Consider the Earth
It is a rock, with a hot interior that will
eventually cool off. [See ASTR 101]
Even when it is stone-cold, it will not
collapse inward.
Its crystalline structure gives it permanent
rigidity.
Electrons in Atoms
are the Reason!
Atoms are held “at arm’s length” by their
surrounding clouds of electrons.
This explains the structure and strengths of
various materials, minerals, etc.
Examples
Sun-like Stars:
Electrons Again - but Differently!
The dense innermost parts of the evolving
sun-like star contains myriads of free
electrons (not attached to individual
atoms).
We reach a stage where these electrons
(not the carbon nuclei) resist being
squashed more closely together.
A New Kind of Resistance:
Pauli Exclusion
Even though they are physically tiny,
electrons cannot be arbitrarily squashed
closer and closer together.
This is not because of their electric charges,
but something more subtle.
Not Like Solid Material in the Earth!
This is surprising new physics, in unfamiliar
regimes of enormously high density! (a
million times that of water)
It is a product of the new (1930s) science of
quantum mechanics (the physics of the
very small), combined with special
relativity.
Incidentally…
The Pauli exclusion principle explains the
ways in which electrons distribute
themselves around ordinary atoms too!
The Cambridge Connection
“Electron Degeneracy”
This new source of resistance against the
pull of gravity is independent of the
temperature of the star.
In other words, the star can now cool off
until it is stone-cold – and yet remain
stable against the enormous inward pull of
gravity. Its electrons prevent its collapse!
This Explains Sirius B
Enormous Gravity Resisted
by Electron Degeneracy
More massive white dwarfs are somewhat smaller (gravity
compresses them even more), but they still resist collapse!
But:
There is a limit to the mass of stars which
may be supported in this way
(the ‘Chandrasekhar limit’, ~1.4 solar
masses)
A sad history… with eventual vindication
One Last Issue
The formation of the very dense core, the growing
importance of the pressure of degenerate
electrons, the halting of the slow contraction: all
of these happen deep in the [Earth-sized] centre
of a huge red giant star , with a thick outer
envelope!
Do we ever get to see the white dwarf ‘cinder’?
If so, why and how?
The Answer
The star obligingly
sheds its skin!
…Producing a Planetary Nebula
The name is a complete
misnomer! It has
nothing to do with
planets!
Note that this is a shell
of gas, not a ring.
This One’s a Perfect Bubble
Shells Can
Look Like
Rings!
Consider these soap
bubbles…
(here, two are joined)
Other Examples
The Shell is ‘Puffed Off’
10-50 % of the mass of the star comes off,
gently, at speeds of perhaps 30 km/sec
Note that this is raw star stuff – not the products
of the nuclear reactions deep within the core!
It is recycling of pristine material, ready to be reused in other stars.
Leaving Behind:
Incandescently Hot Dense Cores
Their Fate?
These intensely hot, luminous cores cool off
thereafter, getting ever fainter, and end
up as
white dwarf stars
They will never succumb to gravity.
Some Real Examples
So: The Sun’s Life In Summary
Its Eventual Fate
Cold carbon clinkers – ‘diamonds in the
sky’
[although not like any diamond you ever encountered!
– a million times the density of water, held up by electron
degeneracy -- but nevertheless full of Carbon nuclei]
There are literally tens of millions of such
stars in our galaxy.
White Dwarfs in Plenty!
The Wonders
of Carbon
“Like a Diamond…” (sort of!)