Download PHYSICS 015

The Death of Sun-Like Stars:
White Dwarfs
As The Core Contracts,
the Envelope Expands
The stars ‘leave’ the main
sequence and become
red giants
They are said to ‘climb the
giant branch’ (no real
implication of motion in
space, of course – just
changing properties)
Reconsider
the Masses
Note that stars of various masses become
red giants of rather similar appearance.
But they don’t stay that way for long!
Meanwhile, in the Shrinking Core
The temperature rises, and finally reaches 108 (one hundred
million) degrees. This ignites “triple-alpha burning.”
As in the p-p cycle, the fusion takes place in a series of steps.
The net result is that helium nuclei are converted principally
to carbon (also oxygen), with a net release of energy.
A Layered Structure Develops
- but remember that this is not because
heavy elements settle to the centre!
Unstable Red Giants
As we have seen, main sequence stars have long
stable lives -- perfect for life on Earth!
By contrast, in the red giant phase, things are not so
placid. The onset of He burning is very vigorous: the
so-called helium flash. The outer properties of the
star can change quite erratically. (This is well
understood by astrophysicists, but we will not
explore the details here.)
The Sun’s Likely
Future Behaviour
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
and O to even heavier elements, but with reduced life
expectancy
For Example:
Suppose we merge Carbon nuclei to form
something heavier (say, C12 + C12  Mg24).
The ‘poor quality’ of this new fuel suggests that it
won’t last very long: remember the binding energy
curve!
And so the cycle should continue, with
progressively poorer fuels being used in turn, for
ever shorter spans of time…
Instead,
Something Amazing Happens!
After the conversion of Helium to Carbon, the Sun
will undergo no more significant nuclear reactions
at all! But why?
Doesn’t this mean that gravity will win, and the
sun will dwindle down to a tiny dense object –
maybe a black hole? With no nuclear reactions to
produce heat, what can prevent such a total
collapse?
Meet Wolfgang Pauli
Nobel prize 1945
…and Chandrasekhar
Nobel prize 1983
Following Pauli, Chandra Developed
an Amazing New Understanding
…including remarkable ‘new physics’ – with a sad
role played by Eddington (seen here with Einstein).
What New Physics?
First, Consider the Earth
It is a rock, with a hot interior that will eventually
cool off. Even when it is stone-cold, it will not
collapse inward, because its crystalline structure
gives it permanent rigidity. [See ASTR 101]
The key players in maintaining this structure are
the electrons that surround atoms.
Electrons in Atoms
In everyday molecules and
materials, atoms are held
“at arm’s length” by their
surrounding clouds of
electrons.
How About
Electrons In Sun-Like Stars?
During the main-sequence phases, the body of
every star contains myriads of free electrons,
torn off the fully ionized nuclei thanks to the
extreme heat. They play no role in the energy
generation and are just ‘part of the background.’
But as the core shrinks, everything is squashed
more densely together. In stars like the sun,
the electrons suddenly behave in an unforeseen
way.
At Extreme Densities
As the density approaches a million times that of
water – unheard of on Earth! – the electrons
suddenly resist being squashed together, but the
resistance is very much more than you would have
expected on classical physics grounds.
It is a product of the new (1930s) science of
quantum mechanics (the physics of the very
small), combined with special relativity.
Pauli Exclusion:
A New Kind of Resistance
Even though they are physically tiny, electrons
cannot be arbitrarily squashed closer and closer
together. This is not because of their electric
charges or physical sizes, but something more
subtle.
Among other things, this
explains the ways in which
electrons distribute themselves
around ordinary atoms too!
“Electron Degeneracy”
In a large body (like a stellar core) that is full of denselypacked electrons, this provides a huge new source of
resistance against the pull of gravity. This resistance is
independent of the temperature of the star.
In other words, the star can now cool off until it is stonecold – and yet remain stable against the enormous
inward pull of gravity. The ‘degenerate electrons’
prevent its further 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!
Chandrasekhar’s Discovery
Chandra developed a full and correct understanding of
this behaviour, but also showed that there is a limit to
the mass of stars which may be supported in this
way: the ‘Chandrasekhar limit’ (~1.4 solar masses)
There are, of course, many stars more massive than
that! They cannot be supported by electron
degeneracy, and gravity might be expected to win
out. Are they fated to collapse?
An Unhappy
Episode
Eddington was very unhappy with that implication, and had
no confidence in Chandra’s findings. Speaking immediately
after Chandra at an important scientific meeting, he
completely undercut him and dismissed his work.
Remarkably, Chandra held no grudge. In the long term, he
was vindicated and won the Nobel prize decades later (for
many important contributions, not just for his white dwarf
work).
So We Understand White Dwarfs
But Why Do We See Them?
White dwarfs, supported by electron degeneracy,
form deep in the cores of sun-like stars when
they are in their red giant phases. In other
words, they are located deep within a hugely
extended low-density envelope of cool gas.
Do we ever get to see the white dwarf ‘cinder’?
If so, why and how?