Transcript Chapter 13 powerpoint presentation

Chapter 13
Post Main Sequence Stellar Evolution
The Sun
The Sun (continued)
Internal Evolution of the Sun
The Sun 5 Billion years from Now
Evolution in the H-R Diagram
Main Sequence Turn-Off indicates
the age of a cluster
Many Mature Stars Pulsate
Which causes periodic changes in
brightness
Which in turn is correlated with the stars
luminosity – extremely useful as a
distance indicator
The Death of Stars Depends on Mass.
For a 1 Mo star…
Planetary Nebulae (with a White Dwarf at the center)
White Dwarfs
W.D progenitors are stars with main sequence masses < 8 M.
W.D’s are Carbon-Oxygen cores, exposed during the planetary
nebula stage.
W.D cores are supported by degenerate electron pressure, according
to the Pauli exclusion principle, not by thermal pressure. So the
pressure is independent of the internal temperature. As a result,
W-D’s have approximately uniform density, so, in this case we can
reliably use the equation of hydrostatic equilibrium to estimate the
the central pressure of a W.D.
Additionally, the mass-radius relation for W.D. is M R3 = constant,
which means that as the mass increases, the size decreases!
The Chandrasekhar Limit
However, there is a maximum mass for W.D’s – the maximum
that can be supported by electron degeneracy pressure - and that
mass corresponds to 1.4 M
Any more mass than 1.4 M will cause further collapse until
Neutron degeneracy is reached, leading to a neutron star. A
Neutron star is essentially a giant iron nucleus comprised of
protons and degenerate neutrons and electrons with enough free
electrons to produce zero net charge.
Neutron stars obey the same mass-radius relation as W.D.’s, so
they too shrink as more mass is added! When the mass of a
Neutron star exceeds 3 M it will collapse into a singularity,
a point of infinite density, where the known laws of physics break
down.
White Dwarfs cool and fade into
obscurity
High Mass Stars (> 5 Mo) process H into
Fe becoming Red Supergiant stars
Endothermic vs. Exothermic Reactions
All thermonuclear reactions occurring in the cores of stars are
exothermic, that is, they release energy, but only up until the
Fusion of Iron (Fe). Iron takes more energy to fuse than can
be obtained from it, and is an example of an endothermic
process, which does not occur in stars.
As stars produce nuclei with masses progressively nearer the
iron peak of the binding energy curve, less and less energy is
produced per kg of fuel, until none is produced at all, marking
the onset of a supernova explosion.
As the star collapses, the core grows until it reaches the
Chandrasekhar limit, and then collapses into a rapidly rotating
neutron star.
The Supernova Explosion
The supernova explosion is caused when the overlying layers
of the stellar atmosphere free-fall onto the core and literally
bounce – off, creating a shock wave that blows off all the other
overlying layers, in a spectacular explosion, that we know as a
Supernova.
The supernova explosion produces so much light that it can
temporarily outshine an entire galaxy. The photons destroy the
heavy nuclei through a process of photodisintegration to produce a
flood of neutrinos.
For all supernovae, the source of light during the decline in
brightness after the explosion is the decay of radioactive isotopes
created in the supernova explosion.
SN 1987A - Before and After
Interestingly, no compact
remnant has been found, only
a flood of neutrinos following
the explosion has been detected.
Supernova Remnants
Pulsars are magnetized Rotating Neutron Stars
The Crab Nebula
Pulsar light curves
Pulsar periods increase as the neutron star looses rotational energy
White Dwarfs, Neutron Stars, and Black Holes
Summary
Maximum mass of a White Dwarf is 1.4 Mo, above that it
collapses into a Neutron star.
If the mass of a Neutron star exceeds 3 Mo, it will likely
collapse into a Black Hole.