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Stellar Evolution
Life Cycle of Stars
Post-Main Sequence Reactions
Red Giant to Horizontal Branch
•After helium burning begins, a star has two
sources of energy, hydrogen fusion in a shell
around the core and helium fusion in the core
•The core of the star becomes rich in carbon
and oxygen nuclei, and the star's surface
temperature goes up to become a horizontal
branch star
•Stars with masses greater than or equal to the
Sun become smaller and hotter at a constant
luminosity. They evolve to horizontal branch
stars by moving across the HR diagram at
constant brightness
AGB Stars
Once the helium in the core of the star is
exhausted, a helium burning shell will
develop beneath the hydrogen burning shell
The electrons in the core again become
degenerate and the star expands and cools
to become an asymptotic giant branch star
Most of the energy is coming from the
hydrogen burning shell. However, the
hydrogen shell is dumping helium ash onto
the helium shell. After sometime, enough
helium is built up so that the helium shell
undergoes an explosive event
A wind develops in the star's envelope
which blows the outer layers into space. It is
in this wind that dust particles are formed.
Planetary Nebula Phase
An asymptotic giant branch star
becomes more luminous and the rate
at which it loses mass increases
For stars less than 8 solar masses, a
strong stellar wind develops and the
outer layers of the star are removed
to expose the hot degenerate core
As the gas is expelled and the core is
visible, the color of the star becomes
much bluer and moves to the left in
the HR diagram
The star begins to emit large
quantities of UV radiation, ionizing
the hydrogen shell of matter. This
shell of ionized hydrogen is a
planetary nebula and the core is a
white dwarf
White Dwarfs
White dwarf stars are much smaller than normal
stars, such that a white dwarf of the mass of the
Sun is only slightly larger than the Earth.
White Dwarf Evolution
Once a white dwarfs contracts to
its final size, it no longer has any
nuclear fuel available to burn.
However, a white dwarf is still
very hot from its past as the core
of a star The white dwarf cools by
radiating its energy outward.
As a white dwarf cools, the ions
can arrange themselves in a
organized lattice structure when
their temperature falls below a
certain point.
The white dwarf will eventually
give up all its energy and become
a solid, crystal black dwarf.
Summary of Stellar Evolution
Low Mass Stars (~Msun)
High (>8 Msun)Mass Stars
Life After Death for White Dwarfs
The best explanation for novae is surface fusion on a white dwarf.
White dwarfs no longer have any hydrogen to burn in a fusion reaction.
A white dwarf in a binary system ‘steals’ extra hydrogen from its companion
by tidal stripping.
Hydrogen gas will build up on the surface of the white dwarf where the
surface gravity is extremely high.
The hydrogen outer shell will reach the point where fusion can begin and the
shell explodes in a burst of energy.
Nova in a Binary System
Nova in Hercules
Post Main Sequence
Supernova 1994D
Supernova 1987A
Supernova 1987A--The two large
rings are not yet completely
understood, though they appear to
be associated with the supernova.
The rings result from something
that the star did before it became a
supernova, probably associated
with strong stellar winds expected
in such stars.
Possibilities for High Mass Stars
Ends for Various Mass Stellar Remnants
Neutron Stars
A neutron star is a star that is
composed solely of degenerate
neutrons.
The mass of a star is squeezed into a
small enough volume that the protons
and electrons are forced to combine
to form neutrons.
For example, a star of 0.7 solar
masses would produce a neutron star
that was only 10 km in radius.
Their extremely small size implies
that they rotate quickly, according to
the conservation of angular
momentum.
Neutron Stars
The interior of a neutron star consists
of neutrons packed into such a dense
state that it becomes a superfluid sea
This dense mixture of neutrons (with
zero electric charge) can become a
friction-free superfluid at high
temperatures
The interior of a neutron star will
consist of a large core of mostly
neutrons with a small number of
superconducting protons.
These superconducting protons,
combined with the high rotation
speeds of the neutron star, produce a
dynamo effect which gives rise to an
enormous magnetic field
Pulsars
A powerful magnetic field, combined
with the rapid rotation, will produce
strong electric currents on the
surface of the neutron star.
Loose protons and electrons near the
surface of the neutron star will be
sweep up and stream along the
magnetic field lines
The magnetic axis of the neutron
star does not necessarily have to be
aligned with the rotation axis (like
the Earth), they can be inclined from
each other
Only when the Earth lies along the
axis of the neutron star is the energy
detected as a series of pulses, and the
object is called a pulsar.
Black Hole Terms
Rs—Schwarzschild radius
Event Horizon—Schwarzschild
Radius—distance beyond which
no event can be seen since light
cannot escape
Photon Sphere—distance at
which light “orbits” a black hole
Frame-Dragging in Black Holes
Black holes are gravity wells
that can not only draw mater
in but can spin it as well.
This effect, called framedragging, is most prominent
near massive, fast spinning
objects.
Matter in this system gets
caught up and spun around
the black hole.
Such discoveries help
scientists better understand
gravity itself.
Detection of Neutron Stars and Black Holes
Black Holes at Galactic Centers?
Recent results by astronomers
using the Hubble Space Telescope
now indicate that most - and
possibly even all - large galaxies
may harbor a black hole.
In all the galaxies studied, star
speeds continue to increase closer
the very center.
This indicates a center millions of
times more massive than our Sun is
needed to contain the stars.
This mass when combined with the
limiting size make the case for the
central black holes.