Transcript 19Nov_2014

Units to cover
• 66, 67,68
Homework 9
Unit 64, problems 4, 5, 9
Unit 65, problems 4, 10
Unit 66, problem 6
Formation of Planetary Nebula
• As a red giant expands, it cools
– Outer layers cool enough for carbon
flakes to form
– Flakes are pushed outward by
radiation pressure
– Flakes drag stellar gas outward with
them
– This drag creates a highspeed stellar wind!
– Flakes and gas form a
planetary nebula
The Hourglass Nebula
White Dwarf Stars
• At the center of the
planetary nebula lies the
core of the star, a white
dwarf
– Degenerate material
– Incredibly dense
• Initially the surface
temperature is around
25,000 K
• Cools slowly, until it
fades from sight.
Figure 64.05e
Our Sun will end its life by becoming
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A. a molecular cloud
B. a pulsar
C. a white dwarf
D. a black hole
Mass Transfer and Novae
• A Roche lobe can
be seen as a sphere
of gravitational
influence around a
star
• Red Giant stars can
fill their Roche
lobes
• In a binary star
system, the Roche
lobes of the two
stars can touch, and
mass can pass
between them.
• If a white dwarf is in orbit around a red giant
companion star, it can pull material off the
companion and into an accretion disk around
itself
• Material in the accretion disk eventually falls to
the surface of the white dwarf
Novae
• If enough material accumulates on the white dwarf’s
surface, fusion can be triggered, causing a massive
explosion
• This explosion is called a nova
• If this process happens repeatedly, we have a recurrent
nova.
A Post-nova expansion
The Chandrasekhar Limit
and Supernovae
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If mass is added to a white dwarf, its
gravity increases
If the white dwarf mass exceeds 1.4 solar
masses (the Chandrasekhar Limit), the end
of the white dwarf is near.
The additional gravity squeezes the
degenerate material in the white dwarf,
causing it to compress by a small amount
• This compression causes the
temperature to soar, and this
allows carbon and oxygen to
begin to fuse into silicon
• The energy released by this
fusion blows the star apart in a
Type 1a supernova
Type 1a Supernova – Another
standard candle!
• The light output from a
Type 1a supernova
follows a very
predictable curve
– Initial brightness
increase followed by a
slowly decaying “tail”
• All Type 1a supernova
have similar peak
luminosities, and so can
be used to measure the
distance to the clusters
or galaxies that contain
them!
Formation of Heavy Elements
• Hydrogen and a little helium were formed shortly after the Big Bang
• All other elements were formed inside stars!
• Low-mass stars create carbon and oxygen in their cores at the end of
their lifespan, thanks to the higher temperatures and pressures present in
a red giant star
• High-mass stars produce heavier elements like silicon, magnesium, etc.,
by nuclear fusion in their cores
– Temperatures are much higher
– Pressures are much greater
• Highest-mass elements (heavier than iron) must be created in
supernovae, the death of high-mass stars
The Lifespan of a Massive Star
Layers of Fusion Reactions
• As a massive star burns its hydrogen,
helium is left behind, like ashes in a
fireplace
• Eventually the temperature climbs
enough so that the helium begins to
burn, fusing into Carbon. Hydrogen
continues to burn in a shell around
the helium core
• Carbon is left behind until it too starts
to fuse into heavier elements.
• A nested shell-like structure forms.
• Once iron forms in the core, the end
is near…
Core Collapse of Massive Stars
• Iron cannot be fused into any heavier element, so it
collects at the center of the star
• Gravity pulls the core of the star to a size smaller
than the Earth’s diameter!
• The core compresses so much that protons and
electrons merge into neutrons, taking energy away
from the core
• The core collapses, and the layers above fall
rapidly toward the center, where they collide with
the core material and “bounce”
• The “bounced material collides with the remaining
infalling gas, raising temperatures high enough to
set off a massive fusion reaction. The star then
explodes.
• This is a supernova!
Before and After – a Supernova
Light Curve for a Supernova
•
The luminosity
spikes at the
moment of the
explosion, and
gradually fades,
leaving behind a…
The Crab Nebula
A Surprise Discovery
Pulsating radio sources were
discovered, and were named
“pulsars”
All pulsars are extremely periodic,
like the ticking of a clock.
– But in some cases, much, much
faster…
An Explanation
• An idea was proposed that solved the
mystery
• A neutron star spins very rapidly about
its axis, thanks to the conservation of
angular momentum
• If the neutron star has a magnetic
field, this field can form jets of
electromagnetic radiation escaping
from the star
• If these jets are pointed at Earth, we
can detect them using radio telescopes.
• As the neutron star spins, the jets can
sweep past earth, creating a signal that
looks like a pulse.
• Neutron stars can spin very rapidly, so
these pulses can be quite close
together in time!
Slowing Down?
• Over time, the spin rate
of a pulsar can decrease
at a small but
measurable rate
• Sometimes the pulsar’s
diameter shrinks
slightly, causing a
momentary increase in
the pulsar’s rotation
• These “glitches” are
short lived, and the spin
rate begins to decrease
again.
Interior Structure of a Neutron Star
Black Holes
• It takes for a test
particle infinite time
to fall onto a black
hole. How can black
holes grow in mass?
Viewing a black hole
• You may be asking, “If
light cannot escape a black
hole, how can we see one?”
• If a black hole is in orbit
around a companion star,
the black hole can pull
material away from it.
• This material forms an
accretion disk outside of
the event horizon and heats
to high temperatures
• As the gas spirals into the
black hole, it emits X-rays,
which we can detect!
Light curves from a black hole
binary system
General Relativity
• Einstein predicted that not
only space would be
warped, but time would be
affected as well
• The presence of mass slows
down the passage of time, so
clocks near a black hole will
run noticeably slower than
clocks more distant
• The warping of space has
been demonstrated many
times, including by
observations of the orbit of
Mercury
• The slowing of clocks has
been demonstrated as well!
Gravitational Redshift
• Photons traveling away from a massive object will
experience a gravitational redshift.
– Their frequency will be shifted toward the red end of the
spectrum
Star Clusters
• Stars form in large groups out
of a single interstellar cloud of
gas and dust
• These groups are called star
clusters
• Open clusters have a low
density of stars – there is lots
of space between the cluster’s
members
• They can contain up to a few
thousand stars in a volume 14
to 40 light years across
• The Pleiades is a very familiar
open cluster
Globular Clusters
• Some clusters are much
more densely packed than
open clusters.
• These globular clusters
can have as many as
several million stars, in a
volume 80 to 320 light
years across!
A snapshot of stellar evolution
• Because all stars in a
given cluster formed at
the same time out of the
same cloud of material,
we can learn a lot about
stellar evolution by
examining a cluster’s stars
• We can locate each star in
a cluster on an HR
diagram and look for the
“turnoff point”, the point
on the main sequence
above which the stars in
the cluster have run out of
fuel and become red
giants
We can deduce the age of a cluster by
finding this turnoff point.
Finding a Cluster’s Age