Download Document

This set of slides
• This set of slides covers the supernova of white
dwarf stars and the late-in-life evolution and death
of massive stars, stars > 8 solar masses, including
supernova of these large mass stars.
• Units covered: 65, 66, 67.
Mass Transfer and Novae
• 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 spirals inward to the
surface of the white dwarf.
Novae
• If enough material accumulates on the white dwarf’s surface,
fusion can be triggered anew at the surface, causing a massive
explosion.
• This explosion is called a nova (new as in new star.)
• If this process happens repeatedly, we have a recurrent nova.
The Chandrasekhar Limit and Supernovae
• If the mass of one of these
accreting white dwarfs exceeds
1.4 solar masses (the
Chandrasekhar Limit), gravity
wins! (momentarily)
• The additional gravity causes
just enough compression…
• 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.
Supernova!
This is a SINGLE
STAR with a
luminosity of
BILLIONS of
stars!
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 life, thanks to the high temperature and
pressure present in a red giant star.
• High-mass stars produce heavier elements like silicon,
magnesium, etc. up through iron, 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
react, fusing into carbon. Hydrogen
continues to fuse 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 (core) 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 – an enormous nuclear explosion.
• This is a Type II, Ib, or Ic supernova. (Ib, Ic subcatagories)
Light Curve for a Supernova
The luminosity
spikes when
the explosion
occurs, and
then gradually
fades, leaving
behind a…
Supernova Remnant
• The supernova has
left behind a
rapidly expanding
shell of heavy
elements that were
created in the
explosion.
• Gold, uranium and
all other heavy
elements all
originated in a
supernova (Type
II) explosion.
Types of Supernovae, Summary
• Type Ia: The explosion that
results from a white dwarf
exceeding the Chandrasekhar
Limit (1.4 solar masses.)
• Type II: Supernovae resulting
from massive star core
collapse.
• Less common:
– Type Ib and Ic: Result from
core collapse, but lacks
hydrogen, lost to stellar winds
or other processes.
Stellar Corpses
• A type II supernova leaves behind the collapsed core of
neutrons that started the explosion, a neutron star.
• If the neutron star is massive enough, it can collapse,
forming a black hole…
A Surprise Discovery
• Jocelyn Bell, a graduate student
working with a group of English
astronomers, discovered a
periodic signal in the radio part of
the spectrum, coming from a
distant galaxy.
• Astronomers considered (briefly)
the possibility of an alien
civilization sending the regular
pulses.
• More pulsating radio sources
were discovered These were
named pulsars.
• All pulsars are extremely
periodic, like the ticking of a
clock. In some cases, this ticking
is amazingly fast!
An Explanation
• An idea was proposed that
eventually solved the mystery.
• A neutron star spins very rapidly
about its axis, due to 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.
The Crab Nebula Pulsar
Interior Structure of a Neutron Star
Density approx.
equal to atomic
nucleus density.
High-Energy Pulsars
• Most pulsars emit both visible and radio
photons in their beams.
• Older neutron stars just emit radio waves.
• Some pulsars emit very high energy
radiation, such as X-rays.
– X-ray pulsars.
– Magnetars.
• Magnetars have very intense magnetic
fields that cause bursts of x-ray and
gamma ray photons.
1015 gauss mag field strength. Earth’s field, about 1 gauss.