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The Death of Stars
Stellar Recycling
• Eventually fusion will
exhaust the hydrogen
supply from the center
of the Sun.
• Internal pressure
decreases.
• Gravity pulls outer
layers in.
• Hydrogen burns in a
shell around the core.
• New energy released
will cause the sun to
expand in size.
• The Sun will swallow
up Mercury and
Venus.
• The surface will be
cooler, so it will look
red
• The Sun has become
a red giant.
The fate of the Sun
Planetary Nebulae
• The core becomes
so hot that helium
fuses to make
carbon.
• The Sun becomes
fainter and smaller.
• As the carbon core
contracts, it heats up
and generates more
energy.
• Hydrogen to Helium
fusion around core
increases again.
• Outer layers of the
Sun drift away
forming a planetary
nebulae.
• This is the Helix
nebula.
Ring Nebula in Lyra
Planetary Nebulae, Keck Telescope
M 27 The Dumbbell Nebula
Double Vortex Nebula
Spirograph
nebula
Stingray Nebula
Hourglass Nebula
NGC6826
NGC7027
NGC7009
NGC6578
White Dwarfs
• Stars less than 1.4
times the mass of the
sun continue to shrink.
• Our Sun will shrink
until it becomes the
size of the earth.
• It becomes very dense
(1 tsp. = 5 tons)
• Small and faint White
Dwarfs are hard to
see.
• They slowly fizzle out
over billions of years.
White dwarf size compared with the Earth
• A nova is a star that
has increased 10
times or so in
brightness.
• Nova occur when a
white dwarf draws off
stellar material from
a Red Giant
companion star.
• This new material
may heat up enough
to cause the star to
brighten dramatically.
• This can happen
many times.
Nova
Red Supergiant Stars
• A star 15 times the mass
of the Sun burns up
faster than the sun and
ends its life in an abrupt
way.
• The core contracts as the
outer layer expands.
• This causes Helium to
fuse to form carbon
• By the time the Helium is
exhausted the outer
layers have expanded
even further
• This creates a huge red
supergiant star.
• Betelgeuse is an
example
• The carbon core of a
supergiant contracts,
heats up, and begins
fusing into even heavier
elements.
• Eventually iron builds up
in the core.
• When iron fuses to make
heaver elements energy
is absorbed not given
off.
• Without this energy the
strong gravitational pull
of the star causes it to
collapse on itself in
seconds.
• The star rebounds in a
tremendous explosions
called a supernovae.
Supernovae
Supernovae cont.
• Type II supernovae
are a result of the
previous slides
process.
• The Crab Nebula is
the result of a
supernova first
seen by the
Chinese in 1054
AD.
• Pictures taken of
this nebula over the
years prove that
the cloud is still
expanding.
Veil Nebula
• Sometimes so much
stellar material is
absorbed by a white
dwarf from its
companion red giant
star that the white
dwarf blows up.
• Type II supernova
take longer to
happen and still have
Hydrogen when they
explode.
• This is a picture of
Supernova 1987a
seen in the Large
Magellanic Cloud,
Feb. 24, 1987
Type II Supernovae
Neutron Stars
• The collapse of the
core after the
explosion causes the
iron in the core to
decay to Helium
nuclei.
• Density becomes so
high that electrons are
squeezed into the
nuclei.
• What’s left is
neutrons.
• The neutron star is
only 20 km across.
• 1 tsp. Would weigh a
billion tons.
• They have a very
strong magnetic field.
Pulsars
• Pulsars are neutron
stars, spinning rapidly
and giving off radio
energy at their poles.
• They were first
detected in 1967 by
Jocelyn Bell.
• The rate of emissions
can be timed and
therefore people can
calculate the rotation
rate of the pulsar.
• Pulsars spin very
rapidly.
•
•
•
•
•
•
•
She was born in 1943.
While at Cambridge, she helped
her advisor, Antony Hewish, to
create a large radio telescope.
She was the first to hear the regular
pulses from what would later be
identified as pulsars.
At first it was thought that these
radio signals came from an alien
civilization, so they were termed
LGM’s (little green men!)
She did this while she was working
on her Ph.D. at Cambridge
University in 1967. She received her
Ph.D. in radio astronomy in 1968.
She is currently a Visiting Professor
of Astrophysics at the University of
Oxford and a Fellow of Mansfield
College.
Dr. Bell Burnell is the current
President of the Institute of Physics.
Jocelyn Bell
Pulsars cont.
Black Holes
• A dying star that
retains more than
three times the
Sun’s mass will
collapse past the
neutron star stage.
• When the mass has
been compressed to
a certain size, its
gravitational
radius, radiation
from the star can no
longer escape into
space.
• A black hole is
formed and from its
surface nothing
escapes not even
light.
Detecting Black Holes
•
•
•
•
•
•
Black Holes attract matter and
matter attracted to a black hole
accelerates as it is pulled
toward the hole.
Some of the matter is pulled
directly into the black hole and
is never seen again.
Some of the matter goes into a
high speed orbit around the
black hole.
This matter forms an accretion
disk around the black hole.
This matter, in the disk, will be
heated to a very high
temperature by friction.
It will radiate x-rays as the
material spins into the black
hole.
Black Holes in Galaxies
• Many people
now believe that
black holes can
be found at the
centers of most
galaxies.
• Many people
feel that a black
hole’s gravity is
necessary to
hold the galaxy
together.
Detecting Black Holes
• When people search
the position of x-ray
sources, they look for
a star that is warped.
• People than look for
an invisible
companion.
• People than check
for the right mass.
• Cygnus X- 1 was one
of the first Black
Holes discovered this
way.