Transcript Slide 1

The death of stars
Learning Objective:
• What happens to stars when they die?
The death of stars
• All of you will state what happens when
hydrogen fusion ends
• Most of you will explain how a star’s fate
depends on its mass
• Most of you will explain how a supernova
leaves a neutron star or a black hole
• Some of you will explain how elements made in
stars become part of new stars and planets
How do stars end?
• Eventually all hydrogen in the Sun’s core will
be used up
• As fusion slows down in the core of any star,
its core cools down and there is less pressure,
so the core collapses
• The star’s out layers, which contain hydrogen,
fall inwards, becoming hot
• This causes new fusion reactions, making the
outer shell expand
How do stars end?
• At the same time, the surface temperature
falls, so that the colour changes from yellow
to red
• This produces a red giant
• In the case of the Sun, calculations suggest
that it may expand sufficiently to engulf the
three nearest planets- Mercury, Venus, and
Earth
Inside a Red Giant
• While the outer layers of a red giant star are
expanding, its core is contracting and heating
up to 100 million K
• This is hot enough for new fusion reaction to
start
• Helium nuclei have a bigger positive charge
than hydrogen nuclei, so there is a greater
electrical repulsion between them
• If they are to fuse, they need greater energy
to overcome this repulsion
Inside a Red Giant
• When helium do fuse, they form heavier
elements such as carbon, nitrogen, and
oxygen, releasing energy
• After a relatively short period (a few million
years), the outer layers cool and drift off
into space
• The collapsed inner core remains as a white
dwarf
• No fusion occurs in a white dwarf so it
gradually cools and fades
Flow chart
Use the text on page 270 to create a flow
chart explaining what happens when the
sun turns into a red giant.
As fusion slows down in the core of any star, the core
cools. This reduces the pressure, so the core collapses
The stars outer layers, which contain Hydrogen, fall
inwards
Life of the Sun
Picture the life of a star like the Sun on the
H-R diagram
• Protostars are to the right of the main
sequence. As it heats up, a protostar moves to
a point on the main sequence, where it stays
for billions of years
• When it becomes a red giant, it moves above
the main sequence
• Finally, as a white dwarf, it appears below the
main sequence
Life of the Sun
Note: these stars are changing the
position they are plotted on the H-R
diagram. This does not mean they
move their position in space
More Massive Stars
• The sun is a relatively small star: its core won’t get
hot enough to fuse elements beyond carbon
• Bigger stars, greater than about 8 solar masses, also
expand, to become supergiants
• In these, core temperatures may exceed 3 billion
degrees and more complex fusion reactions can occur,
forming even heavier elements and releasing yet more
energy
• But this cannot go on forever; even massive stars do
not make elements heavier than iron
Question
1. At what point in its life does a star become a
red giant?
2. What determines whether a star becomes a
red giant or a supergiant?
3. How many helium-4 nuclei must fuse to give a
nucleus of:
a) carbon-12?
b) oxygen-16?
Questions
1. At what point in its life does a star
become a red giant?
When all of the hydrogen in its core has
been fused to helium.
2.What determines whether a star
becomes a red giant or a supergiant?
The mass of the star: only very large
mass stars become supergiants.
Questions
3. How many helium-4 nuclei must fuse
to give a nucleus of:
a) carbon-12?
3 helium-4 nuclei make carbon-12
b) oxygen-16?
4 helium-4 nuclei make oxygen-16
Beyond Iron
• When stars get as far as making iron in its
core, events take a dramatic turn
• So far, fusion of nuclei to make heavier ones
has involved a release of energy
• But when nuclei heavier than iron are made by
fusing lighter nuclei, there is an overall
increase in mass
• This means that some input of energy is
needed (remember: E=mc2)
Supernova Explosion
• A star of about 8 solar masses or more can get as far
as making iron in its core
• Iron nuclei absorb energy when they fuse, and there
is no source of heating to keep up the pressure in the
core
• The outer layers of the star are no longer held up by
pressure of the core, and they collapse inwards.
• The core has become very dense, and the outer
material collides with the core and bounces off, flying
outwards
• This results in a huge explosion called a supernova
Supernova Explosion
• In the course of the explosion, temperatures
rise to 10 billion K, enough to cause the fusion
of medium-weight elements and thus form the
heaviest elements of all- up to uranium in the
periodic table
• For a few days, a supernova can outshine a
whole galaxy
The Next Generation
• The material in the remnants of a supernova
contains all the elements of the periodic table
• As it becomes distributed through space, it
may become part of another contracting cloud
of dust and gas
• A protostar may form with new planets
orbiting it, and the cycle starts over again
Dense and Denser
• The core of an exploding supernova remains
• If its mass is less than about 2.5 solar
masses, this central remnant becomes a
neutron star
• This is made almost entirely of neutrons,
compressed together like a giant atomic
nucelus, perhaps 30km across
Dense and Denser
• A more massive remnant collapses even
further under the pull of its own
gravity, to become a black hole
• Within a black hole, the pull of gravity
is so strong that not even light can
escape from it
Dense and Denser
• Neutron stars are thought to explain pulsars,
discovered by Jocelyn Bell and Anthony Hewish
• As the core of a star collapses to form a neutron
star, it spins fasters and faster
• Its magnetic field becomes concentrated, and this
results in a beam of radio waves coming out of its
magnetic poles
• As the neutron star spins round, this beam sweeps
across space and might be detected a s regular series
of pulses at an observatory on some small, distant
planet
Questions
4. On a sketch copy of an H–R diagram,
draw and label a line tracing out the
life of a Sun-like star from protostar
to white dwarf.
5. Put these objects in order, from least
dense to most dense:
neutron star, protostar, supergiant, black
hole, main-sequence star
Questions
Questions
5. Put these objects in order, from least
dense to most dense:
neutron star, protostar, supergiant, black
hole, main-sequence star
Supergiant < protostar < main-sequence
star < neutron star < black hole
Glossary
• Neutron star: the collapsed remnant of a
massive star, after a supernova explosion.
Made almost entirely of neutrons, they are
extremely dense.
• Black hole: a mass so great that its gravity
prevents anything escaping from it, including
light. Some black holes are the collapsed
remnants of massive stars
The Important Questions
Mark Scheme
3.
2.
1.
5.
4.
6.
7.
Massive stars
• Core temperatures may exceed 3 billion degrees
• No star will fuse elements larger than iron together
• No fusion occurs in White Dwarves, so they will gradually cool
and fade
• For a few days, a Supernova can outshine a whole galaxy
• In the core of a supernova explosion, the temperatures may rise
to 10 billion Kelvin
• Elements larger than hydrogen require higher temperatures to
fuse together
• A neutron star is made up entirely of neutrons, like a giant
atomic nucleus, often only 30km in diameter
• A black hole’s gravity is so great, not even light can escape
Massive stars
• Core temperatures may exceed 3 billion degrees
• No star will fuse elements larger than iron together
• No fusion occurs in White Dwarves, so they will gradually cool
and fade
• For a few days, a Supernova can outshine a whole galaxy
• In the core of a supernova explosion, the temperatures may rise
to 10 billion Kelvin
• Elements larger than hydrogen require higher temperatures to
fuse together
• A neutron star is made up entirely of neutrons, like a giant
atomic nucleus, often only 30km in diameter
• A black hole’s gravity is so great, not even light can escape