The death of starsLearning 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.

Questions3. How many helium-4 nuclei must

fuse to give a nucleus of:

a) carbon-12?3 helium-4 nuclei make carbon-

12b) 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

1.

6.

2. 3.

5.

7.

4.

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