AQA GCSE Physics 3-4
Stars & Space
GCSE Physics pages 266 to 275
April 10th 2010
AQA GCSE Specification
STARS & GALAXIES
13.10 What is the life history of stars?
Using skills, knowledge and understanding of how science works:
• to explain how stars are able to maintain their energy output for millions of years
• to explain why the early Universe contained only hydrogen but now contains a large
variety of different elements.
Skills, knowledge and understanding of how science works set in the context of:
• Our Sun is one of the many millions of stars in the Milky Way galaxy.
• The Universe is made up of at least a billion galaxies.
• Stars form when enough dust and gas from space is pulled together by gravitational
attraction. Smaller masses may also form and be attracted by a larger mass to become
planets.
• Gravitational forces balance radiation pressure to make a star stable.
• A star goes through a life cycle (limited to the life cycle of stars of similar size to the
Sun and stars much larger than the Sun).
• Fusion processes in stars produce all naturally occurring elements. These elements
may be distributed throughout the Universe by the
explosion of a star (supernova) at the end of its life.
The Milky Way
The Milky Way is the
name of our galaxy.
From Earth we can see
our galaxy edge-on. In a
very dark sky it appears
like a ‘cloud’ across the
sky resembling a strip of
spilt milk.
A very dark sky is required to
see the Milky Way this clearly
Galaxies
Galaxies consist of
billions of stars bound
together by the force of
gravity.
There are thought to be
at least 200 billion
galaxies in our Universe
each containing on
average 2 billion stars.
The Andromeda Galaxy
The Sun’s position in the Milky Way
Types of galaxy
Barred-Spiral – NGC 1300
Our galaxy is this type
Spiral – The Whirlpool Galaxy
Elliptical – M32
Irregular – The Small
Magellanic Cloud
Classifying galaxies
Choose appropriate words to fill in the gaps below:
Universe
The ___________
is made up of billions of galaxies which
billions
consist of __________
of stars bound to each other by the force
of ___________.
gravity
galaxy
Sun is
The name of our _________
is The Milky Way. The ______
located towards the outer edge of our galaxy.
spiral
The are different types of galaxy; ________,
barred-spiral,
barred-spiral
elliptical and irregular. The Milky Way is a ____________
Andromeda
galaxy. The _____________
Galaxy is the nearest spiral galaxy
to the Milky Way.
WORD SELECTION:
Andromeda galaxy spiral
Sun Universe
gravity
barred-spiral
billions
Galaxies
Notes questions from pages 266 & 267
1.
2.
3.
4.
5.
6.
7.
(a) What is a galaxy? (b) Name our galaxy. (c) How
many galaxies are there?
Copy and answer question (a) on page 266.
Outline the history of the Universe to the present day.
Explain the part played by gravity in the evolution of the
Universe.
Copy and answer questions (b) and (c) on page 267.
Copy the ‘Key points’ table on page 267.
Answer the summary questions on page 267.
Galaxies
ANSWERS
In text questions:
(a) When we use a powerful
telescope to see a distant
galaxy, we are seeing the
galaxy as it was billions of
years ago because the light
from it has taken billions of
years to reach us.
(b) About 13 billion years.
(c) They are both positively
charged, so they repel each
other. The force of repulsion
is much greater than the
force of gravity between
them.
Summary questions:
1. (a) Expanded, cooled.
(b) Attracted.
(c) Formed.
2. (a) (i) We could not send a
probe far enough.
(ii) Galaxies take millions of
years to form; we couldn’t wait
that long.
(b) (i) Gravitational forces hold
the stars together.
(ii) The Universe has
expanded leaving these vast
spaces.
Stars
A star is a massive,
luminous ball of gas that is
held together by gravity.
The Sun is a typical star that
consists of about 75%
Hydrogen, 24% Helium and
1% other elements such as
carbon and oxygen.
There are about 2 billion
stars in our galaxy.
The Pleiades Star Cluster
Stars colours
The colour of a star depends
on its surface temperature.
The Sun is an average
temperature yellow star.
RED = 3000°C
ORANGE = 5000°C
YELLOW = 6000°C
WHITE = 8000°C
BLUE = 10000°C and
above
Sirius, the brightest star in the
night sky is a blue-white star
The birth of the Sun
Nebula
Stars usually form
inside a nebula.
This is a cloud of
mostly hydrogen
along with smaller
amounts of other
elements ‘dust’.
Many young stars are found
inside the Orion nebula
Protostar
Due to gravitational
attraction, the gas and dust
clumps together.
Gravitational potential
energy is converted into
heat energy.
The gas starts to glow
forming a protostar.
Artist's conception of the birth of
a star within a nebula
Nuclear fusion
When the temperature rises
above about 10 million°C
hydrogen nuclei join together
to form Helium by the process
of nuclear fusion.
Energy is released.
The star becomes stable
when the radiation produced
causes an outward pressure
that prevents further
gravitational collapse of the
star.
Nuclear fusion from hydrogen
isotopes deuterium (H2) and
tritium (H3)
The birth of the solar system
About 99.9% of the original gas and dust formed
the Sun. The remaining 0.1% formed the planets
and other bodies of the solar system.
The future of the Sun
Main sequence
The Sun is about half way through
a 10 billion year period in its life
cycle called ‘main sequence’
During this time hydrogen in the
core of the Sun is converted into
Helium by the process of nuclear
fusion.
The Sun will gradually become
hotter over time so that in about
two billion years time life will no
longer be possible on Earth.
Red Giant
In about 5 billion years time the
hydrogen in the Sun’s core will run
out.
Without outward radiation pressure
the core will collapse under gravity
and become even hotter.
Eventually the temperature will be
high enough to cause the fusion of
helium into heavier elements such as
carbon and oxygen.
The now greater outward radiation
pressure will cause the Sun to
expand into a Red Giant.
When the Sun becomes a Red
Giant it will be nearly as big as
the Earth’s orbit about the Sun.
Planetary Nebula and White Dwarf
After only a few million years the Helium
will also run out in the Sun’s core.
A final collapse of the core occurs to
form a very hot dense object about the
size of the Earth called a white dwarf.
The rest of the Sun is blown away to
form a planetary nebula (from which a
new star might form).
The white dwarf will gradually cool over
billions of years to form a black dwarf.
The Ring Nebula
Low mass stars
The Sun is an average star.
There are many cooler stars of
lower mass called red dwarfs.
These are very faint and can
only be seen through
telescopes.
The nearest star to the Sun is a
red dwarf called Proxima
Centauri. It is just over 4 light
years away.
High mass stars
Most of the stars we can see in the
sky are more massive than the Sun.
Compared with the Sun they:
- are larger
- are brighter
- are bluer when main
sequence
- pass through their life cycles
more quickly
- sometimes end their lives
differently
All the main stars in the
constellation of Orion are
more massive than the Sun.
Red supergiants
Higher mass stars form larger
red giants.
The star Betelgeuse (top left in
Orion) is larger than the orbit of
Mars about the Sun.
Supergiants will also cause
elements such as carbon and
oxygen to undergo nuclear
fusion to form even heavier
elements such as silicon and
iron.
The internal ‘onion’ structure
of a red supergiant star
Supernovae
When a red supergiant star
causes iron in its core to undergo
nuclear fusion energy is absorbed
causing a great implosion.
This rebounds and causes a
massive explosion that can for a
few days outshine a whole galaxy.
This is called a supernova.
Supernovae are very rare. They
can be seen in the daylight sky.
The last observed supernovae in
our galaxy took place in 1604.
The Crab Nebula was
formed after a supernova
observed in 1054
Neutron stars
The core left over from a
supergiant star can be so
massive that gravity causes
electrons and protons to combine
to form neutrons. This is a
neutron star.
A neutron star is only about 10km
in diameter and is extremely
dense. A teaspoon full of neutron
star has a mass of about two
billion tonnes.
Some neutron stars, called
pulsars, emit regular radio
signals.
X-ray image of the neutron
star inside the Crab Nebula
Black holes
The most massive stars
collapse to form black
holes.
The gravity caused by
black holes is so strong
that nothing can escape,
including light.
Black holes can only be
observed from the affect
they have on surrounding
objects such as a
companion star.
The life cycle of the Sun
NOTE:
Due to its relatively low mass the Sun will not become a
red supergiant, supernova, neutron star or black hole.
Star evolution summary
protostar
main
sequence
star
CORE MASS
red
supergiant
> 1.4 ʘ
supernova
red giant
nebula
MASS
< 0.23 ʘ
CORE MASS
planetary
nebula
brown dwarf
(failed star)
< 1.4 ʘ
MASS
0.23 to 4 ʘ
MASS
> 0.05 ʘ
MASS
< 0.05 ʘ
MASS
>4ʘ
ʘ = Sun
white
dwarf
<3ʘ
>3ʘ
neutron
star
black
hole
Choose appropriate words to fill in the gaps below:
hydrogen
Stars are made mostly from __________
and generate their
energy by nuclear _________.
Stars are formed in nebulae
fusion
when ________
gravity causes gas and dust to clump together.
expand to form a
Towards the end of its life, the Sun will _________
red giant after which most of its material will be blown away
nebula leaving behind a small white _____
dwarf star.
as a planetary ______
massive stars than the Sun may undergo a supernova
More ________
neutron
explosion and become _________
stars or black holes.
WORD SELECTION:
expand
neutron
gravity
fusion
massive
hydrogen
nebula dwarf
The life history of a star
Notes questions from pages 268 & 269
1.
2.
3.
4.
5.
6.
7.
Copy Figure 2 on page 269.
Outline the life history of a star like our Sun. Your account should
include what is meant by (a) protostar, (b) red-giant, and (c) white
dwarf.
Explain the additional stages undergone by the most massive
stars. Your account should include what is meant by (a)
supernova, (b) neutron star, and (c) black hole.
(a) How does a star produce energy? (b) Explain why the Sun is
neither expanding or contracting at the present time.
Copy and answer questions (a), (b), (c) and (d) on pages 268 and
269.
Copy the ‘Key points’ table on page 269.
Answer the summary questions on page 269.
The life history of a star
ANSWERS
In text questions:
(a) The potential energy of
gas and dust decreases
when it gathers and is
transformed into heat
energy.
(b) The outward pressure
of radiation from its
core stops it collapsing.
(c) Gravity.
(d) Gravity.
Summary questions:
1. (a) B, A, C, D.
(b) (i) A
(ii) It will fade out and go
cold.
2. (a) (i) Expand, collapse.
(i) Explode, collapse.
(b) (i) The neutron star
must have sufficient
mass.
(ii) The gravitational field
is so strong that nothing
can escape from it.
The formation of elements
Hydrogen and helium
Hydrogen and some helium
was formed at the time of
the Big Bang.
Helium is also formed by
nuclear fusion in main
sequence stars like the
Sun.
Lighter elements
Elements such as carbon,
oxygen and silicon are
formed by nuclear fusion
in red-giant stars.
The heaviest element
formed in red-giants is iron.
The internal ‘onion’ structure
of a red supergiant star
Heavier elements
All elements heavier than
iron are thought to have
been formed during
supernovae explosions.
The fact that such elements
exist on Earth is evidence
that our Sun and the entire
solar system has been
formed out of the supernova
explosion of an earlier star.
Elements such as copper, gold
and uranium were formed in
supernovae explosions
Choose appropriate words to fill in the gaps below:
The lightest and most common element in the Universe is
hydrogen
helium
___________.
Hydrogen and some __________
were formed
from the Big Bang.
fusion
Most elements have been formed by nuclear _________
in the
lighter
cores of stars. Helium and the __________
elements such as
carbon
_________
are formed by stars like the Sun.
iron
Elements up to ______
are formed in the core of red
supergiant stars. The heaviest elements are formed in
supernovae explosions.
___________
WORD SELECTION:
carbon
hydrogen
fusion
helium
lighter
iron
supernovae
How the chemical elements formed
Notes questions from pages 270 & 271
1.
2.
3.
4.
5.
6.
Explain the two different processes by which (a) lighter
and (b) heavier elements were formed.
Copy and answer questions (a) and (b) on pages 270
and 271.
Outline the ways of trying to discover the presence of
extra-terrestial life.
Copy and answer question (c) on page 271.
Copy the ‘Key points’ table on page 271.
Answer the summary questions on page 271.
How the chemical elements formed
ANSWERS
In text questions:
(a) In a supernova
explosion.
(b) Its half-life is very short
compared with the age
of the Sun. Any
plutonium formed when
the Sun formed would
have decayed long ago.
(c) Carbon atoms are in all
the molecules that
make up living objects.
Summary questions:
1. (a) Hydrogen.
(b) Uranium.
(c) Helium, iron.
(d) Hydrogen.
2. (a) Stars, supernova.
(b) Supernova, galaxy.
(c) Stars, supernova.
The light year
A light year is the distance travelled by light in
one year.
Light travels at 300 000 000 metres per second
= 300 000 kilometres per second
= 18 million kilometres per minute
= 1.08 billion km per hour
= 26 billion km per day
= 9.5 trillion km per year!
Question
Calculate the distance to the nearest star to the
Sun, Proxima Centauri, in kilometres and how long
it would take to reach it travelling at 100 km/h (60
mph) if this star is 4.2 light-years away.
Distance = 40 trillion kilometres (4.0 x 1012 km)
It would take about 45 million years to reach
this star.
Universal issues
Notes questions from pages 272 & 273
1. Answer questions (a), (b) and (c) on
page 272.
Universal issues
ANSWERS
(a) 2 km
(b) 9000 km
(c) over 30 million km
How Science Works
ANSWERS
a)
b)
c)
d)
Myths: how the Earth was
created by Phan Ku.
Observations: that the Sun,
moon, planets and stars move
across the sky.
The data concerning the size of
the Moon. Ptolomy’s Earthcentred model required the
Moon to speed up and slow
down and hence therefore to
change its size as seen from
Earth. The Moon, when
measured, did not show these
changes in apparent size.
Example of hypothesis:
Anaxagoras hypothesised that
the Sun and the Moon were
made of rocks.
e) Ptolomy’s Earth-centred theory of
the Universe.
f) Copernicus’ theory of the universe
because it is supported by much
evidence, but theories are not
completely proven in all instances
and therefore always open to
being disproved. This is more
obviously shown by Bruno.
Planets are being discovered
around stars, but there is no
evidence of life outside of the
Earth.
g) Anaxagoras and Bruno were
examples of political influences on
science.