Cambridge (CIE) IGCSE Your notes
Physics
Stars & The Universe
Contents
The Sun as a Star
The Scale of the Universe
Star Formation
Life Cycle of a Star
Galactic Redshift
The Big Bang Theory
Age of the Universe
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The Sun as a Star
Your notes
The Sun as a star
The Sun is a medium-sized star which lies at the centre of the Solar System
It consists mostly of the two elements hydrogen and helium
It radiates most of its energy in the infrared, visible and ultraviolet regions of the
electromagnetic spectrum
Our Sun
The Sun is a medium-sized star consisting of mostly hydrogen and helium
Nuclear fusion in stars
Extended tier only
Stars are huge balls of (mostly) hydrogen gas
In the centre of a star, hydrogen nuclei undergo nuclear fusion to form helium nuclei
An equation for a possible fusion reaction is:
2H
1
+ 31H → 42He + 10n
Where 2H (deuterium) and 3H (tritium) are both isotopes of hydrogen
1 1
These are formed through other fusion reactions in the star
A huge amount of energy is released in the reaction
All stable stars are powered by nuclear fusion reactions
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Examiner Tips and Tricks
Your notes
It is useful to remember that hydrogen is the fuel within stars, but the details of the
reaction between deuterium and tritium is not required at this level.
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The Scale of the Universe
Your notes
The Milky Way
The Universe
The Universe is defined as
A large collection of billions of galaxies
It is also the name given to the entirety of space
Galaxies
A galaxy is defined as
A large collection of billions of stars
Stars are large astronomical objects, such as the Sun
The Milky Way
The Milky Way is one of many billions of galaxies making up the Universe
The Sun is one of many billions of stars making up the Milky Way
Other stars in the Milky Way galaxy are much further away from Earth than the Sun is
Some of these stars also have planets which orbit them
Hierarchy of the Universe
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Your notes
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The Universe is a large collection of galaxies and a galaxy is a large collection of stars. The
Sun is a star at the centre of our Solar System in the Milky Way galaxy
Your notes
Astronomical distances
Astronomical distances, such as the distances between stars and galaxies, are
extremely large
To describe these distances, astronomers use a special unit called the light year
One light-year is defined as:
The distance travelled by light in one year
The diameter of the Milky Way is approximately 100 000 light-years
This means that light would take 100 000 years to travel from one side of the Milky Way
to the other
Extended tier only
One light year is equal to 9.5 × 1012 km, or 9.5 × 1015 m
Worked Example
The centre of our galaxy is 30 000 light years away.
(a) How long does it take light to reach the Earth?
Extended tier only
(b) Calculate this distance in km.
Answer:
(a)
The centre of our galaxy is 30 000 light years away
It takes light 30 000 years to reach the Earth from the centre of our galaxy
(b)
Extended tier only
Step 1: Write down the known quantities:
The centre of our galaxy is 30 000 light years away
1 light year = 9.5 × 1012 km
Step 2: Calculate the distance in km:
distance = 30 000 × (9.5 × 1012)
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distance = 2.85 × 1017 km
Your notes
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Star Formation
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Star formation
Extended tier only
Stars go through a sequence of evolutionary stages, known as the life cycle of a star
All stars follow the same initial stages:
nebula → protostar → stable star
Nebula
Stars form from a giant interstellar cloud of gas and dust called a nebula
Protostar
The gravitational attraction within a nebula pulls the particles closer together until a hot
ball of gas forms, known as a protostar
As the particles are pulled closer together the density of the protostar increases
This results in more frequent collisions between the particles which causes the
temperature to increase
Stable star
Once the protostar becomes hot enough, nuclear fusion reactions occur within its core
The hydrogen in the core of the star is converted into helium
Every fusion reaction releases heat and light which keeps the core hot
Once a star initiates fusion, it is known as a stable star
During this stage, the star is in equilibrium as the forces acting on it are balanced
Gravitational forces act inwards
This is an attractive force which pulls the outer layers inwards
Thermal pressure acts outwards
This is exerted by the expanding hot gases inside the star as energy is released
during fusion
Balanced forces in a stable star
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Your notes
The outwards and inwards forces within a star are in equilibrium. The centre red circle
represents the star's core and the orange circle represents the star's outer layers
Once a stable star is formed, the final stages of its life cycle depend on its mass
The different life cycles are shown below
Summary of the life cycles of stars
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Your notes
Flow diagram showing the life cycle of a star which is the same size as the Sun (solar mass)
and the lifecycle of a star which is much more massive than the Sun
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Life Cycle of a Star
Your notes
Life cycle of low mass stars
Extended tier only
A low-mass star is one with a mass of up to about eight times that of the Sun
After the main sequence, a low-mass star finishes its life cycle in the following
evolutionary stages:
red giant → planetary nebula → white dwarf
Red giant
After several billion years, the hydrogen fuel used for nuclear reactions begins to run out
Once this happens, the rate of fusion decreases, which causes the core to shrink and
heat up
As the energy produced by fusion decreases, the inward force due to gravity
becomes greater than the outward force due to the thermal pressure
Eventually, the star becomes a red giant when the core becomes hot enough for helium
to fuse into carbon
The energy released by re-ignited fusion reactions causes the outer layers of the star to
expand and cool
Planetary nebula
Once the helium in the core runs out, fusion reactions cannot continue
The star becomes unstable and the core collapses under its own gravity
The outer layers are ejected into space as a planetary nebula
White dwarf
The collapsed core of the red giant is called a white dwarf
The white dwarf cools down over time and as a result, the amount of energy it emits
decreases
The life cycle of a low-mass star
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Your notes
The life cycle of a star that is similar to our Sun
Examiner Tips and Tricks
A low mass star is any star that will eventually become a white dwarf. You may see
different sources giving different ranges of masses for stars within this category, or
terms such as low mass stars (up to 2 solar masses) or intermediate mass stars
(between 2 and 8 solar masses). Note that you do not need to know these numbers or
categories, only that all these stars will follow the same evolutionary stages.
Life cycle of high mass stars
Extended tier only
A high-mass star is one with a mass of more than about eight times that of the Sun
After the main sequence, a high-mass star finishes its life cycle in the following
evolutionary stages:
red supergiant → supernova → neutron star (or black hole)
Red supergiant
After several million years, the hydrogen in the core begins to run out
Similar to a low-mass star, the rate of fusion decreases and the core shrinks and heats up
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The star becomes a red supergiant when the core becomes hot enough for helium
fusion to start
Your notes
This causes the outer layers of the star to expand and cool
In the core of the star, helium fuses into carbon
This is followed by further fusion reactions in which successively heavier elements,
such as nitrogen and oxygen, are formed
During this stage, the core collapses and expands repeatedly as fusion reactions start
and stop
Supernova
Eventually, fusion reactions inside the red supergiant cannot continue once iron is
formed
The core of the star will collapse rapidly and initiate a gigantic explosion called a
supernova
At the centre of this explosion, a dense body called a neutron star will form
The outer layers of the star are ejected into space forming new clouds of dust and gas
(nebula)
The nebula from a supernova may form new stars with orbiting planets
The heaviest elements (elements heavier than iron) are formed during a supernova
and are ejected into space
These nebulae may form new planetary systems
Neutron star (or black hole)
In the case of the most massive stars, the neutron star that forms at the centre will
continue to collapse under the force of gravity until it forms a black hole
A black hole is an extremely dense point in space that not even light can escape from
The life cycle of a high-mass star
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Your notes
The life cycle of a star much larger than our Sun
Examiner Tips and Tricks
A high mass star is a one that will not eventually become a white dwarf. Make sure you
understand that most high mass stars become neutron stars and only the highest
mass stars become black holes.
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Galactic Redshift
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Galactic redshift
When a stationary object emits waves, they spread out symmetrically
If the source of the waves moves, the waves can become squashed together or spread
out
This change in wavelength is known as the Doppler effect
This effect applies to both sound waves and electromagnetic radiation (i.e. light waves)
Doppler effect of light
If a source of light moves towards an observer, the observed wavelength decreases
The wavelength of light shifts towards the blue end of the spectrum
This is called blueshift
If a source of light moves away from an observer, the observed wavelength increases
The wavelength of light shifts towards the red end of the spectrum
This is called redshift
Redshift and blueshift of light
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Your notes
When a light source moves away from an observer, redshift is observed, and when a light
source moves towards an observer, blueshift is observed
Observing redshift in distant galaxies
When astronomers compare light from glowing hydrogen in distant galaxies with light
from glowing hydrogen on Earth, the light appears to be redshifted
This means the observed wavelength has increased as the light travelled from the galaxy
to the Earth
This shows that distant stars and galaxies are moving away (receding) from the Earth
Redshift of light from a distant galaxy
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Your notes
Light emitted from glowing hydrogen in distant objects appears to be shifted towards the
red end of the spectrum, showing they are moving away from Earth
The greater the observed redshift:
the greater the distance to a galaxy
the faster the galaxy is moving away from Earth
Examiner Tips and Tricks
You need to know that in the visible light spectrum, red light has the longest
wavelength and the smallest frequency.
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The Big Bang Theory
Your notes
Evidence for the Big Bang Theory
Around 14 billion years ago, the Universe began from a single point that was extremely
hot and dense
A giant explosion, known as the Big Bang, caused the Universe to expand outwards
As each point moved away from the others, the Universe began to cool
As a result of the initial explosion, the Universe continues to expand
The Big Bang Theory
Tracing the expansion of the Universe back to the beginning of time leads to the idea it
must have begun with a “Big Bang”
Evidence from galactic redshift
Galactic redshift indicates that distant galaxies are moving away from us
If galaxies are moving away from us, this means the Universe must be expanding
Expansion of the Universe
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Your notes
Observations of light from galaxies show they are moving away from us which means the
Universe is expanding
Redshift provides evidence for the Big Bang because:
1. Observations show that distant galaxies are all moving away from us
We see that light from glowing hydrogen in stars from distant galaxies is redshifted in
comparison with light from glowing hydrogen on Earth
2. Observations show that the further away a galaxy is, the faster it is moving away from
us
The spectra of light from more distant galaxies are more redshifted than closer galaxies
due to the expansion of space itself
Galactic redshift spectra
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Your notes
The dark lines (representing glowing hydrogen) have shifted towards red wavelengths
due to the stretching of light as it travelled through space that was expanding
Examiner Tips and Tricks
Make sure that you understand that the stretching of the wavelength of light is due to
the expansion of the Universe, not the motion of stars and galaxies themselves.
This can be visualised by imagining a balloon with equally spaced points on it. The
balloon represents space and the points represent galaxies.
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When the balloon is deflated (i.e. the Universe was smaller), the points (galaxies) are
closer together and are at an equal distance apart.
Your notes
As the balloon (Universe) expands, all the points (galaxies) become further apart by
the same amount.
This is because the space between the galaxies itself has expanded.
Cosmic microwave background radiation
Extended tier only
Cosmic microwave background radiation (CMBR) is a form of electromagnetic radiation
that was emitted shortly after the beginning of the Universe
It is detected everywhere throughout the Universe
The CMBR map is the closest image that exists to a map of the Universe
It shows that the temperature of the Universe, and therefore the objects in it, are more or
less uniformly spread out
CMBR map of the Universe
The CMBR map shows areas of higher and lower temperature in the Universe. Regions with
higher temperatures have a higher concentration of galaxies, Suns and planets
Evidence from cosmic microwave background
radiation
Cosmic microwave background radiation provides evidence for the Big Bang because:
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1. Theory predicts the early Universe was extremely hot and dense
Therefore, CMBR would have initially existed as short-wavelength gamma radiation Your notes
The shorter wavelength in the past indicates the Universe must have been very hot in the
beginning
2. CMBR is consistent with radiation that has been stretched over time
The Big Bang would have released a lot of energy in the form of extremely high-energy
gamma radiation
As the Universe expanded, the wavelength of the radiation increased
Over time, it has been redshifted so much that it is now in the microwave region of the
spectrum
3. CMBR can be interpreted as the radiation left over from the Big Bang
The CMBR is extremely uniform which indicates the Universe was initially much smaller
than it is now
Redshift of CMBR
CMBR is a result of high-energy radiation being redshifted over billions of years
Worked Example
Describe and explain what can be deduced about the history of the Universe from the
CMBR.
Answer:
Step 1: Recall the features of the CMBR
Microwave radiation is detected from all directions at a similar intensity
Step 2: State the source of this radiation
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This is the radiation produced just after the formation of the Universe
Step 3: Describe how the wavelength has changed and explain why
Your notes
When the Universe was formed, the radiation was high in energy and short in
wavelength
Now it has less energy and a longer wavelength
This is because the Universe has expanded and cooled, causing the wavelength
to increase
Step 4: Suggest what this tells us about the Universe in the beginning
This suggests the Universe was initially very small and very energetic and has been
expanding since
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Age of the Universe
Your notes
The Hubble constant
Extended tier only
When Edwin Hubble looked at the absorption spectra of distant galaxies, he determined
a relationship between the speed of a galaxy and its distance from Earth
Comparing redshifts of galaxies
Hubble discovered that all galaxies show redshift, but the galaxies that are further away
show a greater increase in redshift
This is Hubble's law, which states
The speed of recession is proportional to the distance of the galaxy away from Earth
'Recession' speed means the speed at which something is moving away
This means that the further away a galaxy is from Earth:
the faster it is moving away
the greater the increase in redshift
Relationship between redshift and galaxy distance
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Hubble's law tells us the greater the distance to a galaxy, the greater the redshift, or the
speed it moves away from Earth
Your notes
Hubble’s law can be expressed as an equation:
v
H0 = d
Where:
H0 = Hubble constant (per second)
v = recessional velocity of an object, the velocity of an object moving away from an
observer (km/s)
d = distance between the object and the Earth (km)
From this equation, the Hubble Constant H can be defined as:
0
The ratio of the speed at which the galaxy is moving away from the Earth, to its distance
from the Earth
The accepted value of the Hubble constant is H = 2.2 × 10–18 per second
0
Examiner Tips and Tricks
Make sure to learn the currently accepted value of the Hubble constant.
You will be expected to know that the current estimate for H0 is 2.2 × 10–18 per second
Measuring recession speed & distance
Extended tier only
The Hubble constant H can be determined from measurements of:
0
redshift of the light emitted by a galaxy
the brightness of supernovae in the galaxy
Measuring recession speeds of galaxies
The speed of recession v of a galaxy (i.e. how fast it is moving away from the Earth) can
be found from the change in wavelength of the galaxy’s starlight due to redshift
Measuring distance using supernovae
The distance d to a galaxy can be determined using the brightness of a supernova in that
galaxy
Supernovae are exploding stars
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Certain types of supernovae have the same peak level of brightness (absolute
magnitude), meaning they can be used as standard candles
Your notes
These supernovae are so bright that they can be used for measuring distances to
the most distant galaxies
Age of the Universe
Extended tier only
Hubble's law can be rearranged to give the expression:
1 d
H0
= v
1
Since time is equal to distance divided by speed, the term represents an estimate
H0
of the age of the Universe
Hubble's law provides further evidence for the Big Bang
It shows that the Universe has been expanding since the beginning of time
If we looked at time in reverse, we would see galaxies were closer together in the
past
This suggests that the Universe must have originated from a single point and has
been expanding outwards ever since
Hubble's law graph
Using measurements from galactic redshift and brightness of supernovae, a graph of
recession velocity against distance can be plotted
Graph of galaxy recession velocity against distance
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A key aspect of Hubble’s law is that the furthest galaxies appear to move away the fastest
The gradient of the graph represents the age of the Universe Your notes
When the distance equals zero, this represents all the matter in the Universe being at
a single point
This is the singularity that occurred at the moment of the Big Bang
Astronomers have used this formula to estimate the age of the Universe is about 13.7
billion years
Worked Example
A distant galaxy is 20 light-years away from Earth.
Use Hubble’s law to determine the velocity at which the galaxy moves away from
Earth.
Answer:
Step 1: List the known quantities
Distance to the galaxy, d = 20 light-years
1 light year = 9.5 × 1015 m
Hubble constant, H = 2.2 × 10−18 per second
0
Step 2: Convert 20 light-years to m
d = 20 ly = 20 × (9.5 × 1015) = 1.9 × 1017 m
Step 3: Substitute values into Hubble's Law
v = H 0d
v = 2 . 2 × 10−18 × 1 . 9 × 1017 = 0 . 418 m/s
( ) ( )
The galaxy moves away from Earth at a velocity of 0.42 m/s
Examiner Tips and Tricks
If you are taking the Extended paper, remember that you have to learn the values for a
light year and the Hubble constant!
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