Edexcel IGCSE Physics Your notes
Stellar Evolution
Contents
Classification of Stars
The Life Cycle of Solar Mass Stars
The Life Cycle of Larger Stars
The Brightness of Stars
Hertzsprung-Russell Diagrams
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 1
Classification of Stars
Your notes
Classification of stars
Stars come in a wide range of sizes and colours, from yellow stars to red dwarfs, from
blue giants to red supergiants
These can be classified according to their colour
Warm objects emit infrared and extremely hot objects emit visible light as well
Therefore, the colour they emit depends on how hot they are
A star's colour is related to its surface temperature
A red star is the coolest (at around 3000 K)
A blue star is the hottest (at around 30 000 K)
Star colour and surface temperature
The colour of a star correlates to its temperature. The bluer the star, the hotter its surface
temperature. The redder the star, the cooler its surface temperature
Astronomical objects cool as they expand and heat up as they contract
This means that their colour will also change according to their surface temperature
When a star becomes a red giant it becomes redder as it expands and cools
When a star becomes a white dwarf it becomes whiter as it contracts and heats up
Examiner Tips and Tricks
We often remember red as being hot and blue as cool in everyday life, but remember
this is the other way around when describing the temperature of stars!
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 2
The Life Cycle of Solar Mass Stars
Your notes
The life cycle of solar mass stars
All stars, including the Sun, began as a cloud of dust and gas
Once a star has formed, it will spend its life going through a sequence of evolutionary
stages, known as the life cycle of a star
Summary of the life cycles of stars
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
Star formation
All stars follow the same initial stages:
Nebula → protostar → main sequence star
Nebula
Stars form from a giant interstellar cloud of gas and dust called a nebula
Protostar
The force of gravity 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 will increase
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 3
This results in more frequent collisions between the particles which causes the
temperature to increase
Main sequence star Your notes
Once the protostar becomes hot enough, nuclear fusion reactions occur within its core
Once a star initiates fusion, it is known as a main-sequence star
During the main sequence, the star is in equilibrium and said to be stable
The life cycle of a solar mass star
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 causing the fusion reactions in the star will begin
to run out
Once this happens, the fusion reactions in the core will start to die down
The star will begin to fuse helium which causes the outer part of the star to expand
As the star expands, its surface cools and it becomes a red giant
White dwarf
Once the helium fusion reactions have finished, the star collapses and becomes 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
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 4
Your notes
The life cycle of a star that is similar to our Sun
Examiner Tips and Tricks
Make sure you remember the life cycle for a solar mass star and ensure you can
describe the sequence in a logically structured manner in case a 6 marker comes up in
the exam!
Ensure you can remember the end stages for a solar mass star clearly (red giant,
planetary nebula, white dwarf) as this is different for a star that is much larger than our
Sun!
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 5
The Life Cycle of Larger Stars
Your notes
The life cycle of larger stars
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)
The key differences between a lower mass and higher mass star at this stage are:
A higher mass star will stay on the main sequence for a shorter time before it
becomes a red supergiant
A lower mass star fuses helium into heavy elements, such as carbon, whereas a
higher mass star fuses helium into even heavier elements, such as iron
Red supergiant
After several million years, the hydrogen causing the fusion reactions in the star will
begin to run out
Once this happens, the fusion reactions in the core will start to die down
The star will begin to fuse helium which causes the outer part of the star to expand
As the star expands, its surface cools and it becomes a red supergiant
Supernova
Once the fusion reactions inside the red supergiant cannot continue, the core of the star
will collapse suddenly and cause a gigantic explosion called a supernova
At the centre of this explosion, a dense body called a neutron star will form
The outer remnants of the star are ejected into space forming new clouds of dust and
gas (nebula)
The heaviest elements are formed during a supernova, and these 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
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 6
Your notes
The life cycle of a star much larger than our Sun
Examiner Tips and Tricks
Make sure you remember the life cycle for a high-mass star and that you can describe
the sequence logically in case a 6-marker comes up in the exam!
Ensure you can clearly remember the end stages for a high-mass star (red supergiant,
supernova, neutron star/black hole) as this is different for a star that is a similar size to
the Sun!
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 7
The Brightness of Stars
Your notes
Apparent and absolute magnitude
Luminosity
The luminosity of a star is defined as
The total amount of light energy emitted by the star
Luminosity is a measure of a star's brightness or power output
Apparent magnitude
The brightness, or apparent magnitude, of a star depends on two main factors:
the luminosity of the star
the distance the star is from Earth (more distant stars are usually fainter than nearby
stars)
Apparent magnitude is defined a
The perceived brightness of a star as seen from Earth
The apparent magnitude scale runs back to front:
the brighter the star, the lower the magnitude
the dimmer the star, the higher the magnitude
The apparent magnitude scale
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 8
Your notes
Examples of the apparent magnitude of different astronomical bodies
Absolute magnitude
Astronomers describe the brightness of stars at a standard distance using the absolute
magnitude scale
a bright star which is far away can look the same as a dim star which is nearby
therefore, it is difficult to measure the brightness of stars directly
Absolute magnitude is defined as
A measure of how bright stars would appear if they were all placed the same distance
away from the Earth
The standard distance astronomers use is 10 parsecs, 32.6 light-years or 3.04 × 1014 km
away from the Earth
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 9
Hertzsprung-Russell Diagrams
Your notes
Hertzsprung-Russell diagrams
The properties of stars can be classified using the Hertzsprung-Russell (HR) diagram
This is a plot of luminosity on the y-axis and temperature on the x-axis
Usually, it is given in solar units, where the luminosity of the Sun = 1, so
For stars which are brighter than the Sun, luminosity > 1
For stars which are dimmer than the Sun, luminosity < 1
Surface temperature is measured in kelvin (K) and is plotted backwards from hottest to
coolest
It can also be displayed as a colour where
The hottest stars are blue
The coolest stars are red
The Hertzsprung-Russell diagram
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 10
The Hertzsprung-Russell (HR) diagram is a way of displaying the properties of stars and
representing their life cycles
Your notes
The key areas of the H-R diagram are:
The brightest stars (high luminosity) are found near the top
The dimmest stars (low luminosity) are found near the bottom
The hottest stars (high temperature) are found towards the left
The coolest stars (low temperature) are found towards the right
The life cycle of a star can be shown on a Hertzsprung-Russell diagram
The main features of the Hertzsprung-Russell diagram are:
Most stars are found to lie on the main sequence. This is the band of stars going
from top left to bottom right
Below the main sequence (and slightly to the left) are the white dwarfs
Above the main sequence on the right-hand side are the red giants
Directly above the red giants are the red supergiants
This means that
The hottest, brightest stars are the largest main sequence stars, also called
supergiant stars
The coolest, brightest stars are red supergiants
The hottest, dimmest stars are white dwarfs
the coolest, dimmest stars are the smallest main sequence stars, also called red
dwarfs
Worked Example
Stars can be classified using the Hertzsprung-Russell (H-R) Diagram.
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 11
Your notes
(a) State the types of stars found in areas A, B, C and D
(b) On the H-R diagram, plot the star with a surface temperature of 20 000 K and a
luminosity 10 000 times greater than the Sun and label it Star X.
Answer:
(a)
A = white dwarf stars
B = main sequence stars
C = red supergiant stars
D = red giant stars
(b)
Step 1: List the known quantities
Surface temperature of Star X = 20 000 K
Luminosity of Star X = 10 000 times that of the Sun
Step 2: Use the graph to find the value for the luminosity of the Sun
Use a ruler and pencil to draw a line from the position of the sun to the luminosity
axis (y-axis)
The Sun’s luminosity on this scale is 1 because the luminosities given are relative to
the luminosity of the sun
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 12
Your notes
Step 3: Calculate the luminosity of Star X
Star X is 10 000 times that of the Sun
The luminosity of the Sun is 1
10 000 × 1 = 10 000 or 104
Step 4: Plot the position of Star X on the HR diagram
Locate the surface temperature of Star X at 20 000 K
Locate the luminosity of Star X at 104
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 13
Your notes
Plot the point and label it Star X:
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 14
Examiner Tips and Tricks Your notes
Make sure you remember the key components of this diagram to be able to draw it
from memory, more specifically which way around the axis goes - the x-axis is the
opposite way round to what you might be used to!
If you forget how where the different types of stars are found on the HR diagram, try
and think about it logically:
The main sequence is the easiest to recognise as it is the long band diagonally
central to the diagram where the majority of stars are found
White dwarf stars are hot, but very small, so they are not very luminous, so you
need to identify the area with a lower luminosity than the main sequence
Red giants and supergiants have clues in their names - they are giant, so they are
very luminous and they are red, so they tend to be on the cooler end of the
temperature scale
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 15
