Doc Brown's GCSE 9-1 Physics Revision
Suitable for GCSE/IGCSE Physics/Science courses
Cosmology - the Big Bang Theory of the Universe
solar system and satellites for more detailed notes
Introduction to the life cycle of stars - obtaining experimental data
The diagram on the right summarises all you
need to know
.This page will tell you all about the life
cycle of stars ...... describing and explaining about the sequences involving
1. clouds of dust and gas, formation of a 2. protostar, a 3. main
sequence star, a 4. red giant or a red supergiant (red super
giant, super red giant), the diffuse 5. planetary nebula, a 6. white
dwarf, a 7. black dwarf (love the names!), the explosive 8.
supernova, the dense whizzing 9. neutron star and the ultimate 10.
black hole - the destination of no return!
The detection and analysis of various
types of electromagnetic radiation have contributed greatly to our understanding
of the life cycle of stars and lots of theoretical calculations too!
Different sorts of telescopes are used
to detect the different frequencies of EM radiations.
This enables you to get the most
comprehensive picture of the structure of the Universe and not just the type
and structure of stars.
Generally speaking, the bigger the
telescope, the greater the resolution of the image produced.
Bigger telescopes can gather more EM
radiation for a computer to produce an image, so a powerful computer
connected to a powerful telescope is the best you can do.
This enables faint objects to be
detected and some the fine structure of aspects of the Universe less
It also means with increased
magnification, we can see further into the space we call the Universe as
we discover more and more galaxies and other structures.
Powerful high screen resolution
computers can produce the finest of images AND store them - in fact they
enable databases to built up over time for further analysis at any time
in the future.
Computers can automatically record
millions of images all the time without having to rely on an astronomer
working the telescope.
Artificial intelligence computer
programmes are being developed that can make it easier for an astronomer
to analyse data more easily and much quicker than the human eye and
From the 1940s onwards and the
development of radar technology, giant dishes called radio telescopes
can collect radio waves
and give us 'hidden' information that other EM wavelengths cannot.
radio waves can penetrate through star dust that scatters visible light so
we see other features of star systems e.g. detecting stars being born.
The discovery of cosmic microwave
background radiation was made using a radio telescope.
Infrared radiation has the same
advantage as radio waves - it penetrates gas and dust, so seeing objects
behind the 'stellar debris' like the early stages of star formation.
You can make special detectors,
special infrared cameras, to produce an infrared picture of an object
from a planet to the whole of the Universe. You need special optical
lenses to do this.
Optical telescopes using glass lenses or
metal surface mirrors, to detect visible
light which can be used to measure the visible size of the 'hot' volume
of a star and the spectral lines can identify the elements in it.
visible light emitted depends on the age and type of star.
Optical telescopes were the earliest
types used to examine the 'universe'.
They can be used to examine near
objects and galaxies.
Ultraviolet is also used to study
young star development and the shapes of galaxies and also identify elements
from uv spectral lines. Special uv cameras form part of a uv telescope.
X-ray telescopes give information
on very high energy particle interactions happening at the highest
e.g. when the temperature is many
millions of degrees centigrade in the violent explosions of supernovae.
The sequences in the life cycle of
Dust clouds and gas:
Stars and their planetary systems are formed from congregations
of clouds of dust and gas that occur in interstellar space (the space
Until fusion begins in stars, the
early Universe contained only hydrogen, but now contains a large
variety of different elements.
It is the fusion processes in stars
that produce all of the naturally occurring elements from
hydrogen (1) to uranium (92) - most of the periodic table!
Very slowly, due to gravity, the more
dense regions of dust and gas can come together to form a protostar, but
there is no nuclear fusion for some time and it mainly consists of
hydrogen - plus small amounts of other elements that were once part of
stars themselves - remember this page is about the life cycle of stars!
In the protostar, as the mass and density increases
so does the gravitational pull. As a result
particle collisions increase in frequency and more forcefully, and heat is released and the protostar begins
to glow, emitting lots of infrared and microwave radiation.
The force of gravity is doing
work to compress the gases and dust so the gravitational potential
energy increases the kinetic energy store of the particles
increasing the temperature and pressure of the core of a protostar.
BUT, stars undergoing nuclear
fusion reactions can only form when
enough dust and gas from space is pulled together by gravitational
attraction in the protostar AND the temperature rises to at least 15
millions degree Kelvin for fusion to start.
Other parts of the gas and dust
further out from the core still get hotter and denser and if the mass is
great enough, gravity will pull them together to form a planets
and orbit the star - held in nearly circular orbits by the gravitational
pull of the central star e.g. like our Sun and its eight planets (and a
lot of other stuff too!).
Main sequence star:
When the temperature of the protostar gets high enough the
nuclei of hydrogen atoms fuse together to form helium nuclei and the
true main sequence star is formed. All of these nuclear fusion
reactions release enormous amounts of energy and temperatures finally
15 000 000oC in centre of the protostar as a star like
our Sun is born and emits vast amounts of energy in the form high
speed particles and all the frequencies of electromagnetic radiation.
There are several possible
nuclear fusion reactions and the most abundant initial fusion
product is helium e.g.
the fusion of hydrogen-1 with hydrogen-2 to form helium-3, or,
the fusion of hydrogen-2 and
hydrogen-3 to give helium-4
Nuclear fusion is the joining
of two atomic nuclei to form a larger one and is the process by
which energy is released in stars.
These are known as
Nuclear fusion releases huge
amounts of energy to keep the star's core at a very high temperature
- high enough to keep fusion going for a long time!
Nuclear energy store ==>
thermal energy store an EM radiation
After the initial formation of the star it becomes it
enters a period of equilibrium - a state of balance between
two competing factors, and at this stage it is called a 'main
Due to the enormous energy release from nuclear
fusion the star tries to expand outwards because of pressure created by the
kinetic energy of the particles and the intensity of the radiation
emitted (thermal expansion - think of a balloon expanding on being warmed).
the case of the 'heated balloon', there is a counter force of gravity
pulling the particles together inwards - this pulling of everything together is
referred to as gravitational collapse.
This creates a balance of inward
and outward forces i.e. an equilibrium which lasts for billions of
years because there is so many hydrogen nuclei to fuse together to form
So, due to nuclear fusion processes, stars are able to maintain their
energy output for billions of years.
Our Sun is ~4.5 billion years old and around
half-way through its stable main sequence star stage.
The greater the mass of the star,
the shorter it's time as a main sequence star.
masses may also form and be attracted by a larger mass to become planets
around a star (a much larger mass) - also so formed by possible spin-off
from the star?
Red giant star
After the main sequence stage comes the first instance when two 'life
cycle' pathways are possible depending on the initial mass of the
main sequence star.
It gets complicated and giant red
stars expand and contract several times before they enter their
final phases - white dwarf ==> black dwarf OR supernova ==> neutron
After billions of years, the hydrogen
in the core begins to run out and the force of gravity is greater than
the pressure of thermal expansion. The star is compressed until it is
dense and hot enough so that the outer layers expand to form a red
giant or red supergiant.
Other nuclear reactions occur because
most of the hydrogen in the core was consumed in
nuclear fusion to helium, the temperature is still high enough for
fusion to continue, but now the initial nuclear fuel is helium, so
fusion to form heavier elements begins.
As the change in fusion reactions are
taking place, the star swells up and enters the red giant or
red supergiant phase of its life - the outer layers are cooler
which is why it glows red and not bright white.
Small to medium sized stars
form red giants.
More massive stars form red
Nuclear reactions like
Heavier elements from lithium
(3Li) to iron (26Fe), atomic numbers
3 to 26, can only be formed during the red giant or super red giant period of
the star's life - the bigger the mass of the star, the hotter and more
unstable it becomes, and the more heavier elements you can form.
In fact the elements heavier
than iron (cobalt to uranium, atomic numbers 27 to 92) can only
be formed in
red supergiant to supernova phase,
where the temperatures are VERY much higher (see
In the core of red giants or red
supergiants, the temperatures can exceed 100 million degrees Kelvin -
the minimum temperature needed to produce the heavier elements from
For suns about the size and mass of our Sun
(a medium sized star), a red giant
the sequence is
When a red giant runs out of suitable
nuclear fusion fuel it becomes unstable and ejects the outer
layers of gas and dust to form a glowing planetary nebula
(nothing to do with planets!).
This stellar debris will eventually
end up in other star systems.
After the planetary nebula is formed
the remnants of the red giant's core come together due to the pull of gravity to form
a dense solid core called a white dwarf, which is still quite hot and glows white from thermal
energy. Nuclear fusion is no longer taking place.
As the white dwarf loses energy from its
thermal energy store, it gradually cools down until the residue no
longer emits visible light. So, it gradually fades away and eventually becomes invisible through
supergiant star (red super giant, super red giant):
For suns much more massive than our Sun a red supergiant is
From main sequence stars, converting
hydrogen to helium in nuclear fusion, you get red giants (described
above) in which elements from lithium (3Li) to iron (26Fe)
are formed from fusing heavier nuclei, BUT, ...
As a result the star begins to
glow brightly again and may expand and contract several times due to
the opposing forces from nuclear energy release and gravitational
The positive nuclei of heavier
elements need enormous kinetic energies to overcome the massive repulsion
forces between the positively charged nuclei when they collide,
prior to fusion to making even larger nuclei.
the sequence is
A supernova is a massive explosion of
a red supergiant.
Eventually a red supergiant itself
runs out of fuel and becomes unstable, and collapses in on its
self, and then undergoes a
massive explosion, shining incredibly brightly for a short
period of time.
The exploding supernova throws out
the outer layers of dust and gas into space leaving a very dense
residual core ...
... so, the residues of the supernova
explosion come together due to gravity to form relatively small but
incredibly dense objects - the neutron star or a black hole.
Elements heavier than
iron, (cobalt to uranium, atomic numbers 27 to 92), are only formed in a supernova
explosion of a red supergiant in a truly 'cosmic' scale explosion, where the
temperatures are much higher than the 15 million degrees of our Sun!
Reminder: The positive nuclei of
the heaviest elements need enormous kinetic energies to overcome the massive repulsion
forces on collision between the even more positively charged nuclei
prior to fusion to making even larger nuclei.
The debris from a supernova explosion
contains all the elements from H to U and become the 'star dist' for future star
So, the elements formed from nuclear fusion may be
distributed throughout the Universe by the explosion of a massive star
(supernova) at the end of its life and eventually will be recycled in other
A neutron star is very small and extremely dense as
the particles are squeezed together by gravitational attraction.
Neutron stars are only 20-30 km in
diameter and are so dense that a few cm3 can have a mass of
Try and imagine the following idea.
The nucleus of an atom is tiny compared to the rest of the atom which is
almost empty apart from the orbiting electrons. The nucleus is < 1/10
000 th of the diameter of an atom. Now, imagine negative electrons are
forced to combine with positive protons to form neutral neutrons. All
that space the electrons occupied has gone, hence the massive increase
in density on the formation of a neutron star. This is only part of the
neutron star story - but it makes you think! I hope?)
If the mass of the remnants from a
supernova explosion are even greater than that required to form a
neutron star, a black hole is formed.
A black hole's mass is so great, and
its density is so great, nothing appears to escape from it, including
Therefore it is invisible to
telescopes - black holes are detected by their gravitational effect on other objects nearby -
in fact its gravitational pull is great enough to 'suck in' matter that
comes near it, so a black hole can increase in size and mass.
Cosmology - the Big Bang Theory of the Universe
solar system and satellites for more detailed notes
TOP OF PAGE
WAVES - electromagnetic radiation, sound, optics-lenses, light and astronomy revision notes index
introduction to the types and properties of waves, ripple tank expts, how to do
Illuminated & self-luminous objects, reflection visible light,
ray box experiments, ray diagrams, mirror uses
Refraction and diffraction, the visible light
spectrum, prism investigations, ray diagrams explained
sources, types, properties, uses (including medical) and dangers gcse physics
The absorption and emission of radiation by
materials - temperature & surface factors including global warming
Global warming, climate change,
reducing our carbon footprint from fossil fuel burning gcse
Optics - types of lenses (convex, concave, uses),
experiments and ray
diagrams, correction of eye defects
The visible spectrum of colour, light filters and
explaining the colour of objects gcse physics revision notes
Sound waves, properties explained, speed measure,
uses of sound, ultrasound, infrasound, earthquake waves
The Structure of the Earth, crust, mantle, core and earthquake waves (seismic wave
Astronomy - solar system, stars, galaxies and
use of telescopes and satellites gcse physics revision notes
The life cycle of stars - mainly worked out from emitted
electromagnetic radiation gcse physics revision notes
Cosmology - the
Big Bang Theory of the Universe, the red-shift & microwave background radiation gcse
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key words: life cycle of stars clouds of dust gas protostar main
sequence star red giant supergiant planetary nebula black dwarf
white dwarf supernova neutron star black hole igcse/gcse 9-1
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