THE LIFE CYCLE OF STARS
Where does it all start? How
does it all end?
How elements are formed by nuclear
fusion in the Sun
school physics revision notes: GCSE physics, IGCSE physics, O level
physics, ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old
students of physics
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.
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The sequences in the life cycle of
Beyond the Earth and beyond our own solar
system and galaxy, outer space is far from being a vacuum of 'emptiness'.
Interstellar space (between stars)
contains huge clouds of dust and a mixture of gases, mainly hydrogen and
helium gases, but traces of lots of other molecules including organic
These clouds are where stars are formed
and are called
is an enormous cloud of dust and gas occupying the space between
stars and acting as a nursery for new stars ... so read on after the
Dust clouds and gas - nebulae:
Stars and their planetary systems are formed from
A nebula is a huge congregation
of clouds of dust and gases (mainly hydrogen and helium and other
ionised gases) 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, over millions of years, 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, more gas and dust is
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
thermonuclear reactions and can only happen in the core of a sun
where the temperature is high enough to facilitate fusion.
At this stage, no heavier
elements beyond helium are made - this happens in the next
red giant stage of the life cycle of a main sequence star.
An extraordinary high
temperature (~15 x 106 oC) is needed to raise the kinetic energy of
ANY positive nuclei, so that the force of any collision is
sufficient to overcome the massive repulsive forces exerted
between ANY two positive nuclei (remember like charges
Hydrogen is fused mainly into
helium, and much smaller quantities of lithium, carbon and
nitrogen can be formed in medium sized stars as the hydrogen
fuel runs out, but you super red giant stars to make even
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
Despite the use of the word
'equilibrium', correctly implying a state of 'balance', inside the
Sun things are very turbulent and one manifestation of this are sun
spots - areas where the intensity and nature of the emitted
electromagnetic radiation varies.
Bearing in mind that most of a
star's life is spent in this state, what is this equilibrium
stability due to?
(i) Due to the enormous energy release from nuclear
fusion reactions (2H ==> He) in the core of the star the temperature
rises an enormous 15 million degrees. Therefore the plasma tries to
expand outwards because of pressure created by the
huge kinetic energy of the particles and the intensity of the radiation
emitted - its a sort of thermal expansion - think of a balloon expanding on being warmed.
(ii) However, unlike
the case of the 'heated balloon', because of the huge mass of the Sun
creating a powerful gravitation field, there is a counter force of gravity
pulling the particles together inwards - this pulling of everything together is
referred to as gravitational collapse to the smallest possible
(i) and (ii) create a balance of
and inward directing forces i.e. an equilibrium (a 'stability') 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 - as long as there is lots of hydrogen
to fuse into hydrogen - but eventually the nuclear fuels runs out.
Our Sun is ~4.5 billion years old
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?
Extra note on the early universe
The early universe only contained
very hot dense clouds of hydrogen with a little helium. These would
be the source of the first main sequence stars as the hydrogen atoms
condense together and compressed under gravity to such an extent the
temperature rises and hydrogen to helium fusion begins.
Extra note on the surface
temperature and brightness of stars
The size of a star influences it
surface temperature and how brightly it shines from the emitted
electromagnetic radiation - obviously we only see the visible
spectrum and feel the warmth from the infrared radiation.
Because we are dealing with such
large masses, astronomers use our Sun as a unit of mass itself and
its mass is referred to as 1 solar mass (its actually ~2 x 1030
Generally speaking, the bigger
the mass of a sun, the higher the surface temperature, due to the
greater nuclear energy release per unit of surface area.
(surface area / volume = 4πr2/4/3πr3
= 3/r, the smaller r, the relatively greater surface area over
which the energy is released outwards)
The surface temperature of our
own Sun is around 5500oC (for 1.0 solar
A main sequence star with a mass
of 0.1 solar masses has a surface temperature of ~2600oC.
A main sequence star with a mass
of 10 solar masses has a surface temperature of ~21700oC.
Extra note on the Sun's activity
The energy release from the Sun
is enormous and not just in the form of electromagnetic radiation.
Huge masses of particles are
ejected into space every second. This causes a solar wind that
creates the Northern Lights (Aurora Borealis) when these particles
hit the Earth's atmosphere.
There are periods of extra
intense solar activity and the ejection of massive amounts of
charged particles (called plasma) from solar flares that the effect
can be damaging to satellites causing them to malfunction AND can
even affect electrical supply systems on the Earth's surface causing
power supply failures.
Red giant star (aspects
overlap with red super giant stars)
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.
For suns about the size and mass of our Sun
(a small to medium sized star), a red giant
Much larger stars form red
After billions of years, the hydrogen
'nuclear fuel' in the core begins to run out and the force of gravity is greater than
the pressure of thermal expansion from the energy release. 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 (4b).
Now the sequence of events gets
complicated as giant red
stars expand and contract several times before they enter their
final phases - white dwarf ==> black dwarf OR supernova ==> neutron
Other nuclear reactions occur because
most of the hydrogen in the core was consumed in
nuclear fusion to helium, the temperature is even higher for fusion to
continue, but now the initial nuclear fuel is helium, so
fusion to form heavier elements begins, but it has its limits.
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
the sequence is
When a red giant runs out of suitable
nuclear fusion fuel (e.g. hydrogen and helium) it becomes unstable and the outer
layers of ionised gas expand to form a glowing planetary nebula
(nothing to do with planets!).
The nuclear fusion has stopped and
little energy is emitted.
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, but cooling continues as 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
optical telescope to become the aptly named black dwarf.
(red super giant, super red giant):
For suns much more massive than our Sun a red supergiant is
formed - we are talking 3 to 50 times the mass of our own Sun.
These stars use up the hydrogen fuel
faster, and, so have a shorter and ultimately more violent life!
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
attraction and all the time lighter elements are being fused into
heavier ones from lithium 3Li to iron 26Fe.
Most of the quantities of heavier elements from lithium
(3Li) to iron (26Fe), atomic numbers
3 to 26, can only be formed during the super red giant period
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 (27Co cobalt to 92U uranium, atomic numbers 27 to 92) can only
be formed in
, the supernova phase,
where the temperatures are even VERY much higher.
In the core of red
supergiants, the temperatures can exceed 100 million degrees Kelvin -
the minimum temperature needed to give lighter nuclear sufficient
kinetic energy to overcome the repulsion forces between positive nuclei
and produce the heavier elements from
Each circular layer in the diagram
represents a different set of nuclear fusion reactions and increasingly
heavier nuclei products the nearer you get to the core.
The diagram on the right shows the
layers of elements formed by fusion of lighter nuclei into heavier
nuclei in super red giant star.
Each layer in the diagram represents
a different set of nuclear fusion reactions and increasingly heavier
nuclei products the nearer you get to the core.
Hydrogen to helium, then beryllium,
carbon, nitrogen and oxygen and eventually in the core you get iron and
nickel. The further you get from the core,
the lighter the element formed at lower fusion temperatures.
A few details of the nuclear
The nuclear changes indicated are not
meant to be balanced nuclear equations, I'm just indicating examples of
what fuses into what - showing element symbols and atomic (proton)
numbers to match the diagram above.
In super red giant stars the highest
mass stars can make all the elements up to and including iron in their
BUT, iron is the heaviest element
super red giants can make because fusion of iron with other elements
does not create energy, and without an extra energy supply, the star
will soon die
Some of the possible sequences in a super red giant from a
very large main sequence star:
In sequence: Outer layer of non
fusing 1H; 1H fusion ==> 2He;
2He fusion ==> 6C
6C + 2He fusion
==> 6O and
6C + 8O fusion ==> 10Ne, 14Si
8O + 2He fusion
==> 10Ne and 8O
+ 8O fusion ==> 16S
10Ne + 2He
fusion ==> 12Mg; Mg fusion ==> ?;
14Si + 14Si fusion ==> 26Fe (limit);
finally an inert iron core
(i) 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. These huge kinetic
energies arise because the core temperature of a red giants is over 100
million degrees oC.
(ii) In the larger main sequence
stars the iron 26Fe formation limit is because there is no
net energy created by fusion and you need an extra energy input -
this you get in supernova explosion where elements 27Co to
92U can be synthesised in higher energy nuclear fusion
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 because there is no
longer the same huge release of nuclear energy, it collapses in on its
self due to gravity, 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 ...
... and, the residues of the supernova
explosion come together due to gravity to leave behind a relatively small, but
incredibly dense objects - the neutron star or a black hole.
Elements heavier than
iron, (cobalt 27Co to uranium 92U, 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 in the 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 dust' nebulae 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.
After the supernova explosion, if the
surviving core is between 1.5 and 3.0 solar masses, a tiny dense neutron
star will form.
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
000th 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?)
There is a neutron star at the centre
of the famous Crab Nebula which was observed by Chinese astronomers in
AD 1054 and was so bright it was visible in daylight.
If the mass of the remnant core from a
supernova explosion is even greater than that required to form a
neutron star, over 3.0 solar masses (9 above), 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
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