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GCSE Physics notes: Explaining the life cycle of stars

summary of the life cycle of starsTHE



Doc Brown's GCSE 9-1 Physics Revision Notes

Suitable for GCSE/IGCSE Physics/Science courses

See also Cosmology - the Big Bang Theory of the Universe and

 Astronomy, 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 readily detected.

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 brain.

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.

e.g. 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.

The 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 temperatures.

e.g. when the temperature is many millions of degrees centigrade in the violent explosions of supernovae.

The sequences in the life cycle of stars

life cycle of stars diagram graphic image

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 between stars).

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 reach 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.

(c) doc b 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   (c) doc b  

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.

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 sequence star'.

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).

However, unlike 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 helium nucleus.

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.

Smaller 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 star/black hole

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 supergiants.

Nuclear reactions like (c) doc b happen etc.

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 , the red supergiant to supernova phase, where the temperatures are VERY much higher (see further down).

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 nuclear fusion.

For suns about the size and mass of our Sun (a medium sized star), a red giant is formed.

From the sequence is , and described below.

Planetary nebula:

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.

White dwarf:

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.

Black dwarf:

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 an optical telescope.

Red supergiant star (red super giant, super red giant):

For suns much more massive than our Sun a red supergiant is formed

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 attraction.

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.

From the sequence is then or described below.


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 formation.

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 stars!

Neutron star:

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 1012 kg!

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?)

Black hole:

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 light.

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.

See also Cosmology - the Big Bang Theory of the Universe and Astronomy, solar system and satellites for more detailed notes


WAVES - electromagnetic radiation, sound, optics-lenses, light and astronomy revision notes index

General introduction to the types and properties of waves, ripple tank expts, how to do wave calculations

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 gcse physics

Electromagnetic spectrum, sources, types, properties, uses (including medical) and dangers gcse physics

The absorption and emission of radiation by materials - temperature & surface factors including global warming

See also Global warming, climate change, reducing our carbon footprint from fossil fuel burning gcse chemistry

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 analysis) gcse notes

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 physics

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