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School Physics Notes: Nuclear fusion and explaining the life cycle of stars


Where does it all start?  How does it all end?  How heavier elements are formed by nuclear fusion of nuclei in stars like the Sun and supernova explosions

See also

Cosmology: Big Bang Theory of the Universe

and Astronomy, solar system and satellites

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

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 molecules.

These clouds are where stars are formed and are called nebulae. A nebula 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 'picture' summary.

life cycle of stars diagram graphic image

Dust clouds and gas - nebulae:

Stars and their planetary systems are formed from a nebula.

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 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, 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 attracted, 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 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 strongly repel).

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 heavier elements.

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'.

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 volume.

(i) and (ii) create a balance of outward 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 helium nucleus.

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 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?

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 kg !!!!).

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 masses).

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.

summary of the life cycle of stars 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 is formed.

Much larger stars form red supergiant stars .

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 star/black hole

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 supergiants.

From the sequence is , and described below.

summary of the life cycle of stars Planetary nebula:

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.

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, but cooling continues as 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 to become the aptly named black dwarf.

summary of the life cycle of stars Red supergiant star (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 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 (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 nuclear fusion.

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 fusion reactions

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 cores.

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 reactions.

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 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 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!

summary of the life cycle of stars Neutron star:

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 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 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.

Black hole:

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 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.

Keywords, phrases and learning objectives for the reflection of visible light rays

Be able to describe, or interpret a diagram of the life cycle of a star.

Know that clouds of dust gas condense together under gravity to form a protostar.

Know this is followed by the main sequence star, red giant or supergiant, planetary nebula, black or dwarf white dwarf, or alternatively a supernova, neutron star and black hole.


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