The life cycle of a star — AQA GCSE Physics
Test yourself on The life cycle of a star with AQA GCSE practice questions.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
The life cycle of a star explained
Every star follows the same opening: nebula, protostar, main sequence.
Read the full explanation
What happens afterwards is decided by the mass of the star, which is what the specification means by size. Stars about the size of the Sun swell into a red giant, then shed their outer layers and shrink into a white dwarf, which slowly cools into a black dwarf. Stars much more massive than the Sun swell into a red supergiant and then explode as a supernova, leaving behind either a neutron star or, for the most massive of all, a black hole. Mass also sets the pace, because a more massive star uses up its hydrogen faster and has a shorter main sequence life, so the biggest stars are also the shortest lived.
Students should be able to describe the life cycle of a star: the size of the Sun much more massive than the Sun.
This is a classic comparison, so set it out as a shared opening and then matched pairs. Shared: both stars form in a nebula of dust and gas pulled together by gravity, both pass through a protostar stage, both begin fusing hydrogen into helium, and both spend most of their lives as a stable main sequence star. Then take the differences in matched pairs. A star like the Sun becomes a red giant, while a much more massive star becomes a red supergiant. The Sun-like star then contracts into a white dwarf, while the massive star explodes as a supernova. The white dwarf cools into a black dwarf, while the supernova leaves a neutron star or black hole.
Fusion processes in stars produce all of the naturally occurring elements. Elements heavier than iron are produced in a supernova.
Fusion builds heavier nuclei out of lighter ones. A main sequence star fuses hydrogen into helium; later, in the giant stages, helium fuses into carbon and oxygen, and a massive star goes on building heavier nuclei as far as iron. Each of these reactions releases energy, because some of the mass of the nuclei is converted into energy. Iron is where it stops, because fusing nuclei heavier than iron takes energy in rather than giving it out, so no star can make gold or uranium in its core. Those elements are made in the enormous energies of a supernova explosion instead. Every element above hydrogen was assembled inside a star or in a supernova, including the carbon in you and the iron in your blood.
The explosion of a massive star (supernova) distributes the elements throughout the universe.
A supernova is the explosion that ends the life of a massive star, and it does two jobs at once. It creates the elements heavier than iron in the extreme temperatures and pressures of the explosion, and it throws all of the star's elements outwards into space at enormous speed. Those scattered elements mix into the clouds of dust and gas between the stars, so the next nebula to collapse begins with heavy elements in it rather than pure hydrogen. That is the evidence that our own Sun is a later generation star: the solar system contains rocky planets and iron cores, which could not have come from the original hydrogen alone. The core left behind becomes a neutron star, or a black hole.
Students should be able to explain how fusion processes lead to the formation of new elements.
An element is defined by the number of protons in its nucleus, so any process that changes that number makes a new element, and that is exactly what fusion does. Two light nuclei are forced together into a single heavier nucleus, and the protons of both end up in the same nucleus, so the atomic number goes up and the element changes. Getting them together is the hard part, because both nuclei are positively charged and repel one another, so only the enormous temperature and pressure in a star's core give them enough kinetic energy to collide and join. The new nucleus has slightly less mass than the parts that made it, and that missing mass is released as energy, which keeps the core hot enough for the next reaction.
Your focus
- Decide, from the mass of a star given in a question, which route it takes after the main sequence and name the object it ends as.
- Describe the stages that every star passes through before the two routes separate.
- Explain why the most massive stars have the shortest main sequence lives.
Show all 15 objectives
- Describe, in order, the stages in the life of a star with a mass similar to that of the Sun.
- Describe, in order, the stages in the life of a star much more massive than the Sun.
- Compare the two life cycles by pairing each stage with its counterpart.
- Explain how fusion inside stars produces elements heavier than hydrogen.
- State why elements heavier than iron cannot be produced by fusion in a star's core.
- Decide for a named element, such as helium, iron or uranium, whether fusion in a star or a supernova produced it, using iron as the dividing line.
- Trace an iron atom from the core of a massive star, through the supernova, into the cloud of dust and gas that formed the solar system.
- Explain how the presence of heavy elements on Earth is evidence of an earlier supernova.
- State the two possible remnants left behind by a supernova.
- Explain why joining two nuclei produces an atom of a different element.
- Explain why fusion only takes place at very high temperatures and pressures.
- Describe where the energy released in a fusion reaction comes from.
The life cycle of a star exam tips
Marking Points
- one mark for all stars starting as a nebula and passing through the protostar and main sequence stages
- one mark for the mass of the star deciding which route it takes afterwards
- one mark for a correct route for a star of about the mass of the Sun
- one mark for a correct route for a star much more massive than the Sun
- stating that both stars form from a nebula pulled together by gravity
- describing both passing through a protostar stage and beginning fusion
- identifying the main sequence as the stable stage where the inward and outward effects are balanced
- comparing the red giant stage with the red supergiant stage
- comparing the contraction into a white dwarf with the explosion of a supernova
- comparing the final stages: black dwarf versus neutron star or black hole
- one mark for lighter nuclei fusing to form heavier nuclei
- one mark for energy being released because some of the mass is converted into energy
- one mark for fusion inside stars producing the elements up to iron
- one mark for elements heavier than iron being produced in a supernova
- one mark for the supernova being the explosion of a massive star
- one mark for the elements being thrown outwards into space
- one mark for those elements becoming part of the clouds from which new stars and planets form
- one mark for using the presence of heavy elements in the solar system as evidence of an earlier supernova
- one mark for two light nuclei joining to form one heavier nucleus
- one mark for the number of protons in the nucleus increasing, so a different element is formed
- one mark for the very high temperature and pressure needed to overcome the repulsion between the nuclei
- one mark for some of the mass being converted into energy, which is then released
Examiner Tips
- 💡Sketch the two branches as a flow chart and learn them as a pair, not as one long list.
- 💡Read the stem for the mass of the star before choosing which route to describe.
- 💡Use the full names, red supergiant and white dwarf, because giant on its own is ambiguous.
- 💡In a compare question use linking words such as whereas, so that every comparison is explicit.
- 💡Cover similarities as well as differences to provide a complete comparison.
- 💡Write the two routes in the same order so that the matched pairs line up on the page.
- 💡State fusion as light nuclei joining to form heavier nuclei, because that phrasing is the first mark.
- 💡Name iron as the dividing line, and the supernova as the source of everything above it.
- 💡Keep fusion and fission apart by remembering that fusion needs extreme temperature and pressure.
- 💡Answer where did the Earth's heavy elements come from with a supernova, not with the Sun.
- 💡Link the explosion to the next generation of stars, because that is what distributes means here.
- 💡Name what remains after the explosion if the question asks about the fate of the star.
- 💡Use the word nuclei throughout, because atoms fuse loses the mark in several mark schemes.
- 💡Quote both conditions, high temperature and high pressure, when explaining why fusion needs a star.
- 💡Remember that an element is fixed by its proton number when a question asks why a new element appears.
Common Mistakes
- thinking a bigger star lives longer, when a more massive star uses its hydrogen up faster
- saying that the Sun will end its life as a supernova
- putting red giant and white dwarf in the wrong order
- writing that the life cycle depends on the star's temperature or brightness rather than on its mass
- describing one star's whole life and then the other's, without ever comparing them; correct this by using linking words like whereas to form matched pairs.
- giving a black hole as the end point of every massive star, when a neutron star is the other possibility; correct this by stating both possible outcomes.
- putting the supernova before the red supergiant stage; correct this by memorising the sequence of expansion then explosion.
- leaving out the shared beginning, which is essential in a compare question; correct this by always starting with the nebula and protostar stages.
- saying that fusion splits large nuclei, which is fission
- claiming that a star can fuse elements heavier than iron in its core
- writing that elements are burned into existence, which is a chemical rather than a nuclear description
- saying that fusion creates energy, rather than releasing it from the mass of the nuclei
- saying a supernova destroys the elements rather than distributing them
- attributing a supernova to a star like the Sun
- claiming that the heavy elements in the Earth were made by the Sun
- forgetting that something is left behind, either a neutron star or a black hole
- describing whole atoms joining rather than nuclei, so the role of the protons is lost
- saying fusion makes isotopes rather than new elements, when it is the proton number that changes
- leaving out the repulsion between the positive nuclei, so there is no reason why extreme conditions are needed
- saying that energy is created in fusion rather than released from the mass of the nuclei