The Life Cycle of a Star: a Physics GCSE Revision Guide
Written by: Katie M
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The grandeur and mystery of space naturally captivates students, making it a highlight of any GCSE physics course. It’s undoubtedly my favourite to teach as a physics teacher; however, I might be biased as astrophysics was my specialism at university.
For years I have strived to simplify the physics of cosmic phenomena for students. In my experience, the life cycle of stars is one of the most commonly asked extended response questions in the GCSE physics exam. This guide will take you through the key stages in a star’s life cycle and give you top tips on tackling exam questions so that you never miss a mark.
Understanding the formation of stars
The life cycle of a star involves a series of evolutionary stages. Each stage can last anywhere from tens of thousands of years to billions of years. Because of the huge time scales involved, astronomers cannot see the changes that a star undergoes, but from observations of the stars that they can currently observe, astronomers can deduce that different stars are at different stages within a life cycle. All stars follow the same initial stages as they form. You will be expected to know these stages and their key features.
The role of gravity
Forces due to gravity are always attractive and act to pull masses together. The greater the mass, the greater the gravitational attraction. In your GCSE exam, you will be expected to be able to describe the role of gravity in the different stages of a star’s life cycle.
Nebulae: the birthplace of stars
Stars are formed from interstellar clouds of dust and gas, known as nebulae. In a nebula containing billions upon billions of hydrogen and helium atoms, gravitational attraction will gradually cause them to clump together. The larger clumps of material will then begin to pull in smaller clumps of material until the nebula effectively collapses in on itself.
As the material falls inward, the nebula spins faster. In my experience, students often struggle to explain why this happens, so I describe this as similar to sitting on a spinning chair; when you stretch your arms and legs out the chair spins slower, but when you tuck your arms and legs in the chair spins faster. This spinning motion causes the nebula to flatten into a disc. At the centre of the disc, the hot, dense ball of hydrogen and helium gas that forms is called a protostar.
The protostar stage
As the protostar collapses under gravity, it gets denser and hotter. This temperature increase occurs because hydrogen and helium atoms convert gravitational potential energy into kinetic energy as they fall inwards. The highest temperature increase happens in the protostar’s core, where it sores to several millions of degrees. Eventually, the temperature becomes hot enough to initiate nuclear fusion reactions. At this point, it enters a long, stable phase called the main sequence.
Star size determines its fate
The fate of a star depends on its mass. While the initial stages are similar for all stars, the more massive a star, the more dramatic its final stages. In your GCSE exam, you could be asked to describe the life cycle of a small to medium star or a massive star.
Main sequence phase
During the main sequence phase, hydrogen nuclei fuse into helium nuclei in the star’s core. This process generates massive amounts of energy and prevents the star from collapsing. and allows it to remain stable for billions of years.
You will be expected to know that the stability of a main sequence star is due to the equilibrium of two forces:
the inward force due to gravity
the outward force due to pressure from fusion reactions.
As long as the inward pull of gravity and the outward pressure acting on the star are equal, the star will be in equilibrium.
Small to medium stars
The main stages of the life cycle of a small to medium star are summarised in the table below:
Stage | Fusion | Key features |
|---|---|---|
Main sequence | Hydrogen fuses into helium | Energy from fusion is released as heat and light Inward and outward forces are balanced |
Red giant | Helium fuses into heavier elements | Hydrogen in the core begins to run out Core contracts and heats up Outer layers expand and cool |
Planetary nebula | Fusion reactions stop | Helium in the core begins to run out Outer layers drift away into space |
White dwarf | No fusion reactions | Hot, dense remnant of the core Energy is radiated into space |
Black dwarf | No fusion reactions | Dark cold remnant of the core All energy has been released |
Massive stars
The main stages of the life cycle of a massive star are summarised in the table below:
Stage | Fusion | Key features |
|---|---|---|
Main sequence | Hydrogen fuses into helium | Energy from fusion is released as light and heat Inward and outward forces are balanced |
Red supergiant | Helium fuses into elements up to iron | Hydrogen in the core begins to run out Core contracts and heats up repeatedly Outer layers expand and cool |
Supernova | Fusion reactions stop | Outer layers collapse Energy is released in a gigantic explosion |
Neutron star | No fusion reactions | Very small, dense core remnant Made out of neutrons |
Black hole | No fusion reactions | Extremely small, dense core remnant Strong gravity prevents light and heat emissions |
The life cycle of the Sun and different stars
The Sun is a medium-sized main sequence star. It is about 4.5 billion years old and is expected to continue fusing hydrogen into helium in its core for another 5 billion years.
After the main sequence, the Sun’s fate will be as follows:
1. Red giant stage:
The hydrogen supply in the Sun’s core will start to run out and fusion reactions will decrease
The core will collapse under gravity until it is hot enough for helium to fuse into heavier elements
The energy released by helium fusion will:
provide the outward force required to prevent the Sun from collapsing
cause the outer layers of the Sun to expand and cool
When the Sun becomes a red giant it will become about 250 times larger than its current size
2. Planetary nebula phase
The helium supply in the Sun’s core will eventually run out and fusion reactions will stop permanently
At this point, its outer layers will drift into space as a planetary nebula
3. White dwarf stage
As gravitational forces take over, the Sun’s core will collapse into a white dwarf
It will not get hot enough for fusion reactions to continue
Over time, it will cool down and become a black dwarf
In your GCSE exam, you will be expected to describe the life cycle of the Sun or stars of a similar size. You should also be able to compare and contrast it to the life cycles of much larger stars, the key points are summarised in the table below:
Similarities in both life cycles | Difference between life cycles | |
|---|---|---|
Post-main sequence | The main sequence stage ends when hydrogen fusion stops The core contracts and heats up until helium fusion starts The outer layers of the star expand and cool | Small to medium stars become red giants Large stars become red supergiants Red supergiants are much larger than red giants and run out of nuclear fuel much faster |
Final stages | The red giant/supergiant stages end when fusion reactions in the core stop permanently The outer layers of the star spread out into space | In small to medium stars, the star’s life is over when helium fusion stops In large stars, the star’s life is over when an iron core forms Small to medium stars become planetary nebulae and white dwarfs Large stars explode in a supernova explosion and either become a neutron star or black hole |
The impact of star life cycles on the universe
At the beginning of the Universe, the only elements present were hydrogen and helium produced by the Big Bang. The Universe we observe today contains a vast array of elements heavier than hydrogen and helium, suggesting they must have formed at a later time.
For your GCSE exam, you should be aware of the role of nuclear fusion in the formation of elements and how supernovae contribute to their distribution across the Universe.
Formation of elements in stars
Stars are the synthesisers of all naturally occurring elements in the Universe. During the main sequence of a star's life, hydrogen nuclei fuse together to form helium nuclei. The nuclear equation for this process is as follows:
In larger stars, fusion reactions produce successively heavier elements up to iron. The fusion of elements beyond iron requires an additional energy input, and the only place this is known to happen is in a supernova explosion.
Supernovae and new element distribution
A supernova is a bright and powerful explosion that happens at the end of the lives of stars which are much larger than the Sun. During a supernova, a large amount of energy is released and elements heavier than iron are produced as nuclei combine with neutrons. These elements are ejected into the Universe by the explosion and go on to form new planets and stars.
Shine bright in your Physics exams!
In this guide, we have revised star formation, the stable period of a star’s life and the ultimate fates of small to medium stars, such as our Sun, and of much larger stars. If you take the time to master the knowledge outlined here, you’ll be well-prepared to tackle any questions about the life cycles of stars that may arise in your GCSE Physics exam.
Whichever exam board you are taking, we have you covered with detailed revision notes, past papers and plenty of practice questions:
Once you’re feeling confident in your knowledge, try some of the following:
Make flashcards that test you on each stage of a star’s life cycle including the roles of gravity and nuclear fusion
Try some practice exam questions to learn how marks are allocated
Create a flow diagram of the stages in a star’s life cycle
Give yourself 5 minutes to plan a response to a 6-mark extended response question
Good luck and shine bright in your Physics exams!
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