How do stars form, live and die, and how does this depend on their mass?
The life cycle of a star from nebula to main sequence, and the different fates of low-mass and high-mass stars.
A focused answer to WJEC GCSE Physics topic 2.5 on the life cycle of stars, covering star formation from a nebula, the main sequence, and the different fates of low-mass stars (red giant, white dwarf) and high-mass stars (red supergiant, supernova, neutron star or black hole).
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What this topic is asking
WJEC wants you to describe how a star forms and the different ways stars end their lives depending on mass. This is part of topic 2.5 Stars and planets in Unit 2 of WJEC GCSE Physics (3420).
Star formation
The elements heavier than hydrogen and helium were made inside stars and scattered into space by supernovae, so the gas and dust of a new nebula already contains them. This is why the same materials are recycled from one generation of stars to the next, and why the atoms in your body were once made inside ancient stars.
While a star sits on the main sequence it fuses hydrogen into helium at a steady rate, and the energy released keeps it in equilibrium against gravity (see The solar system and stars). The more massive a star is, the more brightly it burns and the faster it uses up its fuel, so the most massive stars have the shortest lives. It is when this hydrogen fuel runs low that the star leaves the main sequence and its fate, decided by its mass, begins to unfold.
The fate of a low-mass star
The fate of a high-mass star
Try this
Q1. State the first stage in the life cycle of every star. [1 mark]
- Cue. A nebula (a cloud of gas and dust) pulled together by gravity.
Q2. Name the two possible remnants left after a supernova. [2 marks]
- Cue. A neutron star, or a black hole (if the star is massive enough).
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20194 marksDescribe the stages in the life cycle of a star with a similar mass to the Sun, from its formation onwards.Show worked answer →
A topic 2.5 Describe question. A star forms when gravity pulls together a nebula (cloud of gas and dust), heating it until fusion begins as a protostar (1 mark). It then spends most of its life as a stable main sequence star (1 mark). When the hydrogen runs out it swells into a red giant (1 mark), then sheds its outer layers and the core is left as a hot, dense white dwarf that slowly cools (1 mark). Markers reward the nebula, main sequence, red giant and white dwarf in order. A common error is to give the high-mass route instead.
WJEC 20224 marksExplain how the end of a high-mass star differs from the end of a star like the Sun.Show worked answer →
A topic 2.5 Explain question. A high-mass star becomes a red supergiant rather than a red giant (1 mark) and ends its life in a huge explosion called a supernova (1 mark). The remaining core forms either a very dense neutron star or, if massive enough, a black hole (1 mark). A star like the Sun ends much more gently as a red giant then a white dwarf, with no supernova (1 mark). Markers reward the supergiant, the supernova, the neutron star or black hole, and the contrast with the Sun. A common error is to give a supernova for the Sun.
Related dot points
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Sources & how we know this
- WJEC GCSE Physics specification (3420) from 2016 — WJEC (2016)