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16.2 The Life History of a Star

GCSE Physics · AQA · Chapter 16: Space

A free GCSE Physics lesson for AQA (8463) from OBA School: why stars leave the main sequence, the life cycles of stars of different masses and how the elements are made (physics only). Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain why a star leaves the main sequence when the hydrogen in its core runs out
  • Describe the life cycle of a star about the same mass as the Sun
  • Describe the life cycle of a star much more massive than the Sun, including supernova, neutron star and black hole
  • Explain how fusion in stars makes elements up to iron, and how supernovae make and spread heavier elements
  • Use ρ = m/V and data about stars to compare the stages

Key words

  • Red giant: A huge, cool, red star that a Sun-like star becomes when the hydrogen in its core runs out. Helium fuses in its core.
  • White dwarf: The small, very hot, dense core left when a red giant sheds its outer layers. No fusion happens in it.
  • Black dwarf: A cold, dark white dwarf that has given out all its energy. The final stage for a Sun-like star.
  • Red supergiant: An enormous star formed when a star much more massive than the Sun leaves the main sequence.
  • Supernova: The gigantic explosion of a red supergiant after its core collapses. It can outshine a whole galaxy for weeks.
  • Neutron star: The tiny, extremely dense core left after a supernova, made almost entirely of neutrons.
  • Black hole: What is left if the core after a supernova is massive enough. Its gravity is so strong that not even light can escape.

The end of the main sequence

A main-sequence star is stable because the inward pull of gravity is balanced by the outward pressure from fusion energy in its core. Eventually the hydrogen in the core runs low. Fusion in the core slows down, so the outward pressure falls and the forces are no longer balanced.

  • Gravity makes the core contract and heat up, so new fusion reactions can start.
  • The extra energy makes the outer layers swell enormously and cool, so the star glows red.

What happens next depends on the mass of the star. Massive stars fuse their hydrogen much faster, so they have much shorter lives, even though they start with more fuel.

Stars about the same mass as the Sun

nebula → protostar → main sequence star → red giant → white dwarf → black dwarf

  • Red giant: the star swells and cools. Helium nuclei in the core fuse to make heavier nuclei such as carbon and oxygen.
  • When the light elements in the core are used up, fusion stops. The outer layers drift off into space and gravity makes the core collapse.
  • White dwarf: the leftover core is small (about the size of the Earth), very hot and very dense. It shines white but no fusion happens in it, so it slowly cools.
  • Black dwarf: after a very long time the white dwarf has cooled so much that it no longer gives out light.

The future of the Sun: the Sun is about 4.6 billion years old, about halfway through its main-sequence life. In roughly 5 billion years it will become a red giant large enough to swallow the inner planets, then a white dwarf, and finally a black dwarf. The Sun is not massive enough to become a supernova.

Stars much more massive than the Sun

nebula → protostar → main sequence star → red supergiant → supernova → neutron star (or black hole if the mass is great enough)

  • Red supergiant: the star swells even more. Fusion in its core makes heavier and heavier nuclei, up to iron.
  • When fusion can go no further, the core collapses suddenly under gravity. The collapse then rebounds in a gigantic explosion: a supernova.
  • Neutron star: the explosion squeezes the core into a ball only about 20 km across, made almost entirely of neutrons. It is one of the densest objects known.
  • Black hole: if the remaining core is massive enough, it collapses even further. Its gravitational field is so strong that nothing, not even light or any other electromagnetic radiation, can escape.

Worked example: how dense is a neutron star?

A neutron star has a mass of 3.4 × 10³⁰ kg and a volume of 4.0 × 10¹² m³.
density ρ = m ÷ V = 3.4 × 10³⁰ ÷ 4.0 × 10¹²
ρ = 8.5 × 10¹⁷ kg/m³ (water is 1000 kg/m³, so this is nearly 10¹⁵ times denser)

Answer: 8.5 × 10¹⁷ kg/m³

Where the elements come from

Every element in your body, apart from hydrogen, was made inside a star. Fusion builds bigger nuclei from smaller ones and releases energy – but only up to iron. Making nuclei bigger than iron needs more energy than the fusion releases, so a star cannot do it by ordinary fusion.

  • Big Bang (the early universe); What is made Hydrogen and helium (almost all of it)
  • Main sequence stars; What is made Hydrogen nuclei fuse to make helium
  • Red giants and red supergiants; What is made Helium and other light nuclei fuse to make heavier ones: carbon, oxygen … up to iron (supergiants only go as far as iron)
  • Supernova; What is made Elements heavier than iron, e.g. gold, lead, uranium – made by the enormous energy of the collapse and explosion

The supernova explosion scatters all these elements through space. Gravity later pulls the debris together into new nebulae, which form new stars and planets. That is why the Earth contains heavy elements such as iron, gold and uranium.

Exam tip: Heavy elements (heavier than iron) were not made in the Big Bang and are not made in main sequence stars. They come from supernovae.

Common mistakes

  • Saying the Sun will explode as a supernova or become a black hole. It is not massive enough: it will end as a white dwarf, then a black dwarf.
  • Writing “red giant” for a massive star. Massive stars become red supergiants.
  • Saying fusion happens in a white dwarf. It shines only because it is still hot from before.
  • Saying stars make all the elements by fusion. Normal fusion stops at iron; heavier elements come from supernovae.

Revision questions

Try these, then check your answers below each one.

Question 1. What will be the final stage in the life of the Sun? (Choose from: black dwarf, black hole, neutron star, supernova.)

Answer: black dwarf

Working: The Sun will become a red giant, then a white dwarf, which cools to a black dwarf.

Question 2. True or false? Elements heavier than iron, such as gold, are made by fusion in main-sequence stars.

Answer: False

Working: False. Fusion in stars only makes elements up to iron. Heavier elements are made in supernovae, which also scatter them through space.

Question 3. What name is given to the object left after a supernova whose gravity is so strong that not even light can escape?

Answer: black hole

Working: A black hole. If the core is less massive, a neutron star forms instead.

Related lessons

Continue in the full course

This lesson is section 16.2 (AGCPH1602) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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