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IB DemystifiedMYP Sciences

Stars and the universe

Every atom of carbon in your body and of iron in your blood was made inside a star that died before the Sun was born. Astronomy traces that story, from clouds of gas to exploding stars and an expanding universe that began 13.8 billion years ago.

Recommended for MYP 5 · eAssessment priority · About 3 lessons · Criteria A, B, C and D

NebulaProtostarMain-sequence starRed giantWhite dwarfRed supergiantSupernovaNeutron starBlack holestars like the Sunstars much more massive than the Sun
Figure 1. A star's mass decides how it lives and how it dies.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding the ideas
  5. Evidence for the Big Bang
  6. Astronomy in the real world
  7. Worked examples
  8. In the eAssessment
  9. Check your understanding
  10. Practice questions
  11. Investigation
  12. Criterion-linked questions
  13. Challenge questions
  14. Topic check
  15. Review your mistakes
  16. Your progress

Learning objectives

By the end of this topic you should be able to:

  • describe the life cycles of stars of different masses
  • explain how stars release energy by nuclear fusion
  • use light-years and describe the scale of the universe
  • explain redshift and the evidence for the Big Bang
  • relate star colour to temperature
  • discuss the costs and benefits of space science and dark skies

Before you start

You will use these skills. If any feel shaky, review them first.

  • atoms, nuclei and radioactivity (see Radioactivity)
  • waves and the electromagnetic spectrum (see EM spectrum)
  • gravity, speed and standard form

Key vocabulary

Nebula
A cloud of gas and dust in space where stars form.
Main-sequence star
A stable star fusing hydrogen into helium, like the Sun.
Supernova
The explosion of a massive star at the end of its life.
Light-year
The distance light travels in a year, about 9.46 × 10¹² km.
Redshift
An increase in the wavelength of light from a galaxy moving away from us.
Cosmic microwave background (CMB)
Radiation left over from the hot early universe, detected from all directions.

Understanding the ideas

  1. What is it?

    Stars are enormous balls of hot gas, mostly hydrogen and helium. They form when gravity pulls a nebula together, and they shine because nuclear fusion in their cores turns hydrogen into helium, releasing vast amounts of energy.

  2. Why does it happen?

    Throughout a star's life, gravity pulling inwards is balanced by pressure from fusion pushing outwards. When the hydrogen in the core runs out, this balance changes. The star's mass decides the rest: Sun-like stars become red giants and then white dwarfs; massive stars become supergiants, explode as supernovae and leave neutron stars or black holes.

  3. How do we know?

    Astronomers analyse starlight with spectra to find temperature, composition and motion. In the 1920s, Edwin Hubble found that more distant galaxies are moving away faster, and in 1965 the cosmic microwave background was discovered, confirming a prediction of the Big Bang theory.

  4. Why does it matter?

    Supernovae created the heavy elements that make up planets and living things. Satellites and space science support weather forecasting, communications and navigation, and studying the universe answers some of the deepest questions people ask.

  5. What does it connect to?

    This topic links to nuclear physics, waves and gravity in physics; to the origin of elements in chemistry; and to the conditions for life in biology.

Evidence for the Big Bang

  • Redshift: light from distant galaxies is shifted to longer wavelengths, so they are moving away.
  • Hubble's law: recession speed is proportional to distance, so space is expanding uniformly.
  • Cosmic microwave background: faint microwave radiation from all directions, the cooled glow of the hot early universe.
  • Light elements: the amounts of hydrogen and helium match predictions for a hot early universe.

Running the expansion backwards gives an age of about 13.8 billion years.

Astronomy in the real world

Space telescopes such as the James Webb Space Telescope see galaxies whose light left them more than 13 billion years ago. In 2024, a Pakistani CubeSat, ICUBE-Q, travelled to the Moon on board China's Chang'e 6 mission. Dark-sky reserves around the world protect places where the Milky Way is still visible.

Worked examples

Example 1: light travel time

Mars is about 2.25 × 10⁸ km from the Sun. How long does sunlight take to reach it? (Light speed = 3.0 × 10⁵ km/s.)

  1. t = distance ÷ speed = 2.25 × 10⁸ ÷ 3.0 × 10⁵.
  2. t = 750 s.
  3. 750 ÷ 60 = 12.5 minutes.

Example 2: using Hubble's law

A galaxy is 150 million light-years away. Using 21.5 km/s per million light-years, estimate its recession speed.

  1. speed = 21.5 × 150.
  2. speed ≈ 3200 km/s.

In the eAssessment

Space questions combine diagrams, data and evaluation of evidence. Expect:

  • Describe and compare the life cycles of stars using flow diagrams.
  • Process data on galaxy distances and speeds, and on star temperatures.
  • Calculate light travel times and distances in standard form.
  • Evaluate evidence for the Big Bang and claims about space.

Common ways to lose marks: calling a light-year a unit of time; saying the Sun will become a supernova or black hole; saying stars "burn" fuel like a fire; and treating "theory" as meaning "guess".

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: modelling an expanding universe

Partially guided investigation · about 40 minutes · pairs

Research question
In a balloon model of the universe, how does the distance of a dot from a chosen "home" dot affect how fast it moves away as the balloon is inflated?
Scientific background
If space expands uniformly, every galaxy moves away from every other, and more distant galaxies recede faster (Hubble's law). Dots on an inflating balloon behave in the same way.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
Round balloon, permanent marker, flexible tape measure or string and ruler, stopwatch or a pump that adds a fixed volume per stroke.
Method
  1. Partly inflate the balloon and mark a "home" dot and 5–6 other dots at different distances from it.
  2. Measure the distance from home to each dot along the surface.
  3. Inflate the balloon by a fixed amount (for example three pump strokes) and measure the distances again.
  4. Calculate how far each dot moved away, plot this against its starting distance, and repeat with another home dot.

Safety. Do not over-inflate the balloon near faces and eyes; anyone with a latex allergy should use a latex-free balloon.

Then evaluate: in what ways is the balloon a good model of the universe, and where does it break down?

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

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