Stars and the Universe

O Level / IGCSE Physics · Space physics. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

Test this topic freeAll O Level / IGCSE Physics topics

The idea

The Sun is a medium-sized star made mostly of hydrogen and helium. It is powered by nuclear fusion of hydrogen into helium, and it gives out most of its energy as infrared, visible light and ultraviolet. It is one of many billions of stars in our galaxy, the Milky Way, which is one of many billions of galaxies.

A star forms when a cloud of gas and dust collapses under gravity and heats up. It is stable while the inward pull of gravity is balanced by the outward push from its hot centre. What happens when the hydrogen runs out depends on its mass.

Light from distant galaxies shows redshift: its wavelength is longer than expected, because the galaxies are moving away. This supports the Big Bang theory.

Rules to remember

Common mistake

Treating the light-year as a unit of time. It is a distance: how far light travels in one year.

Worked example

Light travels at 3.0 × 108 m/s in a vacuum. How many metres are there in one light-year?

  1. Time: one year = 365 × 24 × 3600 = 31 536 000 s, which is about 3.15 × 107 s.
  2. distance = speed × time = 3.0 × 108 × 3.15 × 107.
  3. 3.0 × 3.15 = 9.45, and 108 × 107 = 1015, so the distance is 9.45 × 1015 m.

Answer: About 9.5 × 1015 m

Practice questions

Try each one, then open the answer.

1. Which statement about the Milky Way is correct?

  1. A
    it is the only galaxy in the Universe
  2. B
    it is one of many billions of galaxies, each containing many billions of stars
  3. C
    it is a star
  4. D
    it is a planet in the Solar System
Show answer

Answer: B. The Milky Way is our galaxy, one of billions; the Sun is one of its many billions of stars.

2. What happens to a star with a mass similar to the Sun at the end of its main-sequence life?

  1. A
    it explodes as a supernova and leaves a black hole
  2. B
    it expands into a red giant, then becomes a white dwarf
  3. C
    it collapses straight into a neutron star
  4. D
    it becomes a new protostar
Show answer

Answer: B. Sun-like stars swell into red giants when the core hydrogen runs out, shed their outer layers, and leave a white dwarf. Much more massive stars become supernovae.

3. Light from distant galaxies is redshifted, and the more distant the galaxy, the greater the redshift. What does this suggest?

  1. A
    galaxies are moving towards us
  2. B
    the Universe is expanding, supporting the Big Bang theory
  3. C
    distant galaxies are cooler
  4. D
    light slows down over large distances
Show answer

Answer: B. Greater redshift with distance means the galaxies are receding faster the further away they are: the Universe is expanding from a much smaller, hotter state.

More questions on this topic

Keep going

← Earth and the Solar SystemAll rules on one pageStuck? Ask a question