Space physics: life cycle of stars, orbits and red-shift — AQA GCSE Physics revision

Life cycle of stars

• Stars form from clouds of dust and gas (nebulae) pulled together by gravity; as the protostar contracts, gravitational energy converts to heat until fusion begins, forming a main sequence star.
• Main sequence stars fuse hydrogen into helium in their core, releasing energy for millions to billions of years, remaining stable due to a balance between gravitational collapse and the outward pressure from fusion.
• A star like the Sun will expand into a red giant once hydrogen fuel runs low, then shed its outer layers as a planetary nebula, leaving a white dwarf that cools over time.
• A much larger star expands into a red supergiant, then explodes as a supernova, which can leave behind a neutron star or, for the most massive stars, a black hole.
• Elements heavier than iron are formed during a supernova explosion and are scattered into space, eventually forming new stars, planets and even living things.

Orbital motion

• Gravitational force provides the centripetal force that keeps planets, moons and satellites in roughly circular orbits.
• In a circular orbit, the object's speed stays constant, but its velocity (and direction) is continually changing, so it is still accelerating towards the centre.
• A stronger gravitational force (closer orbit, more massive central body) requires a greater orbital speed to maintain a stable orbit.
• Satellites in a smaller orbital radius must travel faster and complete their orbit more quickly than those further away.

Key terms

Nebula

A cloud of dust and gas from which stars form under gravity.

Main sequence star

A stable star fusing hydrogen into helium in its core.

Red giant/supergiant

An expanded, cooler star formed after a main sequence star runs out of hydrogen.

White dwarf

The hot, dense remnant core left after a Sun-like star sheds its outer layers.

Supernova

A massive explosion at the end of a large star's life.

Neutron star

A very dense stellar remnant left after a supernova, made almost entirely of neutrons.

Common exam traps

• Thinking all stars end as black holes — only the most massive stars can form black holes; smaller ones become white dwarfs or neutron stars.
• Confusing red-shift (galaxies moving away) with blue-shift (moving towards, less common at large scale).
• Believing constant speed in a circular orbit means no acceleration — direction changes constantly, so there is centripetal acceleration.

Practice questions with answers

1. 1. Stars initially form from:

   • • Solid rock
   • • Clouds of dust and gas (nebulae)
   • • Pure energy
   • • Black holes

   Answer: Clouds of dust and gas (nebulae)

   Gravity pulls together dust and gas in a nebula to form a protostar, which becomes a star.

2. 2. What keeps a main sequence star stable?

   • • No forces acting on it
   • • Balance between gravitational collapse and outward pressure from fusion
   • • Constant cooling
   • • Magnetic repulsion

   Answer: Balance between gravitational collapse and outward pressure from fusion

   The star remains a stable size when the inward pull of gravity balances the outward pressure from nuclear fusion.

3. 3. What does a Sun-like star become after the red giant stage?

   • • Black hole
   • • Neutron star
   • • White dwarf (via planetary nebula)
   • • Supernova remnant only

   Answer: White dwarf (via planetary nebula)

   A Sun-like star sheds its outer layers as a planetary nebula, leaving behind a hot, dense white dwarf core.

4. 4. What happens to a very massive star after the red supergiant stage?

   • • It quietly fades away
   • • It explodes as a supernova
   • • It becomes a white dwarf
   • • It turns into a nebula only

   Answer: It explodes as a supernova

   Massive stars explode as supernovae, which can leave a neutron star or black hole behind.

5. 5. What can remain after a supernova of the most massive stars?

   • • A white dwarf
   • • A black hole
   • • A protostar
   • • A planetary nebula

   Answer: A black hole

   The most massive stellar remnants can collapse further into a black hole after a supernova.

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