4.8 Space physics (physics only) — coverage pack

4 specification leaves · notes, questions, answers and worked methods

4.8.1.1 · Our solar system

The solar system contains the Sun, eight planets and dwarf planets orbiting it, plus natural satellites orbiting planets; it is a small part of the Milky Way galaxy.
A star begins when gravity pulls gas and dust in a nebula together into a protostar; compression raises the temperature until fusion reactions start.
During the stable main-sequence stage, the inward effect of gravitational collapse is in equilibrium with the outward expansion caused by energy from fusion.
A common error is to call the Milky Way the universe or the solar system: the solar system lies within the Milky Way, which is one galaxy in the universe.

Tier 1 · Easy

1. Place these structures in order from smallest to largest: the Milky Way galaxy, Earth, the solar system.[2 marks]
Answer
- Earth, solar system, Milky Way galaxy.
Method: Earth is one planet in the solar system, and the whole solar system is only a small part of the Milky Way. Therefore the size order is Earth, solar system, Milky Way.

Tier 2 · Standard

1. Explain how a cold cloud of gas and dust can develop into a main-sequence star.[4 marks]
Answer
- Gravity pulls the gas and dust together.
- The contracting material forms a protostar and becomes hotter.
- At a sufficiently high temperature, nuclei begin to fuse.
- The star enters a stable main-sequence stage when fusion is established.
Method: Link the stages causally: gravitational attraction causes collapse and compression, compression raises temperature, and a high enough core temperature allows nuclear fusion to begin.

Tier 3 · Hard

1. A young star stops contracting rapidly after sustained fusion begins. Explain the equilibrium that makes the star stable and predict what happens if its fusion output temporarily decreases.[5 marks]
Answer
- Gravity acts inward and tends to collapse the star.
- Energy released by fusion produces an outward expansion effect or pressure.
- In a stable star, the inward and outward effects are balanced.
- If fusion output decreases, the outward effect becomes smaller than gravity.
- The star begins to contract until a new balance can be established.
Method: Identify the two opposing effects, then compare them. Equal inward gravity and outward fusion-driven expansion give equilibrium; reducing only the outward effect leaves a resultant inward effect and causes contraction.

4.8.1.2 · The life cycle of a star

All stars follow nebula $\rightarrow$ protostar $\rightarrow$ main-sequence star; their later path is determined by the star's mass.
A Sun-sized star becomes a red giant, then a white dwarf and finally a black dwarf; a much more massive star becomes a red supergiant, then a supernova, leaving a neutron star or black hole.
Fusion makes progressively new elements in stars, while elements heavier than iron are made in a supernova; the explosion distributes elements through the universe.
A common error is to give one ending for every star: only the much-more-massive route includes a supernova and can leave a neutron star or black hole.

Tier 1 · Easy

1. Complete the late-life sequence for a star about the size of the Sun: red giant $\rightarrow$ ______ $\rightarrow$ ______.[2 marks]
Answer
- White dwarf, then black dwarf.
Method: Follow the Sun-sized branch of the life-cycle diagram: after red giant comes white dwarf, which cools to become a black dwarf.

Tier 2 · Standard

1. Describe the stages followed by a star much more massive than the Sun after its main-sequence stage, including both possible remnants.[4 marks]
Answer
- It expands into a red supergiant.
- It explodes as a supernova.
- The remnant becomes either a neutron star or a black hole.
Method: Select the high-mass branch rather than the Sun-sized branch. Preserve the order red supergiant, supernova, then give the two alternative remnants: neutron star or black hole.

Tier 3 · Hard

1. Explain how stellar processes both create new elements and spread them into the material from which later stars and planets can form.[6 marks]
Answer
- Fusion joins lighter nuclei to make heavier nuclei during a star's life.
- Successive fusion processes produce naturally occurring elements up to iron.
- A massive star later undergoes a supernova explosion.
- Elements heavier than iron are produced in the supernova.
- The explosion ejects and distributes elements through the universe.
- The dispersed material can become part of later clouds of gas and dust from which stars and planets form.
Method: Separate creation from distribution. Fusion within stars builds new nuclei; the supernova stage both produces nuclei heavier than iron and ejects material, enriching later nebulae.

4.8.1.3 · Orbital motion, natural and artificial satellites

Gravity supplies the inward force that keeps planets, natural satellites and artificial satellites in circular orbits.
Classify by what is orbited and origin: planets orbit the Sun, natural satellites are moons orbiting planets, and artificial satellites are human-made objects placed in orbit.
Higher only: in a circular orbit gravity changes velocity direction while speed can stay constant; if speed changes in a stable orbit, the orbital radius must also change.
A common error is to say orbiting objects have no force acting on them; without the inward gravitational force they would not follow a circular path.

Tier 1 · Easy

1. Name the force that maintains the Moon's orbit around Earth and state whether the Moon is a natural or artificial satellite.[2 marks]
Answer
- Gravity maintains the orbit.
- The Moon is a natural satellite.
Method: An orbit requires an inward gravitational force. The Moon was not placed in orbit by humans, so it is classified as a natural satellite.

Tier 2 · Standard

1. Compare a planet, one of its moons and a weather satellite by stating one similarity and two distinctions.[3 marks]
Answer
- All three are kept in orbit by gravity.
- A planet orbits the Sun, whereas the moon and weather satellite orbit a planet.
- The moon is natural, whereas the weather satellite is artificial and human-made.
Method: Use gravity as the shared feature. Then distinguish the central body orbited and distinguish natural origin from deliberate human placement.

Tier 3 · Hard

1. Higher only: a satellite travels at constant speed in a stable circular orbit. Explain how gravity changes its velocity without changing its speed, and state what must happen to the orbital radius if the speed changes but the orbit remains stable.[5 marks]
Answer
- Gravity supplies a force towards the centre of the orbit.
- This inward force continually changes the direction of motion.
- Velocity changes because velocity includes direction, even though speed stays constant.
- The satellite is therefore accelerating towards the centre.
- If its speed changes in a stable orbit, its orbital radius must also change.
Method: Separate speed from velocity: an unchanged speed can accompany a continuously changing direction. Gravity provides that directional change, and the stable-orbit condition links any speed change to a change in radius.

4.8.2 · Red-shift (physics only)

Light from most distant galaxies is observed at longer wavelengths than expected; this red-shift is the qualitative signature of galaxies receding from us.
Compare spectral lines with laboratory wavelengths: a shift towards longer wavelengths indicates recession, and a larger shift indicates a greater recession speed.
More distant galaxies generally recede faster and show greater red-shift, providing evidence that the universe is expanding and supporting a hot, dense beginning in the Big Bang model.
A common error is to present the Big Bang as unchangeable fact: scientific theories are built from observations and may be refined as new evidence appears; dark mass and dark energy remain incompletely understood.

Tier 1 · Easy

1. A spectral line from a galaxy is observed at a longer wavelength than the same line measured in a laboratory. Name this effect and state what it indicates about the galaxy.[2 marks]
Answer
- The effect is red-shift.
- It indicates that the galaxy is receding.
Method: A shift of known spectral lines towards longer wavelengths is red-shift. In this context, red-shift indicates motion away from the observer.

Tier 2 · Standard

1. Galaxy R is farther away than galaxy S and its spectral lines show a larger red-shift. Explain the conclusions astronomers draw from these observations.[4 marks]
Answer
- Both galaxies are receding because their light is red-shifted.
- R is receding faster because it has the larger red-shift.
- The pattern that more distant galaxies recede faster is evidence that the universe is expanding.
- This expansion evidence supports the Big Bang model.
Method: Translate red-shift into recession, compare shift size to compare speeds, then connect the speed-with-distance trend to an expanding universe and the Big Bang model.

Tier 3 · Hard

1. New supernova observations suggest that very distant galaxies are receding ever faster. Explain how scientists use such observations when evaluating the Big Bang model, and why unresolved dark mass and dark energy do not make the model unscientific.[6 marks]
Answer
- Scientists compare predictions or implications of a model with repeatable astronomical observations.
- Red-shift and the recession-speed trend provide evidence that the universe is expanding.
- Expansion from an earlier state supports the Big Bang model of an initially very hot, dense, small region.
- The newer supernova observations can refine the model by indicating accelerating recession.
- Scientific theories remain open to revision when further evidence is collected.
- Unexplained dark mass and dark energy identify limits of present understanding rather than removing the existing observational evidence.
Method: Treat the model as an evidence-based explanation, not a final certainty. Link red-shift to expansion and hence to the Big Bang, then explain that new acceleration evidence and unresolved phenomena motivate refinement and further testing.