AQA GCSE Physics 8463: Space physics – Study Guide

This study guide teaches Topic 8, Space physics (sections 4.8.1 and 4.8.2), of the AQA GCSE Physics (8463) specification, for first teaching 2016 with exams from June 2018 (version 1.1). Space physics is assessed on Paper 2, alongside Forces, Waves, and Magnetism and electromagnetism. Both tiers sit this topic; the two Higher tier only statements on circular orbits (in 4.8.1.3) are labelled below. The exams run every May/June for the life of the specification.

Use it with the Space physics revision notes and the Space physics practice questions. The course hub is AQA GCSE Physics and the printable checklist lists every statement. For paper-level advice, see the AQA GCSE Physics exam preparation guide.

What this topic covers

Spec	What you must be able to do	Tier
4.8.1.1	Describe the solar system and its place in the Milky Way; explain how the Sun formed from a nebula; explain how gravity starts fusion and how fusion balances gravitational collapse	Both
4.8.1.2	Describe the life cycle of a star the size of the Sun and of a star much more massive than the Sun; explain how fusion forms new elements and how a supernova spreads them	Both
4.8.1.3	Explain that gravity keeps planets and satellites in circular orbits; describe similarities and differences between planets, moons and artificial satellites	Both
4.8.1.3	Explain qualitatively why a circular orbit has changing velocity but constant speed, and why the radius of a stable orbit must change if the speed changes	Higher tier only
4.8.2	Explain red-shift qualitatively; explain how the speed–distance pattern shows the universe is expanding; explain how red-shift supports the Big Bang; explain how observations lead to theories; recognise what is not yet understood (dark mass and dark energy)	Both

Space physics is mostly explanation. There are no equations that belong only to this topic, but questions can use equations from elsewhere in the course, such as distance travelled = speed × time (s = v t) and wave speed = frequency × wavelength (v = f λ).

4.8.1.1 Our solar system

What is in it

The solar system has one star, the Sun. Around the Sun orbit eight planets and the dwarf planets. The moons that orbit planets are natural satellites, and they are part of the solar system too.

The solar system is a small part of a much bigger system, the Milky Way galaxy. A galaxy is a huge group of stars held together by gravity.

How the Sun formed

The Sun formed from a cloud of dust and gas called a nebula. Gravitational attraction pulled the dust and gas together. As the material fell inwards, it became denser and hotter.

Explain this in a chain of steps:

1. Gravity pulls the dust and gas in the nebula together.
2. The material gets more and more concentrated, so the temperature and pressure at the centre rise.
3. When the core is hot and dense enough, light nuclei (such as hydrogen) join together. This is nuclear fusion.
4. Fusion releases energy, and the star begins to shine.

Why a star is stable for a long time

Two effects act on a star at the same time:

• Gravity pulls the material inwards (gravitational collapse).
• Energy released by fusion pushes the material outwards (expansion).

While these two effects balance, the star is in equilibrium. Its size stays roughly the same. This stable stage is called the main sequence, and the Sun is in it now.

Use “balance” or “equilibrium” in your answer. Saying “the star is stable” without naming both effects will not score the marks.

4.8.1.2 The life cycle of a star

The size (mass) of a star decides its life cycle. Every star starts the same way: a nebula collapses to form a protostar, and fusion then starts, giving a main sequence star. What happens next depends on the mass.

A star about the size of the Sun

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

• Red giant: when the hydrogen in the core begins to run out, the star swells and its surface cools, so it glows red.
• White dwarf: the outer layers drift away and the hot core that remains is small and dense. It shines white hot.
• Black dwarf: the white dwarf cools until it no longer gives out significant light.

A star much more massive than the Sun

nebula → protostar → main sequence star → red super giant → supernova → neutron star or black hole

• Red super giant: a massive star swells into a red super giant.
• Supernova: the star collapses and then explodes.
• What is left depends on the mass: a very dense neutron star, or, for the most massive stars, a black hole, where gravity is so strong that not even light can escape.

How stars make the elements

Fusion processes in stars produce all of the naturally occurring elements:

• In the main sequence, hydrogen nuclei fuse to form helium.
• Later, in larger stars, heavier nuclei fuse to form heavier elements, up to iron.
• Elements heavier than iron are produced in a supernova.
• The supernova explosion distributes the elements throughout the universe. That material can later form new stars and planets.

4.8.1.3 Orbital motion, natural and artificial satellites

Gravity keeps things in orbit

Gravity provides the force that keeps planets and satellites moving in their (nearly) circular orbits. Without gravity, each object would move off in a straight line.

• Planets orbit the Sun.
• Moons (natural satellites) orbit planets.
• Artificial satellites are made by people and launched into orbit, usually around the Earth.

Planets, moons and artificial satellites compared

Feature	Planet	Moon	Artificial satellite
Orbits	the Sun	a planet	usually the Earth
Natural or made?	natural	natural	made by people
Force keeping it in orbit	gravity of the Sun	gravity of the planet	gravity of the Earth
Example uses	–	–	communications, weather monitoring, observing Earth or space

Similarities: all three move in orbits, all are kept in orbit by gravity, and none of them is a star, so they do not produce their own light (they reflect light).

Distinctions: what they orbit, whether they are natural or made by people, and their size (planets are generally much larger than artificial satellites).

Higher tier only: changing velocity, constant speed

This part is Higher tier only. Speed is a scalar; velocity is a vector, so it has a direction.

In a circular orbit:

1. The object moves at a constant speed around the circle.
2. Its direction changes all the time.
3. Because the direction changes, its velocity changes.
4. A changing velocity means the object is accelerating.
5. The acceleration is caused by the resultant force, which is gravity, acting towards the centre of the circle.

Gravity acts at right angles to the motion, so it changes direction but not speed.

Higher tier only: speed and radius of a stable orbit

This part is also Higher tier only. For a stable orbit, each speed goes with one particular radius. If the speed changes, the radius must change too.

• Objects in smaller orbits (closer in) must move faster.
• Objects in larger orbits (further out) move more slowly.

The reason, qualitatively: closer to the Earth, gravity is stronger, so an object must move faster to stay in orbit rather than fall inwards.

Worked example 1: speed of an orbiting satellite

A satellite moves in a circular orbit of radius 6.90 × 10⁶ m. It takes 5700 s to complete one orbit. Calculate its orbital speed.

Distance in one orbit = circumference = 2 × π × r
                      = 2 × π × 6.90 × 10⁶
                      = 4.34 × 10⁷ m

s = v t  →  v = s / t
v = 4.34 × 10⁷ / 5700
v = 7610 m/s  (3 s.f.)

The speed is about 7.61 km/s. Note that this is the speed, not the velocity: the direction keeps changing.

4.8.2 Red-shift

What red-shift is

When light from most distant galaxies is observed, its wavelength is longer than expected. The dark absorption lines in the spectrum appear shifted towards the red end. This increase in observed wavelength is called red-shift.

• The galaxy is moving away from us (receding).
• The further away the galaxy, the faster it is moving away, and the bigger the increase in wavelength.

Qualitatively: as the galaxy moves away, each wave crest is emitted from further away than the last, so the crests arrive more spread out and the received wavelength is longer.

Worked example 2: comparing red-shifts

In a laboratory, a dark line in the spectrum of hydrogen has a wavelength of 656 nm. In the light from galaxy A the line is at 689 nm. In the light from galaxy B it is at 722 nm.

Galaxy A: increase = 689 − 656 = 33 nm
          percentage increase = 33 / 656 × 100 = 5.0 %

Galaxy B: increase = 722 − 656 = 66 nm
          percentage increase = 66 / 656 × 100 = 10 % (10.1 %)

Galaxy B has the bigger red-shift, so it is moving away faster and is further away. Always divide by the original (laboratory) wavelength.

Evidence for an expanding universe

Measurements show that a galaxy’s speed away from us increases with its distance. This pattern is evidence that space itself is expanding. If every part of space is stretching, galaxies that are further apart separate more quickly.

Worked example 3: using speed–distance data

The table gives data for four galaxies.

Distance (× 10²⁴ m)	1.0	2.0	3.0	4.0
Speed away (km/s)	2300	4600	6900	9200

Speed ÷ distance is 2300 for every galaxy, so the speed is directly proportional to the distance. A galaxy at 2.5 × 10²⁴ m would be expected to recede at 2300 × 2.5 = 5750 km/s.

Red-shift and the Big Bang

The Big Bang theory suggests that the universe began from a very small region that was extremely hot and dense.

How red-shift supports it:

1. Galaxies are moving away, and the most distant ones fastest, so the universe is expanding.
2. If it is expanding now, it was smaller in the past.
3. Running the expansion backwards leads to everything starting in a very small, very hot, very dense region.

How scientists reach theories

Scientists measured red-shift for many galaxies, found the speed–distance pattern, and proposed a model (the Big Bang) that explains the data. A theory is accepted when it explains the observations, and it can change when new observations appear.

Since 1998 onwards, observations of supernovae suggest that distant galaxies are receding ever faster: the expansion is speeding up.

What we still do not understand

There is still much about the universe that is not understood:

• Dark mass (dark matter) bends light and holds galaxies together, but does not emit electromagnetic radiation. Nobody knows what it is.
• Dark energy is the name given to whatever is causing the universe to expand ever faster.

Common errors

• Writing that the Sun formed from “a gas cloud exploding”. Gravity pulled the nebula together.
• Naming only gravity (or only fusion) for a stable star. You need both, and “balance”.
• Giving the massive-star sequence as “red giant”. Massive stars become red super giants.
• Saying a supernova forms from a star like the Sun. Only much more massive stars end in a supernova.
• Stating that all elements are made in supernovae. Elements up to iron form by fusion in stars; only heavier than iron need a supernova.
• Writing “the speed changes” in a circular orbit. The velocity changes; the speed stays the same.
• Thinking a faster satellite goes further out. For a stable orbit, faster means smaller radius.
• Describing red-shift as “light turns red”. The wavelength increases, and further galaxies show a bigger increase.

Next steps

Go to the revision notes for condensed recall and a quick self-test. Then work through the practice questions. The free diagnostics help you find weak topics across the course.

Official syllabus

AQA GCSE Physics (8463) specification, for first teaching 2016, exams from June 2018, version 1.1, published by AQA – section 4.8 Space physics (4.8.1 Solar system; stability of orbital motions; satellites, and 4.8.2 Red-shift).