Understanding Space physics: IGCSE Physics Explained

Why space physics matters for your IGCSE

Picture yourself standing outside on a clear night, looking up at a sky full of stars. Maybe you've noticed how the Moon changes shape over a few weeks, or wondered why summer days are so much longer than winter ones. These aren't random quirks of nature. They're the result of precise, predictable motions that you can understand and explain once you know the physics behind them.

Space physics is the final section of the IGCSE Physics (0625) syllabus, and it ties together some genuinely fascinating science. You'll cover two broad areas: the Earth and the Solar System, and stars and the Universe. The ideas here are more conceptual than calculation-heavy (though Extended candidates do get an orbital speed formula to work with), and examiners love testing whether you can clearly explain what you observe. The good news? Once the core ideas click, this topic feels satisfying to revise because everything connects.

Earth's rotation and the cycle of day and night

The Earth spins on its axis once every 24 hours (approximately). This rotation is what gives us day and night. The half of the Earth facing the Sun is lit up and experiences daytime, while the half facing away sits in darkness.

From where you're standing on Earth's surface, this rotation makes the Sun appear to travel across the sky from east to west. It rises in the east each morning, climbs to its highest point around midday, and sets in the west each evening. But the Sun isn't actually moving around us. It's our planet that's spinning. This is called the apparent daily motion of the Sun, and you'll need to explain it precisely in exam answers.

Here's a detail that catches people out: Earth's axis isn't straight up and down relative to its orbit. It's tilted at about 23.5 degrees. That tilt doesn't affect the daily cycle of day and night directly, but it plays a huge role in what we'll look at next.

Earth's orbit and the seasons

The Earth orbits the Sun once in approximately 365 days. As it travels around this orbit, the tilt of its axis means that different hemispheres receive different amounts of direct sunlight at different times of year.

• When the Northern Hemisphere is tilted towards the Sun, it receives more direct sunlight and experiences summer. Days are longer, and the Sun appears higher in the sky at midday.
• Six months later, the Northern Hemisphere tilts away from the Sun. Less direct sunlight reaches the surface, days are shorter, and that hemisphere experiences winter.
• The Southern Hemisphere experiences the opposite pattern at the same time. When it's summer in London, it's winter in Sydney.

The Moon and its phases

The Moon orbits the Earth once in approximately one month (about 29.5 days for a full cycle of phases). The Moon doesn't produce any light of its own. What you see in the night sky is sunlight reflecting off its surface. As the Moon moves around Earth, you see different amounts of its sunlit side, which gives you the familiar cycle of phases.

Be careful with this in exams: Moon phases are not caused by Earth's shadow falling on the Moon. That's a lunar eclipse, which is a separate phenomenon. Phases happen because of the Moon's changing position relative to Earth and the Sun.

The Solar System

Your IGCSE syllabus asks you to describe the Solar System and know its contents. Here's what you need:

• One star: the Sun, at the centre of the system.
• Eight planets (in order from the Sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
• Minor planets: these include dwarf planets like Pluto and the rocky asteroids found mostly in the asteroid belt between Mars and Jupiter.
• Moons: natural satellites orbiting the planets. Earth has one; Jupiter has more than 90.
• Smaller Solar System bodies: comets (icy bodies with highly elliptical orbits that develop tails near the Sun) and other natural satellites.

A handy mnemonic for the planet order: My Very Excellent Mother Just Served Us Nachos.

Gravity and orbits

Gravity is the force that holds the Solar System together. The Sun's gravitational pull keeps the planets in their orbits. Earth's gravity keeps the Moon orbiting us. In each case, gravity acts as the centripetal force, constantly pulling the orbiting body towards the central mass and keeping it on a curved path rather than flying off in a straight line.

A few key points about gravitational field strength:

• Gravitational field strength (g) varies between planets. It depends on the planet's mass and radius.
• On Earth's surface, g is approximately 9.8 N/kg.
• The further you move from a massive body, the weaker the gravitational field becomes.

Orbital speed (Extended/Supplement)

If you're taking the Extended paper, you need to know and use the orbital speed equation:

v = 2πr / T

where v is the average orbital speed, r is the average radius of the orbit, and T is the orbital period (the time for one complete orbit).

Worked example 1: Earth's orbital speed

Earth orbits the Sun at an average radius of 1.50 × 10¹¹ m. Its orbital period is 365.25 days. Calculate Earth's average orbital speed.

1. Convert T to seconds: T = 365.25 × 24 × 3600 = 3.156 × 10⁷ s
2. Apply the formula: v = 2π × (1.50 × 10¹¹) / (3.156 × 10⁷)
3. v = 9.42 × 10¹¹ / 3.156 × 10⁷
4. v = 29 900 m/s (approximately 30 km/s)

That means you're hurtling through space at about 107,000 km/h right now, even though you can't feel it at all.

Worked example 2: The Moon's orbital speed

The Moon orbits Earth at an average radius of 3.84 × 10⁸ m with a period of 27.3 days. Find its orbital speed.

1. Convert T: 27.3 × 24 × 3600 = 2.359 × 10⁶ s
2. v = 2π × (3.84 × 10⁸) / (2.359 × 10⁶)
3. v = 2.413 × 10⁹ / 2.359 × 10⁶
4. v = 1020 m/s (approximately 1.0 km/s)

Notice the Moon moves much more slowly than Earth in its orbit. That makes sense: it's orbiting a much less massive body (Earth vs the Sun) at a smaller radius.

Stars and the Universe

The Sun is a star. That might sound obvious, but it's easy to forget when stars look like tiny pinpricks and the Sun fills the sky. Our Sun is a fairly ordinary star sitting in the Milky Way galaxy, which contains somewhere between 100 billion and 400 billion other stars. And the Milky Way is just one galaxy among billions in the observable Universe.

The life cycle of a star

Stars don't last forever. They form, they burn, and eventually they die. The path a star follows depends on how massive it is.

For a star about the size of our Sun:

1. A cloud of gas and dust (a nebula) begins to collapse under its own gravity.
2. As the material contracts and heats up, it forms a protostar.
3. When the core temperature reaches about 15 million degrees, hydrogen nuclei begin to fuse into helium. The star joins the main sequence, where it spends most of its life, steadily converting hydrogen to helium.
4. When hydrogen fuel in the core runs low, the outer layers expand and cool. The star becomes a red giant.
5. The outer layers eventually drift away, and the remaining core contracts into a small, hot, dense white dwarf that gradually cools over billions of years.

For a star much more massive than the Sun:

1. The early stages are similar: nebula, protostar, main sequence. But the star is larger, hotter, and burns through its fuel much faster.
2. It expands into a red supergiant.
3. The core collapses violently, triggering a supernova explosion that blasts the outer layers into space.
4. What remains depends on the mass left behind: either a neutron star (unimaginably dense) or, if massive enough, a black hole.

Distances in space

The distances between stars are so vast that kilometres become impractical. Instead, astronomers use the light-year: the distance that light travels in one year. Light moves at about 3.0 × 10⁸ m/s, which gives one light-year a value of roughly 9.5 × 10¹⁵ m. The nearest star to our Sun, Proxima Centauri, sits about 4.2 light-years away.

The expanding Universe

A remarkable discovery of twentieth-century astronomy is that the Universe is expanding. The evidence for this comes from studying distant galaxies.

Redshift

When a galaxy is moving away from us, the light it emits gets stretched to longer wavelengths. This pushes the light towards the red end of the visible spectrum, a phenomenon called redshift. The faster the galaxy recedes, the greater the redshift.

Observations show that nearly all distant galaxies are redshifted, and the more distant the galaxy, the larger the redshift. This pattern tells us the Universe is expanding: galaxies are moving apart from each other, like dots on the surface of a balloon as it inflates.

Hubble's law (Extended/Supplement)

Edwin Hubble discovered a direct proportional relationship between a galaxy's recession speed and its distance from us:

v = H₀d

where v is the galaxy's recession speed, d is its distance, and H₀ is the Hubble constant. This equation means that the further away a galaxy is, the faster it's moving away from us. It provides strong evidence that the Universe has been expanding from a common origin.

The Big Bang theory

The Big Bang theory proposes that the Universe began from an extremely hot, dense state approximately 13.8 billion years ago and has been expanding and cooling ever since. Two key pieces of evidence support this:

• Redshift of galaxies: the observation that galaxies are moving apart is consistent with an expansion from a common starting point.
• Cosmic microwave background (CMB) radiation: a faint, uniform glow of microwave radiation fills the entire sky in every direction. It's the leftover thermal radiation from the early Universe, cooled by billions of years of expansion. Its existence was predicted by the Big Bang theory before it was actually detected in 1965.

Common mistakes in space physics

Test yourself

Have a proper go at each question before checking the answers below. Writing out your working is good practice for the real exam.

1. Explain why we experience day and night on Earth.
2. A student says "we get summer because Earth is closer to the Sun." Explain why this is incorrect and give the real reason for seasons.
3. Describe the appearance of the Moon at each of these phases: new Moon, first quarter, full Moon.
4. Name the eight planets of the Solar System in order from the Sun.
5. (Extended) Mars orbits the Sun at an average radius of 2.28 × 10¹¹ m with an orbital period of 687 days. Calculate the orbital speed of Mars.
6. Describe the life cycle of a star much more massive than the Sun, from nebula to its final state.
7. What is redshift, and how does it support the Big Bang theory?

Exam strategies for space physics

Space physics questions on the IGCSE Cambridge papers tend to reward clear, precise explanations over heavy calculations. Here are some practical strategies to help you pick up marks:

• Use precise language. "The Earth rotates on its axis" is correct and specific. "The Earth spins around" is vague and could cost you the mark. Examiners want to see that you know the proper terminology.
• Link cause to effect. If a question asks about seasons, don't just say "the Earth is tilted." Explain how the tilt causes one hemisphere to receive more direct sunlight, leading to longer days and higher temperatures.
• Know both pieces of Big Bang evidence. Questions about the Big Bang almost always ask for evidence. Make sure you can name and explain both: the redshift of galaxies and the cosmic microwave background radiation.
• Sketch before you write. For star life cycle questions, draw a quick flow diagram in the margin before writing your answer. This helps you include every stage and keeps your response structured.
• Watch the command words. "State" means give a brief answer without explanation. "Explain" means you need to give a reason. "Describe" means set out what happens, step by step. Matching your answer length and depth to the command word prevents wasted time.
• Don't forget unit conversions. Orbital speed questions almost always give the period in days or years. Convert to seconds before substituting into v = 2πr/T, or you'll get the wrong answer and lose all the marks for the calculation.

Space physics might sit at the end of the syllabus, but it's well worth your revision time. The concepts connect in satisfying ways, and a solid understanding of this topic can earn you reliable marks across both the theory and practical papers. You've got this.