Ch 1 · Earth, the Solar System, Stars and the Universe · IGCSE Physics 0625
Topic 1 of 1 in Space Physics — IGCSE Physics 0625 — 47 lessons.
Introduction
Space physics takes the ideas from the rest of the course — forces, energy, waves and nuclear fusion — and applies them on the largest scales. Why is it summer in Australia when it is winter in Europe? Why are the inner planets rocky and the outer ones giant balls of gas? How do we know the Universe is expanding, and how old it is? The calculations here are short, but they need careful handling of powers of ten and units.
Key ideas — The Earth and the Moon
1. Day and night. The Earth rotates on its axis once in about 24 hours. The half facing the Sun has day; the other half has night. The rotation also makes the Sun appear to move across the sky from east to west each day.
🧑🏫 Teacher's explanation
Imagine you are standing on a spinning carousel at a fairground. As the carousel spins, you see the park lights moving past you in the opposite direction; this is exactly how the Sun 'moves' across our sky because the Earth is rotating on its axis.
📖 Definitions
🔍 Worked example — Standard
A light signal from the Sun takes 500 seconds to reach Earth. If the speed of light is 3.00 × 10^8 m/s, calculate the distance between the Sun and the Earth.
🔍 Worked example — Exam-style
An artificial satellite orbits a planet at a radius of 7000 km with a period of 2 hours. Calculate its orbital speed in km/h.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Calculate the average orbital speed of the satellite. Give your answer in km/h. (b) State the force responsible for keeping the satellite in its orbit. [3 marks]
Key ideas — The Solar System
The gravitational attraction of the Sun keeps the planets, minor planets and comets in orbit. The Sun's huge mass is why everything orbits the Sun.
🧑🏫 Teacher's explanation
Imagine a massive, spinning cloud of smoke and dust in space. Gravity acts like a cosmic magnet, pulling all that dust together. As the cloud spins, it flattens out into a pancake shape called an accretion disc. Most of the 'stuff' clumps in the middle to form the Sun, while the leftovers clump together to form planets. Near the hot Sun, only heavy rocks could stay solid, creating small, rocky planets. Far away, it was cold enough for gases to freeze and clump, creating the giant gas planets.
📖 Definitions
🔍 Worked example — Easy
Calculate the time in seconds for light to travel from the Sun to the Earth, given the distance is 1.5 imes 10^11 m and the speed of light is 3.0 imes 10^8 m/s.
🔍 Worked example — Standard
A planet orbits a star at an average radius of 2.0 × 10^8 km with an orbital period of 4.0 × 10^6 s. Calculate its average orbital speed in m/s.
🔍 Worked example — Exam-style
A satellite orbits a planet at a radius of 7.0 × 10^6 m. If the gravitational field strength at the surface is 9.81 N/kg and the planet's mass is 6.0 × 10^24 kg, calculate the gravitational field strength at the satellite's distance (assume g ∝ 1/r^2 and use g_surface × (R_planet/r)^2, but for this exam style, simply use g = GM/r^2 where G = 6.67 × 10^-11)
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Stars and galaxies
9. The Sun as a star. The Sun is a medium-sized star, made mostly of hydrogen and helium. It radiates most of its energy as **infrared, visible light and ultraviolet*. (S)* Stars are powered by nuclear fusion; in stable stars like the Sun, hydrogen nuclei fuse to form helium.
🧑🏫 Teacher's explanation
Think of the Solar System like a cosmic dance. The Sun is the heavy lead dancer in the center, and its massive gravity pulls on everything else, keeping them in orbit. Because the Sun is so much heavier than everything else, it stays relatively still while the planets circle around it.
📖 Definitions
🔍 Worked example — Easy
A planet orbits a star at an average distance of 1.5 × 10^11 m. Calculate the time in seconds it takes for light to travel from the star to the planet. (Speed of light c = 3.0 × 10^8 m/s)
🔍 Worked example — Standard
A planet has an orbital radius of 2.0 × 10^8 m and an orbital period of 4.0 × 10^6 s. Calculate its average orbital speed v.
🔍 Worked example — Exam-style
A star is located 4.0 light-years from Earth. Calculate the distance in meters. (Use 1 light-year = 9.5 × 10^15 m)
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Calculate the average orbital speed of the planet. [2] (b) State the force that keeps the planet in its orbit. [1] [3 marks]
Key ideas — The expanding Universe
11. Redshift. Redshift is the increase in the observed wavelength of electromagnetic radiation from stars and galaxies that are moving away from us. Light from distant galaxies is redshifted compared with light from the same elements measured on Earth, and more distant galaxies show greater redshift. This shows that galaxies are moving apart — the Universe is expanding — and supports the Big Bang theory: the Universe began from a single, extremely hot, dense point and has been expanding ever since.
🧑🏫 Teacher's explanation
Imagine you are standing on a sidewalk and an ambulance drives past you. As it approaches, the siren sounds high-pitched, but as it moves away, the pitch drops lower. This is the Doppler effect for sound. Light behaves similarly. If a galaxy moves toward us, its light waves get 'squashed' (blueshifted); if it moves away, the waves get 'stretched' out, making the light appear redder. This is called redshift.
📖 Definitions
🔍 Worked example — Easy
A galaxy is observed to be moving away from Earth at a speed of 4.4 × 10^4 m/s. If the distance to this galaxy is 2.0 × 10^23 m, calculate the Hubble constant H_0.
🔍 Worked example — Standard
Using the Hubble constant H_0 = 2.2 × 10^-18 s^-1, estimate the age of the Universe if the recession speed of a distant galaxy is 6.6 × 10^4 m/s.
🔍 Worked example — Exam-style
A galaxy is moving away from us at v = 1.1 × 10^5 m/s. Its distance is d = 5.0 × 10^23 m. Calculate the Hubble constant and use it to estimate the age of the Universe.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Example 1 — orbital speed (Supplement). Mars orbits the Sun at an average radius of 2.28 × 10^11\ m with a period of 687 days. Calculate its average orbital speed.
🔍 Worked example — Easy
A satellite orbits Earth at an average radius of 7.0 × 10^6 m with an orbital period of 6000 s. Calculate its average orbital speed.
🔍 Worked example — Standard
The distance from the Sun to Pluto is approximately 5.9 × 10^12 m. Calculate the time in minutes it takes for sunlight to reach Pluto.
⚠️ Common errors
Common mistakes and examiner tips
Practice
Answers: (1) The Earth rotates on its axis once every 24 hours, so a place spends part of each rotation facing the Sun (day) and part facing away (night). (2) The Earth's axis is tilted. For part of the orbit the northern hemisphere is tilted towards the Sun (longer days, Sun higher, more concentrated radiation) while the southern hemisphere is tilted away. (3) Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune; the rocky planets are Mercury, Venus, Earth and Mars. (4) T = 11.9 × 3.16 × 10^7 = 3.76 × 10^8\ s; v = dfrac2π × 7.8 × 10^113.76 × 10^8 = 1.3 × 10^4\ m/s (13 km/s). (5) 4.2 × 9.5 × 10^15 = 4.0 × 10^16\ m. (6) Near the young Sun it was too hot for gases and ices to condense, so only rock and metal accreted; further out it was cold enough for gases to collect, building large gas giants. (7) When hydrogen in the core runs out it expands to a red supergiant, then explodes as a supernova, forming a nebula with heavier elements and leaving a neutron star or black hole; the nebula may form new stars and planets. (8) d = 1.0 × 10^9 × 9.5 × 10^15 = 9.5 × 10^24\ m; v = H_0 d = 2.2 × 10^-18 × 9.5 × 10^24 = 2.1 × 10^7\ m/s. (9) Microwave radiation of a specific frequency detected from all directions in space; it was produced shortly after the Big Bang and its wavelength has been stretched into the microwave region as the Universe expanded. (10) Fastest when closest to the Sun: as it falls towards the Sun its gravitational potential energy decreases and its kinetic energy increases by the same amount.