HookThe winter a star in Orion started to vanish
In the winter of 2019 to 2020, one of the most familiar stars in the sky began to disappear. Betelgeuse — the red-orange shoulder of Orion, a red supergiant roughly 640 light years away and so enormous that if it sat where the Sun is its surface would reach out past Mars — faded to about a third of its usual brightness by February 2020. Astronomers and headline writers asked the same question: was it finally about to explode? A red supergiant is a star in the last act of its life, and when Betelgeuse does eventually collapse and detonate as a supernova, it will briefly outshine the full Moon. The Great Dimming turned out to be a vast cloud of dust the star had coughed out, not the beginning of the end — but it was a live rehearsal of the physics in this section. Every star is the result of one running battle: gravity pulling inward against the outward push of radiation from fusion, and how that battle ends is decided by nothing more than the star's mass.
Space physics is the one part of GCSE where the numbers stop being human-sized. The Sun turns roughly four million tonnes of its own mass into energy every second; the light reaching us from the far galaxies set out billions of years ago; and the entire universe is flying apart, a fact you can read straight off the colour of starlight. This section runs from our own back garden of the solar system out to the edge of the observable universe: how the Sun and planets formed, how a star lives and dies according to its mass, what keeps a satellite circling the Earth with no engine at all, and how the red-shift of distant galaxies became the evidence for the Big Bang.
ModelOur solar system — and how the Sun switched on
At the centre of our solar system sits the Sun, an ordinary star. Around it orbit the eight planets — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune — most of them circled in turn by natural satellites, or moons. Scattered among them are the dwarf planets, such as Pluto, which orbit the Sun but have not swept their orbits clear of other debris. Wrapped around it all is the human contribution: thousands of artificial satellites we have launched, from the International Space Station to the GPS and weather satellites your phone quietly relies on. Zoom out further and the Sun is just one of many billions of stars in the Milky Way galaxy.
The Sun and everything orbiting it formed from the same cloud. Around 4.6 billion years ago a vast cloud of dust and gas — a nebula, mostly hydrogen left over from earlier stars — was pulled together by its own gravity. The great majority of the material fell to the centre, growing hotter and denser as it collapsed, until the core became hot enough for hydrogen nuclei to fuse. That ignition of nuclear fusion is the moment a protostar becomes a star: it switched the Sun on. The leftover dust and gas, swirling in a disc around the young Sun, clumped under gravity into the planets, moons and everything else. So the solar system is not a set of separate objects that happened to meet — it is the debris and the furnace of a single collapsing cloud.
MechanismThe life cycle of a star
A star's whole life is a tug-of-war between gravity, which pulls its material inward, and the outward pressure of radiation from fusion in its core. While those two are balanced, the star is stable and barely changes for billions of years — this long, steady stage is called the main sequence, and the Sun is about halfway through its ten-billion-year turn on it. During main-sequence life a star fuses hydrogen into helium, and it is this fusion that holds gravity at bay.
When the core hydrogen starts to run low, the balance tips. The star swells and cools at the surface to become a red giant, or, for a much heavier star, a red supergiant like Betelgeuse. What happens next depends entirely on mass. A star about the size of the Sun sheds its outer layers as a glowing shell of gas and leaves behind a hot, dense core called a white dwarf, which slowly cools over aeons. A star much more massive than the Sun ends far more violently: it explodes as a supernova, briefly outshining an entire galaxy, and leaves behind an incredibly dense neutron star — or, if the original star was massive enough, a black hole, whose gravity is so strong that not even light can escape.
There is a beautiful footnote here that examiners love. Fusion inside stars builds up the elements heavier than hydrogen, but only as far as iron. Everything heavier — the gold in a ring, the iodine in your thyroid — was forged in the ferocious energy of supernova explosions and scattered across space to be recycled into new stars and planets. The atoms in your body were quite literally made in stars.
Trace the Sun's life story from beginning to end.
Start: a nebula of dust and gas collapses under gravity, heating as it shrinks into a protostar. When the core is hot and dense enough, hydrogen fusion ignites and the Sun becomes a main-sequence star — stable because gravity inward is balanced by radiation pressure outward. It stays here around ten billion years (it is roughly halfway now).
Middle: as core hydrogen runs low the balance fails; the Sun swells and its surface cools to a red giant, large enough to engulf the inner planets.
End: because the Sun is a low-mass star, it does not explode. It puffs off its outer layers as a shell of gas and is left as a white dwarf — a hot, Earth-sized, super-dense core that simply cools and fades over billions of years.
The whole exam trick is the fork in the road: had the Sun been many times more massive, the same story would have ended not in a quiet white dwarf but in a supernova leaving a neutron star or a black hole. Mass, and nothing else, decides which path a star takes.
MechanismOrbits — falling forever
Nothing pushes a satellite along its orbit; there is no engine and no rope. What holds a moon around a planet, a planet around the Sun, or the ISS around the Earth is a single force — gravity. In a circular orbit the object travels at a constant speed, but its direction is changing every instant, and a changing direction means the velocity is changing, so the object is accelerating. That acceleration points straight in toward the centre, and gravity is the inward force that provides it. The satellite is, in effect, permanently falling toward the Earth and permanently missing, because the ground curves away beneath it just as fast.
The key relationship for Higher tier is that, for a stable circular orbit, there is exactly one speed that fits a given radius. A satellite closer to the Earth must travel faster to stay up; one further out travels slower. It is why Mercury races round the Sun in 88 days while Neptune plods round in 165 years, and why the ISS, skimming low at about 400 km, laps the Earth every 90 minutes, whereas a geostationary satellite far out at 36,000 km takes a full 24 hours and so appears to hover over one spot. If a satellite's speed changes, it cannot stay in the same orbit: speed up and it moves to a larger, slower orbit; slow down and it drops to a smaller, faster one.
The International Space Station orbits at an altitude that puts it a distance of about \(6.77\times10^{6}\ \text{m}\) from the Earth's centre, and it completes one orbit in roughly 92 minutes. How fast is it moving?
In one orbit the ISS travels the circumference of its circle, \(2\pi r\), in one period \(T\), so its speed is \(v = \frac{2\pi r}{T}\).
First make the time consistent: \(92\ \text{min} = 92 \times 60 = 5520\ \text{s}\).
Then \(v = \frac{2\pi \times 6.77\times10^{6}}{5520} = \frac{4.25\times10^{7}}{5520} \approx 7.7\times10^{3}\ \text{m/s}\).
That is about 7.7 km/s, or roughly 27,000 km/h — matching the real figure. The point to hold onto is that this speed is fixed by the orbit: the ISS cannot choose to dawdle at that height. Drop it to a lower orbit and it would have to move faster still; boost it to a higher one and it would necessarily slow down.
DataRed-shift and the expanding universe
When astronomers split the light from a distant galaxy into its spectrum, they find the whole pattern of dark lines shifted toward the red, long-wavelength end compared with the same lines measured on Earth. This is red-shift, and it means the galaxy is moving away from us — the light waves are stretched to longer wavelengths as the source recedes, much as a passing siren drops in pitch once it is heading away.
The decisive observation, made by Edwin Hubble in 1929, is that the further away a galaxy is, the greater its red-shift — so the more distant galaxies are receding faster. Almost every galaxy is rushing away from us, and the pattern is the same in every direction. The natural reading is not that we sit at some unpopular centre that everything is fleeing, but that space itself is expanding, carrying the galaxies apart like ink spots on the surface of an inflating balloon, where every spot sees every other spot moving away from it.
Run that expansion backwards in your mind and everything in the universe was once packed together in an unimaginably hot, dense state that began expanding — the Big Bang. Red-shift is the central piece of observational evidence for it: an expanding universe today implies a tiny, dense origin billions of years ago. This is why a colour measured in starlight can settle a question about the birth of everything.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
In the life-cycle questions, mass is the answer to almost everything. Whenever you are asked how a star will end, split your answer explicitly by mass: a star about the size of the Sun becomes a red giant then a white dwarf; a much more massive star becomes a red supergiant, explodes as a supernova, and leaves a neutron star or a black hole. State the balance that makes a main-sequence star stable — gravity inward against radiation pressure outward — because that one sentence is often worth a mark on its own.
On orbits, resist the trap of saying a satellite moves at 'constant velocity'. It moves at constant speed but continually changing direction, so its velocity is changing and it is accelerating toward the centre, with gravity supplying that force. If you are asked what happens when a satellite's speed changes, say clearly that it must move to a different orbit rather than stay put. For the ISS-style calculation, use \(v = \frac{2\pi r}{T}\), convert the period to seconds first, and keep the radius as the distance from the Earth's centre, not the altitude.
For red-shift, make the logical chain visible and in order: distant galaxies show red-shift → they are moving away → more distant ones move faster → the universe is expanding → wind it back and you reach the Big Bang. Examiners want that reasoning stated as evidence, not just the phrase 'the Big Bang' on its own.