Space physics – GCSE Combined Science Physics

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Tie a ball to a piece of string and swing it round your head. You can feel the string pulling the ball inwards the whole time. Let go and the ball shoots off in a straight line.

Space works a bit like that. The Moon goes round the Earth, and the Earth goes round the Sun. There is no string out there, so what does the pulling?

The answer is gravity. Gravity is a force that pulls objects towards each other. It is the invisible string of the Solar system.

Gravity also decides how stars are born and how they die. The light from faraway galaxies then tells us how the whole universe began.

The distances are hard to picture. Light is the fastest thing there is, yet it still takes about 8 minutes to reach us from the Sun. Light from the nearest other stars takes years.

Here is a way to picture the Solar system. Put a football on the ground for the Sun, and the Earth would be a grain of sand about 25 metres away. The planets are tiny and very spread out, and gravity is what links them all to the Sun.

Some words in this lesson will be new, such as orbit, satellite, nebula, fusion and red-shift. Each one is explained before it is used. If a word will not stick, try saying it in your own words.

The maths is gentle. You will only multiply and divide, and you will use two short equations. Everything else is about explaining clearly, in plain sentences, what is going on.

By the end you will be able to do three things. You will explain how gravity keeps planets, moons and satellites in orbit. You will put the life of a star in order, and you will say what red-shift tells us about the universe.

The Solar system and satellites

The Sun is a star, which is a huge ball of hot gas that gives out light and heat. It looks small only because it is far away, and over a million Earths would fit inside it. Our Sun is one of billions of stars in a galaxy called the Milky Way, and a galaxy is a huge group of stars held together by gravity.

Eight planets travel around the Sun. In order from the Sun they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The path a planet takes round the Sun is called its orbit, and it is close to a circle.

Most planets have moons. A moon is a smaller object that orbits a planet, and the Earth has one while Jupiter has dozens. Other things orbit the Sun too: dwarf planets such as Pluto are smaller than planets, asteroids are lumps of rock, and comets are balls of ice and dust.

Anything that orbits something bigger is called a satellite. A natural satellite was not made by people, so the Moon is a natural satellite of the Earth. An artificial satellite is built by people and launched into space, like a weather satellite or one that sends TV and phone signals.

The nearer a planet is to the Sun, the shorter its year. A year is the time to go once round the Sun, so it depends on the planet's orbit. The bars show that Mercury takes about a quarter of an Earth year, while Mars takes nearly two.

Gravity, mass and weight

Gravity is a force that pulls any two objects with mass towards each other. Mass is the amount of matter in an object, and we measure it in kilograms, kg. The pull between everyday objects, such as two pencils on a desk, is far too small to notice because neither has much mass.

The pull gets bigger when the masses are bigger, and when the objects are closer together. That is why the Earth, which is enormous and right beneath your feet, pulls you down so firmly. The Moon is much smaller, so its pull is much weaker.

Your weight is the force of gravity pulling on you, and we measure it in newtons, N. Mass and weight are not the same thing. A bag of sugar has a mass of 1 kg wherever it is, but the pull on it changes from place to place.

Gravitational field strength, g, tells you how strong gravity is in a place. It is the weight of each kilogram there, so its unit is newtons per kilogram, N/kg. Take g = 10 N/kg on the Earth, and about 1.6 N/kg on the Moon.

In words, weight = mass × gravitational field strength. In symbols, W = m × g, with W in newtons, m in kilograms and g in N/kg, and you use it whenever you know a mass and want a weight. A 60 kg astronaut weighs 60 × 10 = 600 N on the Earth but 60 × 1.6 = 96 N on the Moon, and is still 60 kg.

Why things stay in orbit

Throw a ball sideways and it curves down to the ground. Throw it harder and it lands further away. Now imagine throwing it so fast that the ground curves away beneath it as quickly as the ball falls.

That is what an orbit is. A satellite is always falling towards the Earth, but it is moving sideways so fast that it keeps missing. Astronauts on the International Space Station float because they and the station are falling together.

Gravity points towards the centre of the orbit all the time. Without it the satellite would fly off in a straight line, like the ball when you let go of the string. There is hardly any air in space, so nothing slows the satellite down and it can keep going for years.

Now the tricky part. Speed tells you how fast something is moving, but velocity tells you how fast and in which direction. A satellite in a circular orbit has a steady speed, but its direction changes all the time, so its velocity changes all the time.

A change in velocity is called acceleration. So the satellite is accelerating, even though it is not getting faster. The resultant force is the single force you get after adding up all the forces on an object, and here it is gravity, pointing to the centre of the orbit.

Orbit size, speed and satellites at work

Gravity gets weaker as you move away from a planet, so a close satellite feels a stronger pull than a far one. To stay in a steady orbit, the close satellite has to move faster. The same is true for planets, which is why Mercury races round the Sun and Neptune plods.

Higher tier: for any steady orbit, the speed and the radius go together. The radius is the distance from the centre of the orbit to the satellite. If the speed of an orbiting object changes, the radius of its orbit must change too.

Satellites do useful jobs, and the table shows some of them. Weather satellites photograph the clouds, and communications satellites carry TV, phone and internet signals. A satellite that goes round once every 24 hours above the equator seems to hang over one spot, so a dish on the ground can point at it and stay still.

To work out how fast something is going, use speed = distance ÷ time, or v = s ÷ t. Speed is in metres per second, m/s, distance is in metres, m, and time is in seconds, s. Rearranged, distance = speed × time, and you use this when you know the speed and the time.

Speeds in orbit are huge, often several kilometres every second. For example, a satellite moving at 3 km/s for 100 s travels distance = speed × time = 3 × 100 = 300 km. Always check that the units match, because km/s multiplied by s leaves km.

The life of a star

Stars form from a nebula, which is a huge cloud of dust and gas, mostly hydrogen. Gravity slowly pulls the cloud together into clumps, because every bit of gas pulls on every other bit. This takes millions of years.

As a clump falls together it gets hotter, and the hot ball is called a protostar. When the centre is hot and squashed enough, nuclear fusion starts. Nuclear fusion is when small nuclei join to make a bigger nucleus, and it gives out huge amounts of energy as light and heat.

The star is now a main sequence star, and it stays like this for billions of years. Two effects are balanced. Gravity pulls inwards and the pressure from the hot gas pushes outwards, like air holding a balloon at one size.

When the hydrogen starts to run out, the balance is lost. A star about the size of the Sun swells into a red giant, then sheds its outer layers and shrinks to a white dwarf. The white dwarf is small, hot and dense, and it slowly cools and fades until it becomes a black dwarf.

A star with much more mass than the Sun swells into a red supergiant and then explodes as a supernova. What is left is a neutron star or, for the biggest stars, a black hole, where gravity is so strong that not even light escapes. The explosion scatters new elements into space, and new stars and planets can form from this material, so many of the atoms in your own body, such as carbon and oxygen, were made inside stars.

Red-shift and the Big Bang

Think of an ambulance siren. As it comes towards you the sound is high, and as it drives away the sound drops. The sound waves are squashed together as it approaches and stretched out as it leaves.

Light is a wave too. Wavelength is the distance from one crest of a wave to the next. If a galaxy moves away from us, the light waves from it are stretched, so the wavelength gets longer.

Longer wavelengths of visible light are towards the red end of the spectrum, so the light is shifted towards red, and this is called red-shift. It does not mean the galaxy looks red. It means the wavelength has increased.

Scientists spot the shift in the pattern of dark lines in a galaxy's light, and the whole pattern slides towards the red end. Almost every distant galaxy is red-shifted, so almost every galaxy is moving away from us. The further away the galaxy, the bigger the red-shift, so the faster it is moving away.

Run the film backwards and all the galaxies were once together in one tiny, hot place, which is the Big Bang model, where space has been expanding ever since. A faint glow of microwaves from every direction, the cosmic microwave background, is left over from that hot start. The rival Steady State model cannot explain this glow.

Recap

Gravity pulls masses together and keeps moons, planets and satellites in orbit. Weight = mass × g, so mass stays the same but weight changes from place to place. Take g = 10 N/kg on the Earth.

In a circular orbit the speed is steady, but the direction keeps changing. So the velocity changes, which means acceleration towards the centre. Higher tier: a closer orbit needs a higher speed, so speed and radius go together.

A star begins as a nebula, becomes a protostar, and then a stable main sequence star while fusion pressure balances gravity. A star like the Sun ends as a white dwarf. A much bigger star explodes as a supernova and leaves a neutron star or a black hole.

Red-shift means the wavelength of light from a galaxy has increased because the galaxy is moving away. Further galaxies show bigger red-shifts, so the universe is expanding. This and the cosmic microwave background support the Big Bang model.

Words to know: orbit, satellite, gravity, weight, nebula, fusion, supernova and red-shift. If you can say what each one means in a short sentence, you are in good shape. Unit slips cost easy marks, so write N for weight, kg for mass and N/kg for g.

A quick self-test: cover the page and say out loud why a satellite is accelerating, what keeps a star stable, and what red-shift shows. If you can say each in one clear sentence, you have the main ideas. If one is shaky, go back to that step and read it again slowly.

How this is asked in an exam: a calculation wants the equation, the numbers and the unit. An orbit question wants three links: the direction changes, so the velocity changes, so there is acceleration. A red-shift question wants longer wavelength, moving away, and then expanding universe.