What's in the solar system

Our solar system is held together by the gravity of the Sun — which is itself a star. Orbiting it we find:

• The 8 planets (Mercury → Neptune).
• Dwarf planets, such as Pluto.
• The planets' moons — their natural satellites.
• Asteroids and comets.

Watch out — the Sun IS a star. It only looks different from the night-time stars because it is so much closer. Every other star you can see is a distant sun, so far away that it looks like a tiny point of light. Our solar system is just one tiny part of a much larger galaxy, the Milky Way.

What is the Sun?

From a cloud of gas to a star

A star begins as a nebula — a giant cloud of dust and gas. Gravity pulls this cloud together:

• As it collapses it gets denser and hotter, forming a protostar.
• When it is hot enough, nuclear fusion of hydrogen into helium begins — it becomes a main-sequence star.

A main-sequence star (like the Sun) is stable for a long time because the outward pressure from fusion exactly balances the inward pull of gravity.

Picture it — a cosmic tug-of-war. A main-sequence star is two giant forces pulling against each other: gravity hauling everything inward, and the outward pressure from fusion pushing everything outward. While the two teams are evenly matched the star holds its size and stays stable. When the fuel starts to run out, fusion weakens, gravity "wins" the next tug, and the star begins to change.

Watch out — stars don't "burn" like a fire. A fire is a chemical reaction with oxygen. A star releases energy by nuclear fusion — joining light nuclei (hydrogen → helium) in its core. There is no oxygen and nothing is "on fire".

A stable star

What happens next depends on mass

When a star runs low on hydrogen its future depends on its mass. The shared early stages — nebula → protostar → main-sequence star — then branch into two paths:

Sun-sized star: red giant → sheds its outer layers as a planetary nebula → leaves a white dwarf, which cools to a black dwarf. Much more massive star: red supergiant → supernova → neutron star, or a black hole if the remnant is massive enough.

Sun-like or massive?

For each ending, tap the kind of star it belongs to.

What holds a star up, and for how long?

Stars are element factories

Fusion in a star builds heavier elements from lighter ones — but only up to iron:

• Fusion in stars creates elements up to and including iron.
• Elements heavier than iron are made in the huge energies of a supernova.
• A supernova also scatters these elements out into space, where they can form new stars and planets.

Watch out — fusion stops at iron. A common mistake is to think stars fuse all the way up to gold and uranium. They don't: ordinary fusion can only build elements up to iron. Everything heavier than iron — gold, silver, uranium, the iodine in your thyroid — was forged in extreme events such as a supernova and scattered across space long ago. You really are made of stardust.

Heavy elements

Put the stages in order

Why does the Moon never fall?

Why things stay in orbit

Gravity provides the centripetal force — a force pointing toward the centre — that keeps a planet, moon or satellite in its orbit.

In a circular orbit the speed stays the same, but the velocity is always changing direction. Because velocity changes, the object is constantly accelerating — toward the centre.

Picture it — whirling a ball on a string. Swing a ball round your head on a string: the string tension pulls the ball into a circle, just as gravity pulls a planet into its orbit. Notice the string always pulls inward, never forward. And if you let go (cut the string), the ball flies off in a straight line along its velocity — exactly the direction it was heading. Remove gravity and a planet would do the same.

Watch out — constant speed still means accelerating. Students often say "the speed is constant, so there's no acceleration." But acceleration is any change in velocity, and velocity includes direction. The orbiting object's direction is changing every instant, so it is accelerating — toward the centre — even though its speed never changes.

Constant speed, changing velocity

Why does the Moon never fall?

Smaller orbit means faster

For a stable circular orbit at a given radius there is only one possible speed. The link to remember:

If a satellite speeds up, it can move out to a larger orbit; if it slows down it falls to a smaller one. There are two kinds of satellite:

• Natural satellites — e.g. moons orbiting planets.
• Artificial satellites — e.g. communications and GPS satellites we have launched.

Watch out — closer means faster, not slower. It feels natural to think an inner orbit is "slower" because it is shorter, but the opposite is true: a smaller orbit radius needs a greater speed. Closer to the Sun, gravity is stronger, so the planet must travel faster to stay in a stable orbit. That is why Mercury (innermost) races round the Sun far faster than Neptune (outermost).

Two orbits

What does it orbit?

Tap an object, then tap the box it belongs in.

Are we at the centre of the universe?

Light from distant galaxies

When we look at light from distant galaxies, the dark lines in its spectrum are shifted toward the red (longer-wavelength) end. This is called red-shift. The further the galaxy, the bigger the shift.

Red-shift means the galaxy is moving away from us. The further away a galaxy is, the bigger its red-shift and the faster it is receding.

Picture it — a passing siren, but for light. An ambulance siren drops in pitch as it speeds away: its sound waves get stretched to a lower frequency. Light from a receding galaxy is stretched the same way — to longer (redder) wavelengths. The faster the galaxy recedes, the more its light is stretched, just as a faster ambulance shows a bigger drop in pitch.

Reading red-shift

Evidence for the Big Bang

Because nearly all distant galaxies are red-shifted (and the more distant ones more so), the whole universe is expanding — distant galaxies are all moving away from each other.

This supports the Big Bang theory: the universe began from a very small, hot, dense region and has been expanding ever since.

• The cosmic microwave background (CMB) — faint radiation reaching us from all directions — is further evidence for the Big Bang.
• The model is still incomplete: ideas like dark matter (AQA calls it “dark mass”) and dark energy are needed to explain observations we don't yet fully understand.

Remember: red-shift is the key evidence that the universe is expanding, which is in turn the key evidence for the Big Bang.

What the red-shift tells us

Are we at the centre of the universe?

An unfinished story

The big ideas to know

You've covered the key ideas in AQA 4.8 — the solar system, the life cycle of a star, orbital motion and red-shift. Press Finish to see your score.