The universe

Everything there is. Every measurement points back to a hot, dense start 13.79 billion years ago, though what happened at that start nobody knows. The universe is still getting bigger, and the part of it we can see has an edge — not because the universe stops there, but because light has not had time to reach us from any further.

The size, in steps you can hold

Nobody can picture the universe. What you can picture is a set of steps: the Earth is a ball you could walk round in a couple of years. The Moon is thirty Earths away. The Sun is four hundred times as far as the Moon. The nearest star is a quarter of a million times as far as the Sun. Our galaxy is about 87,000 light-years across, and there are hundreds of billions of galaxies. Each step is a real measurement somebody made, standing on the one before.

• 1 · The size of the Earth — a stick and a shadow
• 2 · The Moon — a curved shadow in an eclipse
• 3 · How the solar system is arranged — two pins and a string
• 4 · The distance to the Sun — Venus crossing it, from two continents
• 5 · The distance to the stars — the Earth’s orbit as a baseline
• 6 · Other galaxies — stars whose pulse gives away their true brightness
• 7 · The scale of the universe — the colour of light from very far away

How it began

Everything was once packed into something unimaginably hot and dense, and it has been expanding and cooling ever since. That is not a guess — three separate pieces of evidence say so, and one of them, the leftover heat, was found by two people who were not looking for it.

• How the universe began, and how anyone could know — the expanding sky, the leftover heat, and the recipe of the light elements

What it is made of

Ordinary matter — every star, planet, person and rock — is about five parts in a hundred. About a quarter is dark matter, which has gravity and nothing else we can detect. The rest, roughly two thirds, is dark energy, which is pushing the expansion faster and which nobody understands at all.

The dark matter halopart of the galaxyBeyond a 150 mm telescope

• What is left to explain — what nobody has worked out yet

The strangest things in it

Stars so dense a teaspoonful would weigh as much as a mountain. A galaxy that is a ring, with an empty gap round its core. Two galaxies mid-collision. A star that seems to have collapsed into a black hole and gone out, with no supernova.

RX J1856.5-3754neutron starBeyond a 150 mm telescopeHoag’s Objectring galaxyBeyond a 150 mm telescopeThe Antennae Galaxies (NGC 4038/4039)colliding galaxies150 mm telescopeN6946-BH1failed supernovaBeyond a 150 mm telescope3C 273quasar150 mm telescopeGW170817kilonovaBeyond a 150 mm telescope

🌙 Plan an evening outsideEvery object, one by one