Structure and scale of the universe

What is in the universe?

The universe is everything that exists: all matter and energy, and space and time themselves. Gravity organises the matter in it into a hierarchy of structures, each one sitting inside the next.

Pluto was called the ninth planet from its discovery in 1930 until 2006. Astronomers then began finding other icy worlds in the same region, one of them (Eris) about as big as Pluto. Rather than add dozens of new planets, the International Astronomical Union wrote the definition in the table, and Pluto became a dwarf planet. That didn't make Pluto less interesting: when the New Horizons probe flew past in 2015 it found mountains of water ice, a huge heart-shaped glacier of frozen nitrogen and a thin hazy atmosphere.

Nebulae

The word nebula is Latin for "mist". Before telescopes could separate them, astronomers called any fuzzy patch a nebula, including galaxies. Today the word only means clouds of gas and dust inside a galaxy, and there are four main kinds.

• Emission nebulae glow pink-red because ultraviolet light from hot young stars makes their hydrogen gas give out light. The Orion Nebula and the Carina Nebula, both visible without a telescope from Australia, are star nurseries like this.
• Reflection nebulae are dust clouds that scatter the light of nearby stars. They look blue for the same reason the sky does: blue light is scattered more than red.
• Dark nebulae are dense dust clouds that block the light behind them. The Coalsack beside the Southern Cross is one, and it forms the head of the Emu in the Sky.
• Planetary nebulae are glowing shells of gas puffed off by dying stars like the Sun (6E). They have nothing to do with planets: early astronomers just thought their round shapes looked like planets.

Our galaxy, the Milky Way

The Milky Way is a barred spiral galaxy about 100 000 light years across, containing somewhere between 100 and 400 billion stars. Most of its stars, gas and dust lie in a thin disc only about 1000 light years thick, with spiral arms where new stars are forming. In the middle is a bulge of older stars crossed by a bar, and at the very centre is a black hole, Sagittarius A*, with about 4 million times the mass of the Sun. Around it all is a faint halo containing old globular clusters and a lot of invisible dark matter (6H).

The Sun sits in a small arm called the Orion Arm, about 26 000 light years from the centre. It travels at about 230 km/s and takes roughly 230 million years to go once around the galaxy. The last time the Sun was in this part of its orbit, the first dinosaurs were appearing.

Because we are inside the disc, the Milky Way appears as a band of light across the sky. It is brightest in the direction of the centre, in Sagittarius and Scorpius, which is high overhead on winter evenings in Australia. The dark lanes along it are dust clouds.

From the Southern Hemisphere you can also see the Large and Small Magellanic Clouds, two small galaxies that orbit the Milky Way, about 160 000 and 200 000 light years away. The nearest large galaxy, Andromeda, is 2.5 million light years away. Together with the Milky Way and dozens of small galaxies it forms the Local Group.

Units for enormous distances

Kilometres work well on Earth, but in space the numbers get out of hand, so astronomers use bigger units and scientific notation. In scientific notation a number is written as a value from 1 to 10 multiplied by a power of ten: 150 000 000 km = 1.5 × 10⁸ km.

A light year is a distance, not a time, even though it has "year" in its name. It's like saying the shops are "ten minutes away": you're describing a distance by how long it takes to travel.

Looking out is looking back in time

Light takes time to reach us, so we always see things as they were when the light left them. You see the Moon as it was 1.3 seconds ago, the Sun as it was 8.3 minutes ago, and Proxima Centauri as it was 4.2 years ago. When you look at the Andromeda Galaxy without a telescope (it's a faint smudge, low in the north in late spring), the light entering your eye left it 2.5 million years ago, before modern humans existed.

This means telescopes are time machines. The most distant galaxies JWST has found are seen as they were less than 300 million years after the universe began. It also explains a strange-sounding number: the universe is 13.8 billion years old, yet the observable universe is about 93 billion light years across. Space has been expanding the whole time the light was travelling, so the galaxies that sent it are now much further away than 13.8 billion light years (6F).