An Interactive Guide to Astronomy
Eight planets orbit the Sun, held in place by gravity — the invisible pull every massive object exerts. Click any planet to learn more.
Gravity is the force that keeps planets orbiting the Sun, the Moon orbiting Earth, and your feet on the ground. The bigger an object's mass, the stronger its gravitational pull. The Sun contains 99.86% of all the mass in the Solar System, which is why everything orbits it.
Stars are giant balls of gas powered by nuclear fusion — smashing hydrogen atoms together to make helium, releasing tremendous energy. How a star lives and dies depends on its mass. Click each stage below.
A light-year is the distance light travels in one year: about 9.46 trillion kilometres. We use it because normal units are impractical at cosmic scales. The nearest star, Proxima Centauri, is 4.24 light-years away — meaning the light you see from it tonight left that star over 4 years ago. Looking at the night sky is literally looking back in time.
A galaxy is a massive collection of stars, gas, dust, and dark matter held together by gravity. The universe contains billions of them — and it's still expanding.
A barred spiral galaxy containing 100–400 billion stars. Our Solar System sits about two-thirds of the way out from the centre, in one of the spiral arms. It takes our Sun about 230 million years to orbit the galactic centre once — a "galactic year."
Contains at least 200 billion galaxies, each with billions of stars. The universe is 13.8 billion years old. Because light takes time to travel, we can only see objects whose light has had time to reach us — this defines the "observable" universe, a sphere about 93 billion light-years across.
Every galaxy is moving away from every other galaxy — like dots on the surface of a balloon being inflated. Edwin Hubble discovered this in 1929. It's one of the key pieces of evidence for the Big Bang: the idea that the universe began from an incredibly hot, dense point 13.8 billion years ago and has been expanding and cooling ever since.
Here's something mind-bending: all the stars, planets, and gas we can see make up only about 5% of the universe. About 27% is "dark matter" — invisible stuff that has gravity but doesn't emit light. The remaining 68% is "dark energy" — a mysterious force causing the universe's expansion to accelerate. We don't know what either of them are. Seriously.
Blow up a balloon partway and draw dots on it. Inflate it more — every dot moves away from every other dot. No dot is at the centre. That's how the universe expands: space itself is stretching, carrying galaxies apart. The galaxies aren't flying through space; space between them is growing.
Visible light is only a tiny slice of electromagnetic radiation. Move your mouse (or tap) across the spectrum below to explore the different types — from radio waves to gamma rays.
Move your mouse across the coloured bar to explore different types of electromagnetic radiation and how astronomers use each one to see the universe.
Sunlight contains all colours. When it hits Earth's atmosphere, shorter wavelengths (blue light) scatter in all directions more than longer wavelengths (red light). So when you look up, you see scattered blue light everywhere. At sunset, sunlight travels through more atmosphere, scattering away most blue, leaving reds and oranges.
Space is big. Really, unimaginably big. Use the slider below to shrink the Sun and watch how the planets compare.
If the Sun were a basketball (24 cm), Earth would be a tiny peppercorn 26 metres away. Jupiter would be a walnut 135 metres away. And Neptune? Over 780 metres away. Most of the Solar System is just empty space. And the nearest star at this scale would be another basketball... 6,700 kilometres away.