Why does the universe work the way it does?
Everything with mass pulls on everything else. You pull on the Earth, and the Earth pulls on you. The bigger something is, the stronger its pull. That's gravity — and it runs the whole universe.
Gravity is the pull between any two objects that have mass. You can't see it or touch it, but it's always there. It's why you stick to the ground, why the Moon goes around Earth, and why Earth goes around the Sun.
Isaac Newton realised that the same force pulling an apple to the ground is the same force keeping the Moon in orbit. Before him, people thought earthly physics and heavenly physics were completely different things.
Albert Einstein showed that gravity is actually massive objects bending space (and time!) around them. Other objects just follow the curves. Imagine a bowling ball on a trampoline making a dip — marbles nearby roll toward it.
Drag the slider to move the objects apart and watch gravity drop
Your mass is how much stuff you're made of — it doesn't change. Your weight is how hard gravity pulls on that mass. On the Moon (1/6 Earth's gravity), you'd weigh 1/6 as much but your mass stays the same. On Jupiter, you'd weigh 2.5 times more. You haven't changed — the gravity has.
Light is how we learn about everything in space. The colour of light, how it bends, and how it stretches tells us what stars are made of, how hot they are, and whether they're moving toward or away from us.
The light your eyes can see — the rainbow from red to violet — is just a tiny slice of all the light that exists. There are types of light with wavelengths too long or too short for us to see.
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Each section is a different type of light that astronomers use to explore the universe.
When you split a star's light through a prism, you get a rainbow — but with dark lines at certain spots. Each chemical element makes its own unique pattern of dark lines, like a fingerprint.
When a star moves away from us, its light gets stretched to longer, redder wavelengths (redshift). When it moves toward us, light gets squished to shorter, bluer wavelengths (blueshift).
A star's colour tells you its surface temperature. Red stars are the "coolest" (about 3,000°C). Yellow stars like our Sun are medium (about 5,500°C). Blue-white stars are the hottest (10,000–50,000°C). It seems backwards — we think of red as "hot" and blue as "cold" — but in physics, blue light has more energy than red light.
Click each star to learn more
An orbit is what happens when something is moving sideways fast enough to keep "falling around" another object instead of crashing into it. The Moon is constantly falling toward Earth — but it keeps missing!
Imagine throwing a ball really hard sideways. It curves down and hits the ground. Now throw it harder — it goes further before hitting. Now throw it SO hard that the curve of its fall matches the curve of the Earth. It never lands. It's in orbit!
Closer planets orbit faster — farther planets orbit slower
Escape velocity is the speed you need to completely break free of an object's gravity. For Earth, it's 11.2 km/s — that's about 40,000 km/h. This is why rockets need to be so powerful!
In the 1600s, Johannes Kepler figured out three laws about how planets orbit — by studying years and years of careful observations.
Think of water spiralling down a drain. Water near the centre spins faster than water at the edges. Orbits work similarly — closer to the massive object, gravity is stronger, and the orbital speed has to be higher to avoid falling in. Mercury zooms at 47 km/s. Neptune ambles at 5.4 km/s.
Stars aren't burning like a campfire. They're powered by nuclear fusion — smashing tiny atoms together so hard they become new atoms, releasing mind-boggling amounts of energy in the process.
Deep inside the Sun's core (15 million degrees!), hydrogen atoms are squeezed together so hard that they fuse into helium. When this happens, a tiny bit of mass disappears — and gets converted into a HUGE amount of energy.
Energy equals mass times the speed of light squared. It means mass and energy are actually the same thing in different forms — and a tiny bit of mass contains an enormous amount of energy.
The Big Bang only made hydrogen and helium. Everything else — carbon, oxygen, iron, calcium, gold — was forged inside stars through fusion, then scattered across space when those stars died.
Click each stage to learn what stars create — and how
Fusion = joining small atoms together (what stars do). Fission = splitting big atoms apart (what nuclear power plants do). Fusion is much more powerful and doesn't produce radioactive waste, but we haven't figured out how to do it reliably on Earth yet. Scientists are working on it — maybe your generation will crack it!
An exoplanet is a planet orbiting a star that isn't our Sun. We've found over 5,700 of them so far — and there are probably billions more in our galaxy alone.
Exoplanets don't glow — they're tiny and dark next to their blindingly bright stars. It's like trying to spot a firefly next to a lighthouse from thousands of kilometres away. So astronomers use clever tricks.
The habitable zone is the range of distances from a star where liquid water could exist on a planet's surface. Too close: water boils. Too far: water freezes. In the sweet spot: it could be just right for life.
Many exoplanets are nothing like anything in our Solar System. Some are genuinely bizarre, and they've forced scientists to completely rethink how planets form.
5,700+ confirmed exoplanets. Billions estimated in the Milky Way. 40 light-years to TRAPPIST-1 (pretty close, cosmically speaking). 4.24 light-years to Proxima Centauri b, the nearest known exoplanet. 1995 — the year the first Sun-like exoplanet was confirmed. The field is younger than most of your teachers.