The Solar System and the Universe In Depth
The iron in our blood and the calcium in our bones were once inside the heart of a star. This lesson shows how the Sun shines, why the planets don’t fall into the Sun, why the Moon changes shape every night, how stars are born and die, how many years light takes to reach our eyes, and how scientists know the Universe is about 13.8 billion years old.
How does the Sun shine?
The Sun doesn’t burn like a campfire. At its centre, enormous heat and pressure squeeze hydrogen nuclei together until they fuse into helium. A tiny bit of mass goes missing along the way and turns into energy, following Einstein’s most famous equation, E = mc². Tap each layer to see what it does.
How big is it?
To line Earths up across the Sun, you’d need about 109 of them. And if you poured Earths into the Sun, about 1.3 million would fit inside. The Sun holds about 99.86 per cent of all the mass in the Solar System, so all the planets put together are just small crumbs orbiting around it.
How much does it burn each second?
Every second, the Sun turns about 4 million tonnes of mass into energy. That sounds like a lot, but it has so much mass that it can keep going like this for about another 5 billion years. The Sun is about 4.6 billion years old now, right in the middle of its life.
Meet the planets one by one
The four inner planets are rocky. The four outer planets are giants made mostly of gas and ice. Between the two groups lies the asteroid belt. Tap to choose a planet.
Try it: shrink the Solar System to fit a school field
Imagine the Sun is a ball 30 centimetres wide, placed in the middle of the field. How small would each planet be, and how far away?
Try it: take your scales to other worlds
Your mass stays the same everywhere, but gravity is different, so the scales show a different number. Type in what the scales show on Earth.
Why isn’t Pluto a planet any more?
In 2006, astronomers around the world agreed that a planet must pass three tests. One: it orbits the Sun. Two: it is big enough for its own gravity to pull it into a round shape. Three: it has cleared its orbit, meaning it owns its path. Pluto passes the first two, but it orbits among thousands of other icy objects in the Kuiper Belt, so it was classed as a dwarf planet. It’s a great example of science updating its definitions as we learn more.
Why don’t the planets fall into the Sun?
In 1687, Isaac Newton explained that everything with mass pulls on everything else. The more mass, the stronger the pull, and if you move twice as far away, the pull drops to a quarter. Planets are actually falling towards the Sun all the time, but they move sideways so fast that they keep missing it. That is what an orbit is. Try firing Newton’s cannon.
Start at 7.9 kilometres per second, then slowly go lower or higher
Why do astronauts float?
The International Space Station is only about 400 kilometres up, where gravity is still about 90 per cent as strong as on the ground. Astronauts float because they and the station are falling together all the time, while shooting sideways at about 28,000 kilometres per hour, so they keep falling around the Earth, just like the cannonball at 7.9 in the simulation.
Kepler’s three laws
| Planet | Distance a | Period T (years) | T² ÷ a³ |
|---|
Distance a is measured in astronomical units, where the distance from Earth to the Sun equals 1. Look at the last column: every planet gets almost exactly 1. Kepler found this law from the numbers alone, before Newton could explain why.
Einstein: space can bend
In 1915, Albert Einstein put forward the general theory of relativity. He said gravity isn’t an invisible pulling force: it is mass bending space and time. Picture a bowling ball on a trampoline. A marble rolling past curves towards it because the surface dips, not because a string is pulling it. The theory predicted that starlight would bend as it passed close to the Sun, which was really measured during the 1919 solar eclipse, and it predicted gravitational waves, which were first detected in 2015.
Why does the Moon change shape every night?
Half of the Moon is always lit by the Sun. What changes is how much of that bright half we can see from here. It is not Earth’s shadow blocking it, a mistake that lots of people, even grown-ups, make. Drag through one whole month, which lasts about 29.5 days.
Why don’t eclipses happen every month?
The Moon’s orbit is tilted about 5 degrees compared with Earth’s orbit, so most of the time the Moon passes above or below the straight line between Earth and the Sun. Eclipses can only happen when all three line up exactly, which comes round about twice a year.
Always the same face, and slowly drifting away
The Moon spins around once in exactly the time it takes to orbit Earth once, so we always see the same face. The far side isn’t dark: it gets sunlight at new moon. The Moon’s pull causes two high tides a day, and the Moon is slowly moving away from Earth by about 3.8 centimetres a year, about as fast as your fingernails grow.
The life of a star, and how we are made of stars
Every star is born from a cloud of gas and dust collapsing under its own gravity, but how it ends depends on how heavy it was at birth. The heavier the star, the hotter, brighter and shorter-lived it is.
Try it: a star’s colour tells its temperature
Kelvin is the temperature unit scientists use. Zero kelvin is the coldest anything can possibly be. Subtract 273 to get degrees Celsius. Red stars are the coolest and blue-white stars are the hottest, the opposite of the red and blue taps we’re used to.
Try it: stars born with different masses
A light year is a distance, not a time
Light travels at about 300,000 kilometres per second, fast enough to go round the Earth seven and a half times in one second. The distance light travels in one year is called a light year, about 9.46 trillion kilometres. And because light takes time to travel, whenever we look far away, we are seeing the past. Pick a destination and compare travel times.
The star you see tonight may be a picture from the Ayutthaya period in Thailand
The light from Betelgeuse reaching your eyes tonight set off over 500 years ago, during the Ayutthaya period in Thailand. If it had exploded yesterday, we wouldn’t find out for hundreds of years. And the light from the Andromeda Galaxy, which you can see with your bare eyes in a very dark place, set off about 2.5 million years ago, before our own species of human even existed.
Our home in the Milky Way
The Milky Way is a spiral galaxy about 100,000 light years across, with hundreds of billions of stars. The Sun is about 26,000 light years from the centre, and it takes about 230 million years to go round the centre once. The last time we were at this spot, the very first dinosaurs were just appearing on Earth.
How did the Universe begin?
Scientists have found that the Universe is expanding. If you run time backwards, everything must once have been packed together, extremely hot and dense, about 13.8 billion years ago. We call that event the Big Bang, but the name is misleading. It wasn’t an explosion flying out into empty space. It was space itself stretching. Try blowing up a balloon.
Seen from our home galaxy
Every dot moves away from every other dot. No dot is the centre, and the farther dots move away faster. This is what Edwin Hubble saw in the light of real galaxies in 1929.
Timeline of the Universe
Three pieces of evidence
What is the Universe made of?
Ordinary matter, which makes up stars, planets and us, is only about 5 per cent. About 27 per cent is dark matter, which we can’t see but which has gravity, and about 68 per cent is dark energy, which makes the Universe expand faster and faster. Nobody knows yet what either of them really is.
How we explore space, and how the body changes
Telescopes carry our eyes billions of light years away, space probes have reached every planet, and people have lived on the space station without a break since 2000. Thailand is taking part too.
Thailand and space
The National Astronomical Research Institute of Thailand (NARIT) runs the Thai National Observatory on Doi Inthanon, with a telescope whose mirror is 2.4 metres wide. Thailand also has its own Earth-observation satellite, THEOS-2, launched in 2023, which takes pictures to help plan for water, forests and disasters.
Exoplanets
Astronomers have confirmed more than 6,000 planets orbiting other stars. Some sit in a zone that is neither too hot nor too cold, where water could be liquid. This is called the habitable zone. But so far, no evidence of life beyond Earth has been found.
Our bodies in space
That’s why astronauts on the station must exercise for about two hours a day, and space doctors check their bodies throughout the mission. Space medicine is one more career for kids who love both the human body and the stars.
Game: sort the space objects
Read the name and choose what it is. Get answers right in a row for bonus points. You have 45 seconds.
Test yourself
Fancy a game on this lesson?
Every question from this lesson is in the Body 101 arena. Race the clock, duel someone at home, play as a team, beat the bosses, or practise with no timer at all.