01The solar system
Our solar system has the Sun at its centre, with eight planets, dwarf planets, moons, asteroids and comets all held in orbit by the Sun's gravity.
Gravity between the Sun and each planet provides the force needed to keep them moving in roughly circular (elliptical) orbits instead of flying off in a straight line.
02Stars and galaxies
A star is a huge ball of mostly hydrogen and helium gas, producing energy through nuclear fusion in its core, which is why stars shine.
A galaxy is a vast collection of billions of stars, gas and dust, held together by gravity. Our Sun is one star inside the Milky Way galaxy.
03The Big Bang theory
The Big Bang theory is the scientific model for how the universe began — from an extremely hot, dense point about 13.8 billion years ago — and has been expanding and cooling ever since.
Evidence for this includes the fact that distant galaxies are moving away from us (and each other), and background radiation left over from the early universe.
04Day, night and seasons
Day and night happen because Earth spins (rotates) on its axis once approximately every 24 hours, so different sides face the Sun.
Seasons happen because Earth's axis is tilted, so as Earth orbits the Sun over a year, different parts receive more or less direct sunlight.
| Term | 한국어 | Meaning |
|---|---|---|
| solar system | 태양계 | The Sun and everything that orbits it |
| orbit | 궤도 | The curved path an object follows around another due to gravity |
| star | 항성/별 | A huge ball of gas producing energy by fusion |
| galaxy | 은하 | A huge collection of stars held together by gravity |
| Big Bang | 빅뱅 | The event thought to mark the start of the universe |
| axis | 자전축 | The imaginary line a planet spins around |