Section 1 of 5Structure of the Universe
Objects in space are grouped inside larger and larger structures.
Universe
- A large collection of billions of galaxies, and also the name given to the whole of space.
Galaxy
- A large collection of billions of stars held together by gravity, such as the Milky Way.
Star
- A large, luminous ball of gas, mostly hydrogen, that produces energy by nuclear fusion in its core, such as the Sun.
Planetary system
- A star together with the planets and other objects that orbit around it.
Planet
- A large body that orbits a star and is not itself a star;
- Earth is the third of eight planets in the Solar System.
Moon
- A smaller body that orbits a planet; the Moon orbits the Earth.
Section 2 of 5Our Place in the Universe
1
Earth orbits the Sun, which is a star at the centre of the Solar System.
2
The Solar System is located within the Milky Way, which is one of billions of galaxies.
3
Together, the billions of galaxies make up the Universe, which is the entirety of space.
Section 3 of 5Other Bodies in the Solar System
Other Bodies in the Solar System
Comet
- An icy body with a highly elliptical orbit that brings it close to the Sun for part of each orbit.
Asteroid
- A rocky body that orbits the Sun, most of them lying in a belt between the orbits of Mars and Jupiter.
Artificial satellite
- A man-made object that orbits the Earth, such as a communications satellite.
Smaller bodies
- Smaller bodies always orbit larger ones, so planets orbit the Sun and moons orbit planets.
Section 4 of 5Worked Example
Question Put these four objects in order from the smallest structure to the largest: the Universe, the Sun, the Moon, the Milky Way.
- Step 1: The Moon orbits a planet, so it is a moon, the smallest structure in the list.
- Step 2: The Sun is a star, which is the centre of the Solar System and a planetary system.
- Step 3: The Milky Way is a galaxy, which is a collection of billions of stars.
- Step 4: The Universe contains billions of galaxies, so it is the largest structure.
Answer Moon, Sun, Milky Way, Universe.
Section 5 of 5Exam Checklist & Tips
8.1.1 – Key Points for the Exam
- Universe The entirety of space, which contains billions of galaxies.
- Galaxy A large collection of billions of stars.
- Solar System The Sun together with the planets, moons and other bodies that orbit it.
- Planetary system A star and all the objects that orbit it.
- Comet An icy body that moves on a highly elliptical orbit around the Sun.
Common Mistakes
- Calling the Moon a planet or a star; it is a moon that orbits the Earth.
- Saying that the Sun orbits the Earth; in the Solar System the planets orbit the Sun.
- Confusing a galaxy, which contains billions of stars, with the Universe, which contains billions of galaxies.
- Saying the Milky Way contains the entire Universe; it is only one galaxy among billions.