Our solar system: Our solar system is a small part of the Milky Way galaxy.

The idea

Scale is the point of this statement. A galaxy is a collection of billions of stars held together by gravity, and the Milky Way is ours. The Sun is one ordinary star among perhaps a hundred billion of them, sitting far out from the centre, so the whole solar system is a speck on that map. Beyond the Milky Way there are billions of other galaxies, separated by distances far larger than the gaps between stars inside a galaxy. The ladder runs in this order: moons orbit planets, planets orbit a star, stars sit within a galaxy, and galaxies together make up the universe. The faint band of light called the Milky Way in a dark sky is the disc of our own galaxy seen edge on from the inside.

What earns the marks

• Defining a galaxy as a very large collection of stars held together by gravity.
• The Sun being one star among billions in the Milky Way.
• Placing the solar system within the galaxy and the galaxy within the universe.
• Stating that there are many other galaxies beyond our own.

Common mistakes and how to put them right

• Using solar system, galaxy and universe as though they meant the same thing.

• Saying the Milky Way orbits the Sun, or that the solar system contains galaxies.

• Believing the Milky Way is the only galaxy.

• Describing the Sun as the largest or the brightest star in the galaxy.

Exam tips

• Write the order of scale in the margin before answering; it stops the classic mix-up.
• Say billions rather than lots when you describe how many stars a galaxy holds.
• If asked where the solar system is, name the galaxy rather than describing a position.