Galaxies and Asteroids

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1. Key words

2. What is a galaxy?

A galaxy is an enormous group of stars, planetary systems and clouds of dust and gas, all held together by gravity. Our Sun is just one star in a galaxy called the Milky Way.

3. Our galaxy — the Milky Way

The Milky Way is a spiral galaxy: from above it would look like a flat, spinning disc with a bulge of stars in the centre and long curved spiral arms. The arms are full of stars, dust and gas. The Sun is in one of the arms, about halfway from the centre to the edge.

• The Milky Way is about 100 000 light-years across. A light-year is the distance light travels in one year — about 9.5 million million km. Light is the fastest thing there is, yet it would take 100 000 years to cross our galaxy.
• The Sun takes about 230 million years to travel once around the centre of the galaxy.
• On a very dark night, far from city lights (for example in the Western Desert or Sinai), you can see the Milky Way as a pale band of light across the sky. That band is the light of billions of distant stars in the disc of our galaxy, seen from the inside.
• The dark patches in the band are clouds of dust blocking the light of the stars behind them.

Other galaxies: the Universe contains at least hundreds of billions of galaxies. They have different shapes:

It helps to see how all these objects fit inside each other:

4. What is an asteroid?

An asteroid is a lump of rock (some also contain a lot of metal, such as iron and nickel) that orbits the Sun. Asteroids are much smaller than planets. Most are not round: they are lumpy, like potatoes, because their gravity is too weak to pull them into a ball.

Compare these with the Earth, which is about 12 700 km across. Even all the asteroids in the asteroid belt put together have only a few per cent of the mass of our Moon.

• Most asteroids orbit the Sun in the asteroid belt, between the orbits of Mars and Jupiter. There are more than a million asteroids bigger than 1 km there.
• In films the asteroid belt is crowded, but really the asteroids are very far apart — spacecraft have flown through it without hitting anything.
• Some asteroids have orbits that come close to the Earth. About 66 million years ago an asteroid roughly 10 km wide hit the Earth, and scientists link it to the extinction of the dinosaurs. Today astronomers track near-Earth asteroids, and in 2022 a spacecraft (the DART mission) crashed into a small asteroid and changed its orbit.

5. How asteroids formed

To understand asteroids, look at how a planetary system such as the Solar System forms. Scientists think this happened about 4.6 billion years ago:

1. A huge cloud of dust and gas (a nebula) in the galaxy began to collapse because of gravity.
2. As it shrank, it began to spin and flattened into a disc.
3. Most of the material was pulled to the centre and became hot enough to form a star — the Sun.
4. In the disc, dust grains stuck together to make small rocks; rocks crashed together and joined to make bigger rocks; the biggest pulled in more material by gravity and grew into planets.
5. Not all the rocks became part of a planet. The rocks left over are the asteroids.
6. Between Mars and Jupiter, the very strong gravity of the giant planet Jupiter kept stirring the rocks up, so they crashed and broke instead of joining together. That is why there is an asteroid belt there instead of another planet.

Because asteroids have hardly changed since then, they are like time capsules. Spacecraft from Japan and the USA have collected tiny samples from asteroids and brought them back to the Earth to learn what the early Solar System was made of.

6. Worked examples

Worked example 1 — Describe what a galaxy is made of. [3]

1. Stars (1).
2. Planetary systems / planets orbiting stars (1).
3. Dust and gas (clouds of stellar dust and gas) (1).
4. Extra detail that helps: they are all held together by gravity.
5. Common mistake: "a galaxy is a group of planets". Planets are only part of it — the main things are stars, plus dust and gas.

Worked example 2 — Write these in order of size, starting with the smallest: galaxy, planet, Universe, star, planetary system. [2]

1. Think about what is inside what: a planet orbits a star; a star with its planets is a planetary system; billions of these make a galaxy; all the galaxies are in the Universe.
2. Answer: planet → star → planetary system → galaxy → Universe (2 marks; 1 mark if only one pair is swapped).
3. Common mistake: putting the Solar System before the Sun. The Solar System *contains* the Sun, so it is bigger.

Worked example 3 — Give two ways in which asteroids are different from planets. [2]

1. Any two: asteroids are much smaller (1); most are not round / irregular in shape (1); there are millions of asteroids but only eight planets (1); most asteroids orbit together in a belt, but each planet has its own orbit (1).
2. Exam tip: "Asteroids are rocks" is not a difference by itself — Mercury, Venus, Earth and Mars are rocky too. Say smaller.

Worked example 4 — Describe how the asteroids in the asteroid belt formed. [3]

1. The Solar System formed from a cloud of dust and gas; the dust stuck together to make rocks (1).
2. Most rocks joined together to make planets, but some were left over — these are the asteroids (1).
3. Between Mars and Jupiter, Jupiter's gravity stopped the rocks from joining to form a planet (1).
4. Common mistake: "asteroids are pieces of a planet that exploded". There is no evidence for this; they are left-over building material.

Worked example 5 — The Earth's diameter is about 12 700 km. Vesta's diameter is about 525 km. How many times wider is the Earth than Vesta? Give your answer to the nearest whole number. [2]

1. 12 700 ÷ 525 (1)
2. = 24.19… ≈ 24 times (1)
3. This shows why asteroids are described as much smaller than planets.

Worked example 6 — A teacher says: "The Milky Way is like a huge city. Each house is a planetary system, and the people in the house are the star and its planets." (a) Give one way the analogy is useful. [1] (b) Give one way it is not like a real galaxy. [1]

1. (a) It shows that a galaxy is made of a very large number of planetary systems grouped together / that a planetary system is a small part of the galaxy (1).
2. (b) Any one: a city has no clouds of dust and gas between the houses / a galaxy is held together by gravity, a city is not / the planetary systems in a galaxy are extremely far apart compared with houses (1).
3. Exam tip: an analogy is a model. For a strength say what it shows correctly; for a weakness say what is different in the real thing.

Practice

All pages in this course

Start here

• Stage 8 Science: The Tests and How to Prepare
• Thinking and Working Scientifically: Models, Investigations and Analysing Results

Biology

• Joints and Antagonistic Muscles
• Blood and Its Components
• Breathing, Gas Exchange and Diffusion in the Lungs
• A Balanced Diet, Energy Stores and Lifestyle
• Aerobic Respiration
• Ecosystems, Bioaccumulation and Invasive Species

Chemistry

• The Structure of the Atom
• Purity, Concentration and Chromatography
• Word Equations, Exothermic and Endothermic Reactions
• The Reactivity of Metals
• Impure Products and Solubility of Salts

Physics

• Speed and Distance–Time Graphs
• Balanced and Unbalanced Forces, and Moments
• Pressure, Particles and Diffusion
• Reflection and Refraction
• Dispersion and Colour
• Magnetic Fields and Electromagnets

Earth and space

• The Earth's Magnetic Field and Earth's Resources
• Climate, Weather and Climate Change
• Galaxies and Asteroids

Practice tests

• Stage 8 Science — Practice Test 1
• Stage 8 Science — Practice Test 2
• Stage 8 Science — Practice Test 3