Stellar Evolution » Star Colour and Temperature
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 8.7, 8.8
- Classify stars by colour: red, orange, yellow, white and blue
- Link a star's colour to its surface temperature
- Put stars in order of colour and temperature
- Recognise the Sun, Betelgeuse and Rigel
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Unlock This CourseStars come in different colours
Look at the night sky for long enough and you will see that stars are not all the same colour. Most look white, but a few look reddish and a few look blue-white. Astronomers use this to sort stars into groups. This is called classifying stars by colour.
Stars are classified into five main colours:
- red
- orange
- yellow
- white
- blue
Two stars you may be asked about are Betelgeuse, a red star in the pattern of stars called Orion, and Rigel, a blue-white star in the same pattern. The Sun is a yellow star, even though sunlight looks white to us.
Key terms:
- Classify: to sort things into groups using a feature they share. Here the feature is the colour of the star.
- Surface temperature: the temperature of the outer layer of a star, the part that gives out the light we see. It is measured in kelvin (K).
Colour tells you the temperature
The colour of a star is linked to its surface temperature. You can see the same thing with a piece of metal in a fire. At first it glows dull red. As it gets hotter it glows orange, then yellow, and when it is very hot it glows white. A star works in the same way.
🔴 Red stars
The coolest stars. Their surface is at roughly 3000 K.
🟡 Yellow stars
Medium temperature. The Sun is a yellow star at roughly 6000 K.
🔵 Blue stars
The hottest stars. Their surface is far hotter than the Sun's.
Learn this
Red stars are the coolest. Blue stars are the hottest. The colour of a star shows its surface temperature.
This is useful because no one can travel to a star and put a thermometer on it. Astronomers look at the colour of the light that reaches the Earth and use it to tell how hot the surface of the star is. A single glance at the colour gives a good clue to the temperature.
The kelvin (K) is the unit for temperature that astronomers use. For this lesson you only need to know that bigger numbers mean hotter stars.
Putting the colours in order
The table below lists the five colours from coolest to hottest. The numbers are rough values to give you a feel for the scale.
| Colour | Surface temperature | Example |
|---|---|---|
| Red | coolest, roughly 3000 K | Betelgeuse |
| Orange | hotter than red, cooler than yellow | |
| Yellow | roughly 6000 K | the Sun |
| White | hotter than yellow | |
| Blue (blue-white) | hottest of all | Rigel |
A memory trick: the order is the same as the order of colours in a hot piece of metal. Red first, blue last, with the temperature going up all the way.
Your Sun sits near the middle of the table. It is hotter than a red star such as Betelgeuse, but much cooler than a blue star such as Rigel.
Worked examples
Worked example 1: put in order
Four stars are described as blue, red, yellow and white. Put them in order of surface temperature, coolest first.
Step 1: the coolest colour is red.
Step 2: next hottest is yellow, then white.
Step 3: the hottest colour is blue.
Answer: red, yellow, white, blue.
Worked example 2: compare two stars
Star A is orange. Star B is blue. Which star has the hotter surface? Explain your answer.
Answer: Star B is hotter. Blue stars are the hottest stars, and orange stars are cooler than yellow stars such as the Sun. The colour of a star shows its surface temperature.
Worked example 3: explain an order
Betelgeuse, the Sun and Rigel are listed in order of surface temperature, coolest first. Explain why this is the correct order.
Step 1: Betelgeuse is red, and red stars are the coolest, at roughly 3000 K.
Step 2: the Sun is yellow, at roughly 6000 K, so it is hotter than Betelgeuse.
Step 3: Rigel is blue-white, and blue stars are the hottest, so it comes last.
Answer: the order red, yellow, blue-white is the order of increasing surface temperature.
Common mistakes
Common mistakes
- Thinking that red means hot because red is the colour we use for fire on a warning sign. In stars, red is the coolest colour.
- Saying the Sun is white. It looks white to us, but it is classified as a yellow star.
- Mixing up Betelgeuse and Rigel. Betelgeuse is red and cool. Rigel is blue-white and hot.
- Saying the colour depends on how far away the star is. The colour is linked to the star's surface temperature.
- Writing the order the wrong way round. Check which end of your list is coolest.
Exam-style question
The table shows information about three stars.
| Star | Colour |
|---|---|
| Betelgeuse | red |
| The Sun | yellow |
| Rigel | blue-white |
(a) State which of the three stars has the hottest surface. [1 mark]
(b) State the colour of the Sun. [1 mark]
(c) Explain how the colour of a star shows its surface temperature. Use an example from the table in your answer. [2 marks]
Model answer
(a) Rigel (1).
(b) Yellow (1).
(c) The colour of a star is linked to its surface temperature (1). Red stars such as Betelgeuse are cooler, at roughly 3000 K, and blue stars such as Rigel are hotter (1). Accept: the Sun is yellow at roughly 6000 K, cooler than blue stars and hotter than red stars.
Exam tip
When a question asks you to explain, name both the colour and the temperature. Say "blue means hotter", not just "blue is different".