🌟 Top left
Hot and bright. These are the blue and white stars at the top of the main sequence.
Stellar Evolution » Absolute Magnitude and the HR Diagram
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 8.11P, 8.12P
- Explain why a star's brightness in the sky depends on its distance
- Use absolute magnitude to compare stars fairly, and read the scale the right way round
- Draw the Hertzsprung-Russell diagram step by step
- Place the main sequence, red giants, supergiants, white dwarfs and the Sun on it
This lesson is examined in Paper 2 only.
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Unlock This CourseWhy a star's brightness can fool you
Look at two street lamps at night. One is right next to you and the other is far down the road. The near one looks much brighter, but they may be exactly the same lamp. How bright something looks depends on how far away it is as well as on how much light it gives out.
Stars are just the same. A faint star in the night sky might be a dim star that is quite close, or a hugely powerful star that is very far away. Just looking at the sky, you cannot tell which. Astronomers need a fair way to compare stars.
Absolute magnitude
The fair way is to imagine every star placed at the same standard distance from us. Then distance can no longer make one star look brighter than another, and only the light the star really gives out matters.
Key terms:
- Absolute magnitude: a number that shows how bright a star would look if it was placed at a standard distance from us.
- Hertzsprung-Russell diagram (HR diagram): a graph that plots the brightness of stars against their surface temperature.
Learn this: the scale runs backwards
The lower the absolute magnitude, the brighter the star. A star with an absolute magnitude of −5 is far brighter than one with an absolute magnitude of +10. Very bright stars have negative numbers.
The scale looks odd because it comes from ancient astronomers, who called the brightest stars "first magnitude" and the faintest "sixth magnitude". The system was kept, so bright means a small number.
| Absolute magnitude | What it means |
|---|---|
| −5 | Very bright star |
| 0 | Bright star |
| +5 | Medium brightness (the Sun is about here) |
| +10 | Faint star |
| +15 | Very faint star |
You do not need to calculate anything with absolute magnitude. You only need to know what it shows and which way round the scale goes.
The Hertzsprung-Russell diagram
If you plot lots of stars on one graph, with their brightness up the side and their surface temperature along the bottom, the stars do not land in random places. They form clear groups. This graph is the HR diagram. Remember from the lesson Star Colour and Temperature that blue stars are the hottest and red stars the coolest.
How to draw an HR diagram
Drawing it step by step
- Draw a pair of axes.
- Label the vertical axis absolute magnitude (or brightness). Put the brightest stars at the top. With absolute magnitude, this means the numbers get smaller going up, for example +15 at the bottom and −5 at the top.
- Label the horizontal axis surface temperature (or colour). Put the hottest on the left and the coolest on the right. So the colours go from blue on the left to red on the right.
- Draw the main sequence as a wide band running from the top left down to the bottom right.
- Mark the red giants and supergiants in the top right.
- Mark the white dwarfs in the bottom left.
- Put a dot for the Sun on the main sequence, in the middle of the band.
🔴 Bottom right
Cool and faint. These are the dim red stars at the bottom of the main sequence.
What each group is
| Group | Where on the diagram | What it is |
|---|---|---|
| Main sequence | Band from top left to bottom right | Stable stars fusing hydrogen, including the Sun |
| Red giants and supergiants | Top right | Cool but very bright, because they are enormous |
| White dwarfs | Bottom left | Hot but faint, because they are tiny |
Most stars are on the main sequence, because stars spend most of their lives there. The groups also match the life cycles you have already met. The lessons Life Cycle of Stars Like the Sun and Life Cycle of Massive Stars name these stages. On the diagram, a star's position shows which stage it has reached.
Why are red giants bright when they are cool? A cool surface gives out less light for each bit of area. But a red giant has a huge surface, so in total it gives out a lot of light. A white dwarf is the opposite. Its surface is hot, but it is so small that it gives out little light overall.
Worked examples
Worked example 1: which star is brighter?
Star X has an absolute magnitude of +8. Star Y has an absolute magnitude of −2. Which star is brighter?
Step 1: the scale runs backwards, so the lower number is brighter.
Step 2: −2 is lower than +8.
Answer: star Y is brighter.
Worked example 2: find the group
A star is cool and red, and has a very low absolute magnitude (very bright). Where is it on the HR diagram, and what is it?
Step 1: cool and red means it is on the right-hand side.
Step 2: very bright means it is near the top.
Answer: top right, so it is a red giant or supergiant.
Worked example 3: describe the Sun's place
Describe where the Sun is on the HR diagram.
Step 1: the Sun is a stable star fusing hydrogen, so it is on the main sequence.
Step 2: it is yellow and medium brightness, so it sits in the middle of the band.
Answer: on the main sequence, about halfway along the band, at an absolute magnitude of about +5.
Common mistakes
Common mistakes
- Thinking a larger absolute magnitude means a brighter star. It is the other way round: lower means brighter.
- Putting the hottest stars on the right. On the HR diagram the hottest are on the left.
- Saying how bright a star looks in the sky is the same as its absolute magnitude. Looks in the sky depend on distance, absolute magnitude does not.
- Swapping the red giants and the white dwarfs. Giants are top right, white dwarfs are bottom left.
- Drawing the main sequence from bottom left to top right. It runs from top left to bottom right.
Exam-style question
The Hertzsprung-Russell diagram shows the absolute magnitude and surface temperature of stars.
(a) State what is meant by absolute magnitude. [1 mark]
(b) Describe how you would draw the axes of an HR diagram. [2 marks]
(c) White dwarfs are hot and faint. Explain where on the diagram they are found. [2 marks]
Model answer
(a) How bright a star would look if it was at a standard distance from us (1).
(b) Absolute magnitude (brightness) goes up the side, with the brightest stars at the top (1). Surface temperature goes along the bottom, with the hottest on the left (1).
(c) They are on the left because they are hot (1). They are at the bottom because they are faint, since they are so small (1).
Exam tip
If a question asks you to draw or describe the HR diagram, always say which end of each axis is hot or bright. The temperature axis is the one most students get the wrong way round.