IB Physics HLTopic 5 — Fusion & StarsPaper 1 & 2the star family map~15 min read
The HR Diagram
Imagine plotting every star in the sky by just two things: how bright it is and how hot it is. When astronomers Hertzsprung and Russell did exactly that, something remarkable appeared — the stars didn’t scatter randomly, they clustered into distinct families. The result, the Hertzsprung–Russell (HR) diagram, is one of the most important graphs in all of astronomy, and reading it is a guaranteed exam skill.
📚 What you need to know
The HR diagram plots stellar luminosity (y-axis) against temperature (x-axis)
Luminosity (relative to the Sun) runs dim at the bottom to bright at the top
Temperature runs hot on the left to cool on the right (the axis is reversed!)
Most stars lie on a diagonal band called the main sequence
Red giants and red supergiants sit above the main sequence (bright but cool)
White dwarfs sit below the main sequence (hot but dim)
The diagram only shows stars in stable phases of their lives
The two axes
The HR diagram has two slightly unusual axes, and getting them right is half the battle:
Luminosity (y-axis): how much power the star radiates, compared to the Sun. It’s a logarithmic scale from very dim at the bottom to very bright at the top.
Temperature (x-axis): the surface temperature in Kelvin — but plotted backwards. Hot stars (~30 000 K) are on the left, cool stars (~3000 K) on the right.
That reversed temperature axis catches everyone out. Hot on the left, cool on the right — the opposite of a normal graph. My trick: remember that temperature decreases as you read left to right, so the hottest blue stars are top-left and the coolest red ones are bottom-right. Get the axis directions right and everything else falls into place.
The three main regions
When you plot real stars, they fall into three main groups — and each one corresponds to a stage of stellar life you already know:
Stars cluster into families. The diagonal main sequence holds most stars; red giants and supergiants sit upper-right (bright but cool); white dwarfs sit lower-left (hot but dim).
Main sequence
Most stars — about 90% — lie on the diagonal main sequence band. For these stars, luminosity increases with surface temperature, so they run from hot-and-bright at the top-left to cool-and-dim at the bottom-right. The Sun sits right in the middle. These are stars fusing hydrogen into helium.
Red giants and supergiants
Above the main sequence sit the red giants and red supergiants. These are odd: they’re cool (so they’d normally be dim) but very bright. The only way a cool star can be so luminous is if it’s enormous — a huge surface area radiating lots of total light. So these stars must be much bigger than main sequence stars.
White dwarfs
Below and to the left are the white dwarfs. These are the opposite: hot but dim. A hot star that gives out so little light must be tiny — a small surface area despite the high temperature. White dwarfs are the dense, cooling cores left behind by low-mass stars.
Read it like this: bright + cool = big (giants, upper right). Hot + dim = small (white dwarfs, lower left). Size explains why stars sit off the main sequence.
WE 1
On an HR diagram, a star lies in the top-right region: high luminosity but low surface temperature. What type of star is it, and how do you know?
Step 1 — read the position
Top = very luminous; right = cool.
Step 2 — what that means
A cool star that’s still very bright must have a huge surface area, i.e. it’s very large.
Step 3 — identify
Large, cool, luminous → a red giant or red supergiant.
Red (super)giant — bright despite being cool means it’s bigAlways link luminosity + temperature to size. Bright but cool can only mean large; that’s the whole logic of the giant region.
WE 2
A star has a surface temperature of 20 000 K and a luminosity 10 000 times that of the Sun. State where it lies on the HR diagram and what type of star it is.
Step 1 — place the temperature
20 000 K is hot → towards the left of the axis.
Step 2 — place the luminosity
10 000 × Sun = 104 → high up the y-axis.
Step 3 — identify
Hot and very bright → upper-left, on the main sequence (a hot, massive main sequence star).
Upper-left of the main sequenceSince luminosities are relative to the Sun, the Sun sits at luminosity 1. A value of 10⁴ is four decades higher — a hot, bright main sequence star.
⚛ Reading the HR diagram
y-axis: luminosity (relative to Sun), dim bottom → bright top.
x-axis: temperature, hot left → cool right (reversed!).
Temperature axis is reversed: hot left, cool right.
The Sun sits at luminosity 1 (values are relative to it).
Bright + cool = large (giants); hot + dim = small (white dwarfs).
The diagram shows only stable phases — not brief transitions.
You must identify the regions out of context in exams.
⚠ Common mistakes
Reading the temperature axis the wrong way (it’s reversed)
Forgetting luminosity is relative to the Sun (Sun = 1)
Putting white dwarfs in the wrong corner — they’re hot but dim (lower left)
Thinking the main sequence shows a star’s whole life — it’s one phase
Not linking off-sequence positions to size
Quick recap: The HR diagram plots luminosity (dim→bright, up) against temperature (hot→cool, left to right — reversed). Most stars lie on the diagonal main sequence; red giants and supergiants sit upper-right (bright but cool, so large); white dwarfs sit lower-left (hot but dim, so small). It shows only stars in stable phases.
The HR diagram needs a star’s temperature — but how do we measure the temperature of something trillions of kilometres away that we can never touch? The answer is hidden in the star’s light: split it into a spectrum and it reveals both the temperature and what the star is made of. Next page: Stellar Spectra.
HR diagram regions confusing you?
Book a free meeting and we’ll drill the reversed axes, the three star families, and the size reasoning behind giants and white dwarfs.
