IB Physics SLTopic 5 — Fusion & StarsPaper 1 & 2Luminosity vs temperature~8 min read
The HR Diagram
Imagine plotting every star in the sky on one graph — brightness up the side, temperature along the bottom. You might expect a random scatter. Instead the stars fall into a few clear groups. That single, famous chart is the Hertzsprung–Russell diagram, and it’s one of the most powerful tools in astronomy. Learn to read it, and you can tell a star’s type at a glance.
📘 What you need to know
The HR diagram plots luminosity (y-axis) against surface temperature (x-axis)
Luminosity is shown relative to the Sun and rises from dim (bottom) to bright (top) — the scale is logarithmic
The temperature axis is reversed: hot on the left, cool on the right
Stars fall into distinct groups, not a random scatter
Most stars lie on the main sequence — a diagonal band where luminosity rises with temperature
Red giants and red supergiants sit above the main sequence: very bright but cool, so they must be large
White dwarfs sit below-left: hot but dim, so they must be small
The diagram only shows stars in stable phases — brief transition stages are too short-lived to appear
What the Axes Mean
Two astronomers, Ejnar Hertzsprung and Henry Norris Russell, independently had the same idea: plot stars by their luminosity against their surface temperature. The axes have two features that trip students up every year, so nail them now.
🧭 Reading the axes
y-axis = luminosity, measured relative to the Sun (so the Sun sits at 1). It runs from dim at the bottom to bright at the top, on a logarithmic scale — each step is a factor of ten
x-axis = surface temperature in kelvin, but drawn backwards: the hottest stars are on the left and the coolest on the right
The reversed temperature axis is the classic trap. Whenever you look at an HR diagram, say to yourself: “hot-left, cool-right.” The high numbers (30 000 K) live on the left, not the right like a normal graph. Everything else falls into place once you’ve got that.
The Groups on the Diagram
When you plot real stars, they don’t scatter randomly — they cluster into a handful of clear regions. Here’s the map you need to be able to recognise, even without labels.
The HR diagram: most stars lie on the diagonal main sequence, giants and supergiants sit upper-right (bright but cool), and white dwarfs sit lower-left (hot but dim).
The main sequence
The obvious diagonal band running from top-left to bottom-right is the main sequence, home to about 90% of stars including the Sun. Along it, luminosity increases with surface temperature — hotter main sequence stars are also brighter.
Giants and supergiants
Above the main sequence sit two groups: red giants and, higher still, red supergiants. These stars are very luminous but relatively cool. The only way a cool star can also be extremely bright is by being enormous — a huge surface area emits a lot of light even at a low temperature. So giants and supergiants are much larger than main sequence stars.
White dwarfs
Below and to the left lie the white dwarfs. These are hot but not very luminous. A hot star that emits little light must have a tiny surface area, so white dwarfs are much smaller than main sequence stars — roughly Earth-sized.
Quick recap: luminosity (log) up the y-axis, temperature reversed on the x-axis; main sequence = diagonal band, giants/supergiants = bright but cool so large, white dwarfs = hot but dim so small.
Why Some Stars Are Missing
The HR diagram only shows stars in stable phases of their lives. The transitions between stages — like a star swelling into a red giant — happen very quickly compared with the millions or billions of years a star spends being stable, so very few stars are ever caught mid-transition. That’s why the diagram has clear clusters and empty gaps rather than a smooth spread. Black holes never appear either, for a simple reason: they emit no light, so there’s no luminosity to plot.
WE 1
On an HR diagram, a star lies in the upper-right region: it has a high luminosity but a low surface temperature. Identify the type of star and explain what this tells you about its size compared with a main sequence star of the same temperature.
Identify
high luminosity but low (cool) temperature, upper right
→ a red giant (or red supergiant)Explain the size
a cool surface emits relatively little light per unit area
yet the star is very luminous overall
so it must have a very large surface area
→ it is much larger than a main sequence star of the same temperature
WE 2
A student sketches an HR diagram and places the Sun (temperature 5800 K, luminosity 1) at the far right of the x-axis. (a) State the error. (b) Star X has a temperature of 25 000 K and a luminosity 10 000 times the Sun. State the region of the diagram where it belongs.
Part (a)
the temperature axis is reversed (hot on the left, cool on the right)
5800 K is a middling temperature, not the coolest
→ the Sun belongs near the middle of the axis, not the far rightPart (b)
25 000 K is very hot → toward the left
luminosity 10⁴ is very bright → high up
→ upper-left: the hot, bright end of the main sequence
💡 Top tips
“Hot-left, cool-right.” Say it every time — the reversed temperature axis is the most common HR mistake
Bright + cool = big; hot + dim = small. Use luminosity and temperature together to reason about a star’s size
Both axes are unusual: luminosity is logarithmic and relative to the Sun (Sun = 1), temperature is reversed
Learn to place four regions blind: main sequence (diagonal), red giants and supergiants (upper right), white dwarfs (lower left)
⚠ Common mistakes
Drawing temperature the normal way round — on an HR diagram it decreases left to right
Treating the luminosity axis as linear — it’s logarithmic, so each gridline is ×10
Putting white dwarfs at the bottom-right — they’re hot, so they sit to the left
Expecting a smooth scatter — only stable phases appear, giving distinct clusters with gaps between them
Up next: Emission & Absorption Spectra in Stars — we’ll see how the light from a star carries a barcode of dark lines that reveals what it’s made of, and how those spectra connect to the temperatures we’ve just been plotting.
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Book a free meeting and let’s work through the tricky bits together.
