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The Pleiades · M45
Nothing else decides it. Not age, not size, not distance — just how hot the surface is. Ten of the stars above are real, at their real positions.
A star's colour is its blackbody peak. Cool surfaces glow red for the same reason an iron bar does; heat one far enough and it runs orange, yellow, white, then blue. The whole of stellar colour is that single line.
Red
2,500–3,500 K
Betelgeuse
Cool, vast, and dying.
Orange
3,500–5,000 K
Arcturus
The commonest star you can actually see.
Yellow
5,000–6,000 K
The Sun
Ordinary, from the inside.
White
6,000–10,000 K
Sirius
The brightest in our sky.
Blue-white
10,000–25,000 K
Vega, Rigel
Hot enough to burn fast.
Blue
25,000 K and up
Spica
Rare, and short-lived.
Each disc is drawn from its class's published surface temperature, so the colour is computed rather than picked. Sizes are the main-sequence radii on a log footing — the true span from O to M is close to ten to one, which would render the red dwarf as a speck.
O
B
A
F
G
K
M
The orange figure is each class's share of all main-sequence stars. Three quarters of every star in existence is a class M red dwarf — and not one is bright enough to see without a telescope. Every star you have ever looked at comes from the thin slice above them.
Not a list of categories — a life. A star does not choose to become a white dwarf; it arrives there, and which ending it gets was decided by its mass on the day it formed.
A cloud still falling inward. Hot from collapse alone, not yet from fusion.
Too light to ever ignite hydrogen. It has been cooling since it formed and will not stop.
The commonest star there is, and invisible to the naked eye. Burns so slowly none has ever died.
Ours. Middle of the sequence, halfway through its hydrogen, unremarkable in every respect but one.
Massive, brilliant, and in a hurry. It spends its fuel thousands of times faster than the Sun.
The core has finished its hydrogen and contracted; the envelope answered by swelling and cooling.
The largest stars that exist. Betelgeuse in Orion's shoulder is one, and it is nearing its end.
So luminous it is blowing its own outer layers off. What is left is the stripped, furious core.
A dead core the size of Earth, held up by electron degeneracy. It has no fuel left and simply cools.
The mass of the Sun and a half, inside a city. A matchbox of it would weigh three billion tonnes.
A neutron star whose beam happens to cross us. We see a pulse each time it turns.
Where even neutron degeneracy failed. Not a dark star — a region nothing leaves, and the ring of light bent around it.
Discs are comparable, not literal. A supergiant is around a thousand solar radii and a neutron star is ten kilometres across — a span of roughly a hundred million to one, at which everything but the supergiant would be invisible. The size figures beside each entry are the real ones. Sources: Wikipedia, Stellar evolution / Stellar classification / Brown dwarf / Neutron star.
SN 1054 · The Crab Nebula
Chinese astronomers recorded this star on 4 July 1054. It was bright enough to see in daylight and stayed visible for about two years. What is left is still flying outward at 1,500 km/s, and still lit from inside by the neutron star it left behind — turning 30 times a second.
Every star in the list is B-type: hot, blue-white, and young enough that none has had time to swell and cool into a red giant. That is not a coincidence, it is the cluster's age — they were born together, around a hundred million years ago, and the massive ones are still burning.
So the warm points scattered through the field above are not sisters. They are foreground and background stars that happen to lie in the same direction, thousands of light years apart from the cluster and from each other. A photograph flattens all of that into one plane.
The spikes are not real either. They are the shadow of the vanes holding a telescope's secondary mirror, which is why only the bright stars have them — and why they all point the same way.
| Star | Magnitude | Type |
|---|---|---|
| Alcyone | 2.87 | B7 IIIe |
| Atlas | 3.62 | B8 III |
| Electra | 3.70 | B6 IIIe |
| Maia | 3.86 | B7 III |
| Merope | 4.17 | B6 IV |
| Taygeta | 4.30 | B6 IV |
| Pleione | 5.05 | B8 Vne |
| Celaeno | 5.44 | B7 IV |
| Asterope | 5.64 | B8 V |
| Asterope II | 6.41 | B9 Vn |
Magnitude is inverted and logarithmic: lower is brighter, and each step of 1 is about 2.5 times the light. Alcyone at 2.87 is roughly thirty times brighter than Asterope II at 6.41.