Explore · interactive
The cluster census
A cluster is a draw from the initial mass function, not a smooth curve painted across the sky. Sample one here from the Maschberger (2013) IMF — the same sampler my progenax code uses — then read it two ways at once, as points in space and as points on the Hertzsprung–Russell diagram. Every colour, temperature and luminosity is computed on the spot from the same validated ZAMS relations my software uses.
Census — how many stars of each mass?
- stars
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- O-type (≥16 )
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- heaviest
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- median mass
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- total mass
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Drag the high-mass slope and the whole character of the cluster shifts: flatten it and a few brilliant blue stars appear in the upper-left of the H–R diagram; steepen it and they vanish into a crowd of faint red dwarfs. Cool red stars are always the many; the rare hot stars are always the few — and yet the few are the whole story, because luminosity climbs so steeply with mass that a handful of O stars outshine, and out-push, everything else combined.
Change N and watch the massive tail flicker. That is not a rendering glitch — it is sampling noise. A small cluster genuinely might not draw a single massive star, which is exactly why the high-mass end of the mass function is so hard to measure and so easy to get wrong. Nothing here is choreographed: the stars are re-sampled from the mass function in your browser, and the diagram is derived, not drawn.
Open the Environment lens and the slope stops being a free knob: a cluster's metallicity and mass set it, through the Marks & Jeřábková prescription — metal-poorer, more massive clusters come out top-heavier — and here the cluster mass also fixes how many stars there are at all.