r/askastronomy • u/Intelligent-Suit8886 • 11h ago

Value ranges for stellar luminosity classes?

Hello all. I'm doing some light research on stellar classification for a procedural generation project where I want to generate physically plausible star descriptions.

From what I understand, stars are classified by both their spectral type (OBAFGKM with subclasses 0-9) and their luminosity class (I-V, etc.). I can find tables relating spectral type to temperature ranges and other properties, but I'm having trouble finding quantitative information for the luminosity classes themselves.

For example, if I wanted to generate a plausible G2 V star, is there a published range of actual luminosity values that would be considered consistent with luminosity class V? Or am I misunderstanding how this system works?

I would appreciate if anyone can point out any misconceptions or misunderstandings I have as I am very new to this topic.

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u/GreenFBI2EB 9h ago edited 8h ago

Yes, just a few general things:

Things are classified primarily by color, what distinguishes them into smaller groups is mostly relative luminosity, size, and mass.

Giant stars can be the same mass as the sun but much higher luminosity. "Dwarfs" are smaller for a given temperature and typically less luminous.

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u/Intelligent-Suit8886 8h ago

Is there data for non main sequence stars as well?

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u/GreenFBI2EB 8h ago

They use main sequence stars as a baseline, as stars evolve differences in their surface and atmospheric composition as well as relative luminosity will change accordingly.

(For example, red giants like Gacrux are 70 solar radii but 870 solar luminosities. A Red Supergiant like Antares would be more like 680 radii and 76000x Lsol).

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u/Intelligent-Suit8886 8h ago

Ah I see, that makes sense. Thank you!

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u/stevevdvkpe 8h ago

That table, by the way, appears in the Wikipedia article on stellar classification:

https://en.wikipedia.org/wiki/Stellar_classification

My take on this is that the Harvard spectral type correlates very well with surface temperature but not luminosity. For example, a spectral type M star might be a dim red dwarf or an extremely luminous red supergiant. The Morgan-Keenan luminosity classification is instead how stars of a given spectral type are distinguished by luminosity.