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SESK: Stuff Everyone Should Know · sesk.info/astro-spectral-classes-obafgkm

What is the order of the main stellar spectral classes from hottest to coolest?

O, B, A, F, G, K, M, from hottest to coolest.

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  1. M, K, G, F, A, B, O
  2. O, B, A, F, G, K, M ✔
  3. A, B, F, G, K, M, O

Why: Annie Cannon rearranged older alphabetical classes by decreasing temperature, giving O B A F G K M. O stars are the hottest and M stars the coolest. The Sun is a G star.

Huh, didn't know that: Cooler objects beyond class M use the added spectral classes L, T and Y.

The dad joke
Why did the O star get kicked out? It was just too hot to handle.
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References

Answer: O, B, A, F, G, K, M. Annie Cannon rearranged older alphabetical classes by decreasing temperature, giving O B A F G K M. O stars are the hottest and M stars the coolest. The Sun is a G star.

  1. Astronomy 2e, 17.3 The Spectra of Stars (and Brown Dwarfs) openstax.org/books/astronomy-2e/pages/17-3-the-spectra-of-stars-and-brown-dwarfs
    From hottest to coldest, these seven spectral classes are designated O, B, A, F, G, K, and M.
    Checked 2026-10-10.

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