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SESK: Stuff Everyone Should Know · sesk.info/aero-star-class-o-hottest

In the star classes O, B, A, F, G, K, M, which stars are the hottest?

Class O stars, which shine blue.

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  1. Class O, which are blue ✔
  2. Class M, which are red
  3. Class G, which are yellow

Why: The letters run from hottest to coolest. O stars are the hottest and look blue, while M stars are the coolest and look red. Our Sun, about 5,800 kelvins, is a G star in the middle.

Huh, didn't know that: A star's surface temperature decides its color, so a blue star is hotter than a red one.

The dad joke
Why did the red star feel left out? Everyone said it was too cool for the O class.
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References

Answer: Class O, which are blue. The letters run from hottest to coolest. O stars are the hottest and look blue, while M stars are the coolest and look red. Our Sun, about 5,800 kelvins, is a G star in the middle.

  1. CAP Aerospace Dimensions Module 5, Space Environment (Alaska Wing copy) akwg.cap.gov/media/cms/Module_5__Space_Environment_C640727DEF762_83c5b.pdf
    with class O stars being the hottest and blue in color. Class M stars are the coolest in temperature and are the color red.
    Checked 2026-10-10.

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