I know that blue stars are the hottest and youngest (according to Universe Today). But what makes them blue, instead of other colors like purple?

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    $\begingroup$ The colour sensitivity of our eyes makes us see a "purple" star blue. There was a question in here asking why we do not see green or purple stars. Try to find it because there was more discussion in that post. $\endgroup$ – Kornpob Bhirombhakdi Dec 30 '18 at 21:35

Our eyes and brains make them blue.

Stars are (close to) a black body, they emit a broad band of radiation. The hottest stars will emit most of their radiation in the ultraviolet but they will still be very bright in visible light.

As they get hotter, they emit even more ultraviolet, and more visible light but the mixture of wavelengths in visible light doesn't change much.

This mixture is interpreted by our brains as being a sort of light blue. No matter how hot the star gets, it will still be emitting lots of visible light in a mixture that looks "light blue". It will never look purple.

Purple can only be seen if either a single short-wavelength visible light is being emitted, or a mixture of short and long wavelengths (red and blue). Neither of the mixtures can occur in light emitted from a black body.

So stars can appear red, orange, yellow, white or light blue, but never green or purple.

  • $\begingroup$ This is inaccurate -- especially considering that we generally allow the violet end of the visible spectrum to be called "purple" . You completely left out the Planck curve, which makes your statements confusing. $\endgroup$ – Carl Witthoft Dec 31 '18 at 15:54

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