Why do hot stars look blue and not ultraviolet?

While their peak emission is in the UV, they still emit plenty of visible blue light, which our eyes detect. We cannot see UV.

What colour is a 3000 K star?

Deep red-orange, like Betelgeuse. The peak is in the near infrared, but enough red visible light is emitted to be easily seen.

Light bulb colour temperatures use the same concept. A 2700 K bulb is warm/yellow; a 6500 K bulb is cool/bluish daylight.

Convert between a star's colour temperature in Kelvin and its peak emission wavelength, plus approximate visible colour description.

Surface Temperature

Peak Wavelength

501.5 nm

Approximate B-V0.69

Peak Wavelength vs Surface Temperature

A star's colour is determined by its surface temperature. Hotter stars appear blue-white; cooler stars appear orange-red.

lambda_max = 2.8978e6 / T (nm)

B-V ≈ -3.684 log10(T) + 14.551

This empirical fit works reasonably for main-sequence stars between 3000 and 30 000 K.

A blue star at 10 000 K.