Star Color and Temperature Calculator

Stars emit light as approximate blackbodies, meaning their color is determined by their surface temperature.

Wien’s Displacement Law gives the wavelength of peak emission:

λ_max = 2,897,771 / T nanometers

Where T is temperature in Kelvin.

Color interpretation:

• λ_max < 380 nm → ultraviolet (star appears blue-white)
• 380–450 nm → violet/blue
• 450–490 nm → blue
• 490–560 nm → green (but stars never look green due to broad spectrum: appear white)
• 560–590 nm → yellow
• 590–625 nm → orange
• 625–740 nm → red

• 740 nm → infrared (star appears deep red or brown dwarf)

B-V color index to temperature: The B-V index measures the difference in brightness in blue (440 nm) vs visual (550 nm) filters. A hot blue star has negative B-V; a cool red star has a large positive B-V.

T ≈ 4600 × (1/(0.92 × B-V + 1.7) + 1/(0.92 × B-V + 0.62))

This is Ballesteros’ 2012 fit, and it is worth knowing where it lands on the one star we know best. Feed it the Sun’s B-V of 0.65 and it returns 5,778 K, six kelvin above the 5,772 K the International Astronomical Union adopted in 2015. Six kelvin is nothing next to the scatter in real B-V measurements, but it explains why this page and the stellar luminosity calculator quote the solar temperature slightly differently: that page needs the value consistent with the IAU’s L☉ and R☉, and this formula was fitted before it existed.

The relation also stops meaning much outside roughly −0.4 to +2.0. Below −0.4 the calculator clamps and says so, because the denominators are heading for zero and the temperature it would report is arithmetic rather than astronomy.

Spectral class temperature ranges:

• O: > 30,000 K (blue)
• B: 10,000–30,000 K (blue-white)
• A: 7,500–10,000 K (white), Sirius, Vega
• F: 6,000–7,500 K (yellow-white), Procyon
• G: 5,200–6,000 K (yellow), Sun, Alpha Centauri A
• K: 3,700–5,200 K (orange), Arcturus, Aldebaran
• M: < 3,700 K (red), Betelgeuse, Proxima Centauri

The human eye does not see most of the photons a star emits. What it registers is the whole spectrum added together, not the peak. This is why even very hot stars appear white rather than violet or ultraviolet, and why no star anywhere looks green: a peak at 520 nm still comes with so much red and blue either side of it that the eye adds the lot up and calls it white.