The B−V Color Index: Reading Star Temperatures by Eye

A star isn't one color. It emits a continuous spectrum, brightest at one wavelength and falling off on either side — close to what physics calls a blackbody curve. The peak of that curve depends on temperature: hot stars peak in the ultraviolet, cool stars peak in the infrared, and the visible band sits somewhere in between.

To turn that curve into a single number, photometers measure brightness through two standard filters defined by Harold Johnson and William Morgan in 1953:

• B (blue) — peaks near 445 nm, in the blue.
• V (visual) — peaks near 551 nm, in the yellow-green where the eye is most sensitive.

The color index is simply the magnitude difference: B − V.

Magnitudes count backwards — smaller numbers mean brighter — so a hot star, which pumps out more blue light than visual, has a smaller (brighter) B than V. Subtract: negative B−V. A cool red star is the reverse: dim in B, bright in V → positive B−V. The further from zero, the more extreme the temperature.

Here's the entire mechanism in one slider. Drag the temperature, watch the Planck curve slide across the visible band, and see B−V emerge as the difference between the two coloured slices.

Try it. Click the pinned reference stars to snap. At T = 9,600 K (Vega), B−V reads exactly 0.00. At 5,778 K (the Sun), it reads about +0.65. At 3,500 K (Betelgeuse), it climbs past +1.8.

The slider is the lesson. A hot star's curve dumps most of its photons short of the V filter — the B slice eats more of the curve, the V slice eats less, and the magnitude difference goes negative. As you cool the star, the peak slides redward, the V slice fattens, and B−V climbs. There's nothing else to it.

A handful of bright stars anchor every region of the index — point a finder at any of them and you can verify the scale yourself.

The same number drives the x-axis of the Hertzsprung–Russell diagram: every dot on a colour-magnitude diagram is just B−V plotted against absolute V magnitude. The two filters that fit on a 1953 photometer turned out to be enough to organize stellar physics for half a century.

The most famous demonstration of B−V in the sky is Albireo, at the head of Cygnus. The brighter component (β¹ Cyg) is a K-type giant with B−V = +1.13 — deep gold. Its companion (β² Cyg) is a B-type dwarf at B−V = −0.10 — vivid blue-white. They sit only 35 arcseconds apart, splittable in any telescope. The colour contrast is so extreme it survives even the eye's poor color discrimination at low light levels.

A runner-up is η Cassiopeiae (Achird) — yellow primary at B−V = +0.57, dusty-orange companion at +1.64. The contrast is subtler than Albireo but visible in steady seeing.

For an even simpler test, look at Orion:

A caveat: don't expect saturated colour at the eyepiece. The eye's cone cells need a lot of light to register hue, and most stars sit near the rod-cone threshold where colour discrimination collapses — which is why a "red giant" so often looks pale orange. The effect is much weaker on bright stars and on tight pairs where the brain can compare the two colours directly. Carbon stars, with B−V > +2.5, are the exception that proves the rule: they're so red that even the eye's washed-out colour vision still flags them as ruby orange.