color index

Glance at the night sky and you can already see that stars are not all white: some are noticeably blue-white, others orange or red. That tint is a clue to temperature, and astronomers turn it into a precise number called the color index. Instead of describing color in words, they measure how bright a star is through two different colored filters and compare.

In practice you measure a star's magnitude through, say, a blue filter (B) and a yellow-green 'visual' filter (V), and take the difference, B minus V. Because the magnitude scale runs backward, a hot blue star is relatively brighter in B, giving a small or negative B minus V; a cool red star is relatively brighter in V, giving a large positive B minus V. So the color index is a direct stand-in for temperature: small or negative means hot and blue, large and positive means cool and red. A very hot star might have B minus V near minus 0.3, the Sun about plus 0.65, a cool red star above plus 1.5.

The color index is beloved because it is cheap and powerful. Measuring two magnitudes is far easier than taking a full spectrum, yet the color index pins down a star's temperature well enough to place it on the Hertzsprung-Russell diagram and to sort thousands of stars at once in a survey. There is one important complication: interstellar dust reddens starlight, making a star's color index larger (redder) than its true temperature would give. This 'reddening' must be measured and removed before color can be trusted as a thermometer.