Other Colors, Other Magnitudes

By the late 19th century astronomers were using photography to record the sky and measure star brightnesses, and a new problem cropped up. Some stars showing the same brightness to the eye showed different brightnesses on film, and vice versa. Compared to the eye, photographic emulsions were more sensitive to blue light and less so to red light. Accordingly, two separate scales were devised. Visual magnitude, or m_vis, described how a star looked to the eye. Photographic magnitude, or m_pg, referred to star images on blue-sensitive black-and-white film. These are now abbreviated m_v and m_p, respectively.

This complication turned out to be a blessing in disguise. The difference between a star's photographic and visual magnitude was a convenient measure of the star's color. The difference between the two kinds of magnitude was named the "color index." Its value is increasingly positive for yellow, orange, and red stars, and negative for blue ones.

But different photographic emulsions have different spectral responses! And people's eyes differ too. For one thing, your eye lenses turn yellow with age; old people see the world through yellow filters. Magnitude systems designed for different wavelength ranges had to be more clearly defined than this.

Today, precise magnitudes are specified by what a standard photoelectric photometer sees through standard color filters. Several photometric systems have been devised; the most familiar is called UBV after the three filters most commonly used. U encompasses the near-ultraviolet, B is blue, and V corresponds fairly closely to the old visual magnitude; its wide peak is in the yellow-green band, where the eye is most sensitive.

Color index is now defined as the B magnitude minus the V magnitude. A pure white star has a B-V of about 0.2, our yellow Sun is 0.63, orange-red Betelgeuse is 1.85, and the bluest star believed possible is –0.4, pale blue-white.

So successful was the UBV system that it was extended redward with R and I filters to define standard red and near-infrared magnitudes. Hence it is sometimes called UBVRI. Infrared astronomers have carried it to still longer wavelengths, picking up alphabetically after I to define the J, K, L, M, N, and Q bands. These were chosen to match the wavelengths of infrared "windows" in the Earth's atmosphere — wavelengths at which water vapor does not entirely absorb starlight.

In all wavebands, the bright star Vega has been chosen (arbitrarily) to define magnitude 0.0. Since Vega is dimmer at infrared wavelengths than in visible light, infrared magnitudes are, by definition and quite artificially, "brighter" than their visual counterparts.