Monday, February 01, 2010

Tread softly


It has become something of a habit here to comment on the Astronomy Picture of the Day (APOD), usually to call attention to something not mentioned in the caption. A few days ago it was Kemble's Cascade, an unusual (and accidental) alignment of about twenty stars in the constellation Camelopardalis (the Giraffe), now high in the northern sky in the evening. (Click to enlarge.)

First, note that none of the stars in this view are likely to be seen with the naked eye. There are no stars in the constellation brighter than the 4th magnitude, which means from a typical light-polluted location Camelopardalis is a big blank part of the sky.

But look what a telescope can see in this view which covers about 4 degrees of the sky -- about 8 times the width of the full Moon. What a delicious serving of stars!

The size of the dots on the photograph have nothing to do with the relative size of the stars; at the distance of the stars they are all effectively points of light. Rather, the size of the dots indicates the relative apparent brightness of the stars; how much light soaked into the film while the shutter was open (think of water dripping onto a paper towel). And keep in mind that the apparent brightness of a star is not necessarily an indication of its distance; the intrinsic brightness of stars varies greatly.

But -- it's the colors of the stars that strike me here. Reddish-orange. Yellow. White. Blue. Like a cascade of jewels, or the rainbow of colors in a waterfall's mist.

The celebrated 19th-century British observer William Henry Smyth professed to see stars the color of sardonyx, damson and smalt, which suggests either especially perceptive vision or a vivid imagination. He listed a dozen shades of white, including pearly, lucid, creamy, silvery, and just plain whitely white. Smyth could have had a career as one of those folks who make up names on paint chips.

Smyth's almost exact contemporary, the Russian-German astronomer Wilhelm Struve, used Latin labels to classify star colors: egregie albae, albaesubflavae, aureae, rubrae, caeruleae, virides, purpureae, and even olivaceasubrubicunda, which translates as something like pinkish-olive. I'm not sure I've ever seen a pinkish-olive star, but maybe you can pick one out in the photograph. By the way, the brightest (double) star in the little cluster at left is named for Struve.

The color of stars tells us how hot they are -- red is cool, blue-white is hot. Match up the color of the star to the color of a filament in a clear light bulb and they'll be the same temperature.

Usually we think of stars as uniformly white, but that's because the human eye is not sensitive to color in dim light -- and maybe because we just don't look closely enough. Anyway, as I look at this photograph, I think of lines from Yeats poem "He Wishes for the Cloths of Heaven":
Had I the heavens' embroidered cloths,
Enwrought with golden and silver light,
The blue and the dim and the dark cloths
Of night and light and the half-light,
I would spread the cloths under your feet.
As now I do.