Why Most Things Look Black-and-White In A Telescope

A color sugar maple lights up the roadside along County H near Webb Lake, Wis. Saturday afternoon. Bob King

I attended a star party in Wisconsin this past weekend during the prime of the fall color season. On the way down I pulled the car over more than once to take photos of the breathtaking maples and oaks set among dark pines. Our eyes delight in natural color. It gets a little motor spinning in the brain. Bright colors in nature jump out from the norm of brown, white, gray and green. And the more vivid and intense the hues the greater our delight.

Looking at photos of nebulas and galaxies we’re similarly struck by the incredible reds and greens of nebulae, the pink knots of star formation in a galaxy’s spiral arms and blue hues of starlight reflected in cosmic dust. But in real life, when gazing through a telescopes, these colors are nearly always absent. Almost everything appears in shades of gray even in larger amateur telescopes that most of us can’t afford.

The Orion Nebula, located about 1,500 light years from Earth, is home to many hundreds of newborn stars created from clumps of gas and dust contracting under the force of gravity. The stars excite the gases in the nebula and cause them to glow red and green. NASA/ESA

There are exceptions. Planets show shades of pink, red, yellow and even blue as do the brighter stars. A precious few of the brightest nebulae, including the famous Orion, glow green and red, but these hues are subtle and come nowhere near what photographs reveal. The brightest planetary nebulae, a group of nebulae named for their small, round shapes resembling planets, glow pale green and blue. Everything else appears in fuzzy shades of gray. The night sky is a black-and-white paradise.

A more subtle color experience — a sketch of the Orion Nebula at low magnification (64x) through a 15-inch telescope. In a smaller 6-inch scope, the bright inner region still looks faintly green, but the outer parts are wispy and gray. In larger telescopes the color is more obvious but never reaches the saturation and intensity of a photograph. Bob King

To appreciate the muted colors in astronomical objects you’ll need at least a 6- or 8-inch telescope. That’s generally where beginners start when buying a good scope, so beyond the planets and moon many are disappointed when they get their first look at everything else — galaxies, nebulae and even star clusters — they’ve grown accustomed to seeing in photographs.

Don’t curse your telescope. It’s just biology. Everything really does glow red, green and gold as Christmas but just not with enough intensity to activate the cone cells in our retinas that sense color. Instead, rod cells do the work. They’re excellent in low light and good at sensing motion but don’t sense color. During the day, our cones are busy serving up fall’s palette of colors, while at night the rods help us find our way in darkness.

Light moves through the eye to the retina which is lined with millions of elongated photoreceptor cells called cones and rods. There are three types of cones, each most sensitive to either blue, green or red, and require much more light to activate compared to the rods, which respond to low light but can’t distinguish color. © Arizona Board of Regents / ASU Ask A Biologist

Cameras are eyes that accumulate light. We see everything instant to instant, where a camera set to time exposure can gather light the way rain fills a bucket. The more light, the more color revealed, hence the reason photos of the Orion Nebula blow us away. Also, the electronic chip that records the picture can be more sensitive to certain kinds of light — especially red — than the human eye, further exaggerated the color effects. Of course, the problem with a photo is that beautiful as it might be, it’s only a facsimile of the real thing. And from our own personal experience we know that the real thing is always better and far more interesting than any copy even though it may lack the glitz and glitter.

Several small, bright planetary nebulae like NGC 6210 show pale pastel hues of green or blue in modest-sized telescopes. You’re looking at the expanding shell of a dying star set aglow by intense radiation from an Earth-sized white dwarf star at its center. DSS2

OK, how about taking a spaceship to the nebula to see it close up. Would that help? First off, seeing them in a telescope is like going part way in a ship, and we know that adds only a little color in some cases. These objects that fit so nicely in a telescope’s field of view are in reality so vast and diffuse that getting closer would only spread them more apart and dilute what color there is.

To see a star cluster or a nebula through a telescope is to viscerally experience the real stuff. Bright or faint, nothing can beat it.

Aurora update: Looks like Europe got a good aurora show last night. The solar blast arrived earlier than forecast during daylight hours for the U.S. By nightfall, activity was subsiding. Oh, well. Next time.

2 Responses

  1. Troy

    The follow up question I have is if you were to fly in your space ship, and get 100 times closer would you still see a black and white image? (The telescope of course is collecting a lot of photons you wouldn’t normally see when close up as well) Is there any distance where the Orion nebula would dazzle your eyes?

    1. astrobob

      Good question. No — large objects like Orion are too big and diffuse to show color even close up, though I imagine the greenish glow in the nebula’s core would be obviously pale green. I should probably include that in the blog in case others are wondering.

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