Colors of the cosmos: red, green, and blue may mean one thing to a scientist and something different to everybody else

Natural History, March, 2002 by Neil deGrasse Tyson

Only a few objects in Earth's nighttime sky emit or reflect enough light to trigger our retinas' color-sensitive cones. The red planet Mars can do it. So can the blue supergiant star Rigel (Orion's left kneecap) and the red supergiant Betelgeuse (Orion's right armpit). But aside from these standouts, the pickings are slim. To the unaided eye, space is a dark and colorless place.

Not until you aim large telescopes at it does the universe show its true colors. Glowing objects such as stars come in shades of red, white, and blue--a cosmic fact that would have pleased the Founding Fathers. Interstellar gas clouds can take on practically any color at all, depending on which chemical elements are present, whereas a star's color follows directly from its surface temperature: Cool stars are red. Tepid stars are white. Hot stars are blue. Very hot stars are ... still blue. How about very, very hot places, like the 15,000,000 [degrees] center of the Sun? Blue. To an astrophysicist, red-hot foods and red-hot lovers both leave room for improvement. It's just that simple.

Or is it?

A conspiracy of astrophysical law and human physiology just about rules out the existence of green stars. How about yellow stars? Some astronomy textbooks, many science fiction stories, and nearly every person on the street belong to the Sun-Is-Yellow Movement. Professional photographers, however, would swear the Sun is blue; daylight film is color balanced on the expectation that the light source (presumably the Sun) is strong in the blue part of the spectrum. The old blue-dot flash cubes were just one example of the attempt to simulate the Sun's blue light for indoor shots when using daylight film. On the other hand, painters with loft studios consider sunlight to be pure white, offering them the most accurate possible view of their pigments.

No doubt the Sun acquires a yellow-orange patina near the dusty horizon during sunrise and sunset. But at 12:00 noon, when atmospheric scattering is at a minimum, the color yellow does not spring to mind. Indeed, light sources that are truly yellow make white things look yellow. So if the Sun were pure yellow, then snow would look yellow--whether or not it had fallen near fire hydrants.

To an astrophysicist, "cool" objects have surface temperatures between 1,000 [degrees] and 4,000 [degrees] Kelvin and are generally described as red. Yet the filament of a white incandescent lightbulb cannot exceed 3,000 [degrees] Kelvin by much--tungsten melts at 3,680 [degrees]. Below about 1,000 [degrees], objects become dramatically less luminous in the visible part of the spectrum. Gaseous orbs with these temperatures happen to be failed stars. We call them brown dwarfs even though they are not brown and they emit hardly any visible light at all.

While we're on the subject, black holes aren't really black. Depending on its mass, a black hole can lose energy across the entire spectrum. In a process that resembles evaporation, black holes emit small quantities of light from their event horizons. Physicist Stephen Hawking was the first to describe this phenomenon, in which the evaporation rate increases as the black hole gets smaller, ending its life in a runaway flash of gamma rays.

Modern scientific images occasionally use a false-color palette. The meteorologists who make TV weather maps might denote heavy rainfall with one color and light rainfall with another. Or better yet, snow with one color, sleet with a second color, and rain with a third. When astrophysicists create false-color images of cosmic objects, they often assign an arbitrary sequence of colors to an image's range of brightness. The brightest parts might be red and the dimmest parts blue. So the colors you see bear no relation to the actual colors of the object. As in meteorology, some of these images have color sequences that relate to other attributes, such as the object's chemical composition or temperature. And it's not uncommon to see an image of a spiral galaxy that has been color coded for its rotation: the parts coming toward you are shades of blue, while the parts moving away are shades of red. In this case, the assigned colors evoke the widely recognized blue and red Doppler shifts that reveal an object's motion.

In the cosmic microwave background (the energetic remnants of the big bang), some areas are hotter than average. And, of course, some are cooler than average. The range spans a mere 0.00001 [degrees]. How do you display this fact on a map? Make the hot spots blue and the cold spots red or cold spots blue and hot spots red. In either case, a teeny fluctuation in temperature shows up as an obvious difference on the picture.

In other cases, we create a full-color image of a cosmic object by using invisible light, such as infrared or radio waves. What we do is assign the three colors to which the human retina is sensitive (red, green, and blue, or RGB for short) to three different parts of the spectrum. This way, we construct the full-color image we would see if we were born with the capacity to see colors in otherwise invisible bands.